711
Augmented Tissue Oxygen Supply during Striated
Muscle Contraction in the Hamster
Relative Contributions of Capillary Recruitment, Functional Dilation, and
Reduced Tissue PO2
Bruce Klitzman, David N. Damon, Richard J. Gorczynski, and Brian R. Duling
From the Department of Physiology, School of Medicine, University of Virginia, Charlottesville, Virginia
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
SUMMARY. To investigate the relative contributions of alterations in blood flow, capillary density,
and tissue PO2 to elevated oxygen delivery in working muscle, we conducted experiments on the
suffused hamster cremaster muscle, using in vivo microscopic techniques. Muscle PO2 was measured
during striated muscle twitch contraction at 1 Hz. Tissue oxygenation was changed by using
suffusion solutions equilibrated with 0%, 5%, 10%, 21%, or 50% oxygen. Contraction caused an
increase in capillary density (capillary recruitment), whose magnitude was related to the equilibration
gas and, thus, to the suffusate PO2. Capillary recruitment first increased as the oxygen content was
raised, peaked with 10% oxygen, and then diminished with higher oxygen content. Arteriolar
functional dilation was also observed; when oxygen was raised above 21%, dilation was decreased.
The data suggest that oxygen supply is increased primarily by arteriolar conductance changes with
low suffusion solution oxygen (0% to 5%), and by capillary recruitment and increased PO2 gradients
above 10% oxygen. When vasomotor tone was increased by addition of norepinephrine to the
suffusion medium, the changes observed were similar to those observed when oxygen was increased.
Therefore, we propose that the altered microvascular responses during vasoconstriction are a
function of vascular tone rather than the levels of tissue PO2. A model is proposed which may
partially explain the relations among vascular tone, functional dilation, and capillary recruitment.
Our data also suggest that tissue PO2 may not be precisely regulated about a narrowly defined set
point in this striated muscle but that, instead, tissue PO2 is a dependent variable controlled by the
integrated effects of capillary recruitment, functional vasodilation, and altered metabolism. (Circ
Res 51: 711-721, 1982)
THE oxygen consumption of working striated muscle
is greater than that of resting muscle, and tissue
oxygen delivery is usually augmented to match the
increase in oxygen demand that occurs in the working
muscle. The coordination of vascular oxygen delivery
and tissue oxygen consumption is not fully understood, but at least three factors are known to contribute to the increased ability of the microcirculation to
supply oxygen to working muscle. These are: arteriolar dilation, which increases flow; capillary recruitment, which reduces diffusion distances within the
tissue; and decreased tissue PO2, which steepens oxygen diffusion gradients (Krogh, 1918; Gray et al.,
1967; Granger et al., 1976; Gorczynski and Duling,
1978; Gorczynski et al., 1978; Honig et al., 1980).
The relative contributions of these elements have
been reported to vary with different experimental
manipulations, such as sympathetic stimulation (Renkin and Rosell, 1962; Lewis and Mellander, 1962),
application of humoral agents (Jarhult, 1971; Alrura,
1973; Vetterlein and Schmidt, 1975; Katz, 1978) and
muscle contraction (Granger et al., 1976; Honig et al.,
1980). Also, relative effects on flow and capillary
exchange capacity during exercise may vary with the
level of muscular work. Renkin et al. (1966) and
Appelgren (1972) have reported that, with progressive
increases in muscle work, capillary exchange capacity
may reach a maximum and then begin to decline
while conductance continues to increase. Capillary
recruitment may also follow a much different time
course than does the flow increase in a working
muscle (Honig et al., 1980). In addition, it is reported
that the level of tissue oxygenation may influence the
vasomotor response. With a low venous PO2, oxygen
supply increments depend primarily on increased
blood flow. In contrast, with high venous PO2, augmentation of oxygen supply is accomplished to a
larger extent by capillary recruitment and a decrease
in the diffusion distance, accompanied by an increase
in oxygen extraction from the blood (Granger et al.,
1976).
Differential responses of arterioles and capillaries
are not easy to reconcile with the anatomy of the
microcirculation of striated muscle since direct microscopic examination of the origins of the capillaries
has failed to disclose any form of precapillary sphincters which might permit independent control of arterioles and capillaries (Wiedeman et al., 1976). Rather,
in this tissue, control of capillary patency and flow
seems to reside either in the terminal arterioles (Er-
Circulation Research/VW. 51, No. 6, December 1982
712
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
iksson and Myrhage, 1972; Gorczynski et al., 1978)
or, possibly to a lesser extent, in the postcapillary
vasculature (Klitzman and Johnson, 1982).
Examination of pressure distributions in the microcirculation leads us to the conclusion that the vascular
resistance is probably distributed over the entire vascular bed, including the terminal arterioles and largei
vessels (Fronek and Zweifach, 1975; Bohlen et al.,
1977). One must therefore ask how it is possible to
exert differential control of capillaries and arterioles
when the effector for such control is to some extent
the same for both microcirculatory elements—that is,
the smooth muscle of the terminal arterioles.
Those data which appear to show differential control of flow and capillary density have been derived
from indirect methods for assessing capillary density:
measurement of fluid filtration (Granger et al., 1976),
measurement of small molecular weight solute extraction by the capillaries (Renkin et al., 1966), or
histological presence of red cells in the capillaries
(Honig et al., 1980). We examined some of these
relations in a superfused microcirculatory preparation.
In vivo microscopic examination of transilluminated muscle provides a tool for directly observing
the postulated interactions between capillary recruitment and arteriolar dilation in effecting the delivery
of oxygen to the muscle. Furthermore, net tissue
oxygen delivery can be manipulated as an independent variable in a microcirculatory preparation by
altering the concentration of oxygen in a suffusion
solution. In addition, tissue PO2 may be measured at
precisely defined locations to determine the net effects of altered supply and demand. The purposes of
this investigation were to determine the relationships
between capillary and arteriolar responses during
muscle contraction and to determine the contribution
of tissue PO2 changes to oxygen delivery. Our findings led us to propose a physical mechanism for the
interaction between arteriolar diameter and capillary
recruitment and to suggest that tissue PO2 is not a
precisely regulated variable.
Methods
Animals and Perfusion Solutions
Male golden hamsters (100 to 150 g) were anesthetized
with an ip injection of sodium pentobarbital (70 mg/kg).
Deep esophageal temperature was maintained at 38°C by
conductive heating. Tracheal and femoral venous catheters
were inserted. Supplemental normal saline containing 9.0
mg/ml sodium pentobarbital was infused at 0.49 ml/hr,
which maintained a stable level of anesthesia and hydration.
Animals prepared in this fashion display stable cardiovascular and microcirculatory function for several hours, as
evidenced by stable microvessel tone and reactivity, as well
as stable femoral arterial blood pressure.
The cremaster muscle was prepared as described by
Gorczynski et al. (1978). It was pinned as a flat sheet on a
Lucite pedestal and suffused (approximately 5 ml/min) with
bicarbonate-buffered physiological saline with the following composition (mM): NaCl, 131.9; KCI, 4.7; CaCi2, 2.0;
MgSC, 1.2; and NaHCO3/ 20.0, at pH 7.35-7.40. The saline
was equilibrated with gas nominally containing 5% CO2,
various percentages of O2, and the balance N2.
The muscle was transilluminated using light from a
xenon arc lamp, monochromatically filtered at 436 nra and
viewed through a Leitz Laborlux II microscope equipped
with a Cohu television camera, Conrac video monitor, and
Sony EV 3650 videotape recorder. A time reference, generated by an Odetics digital timer accurate to 0.01 sec, was
recorded on each video field. The magnification of the
image was 1180X from tissue to TV monitor screen.
Oxygen tensions of solutions and tissues were measured
with recessed-tip oxygen microelectrodes (Whalen et al.,
1967) with outside tip diameters of 2-5 /im. For suffusate
PO2 measurements, the oxygen microelectrode was positioned in the solution 0.3 to 1.5 mm above the muscle. For
muscle PO2 measurements, the oxygen microelectrode was
positioned equidistant from the venous end of a pair of
capillaries, 50 to 150 /im below the muscle surface, a region
previously shown to reflect the minimum tissue PO2 (Gorczynski and Duling, 1978). Electrodes were calibrated
against N2 and room air at 34°C and measurements were
discarded if the calibrations before and after an experimental series differed by more than 10%. Tissue PO2 was also
measured several minutes after the circulation was arrested
to obtain a minimum PO2 of the superfused muscle as an
internal check on the zero reference for the oxygen electrode.
Muscle Stimulation
Phasic contractions of the striated muscle were used to
increase tissue oxygen demand. Contraction of the muscle
for 1 minute was induced by square wave pulses 0.02-0.05
msec in duration, at 6-18 V, and 1 Hz. These pulses were
applied across two Ag-AgCl macroelectrodes; the cathode
was positioned across the narrow proximal portion of the
cremaster, and the anode encircled the distal portion. The
amplitude of the stimulus voltage was adjusted to produce
twitch contractions yielding maximal displacement of the
tissue. In most experiments, the stimulation voltage was
increased 20% above the level producing maximal contraction.
Kjellmer (1965) has shown that motor nerves may be
stimulated at intensities well below threshold for vasoconstrictor fibers. However, more recently, vascular responses
related to stimulation of autonomic nerves have been reported by Hirst (1977), but not when low frequency (1 Hz)
electric field stimulation was used. Thus, we assume that
autonomic neurons were not likely to have been stimulated.
In a few experiments, small isolated groups of muscle
fibers were stimulated using 3 M NaCl-filled micropipettes
to deliver electrical pulses directly to the muscle fibers (0.1
to 1.0 msec at 4 Hz) as described previously (Gorczynski et
al., 1978).
Measurement of Capillary Density
Capillary density was estimated by counting the number
of capillaries containing flowing red cells which intersected
a 250-jim long reference line on the TV monitor. Fields for
observation of capillaries were not confined to any particular portion of the cremaster, with the exception of a band
of tissue approximately 2 mm wide at the tissue edge which
was rarely observed because of the presence of damaged
regions. Within the area studied, regions of obvious damage
or regions free of capillary flow were excluded from study.
The reference line was positioned perpendicular to the
Klitzman et a/./Capillary-ArterioIar Interactions
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
skeletal muscle fiber axis and, thus, to most capillaries.
Capillary density data were recorded in units of number of
capillaries per line (#/250 /um line) and #/mm 2 as described
previously (Klitzman, 1979). Number of capillaries per mm2
tissue cross-section was estimated from the depth of field
of the optical system and the length of the reference line.
The reference line and the depth of field of the objective
were assumed to form a plane, normal to the long axis of
the capillaries, with the dimensions 0.25 mm X 0.05 mm
= 0.0125 mm2. Dividing the number of capillaries with red
cell flow intersecting this plane by the area of the plane
yielded an estimate of capillary density with the units of
#/mm 2 . This method should yield data similar to those
obtained by sectioning the tissue perpendicular to the capillaries and counting the number of capillaries in the crosssection.
Measurement of the capillary density in contracting muscle was not practical because of excessive tissue movement.
Therefore, estimates for the steady state contraction values
of capillary density were obtained during the first 10 seconds after stimulation. We observed little change during
this brief interval in either the number or the rate of
perfusion of capillaries, and therefore assumed steady state
values.
Measurement of Arteriolar Diameter
Arteriolar diameters were measured on-line by positioning two parallel lines, generated by a modified Colorado
Video Image Analyzer, over the inner walls of the vessels
(Gorczynski et al., 1978). An output voltage proportional to
the separation of the two lines was recorded. The system
was accurate to ± 1 jim. Arterioles studied included thirdand fourth-order vessels, and values reported here are the
diameters at the end of 1 minute of stimulation.
Application of Vasoactive Agents
Since exposure to oxygen induces vasoconstriction, responses observed might have been due to either altered
oxygen delivery or to vasoconstriction. To evaluate the
specificity of oxygen, we used topical application of norepinephrine (NE) to induce a degree of vasoconstriction
equivalent to elevating the suffusion solution oxygen from
0% to one of the higher oxygen content gases. Appropriate
NE concentrations were achieved in the suffusion solution
by infusing 10~2 M NE into the line that carried the suffusion
solution to the tissue surface. The vasoconstrictor effect of
suffusion solutions equilibrated with 5 and 10% oxygen
were mimicked by applying NE in approximate concentrations of 0.2-1.0 /XM and 1-3 JUM, respectively. Since the NE
solution contained no anti-oxidant (e.g., ascorbic acid) and
was at room temperature for as much as 3 hours before
application, the true NE concentration that mimicked a
specific vascular response may have been somewhat lower.
We assessed the state of tone of the arteriolar smooth
muscle by observing the effect of topical application of a
drop of 10~4 M adenosine to the suffusate. This usually
elicited a rapid, transient arteriolar dilation; vessels without
tone were excluded from this study. This is not likely to
have induced significant bias in the mean data, since only
a small percentage of vessels in the preparations had no
tone. Under control conditions (rest, suffusion with 0%
oxygen), all arterioles appeared to be patent.
Statistics
Except as noted, unpaired Student's f-tests between
group means were evaluated at a significance level of 5%.
713
Results
General Observations
The microvascular architecture in the cremaster
muscle has been described by others (Smaje et al.,
1970; Gorczynski et al., 1978; Sarelius et al., 1981).
The capillaries throughout much of their length (498
± 250 /xm, SD) were roughly parallel to the muscle
fibers in their plane of travel. The relationship between capillary flow and arteriolar vasomotor activity
was complex, since arrest of capillary flow in one
capillary arising from an arteriole was often observed
at a time when the feed arteriole was patent and red
cells were still entering other daughter vessels originating both up and downstream from a branch capillary with arrested flow.
In the resting muscle, approximately half of all
capillaries were perfused with red cells. Of the remaining capillaries, platelet motion indicating plasma
flow could be seen in only a small fraction, and some
had stagnant plasma. Many of the capillaries with no
flow appeared to contain stationary red cells. Each
terminal arteriole gave rise to several capillaries, and
the ratio of perfused to unperfused capillaries varied
from one terminal arteriole to the next. Some variation
in perfused and unperfused capillaries occurred with
a time course of several minutes, but capillaries that
were perfused at one time in the resting muscle
generally remained perfused and those that were
initially unperfused remained unperfused.
Capillary Recruitment during Muscle Contraction
As nearly as could be ascertained, capillary density
increased within 15-30 seconds after the initiation of
1 Hz stimulation and reached a steady state within 45
seconds. Within 15-30 seconds after the stimulus
ended, capillary density began to decrease, and it had
returned to control after 60-120 seconds.
The capillary densities observed with the different
suffusate PO2 levels during rest and contraction are
graphed in Figure lA. Elevation of the suffusate PO2
. tended to decrease the capillary density significantly
in both the resting and contracting muscle.
Figure IB shows the effect of oxygen on the capillary recruitment; i.e., the difference in capillary density between the contracting and the resting state. An
enhanced capillary recruitment during muscle contraction was observed as solution oxygen contents
were increased up to 10% oxygen. However, as PO2
was elevated further, capillary recruitment diminished
strikingly.
Maximal capillary density was estimated by adding
10~4 M adenosine to the 0% oxygen suffusate during
8 Hz twitch contraction. This caused an increase in
capillary density to 9.1 ± 0.4/line or 728 ± 33/mm2
(SE). Therefore, the capillary densities observed during
1 Hz contraction were submaximal.
Functional Dilation of the Arterioles
Arteriolar diameters during striated muscle contraction are presented in Figure 2. When exposed to
Circulation Research/Vb/. 51, No. 6, December 1982
714
10 "A
_9
|8
*
7
1 5
-1800
(77) (26)
i
\(60
300
-480 »
240
-3201
160
<
! \\ .
a 43'
Ia. 2
E
-640
£
-
.
•
\
00
o
<-> I
i
1
I
0
0
10
20
50
Suffusion Solution 0 2 (%)
80
160 a
O
0
0
10
20
50
Suffusion Solution 0 2 (%)
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
FIGURE 1. Effects of altered suffusion solution oxygen content on
capillary density. Measurements made at rest (O) and during 1 Hz
stimulation of the muscle (0) are shown in panel A. Capillary
densities in the contracting muscles are significantly different from
the resting muscles in all solutions except 50% O?. The magnitude
of the increment in capillary density during the contraction (capillary recruitment) is shown in panel B. At each point, the data
represent the difference between the rest and the contraction
capillary density. Data points are means ± SE; numbers in parentheses are numbers of observations. The maximum capillary density is shown as the dashed line in the upper part of panel A.
0
10
20 "
50
Suffusion Solution 0 2 (%)
0
10
20 "
50
Suffusion Solution 0 2 (%)
FIGURE 2. Effect of altered suffusion solution oxygen content on
arteriolar diameter. Resting diameter is shown by the open circles
(O). Mean diameter was decreased significantly at all suffusate O2
contents. Diameter during contraction is shown by the closed
circles (0). In panel B, the magnitude of the increase in arteriolar
diameter during exercise, the functional dilation, is shown. Data
are mean ± SE; numbers in parentheses are numbers of observations.
the lowest suffusate PO2 (12 mm Hg, see Table 1) the
average arteriolar diameter was 18.6 ± 1.7 jum and
ranged from 10.9 to 28.8 fim. The maximal diameters
for these arterioles, determined under the same conditions as maximum capillary density, averaged 40.7
± 2.4 fim. Elevation of the suffusate PO2 caused a
reduction in the diameter of arterioles at rest and
during contraction (Fig. 2A). The functional dilations
(differences between the arteriolar diameters in the
resting and in the contracting muscle) are shown in
Figure 2B. The magnitude of the functional dilation
is diminished in the presence of the highest suffusate
PO2.
Relative conductances in arteriolar segments of the
cremaster muscle were estimated roughly from our
measurements of arteriolar diameter (conductance is
assumed to be proportional to diameter raised to the
fourth power; see Discussion) and these are shown in
Figure 3. The estimated relative conductances were
reduced significantly as the superfusate PO2 was elevated, both at rest and during contraction. The change
in conductance with exercise is also diminished as
solution oxygen is elevated (panel B). It is interesting
to note that, whereas the diameter change in Figure
2B is not influenced by changes in suffusion solution
oxygen from 0 to 21%, the estimated conductance
change is influenced significantly at all oxygen levels
(Fig. 3B). This reflects the fact that the same arteriolar
diameter increment is induced from a smaller resting
diameter as solution oxygen content is elevated.
TABLE 1
Effects of Suffusion Solution Oxygen Content on Tissue PO 2
Norepinephrine suffusion*
Percent oxygen in equilibration gas
Diam =
5%O 2
Diam =
10% O 2
280.5 ± 22.7
(11)
= 12
=12
62.9 ± 17.0
(10)
221.4 ± 29.8
(6)
9.6 ± 4.8
(4)
6.7 ± 4.2
(4)
22.3 ± 3.4
(26)
37.7 ± 12.3
(10)
154.2 ± 44.2
(6)
5.1 ± 2.5
(4)
5.1 ± 3.6
(4)
19.3 ± 2.6
(26)
25.1 ± 7.8
(10)
67.2 ± 25.9
(6)
4.6 ± 2.6
(4)
1.6 ± 2.0
(4)
0%
5%
10%
21%
50%
12.0 ± 2.3
(21)
42.5 ± 2.7
(19)
73.5 ± 2.9
(18)
133.2 ± 5.8
(14)
14.8 ± 2.6
(23)
24.4 ± 2.2
(25)
37.4 ± 6.0
(26)
Tissue (contraction)
9.2 ± 2.2
(23)
14.2 ± 2.6
(25)
Change in POif
5.7 ± 1.0
(23)
10.2 ± 1.5
(25)
Oxygen tension (mm Hg)
Solution
Tissue (rest)
Data are expressed as means ± SE. Numbers in parentheses = number of observations.
* Norepinephrine suffusion used to mimic O2 induced contraction (Fig. 2). Solutions containing norepinephrine were equilibrated with 0%
O2 and solution PO2 was approximately as in other experiments,
t Contraction minus rest.
Klitzman et a/./Capillary-Arteriolar Interactions
To assure that the arteriolar dilations which we
observed were not a complex response induced by
pressure redistribution secondary to upstream or
downstream microvessel dilation, we stimulated small
groups of muscle fibers, using microelectrodes. From
previous experiments, we knew that a local stimulus
frequency of 4 Hz causes a dilation similar to that
obtained with whole muscle stimulation at 1 Hz (Gorczynski et al., 1978). Figure 4 shows the results of the
localized stimulation experiments. The vessels constricted when the suffusate PO2 was elevated, both at
rest and during muscle contraction, but the functional
dilation during contraction was significantly smaller
(paired f-test, P < 0.05) when vessels were exposed to
5% oxygen than when exposed to 0% oxygen. The
reduced functional dilation with elevated oxygen was
seen only with much higher percentages of oxygen in
the case of whole cremaster stimulation (Fig. 2B).
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
Specificity of Oxygen
Elevated suffusate PO2 caused a significant reduction of capillary density and arteriolar diameter both
prior to and during muscle contraction (Figs. 1, 2, and
4). Therefore, the effects of altered suffusate PO2 on
capillary recruitment or arteriolar dilation might have
been either the specific result of altered availability of
oxygen to the tissues or a nonspecific result of the
vasoconstriction induced by the oxygen. For this reason, initial diameters and capillary densities were
reduced prior to muscle stimulation, using norepinephrine as a vasoconstrictor rather than oxygen. The
premise was that if the effects of elevated solution
PO2 could be mimicked by application of norepinephrine, then part or all of the interaction between
oxygen and the microcirculation might be attributable
715
to alterations in the initial contractile state of the
microvessels prior to striated muscle stimulation.
For each arteriole studied, the magnitude of the
constriction induced by oxygen was matched by appropriate concentrations of norepinephrine, and the
striated muscles then were stimulated. The results of
these experiments are shown in Figure 5 and in the
righthand group of bars in Figure 4. The functional
dilation was diminished by constriction induced by
either oxygen or norepinephrine. This was seen during stimulation of both small muscle fiber groups (Fig.
4) and of the whole cremaster (Fig. 5).
The results of similar studies on capillary density
are shown in Figure 6, in which capillary recruitment
during vasoconstriction induced by oxygen is compared with recruitment observed during vasoconstriction induced by norepinephrine. The diminution of
resting capillary density and the subsequent alteration
of capillary recruitment were almost identical whether
resting capillary density was reduced by norepinephrine or by oxygen.
Tissue PO2
Tissue PO2 was measured, both as an index of the
efficacy of the regulatory process, and as one parameter which determines tissue oxygen diffusion gradients. In Table 1, tissue PO2 measurements are presented as a function of the oxygen content of the gas
equilibrated with the suffusion solution. Also, the
PO2 measurements taken during application of norepinephrine in a 0% oxygen solution are shown for
the resting and working muscles.
[~lRest
V/////A Contraction
tgfi-:t Functional Dilation
I
20
E
o
•6 400
°o '5
o
JD 10
(ll
(il)
J> 100 -
0
10
20
"
50
Suffusion Solution 0 2 (%)
2(5
'I
50
Solution 0 2 (%)
FIGURE 3. Estimated effect of suffusion solution oxygen content on
arteriolar conductance at rest and during muscle contraction. Diameter measurements shown in Figure 2 were raised to the fourth
power. Data are expressed as the percent of the conductance
observed with the 0% oxygen solution at rest. Errors were calculated
according to Bevington (1969). In panel A, elevation of suffusion
solution oxygen content reduces the conductance of the arterioles
of the working muscle more than the arterioles of the resting
muscle. The result is a smaller conductance change during work.
Panel B shows the estimated change in conductance associated with
various oxygen contents. Elevation of the suffusion solution oxygen
content diminishes the magnitude of the conductance change.
0%0*
5%
0%02
NE=5%
FIGURE 4. Effect of changes in suffusion solution oxygen content or
of norepinephrine application on arteriolar diameter response to
localized stimulation. Experiments were carried out by stimulating
small groups of muscle fibers (3 to 5) with NaCl-filled micropipettes. The open bars represent the arteriolar diameter in resting
muscle. The hatched bars represent the absolute diameters during
striated muscle stimulation, and the stipled bars represent the
change in diameter associated with muscle contraction (functional
dilation). The magnitude of the functional dilation is reduced
significantly with the contractions associated with either application
of oxygen or norepinephrine (P < 0.05, paired difference test).
Circulation Research/Vo/. 51, No. 6, December 1982
716
-70 r
,
30
I"
|
20
o
s
15
o
o
g 10
5
0
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
10 20
50
% 0 2 Equivalent of
Equivalent of
Norepinephrine Contraction
Norepinephrine Contraction
FIGURE 5. Effect of norepinephrine application on functional vasodilation. Norepinephrine was applied in quantities sufficient to
produce a constriction of vascular diameter approximately equal to
that induced by the oxygen content shown on the abscissa. The
effect of such a reduction in arteriolar diameter during contraction
is shown in panel B. Symbols are the same as in Figure 2.
Elevation of the suffusate PO2 caused an elevation
of the tissue PO2 in both the resting and contracting
muscles as a result of a greater flux of oxygen from
solution to tissue. The tissue PO2 in the working
muscle was somewhat less sensitive to suffusion solution gas composition than the PO2 in the resting
muscle. This fact was reflected in a fall in tissue PO2
during contraction which was proportional to the
resting tissue PO2. This relationship is reflected in
Figure 7, where the fall in tissue PO2 during contraction increases as a function of the resting tissue PO2
prior to muscle stimulation. It should be noted that
the proportionality between resting tissue PO2 and
the contraction-induced decrease in tissue PO2 appears to extend even to observations made during
• Rest
EH Contraction
10
10% 0 ,
5% Ot
c
^ 8
•
;•
6
1. 2
o
l
0% o2 5%
0%0 2
0% 0*
10% O2
0% Ot
NE=5%
FIGURE 6. Effects of oxygen and norepinephrine on capillary density
in resting and contracting muscle. Norepinephrine was applied to
the cremaster in a dose sufficient to produce a decrease in resting
muscle capillary density equivalent to that observed with either 5%
oxygen, (panel A) or with 10% oxygen (panel B). Stimulation of the
muscle (hatched bars) produced a capillary recruitment whose
magnitude was related to the initial vascular state.
10
20
30
40
50
60
70
220
Rest Tissue P0 2 (mmHg)
FIGURE 7. Relation between the change in PO2 with muscle contraction and the resting tissue PO2 • Squares represent data obtained
with changes in suffusion solution oxygen content. Shaded region
and triangles represent data which were obtained during suffusion
with NE solution in a concentration sufficient to reduce vessel
diameters to those seen with either 5 or 10% oxygen (see Table 1).
application of norepinephrine (lower left section of
Fig. 7, triangles). Application of norepinephrine in a
concentration sufficient to mimic 10% oxygen suffusion caused a statistically significant reduction in the
change in PO2 observed during contraction, when
compared to the response observed in 0% oxygen.
Discussion
In this study, we provide direct evidence that the
relative contribution of different microvascular elements to control of tissue oxygen supply may vary,
depending on the initial state of vascular tone and not
necessarily on the level of tissue oxygenation. We
find that, during striated muscle stimulation, the
change in diameter (Figs. 2, 4, and 5) and estimated
conductance (Fig. 3) is less in constricted vessels than
in dilated vessels. By inference, we assume that this
would have been reflected in a smaller functional
hyperemia had we measured flow. In contrast, the
capillary recruitment is first increased by elevations
of solution oxygen content, and only at very high
oxygen is the recruitment reduced (Figs. 1 and 6).
To compare our findings on arteriolar diameter
with previous reports using flow measurement, we
assumed a simple, direct proportionality between
conductance and the fourth power of the arteriolar
diameter. We recognize that this assumption has been
only partially validated (Lipowsky et al., 1980), and
may not apply in all cases. Furthermore, we have
assumed that total conductance is determined primarily by the diameter of the arterioles (Fronek and
Zweifach, 1975) but it is clear that, under some conditions, other vascular segments can also influence
total vascular conductance (Abboud, 1972). With
these cautions in mind, however, some insights may
be obtained from the computation.
Klitzman et a/./Capillary-Arteriolar Interactions
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
Our findings support other reports that a low tissue
PO2 (presumably causing a more dilated initial state)
is a common feature of a more precise balance between flow and oxygen demand in striated muscle
(Stainsby, 1962; Jones and Berne, 1965; Granger et al.,
1976; Sullivan and Johnson, 1981). However, the results are not consistent with the idea that the level of
tissue oxygenation per se is the main determinant of
the regulatory pattern. The data suggest that it is not
the level of tissue oxygenation that determines the
vasomotor response but, rather, the state of vascular
tone. The data on which this conclusion is based are
shown in Figures 2, 4, and 5, and in Table 1. When
suffusate PO2 was elevated, the tissue received additional oxygen from the solution, tissue PO2 was elevated, and less oxygen would have been required
from the vascular route to meet tissue metabolic
needs. Associated with this was a vasoconstriction
induced by local regulatory processes (Figs. 2, 4, and
5). Norepinephrine, applied in a dose chosen to mimic
the arteriolar constriction induced by oxygen, also
caused reduced functional dilation, but resting tissue
PO2 decreased (Table I). Thus, oxygen and norepinephrine had identical effects on the initial vascular
tone of the arterioles, but opposite effects on the
tissue oxygenation. However, constrictions induced
by both agents reduced the magnitude of the functional dilation. Thus, diminished functional dilation
was correlated with a reduction of initial (resting)
arteriolar diameter, not with a reduction in the initial
tissue PO2.
The parallel between vasomotor tone (rather than
tissue oxygenation) and vascular response to exercise
is emphasized further by the data of Figures 1 and 6.
The degree of capillary recruitment during contraction of the muscle was related to the initial capillary
density, whether constriction was induced by oxygen
or by norepinephrine. Both constrictions caused a
reduction in the capillary density of resting muscle
and both augmented capillary recruitment during exercise, despite opposite effects on tissue oxygenation
(Table 1). Thus, these data also suggest that the initial
state of the vasculature is the common feature determining a regulatory response, not the level of tissue
oxygenation.
Our observations on both the functional dilation of
arterioles and on capillary recruitment during stimulation emphasize the need for considering the initial
state of the vasculature in evaluating the vascular
response to an experimental intervention. A number
of other experimental findings show that the initial
resistance, capillary density, and subsequent vasomotor response are related in complex ways (Emerson
et al., 1973; Myers et al., 1975; Granger et al., 1976),
and these facts emphasize that analysis of vascular
function must be carried out under defined initial
conditions of the vasculature and, wherever possible,
constant initial conditions.
In this regard, it should be noted that the capillary
reserve as well as the degree of capillary recruitment
are highly dependent on the level of tisssue oxygenation (Fig. 1) and on the level of vasomotor tone (Figs.
717
1 and 6) at the time of stimulation. Therefore, discrepant reports on capillary reserve and recruitment
(Krogh, 1919; Gorczynski et al., 1978; Lindbom et al.,
1980; Sarelius et al., 1981; Hudlicka et al., 1982)
should be reexamined in this light. It may be that
these disparities reflect only differences in the state
of the vascular bed at the time of study.
The mechanism that links the magnitude of a functional dilation and the initial vascular diameter is
unknown. It may be that our findings reflect the fact
that the smooth muscle of the constricted arterioles
operates at different points on its stress-strain curve
0ohnson, 1968; Gore, 1972; Herlihy and Murphy,
1973), and thus arteriolar reactivity is changed. Gore
(1972) has proposed that vasomotor tone may alter
the initial wall stress in relation to some optimal stress
level and thereby change the magnitude of the diameter response to a stimulation. Alternatively, the
initial constriction may be interpreted to have altered
the initial length of the contractile elements rather
than wall stress, and this may have been the key
factor in reducing the subsequent response. Further
experimental work in this regard is necessary, as the
role of arteriolar length-tension relations in vascular
reactivity remains an almost unexplored area at this
time. It is clear, however, that under controlled conditions, the initial state of an arteriole can determine
arteriolar reactivity (Baez et al., 1967; Gore, 1972).
The diminished functional dilation induced by vasoconstriction (Figs. 2, 4, and 5) might also reflect a
true alteration in microvessel sensitivity as a result of
the accumulation of some metabolite during the period of reduced flow associated with the vasoconstriction. Our data are insufficient to assess this possibility.
In the absence of precapillary sphincters, it must
be assumed that the shift of response dominance
between arterioles and capillaries shown in Figures 1
and 2 reflects in some way the activity of arterioles
(Eriksson and Myrhage, 1972; Gorczynski et al., 1978;
Lindbom et al., 1980). It might be proposed that the
different effects on capillary perfusion and total flow
resistance may derive from the fact that a significant
fraction of the total resistance lies in arteriolar vessels
from which capillaries do not originate; that is, second- and third-order arterioles (Fronek and Zweifach,
1975). Since capillaries in this preparation originate
largely from third- and fourth-order arterioles, the
findings reported here might to some extent reflect
opening and closing of third- and fourth-order vessels
in response to the various stimuli applied, or what
might be termed "arteriolar recruitment." If the
fourth-order vessels represent only a small part of the
total resistance, then closure of a fraction of them
would not greatly affect total resistance (Mayrovitz et
al., 1977). Our findings might then reflect higher
sensitivity of fourth-order vessels to exercise than of
other arterioles. Final conclusions on this possibility
will require a detailed analysis of comparative reactivities of the arterioles at all levels of the arteriolar
tree under one set of conditions.
We can conclude from our data that regulation of
Circulation Research/Vo/. 51, No. 6, December 1982
718
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
capillary density by closure of terminal arterioles is
not the sole factor involved in the responses we
observe, however. Several facts lead us to this conclusion. First, a greater sensitivity of the terminal arterioles to functional dilation was not observed in earlier
work with this preparation by Gorczynski et al. (1978).
Second, over the low end of the PO2 range shown in
Figures 1 and 2, closure of arterioles was an infrequent
observation, and, thus, a segmental analysis would
not likely produce a major alteration in the conclusions reached here. Another argument against closure
of terminal arterioles as the sole determinant of capillary density is the fact that many of the vessels
observed in the present work were parent vessels for
capillaries. We often observed that red cells stopped
entering individual capillaries originating from an
arteriole when flow in the parent arteriole was still
brisk, and while red cells continued entering capillaries upstream and downstream from the unperfused
capillary. It thus is necessary to reconcile the facts
that: (1) terminal arterioles need not close to regulate
capillary flow; (2) terminal arterioles appear to contribute significantly to vascular resistance; and (3)
flow control seems to be at least qualitatively different
from capillary density control.
There is some evidence that capillaries from amphibians may contract. (Weigelt et al., 1981) but this
has not been observed in mammals. Occlusion of
vessels by either red cells or white cells is another
possible means for blocking capillary flow (Slaaf and
Wiederhielm, 1982; Schmidt-Schoenbein and Engler,
1982), but it is not obvious that these effects should
vary in proportion to the state of arteriolar tone. It is
also possible that the differential responses of capillary recruitment and functional dilation are related to
shifting hydrostatic pressure gradients across the capillaries as a result of changes in arteriolar resistance.
Data are not available to assess this possibility.
On the basis of recent preliminary observations
made on in vitro isolated cannulated arterioles (Duling et al., 1981; R. Dacey, personal communication),
we can propose a hypothetical explanation for an
interaction between arterioles and capillaries which is
not usually considered. It is occasionally possible to
observe the orifice of small microvessels which branch
from a cannulated arteriole; the orifice diameter varies
with the diameter of the parent arteriole. An example
of measurements made on such a preparation is
shown in Figure 8. These data were obtained from a
maximally dilated vessel and, thus, represent purely
passive behavior of the orifice of a microvessel as a
function of the internal diameter of the parent vessel
(terminal arteriole and capillary, for example). As
would be expected, as the internal diameter of the
parent vessel increased, the size of the orifice increased as well, although nonlinearly. This observation has suggested to us a mechanism for regulation
of capillary flow which is diagrammed in Figure 9.
Suppose that, in the maximally dilated state, most
capillary origins are large enough to permit red cells
to enter with a minimum amount of deformation
60
70
80
90
Arteriolar Internal Diameter
100
FIGURE 8. Effect of changes in arteriolar diameter on the size of the
origin of a microvessel branching from a parent vessel. The parent
arteriole was isolated and cannulated as described by Duling et al.
(1981). Orifice diameter was changed by inflation of the parent
vessel. 0 represents diameter at 60 mm Hg, or approximately
normal pressure.
(capillary perfusion = 100%). As the arterioles constrict, the diameters of the capillary orifices decrease
progressively, requiring greater deformation of red
cells entering the capillary. Ultimately, some of the
capillary orifices will become too small for red cells
to enter. Assuming some population distribution of
red cell size and capillary orifice size, the reduction in
the number of capillaries perfused should be progressive and graded.
In Figure 9 we have approximated the shape of the
regulation between orifice diameter and arteriolar
diameter shown in Figure 8. The drawing is scaled so
that, as maximal arteriolar diameter is approached, all
of the capillaries originating from the arteriole are
assumed to be perfused. Analysis of Figure 9 allows
one to predict some of the interrelations which we
have shown between arteriolar diameter and capillary
density. Three different resting (R) and contracting
100
Capillory Recruitment
(0% 02>
.2 °
Capillary Recruitment
(10% O2)
>-'o
o
o
"1 Capillary Recruitment
l(2i%o2)
Arteriolar
Diameter (% Maximal)
,
100
FIGURE 9. Hypothetical relation between arteriolar diameter and
capillary perfusion. It is assumed that capillary perfusion is determined by the diameter of the capillary orifice as shown in Figure
8. "R" represents the rest value of the arteriolar diameter at three
different initial states of vasoconstrictor tone. "C" represents the
diameter during muscle contraction. It is assumed that stimulation
of the striated muscle causes similar dilation of the arteriole from
each of the resting diameters (Fig. 2). Predicted capillary recruitment is not uniform but shows a maximum in the mid-portion of
the arteriolar diameter range.
Klitzman et a/./Capillary-Arteriolar Interactions
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
(C) diameters are plotted on the hypothetical curve.
It is assumed for this case that the functional dilation
causes the same increase in diameter regardless of the
initial diameter of the vessels (Fig. 2B, low PO2 range).
The predicted increments in capillary density corresponding to the changes in diameter are indicated by
the solid vertical lines for each of the rest-contraction
transitions. Because of the shape of the curve, the
theoretical increment in capillary density is smallest
at the upper and the lower regions of the curve and
largest in the center. Note that this is the relation
expected from Figures 1 and 2. Thus, at least qualitatively, the apparent interaction between capillaries
and arterioles could be explained by very simple
geometric considerations or by what might be called
a "passive sphincter" mechanism.
Although the analysis proposed in Figure 9 is obviously a simplification, it serves two useful purposes.
First, it emphasizes that there is likely to be a relation
of the general form shown in Figures 8 and 9 which
will determine, in part, how arteriolar diameter and
capillary density interact. Second, it partially obviates
the need for a precapillary sphincter as classically
defined, which has not been found, while allowing
capillary density to change somewhat independently
of arteriolar flow. Figure 9 also highlights the fact that
a complex interaction observed between microvessel
elements need not reflect precise feedback between
the two separate elements and the tissue. Instead, it
may be that our findings represent an intrinsic behavior pattern generated in part simply by the anatomy
of the microcirculation.
Oxygen delivery to contracting striated muscles
may be increased by at least three processes: increased
flow, increased capillary density, and decreased tissue
PO2. Our findings lead to the conclusion that the
degree of participation of each of these in regulation
depends on the initial state of the vasculature. At high
levels of tone, the changes in capillary density and
tissue PO2 are large, whereas, with low levels of tone,
the change in flow predominates. The efficacy of
these processes in matching supply to demand may
be estimated by examining Table 1 and Figure 7. First,
it is apparent, in Table 1, that in no case is regulation
of tissue PO2 perfect. Elevation of suffusate PO2
causes vasoconstriction and reduced capillary density,
thereby reducing oxygen input to the system via
vascular sources, but this does not prevent a rise in
tissue PO2, either in the resting or in the contracting
muscle. Above 21% oxygen, the tissue PO2 begins to
rise steeply with elevations in suffusate oxygen and,
in fact, in this region the tissue PO2 change is not
different than would be observed in an unperfused
cremaster muscle (Klitzman, 1979). This might be
expected, however, since in this region the vessels are
so greatly constricted by the elevated oxygen that
diameter (Fig. 2) and blood flow (Lindbom et al., 1980)
to striated muscle may be reduced almost to zero. In
this context, it should be noted that the range of
suffusate PO2's between 0 and 21% oxygen include
the range of physiological oxygen tensions.
719
When the striated muscle contracts, the tissue PO2
falls in all cases (Fig. 7), thus demonstrating that the
combination of functional dilation and capillary recruitment is inadequate to meet the elevated oxygen
needs of this tissue without a reduction in tissue PO2.
However, the reduced PO2 is functional in a sense, as
this steepens the diffusion gradient for oxygen and
increases the tissue oxygen delivery.
If the cremaster were unperfused and the resting
oxygen consumptions were the same at all levels of
tissue oxygenation, then diffusion theory would predict that the magnitude of the decrease in tissue PO2
should be the same, regardless of the initial PO2. The
magnitude of the reduction should be directly proportional to the ratio of the oxygen consumption to
the Krogh diffusion coefficient 0acobs, 1967; Klitzman et al., in press). We have carried out a few
experiments with very high oxygen compositions
(21%, 50%, 95% oxygen) and these appear to fit the
predictions of diffusion theory (unpublished observations; Klitzman et al., in press).
It is therefore somewhat curious that the contraction-induced fall in tissue PO2 was closely related to
the initial tissue PO2 (Fig. 7). This relation appeared
to hold for both the experiments in which tissue PO2
was raised by changes in suffusion solution oxygen,
and those in which tissue PO2 was lowered by application of norepinephrine (Fig. 7, shaded area). A
similar, though weaker, correlation has been observed
to extend across animals of various ages (Proctor et
al., 1981). This finding is remarkable because it is not
what would be predicted from simple diffusion theory, as mentioned above.
These facts are worth noting, since a possible explanation for the behavior shown in Figure 7 is that
the increase in oxygen consumption of the cremaster
muscle during stimulation is proportional to the ambient PO2, i.e., the consumption of oxygen by the
working muscle might be supply limited. This possibility should be examined in future work, as it would
substantially complicate our understanding of the
regulatory process.
At the low end of the PO2 range, which should be
of greater physiological relevance, the slope relating
tissue PO2 and suffusate oxygen content is more
shallow (Table 1). This end of the range shows the
combined effects of increasing blood flow and decreasing diffusion distance (Figs. 1 and 2) as the
solution PO2 falls. In contrast to earlier work (Duling,
1972), tissue PO2 was not constant either in the resting
or in the contracting muscles as the suffusate PO2 was
altered. Thus, either the muscles were incapable of
regulating PO2 in the face of the altered solution PO2,
or alternatively, tissue PO2 is poorly controlled in this
striated muscle. It should be pointed out that our
results were obtained from preparations which were
stimulated at relatively low frequencies and on a
muscle which has a high population of glycolytic
fibers (Sarelius, Maxwell, Gray, Duling, unpublished
observations). The work load and associated oxygen
consumptions at 1 Hz are likely to have been low, and
720
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
this may have influenced our findings. It would be of
interest to explore the interactions reported here at
higher work loads and in more oxidative tissues.
The results shown in Table 1 also provide some
hints about the nature of the oxygen-linked feedback
system relating vascular responses and tissue oxygenation. First, the data suggest that it may be inappropriate to consider tissue PO2 a regulated variable in
the idealized sense of a proportional or integral control model with a predetermined set point (Granger
and Shepherd, 1979). This follows from the fact that
tissue PO2 is not fixed and held at a precise level as
oxygen availability changes. Rather, the tissue-vessel
feedback seems to buffer the change in PO2 instead
of seeking a single value of tissue PO2 (i.e., a set
point). Furthermore, the tissue PO2 is not held constant during either norepinephrine suffusion or muscle contraction. The vessels clearly possess the capacity to respond to tissue oxygenation in both cases, but
there is no real evidence that a fixed reference tissue
PO2 is sought. This concept is quite consistent with
a previous observation made on the cheek pouch
microcirculation during alterations in suffusate oxygen and/Carbon dioxide (Duling, 1973).
Thus, we feel that the data are consistent with the
idea that there is a feedback between tissue oxygenation and vascular tone, but the feedback simply
increases vessel tone as tissue PO2 rises; it does not
seek a particular level of tissue oxygenation. It may
be more appropriate to view the smooth muscle of
the arteriole as analogous to a summing amplifier,
with tissue PO2 being one of a multitude of neural
and hormonal inputs acting in concert to determine
overall vascular behavior.
The second fact which emerges from consideration
of the data in Table 1, in combination with the data
showing the arteriolar sensitivity to oxygen in Figure
2, is that the vasoconstrictor response does not appear
to be explicable on the basis of some critical tissue
element hovering on the brink of anoxia (Thews,
1960). Vasoconstriction was seen over the full range
of tissue PO2. The location chosen for our electrodes
in these studies was a point at which the minimum
PO2 in the tissue was observed, and thus, we can
conclude that the muscle circulation can be influenced
by oxygen at striated muscle cell PO2 levels as high
as 30-40 mm Hg. Isolated mitochondria appear to be
limited by oxygen availability only at PCVs less than
1 mm Hg (Chance, 1969), and thus, simple limitation
of oxygen availability as a factor in regulation is
unlikely to occur until the solution oxygen falls below
5%. Even in the 0% oxygen solution, a gradient of
almost 15 mm Hg would have to exist between the
electrode measuring site and the mitochondrion to
lower PO2 at the cytochrome chain to the required
critically low values. Three plausible explanations for
these data may be advanced. First, some chemical
process other than the cytochromes may be acting in
the feedback between oxygen and vessel tone. Second, the critical PO2 for mitochondria in intact skeletal muscle may be different than is thought 0obsis,
Circulation Research/Vo/. 51, No. 6, December 1982
1972; Wilson et al., 1979). Third, there may be steep
local gradients in PO2 within the cells, for example,
due to clustering of mitochondria (Hoppeler et al.,
1981) or unexpectedly low diffusion coefficients
within the cells (Longmuir et al., 1978). We cannot
distinguish these possibilities, and future experiments
aimed at answering these questions more directly are
needed.
This study was supported by NIH Grants HL 12792 and HL
05815. The work was conducted during Dr. Duling's tenure as an
Established Investigator of the American Heart Association.
Preliminary reports of this work appeared in: Adv. Physiol. Sci.
(Hungary) 7: 1-16, 1980, Fed. Proc. 36: 451, 1977, and The Physiologist 20: 51, 1977.
The present address for Dr. Klitzman is: Department of Physiology and Biophysics, School of Medicine, Louisiana State University at Shreveport, Shreveport, Louisiana 71130.
Dr. Corczynski's present address is: American Critical Care,
1600 Waukegan Road, McCaw Park, Illinois 60085.
Address for reprints: Brian R. Duling, Box 449, Department of
Physiology, University of Virginia School of Medicine, Charlottesville, Virginia 22908.
Received July 6, 1982; accepted for publication September 9,
1982.
References
Abboud FM (1972) Control of the various components of the
peripheral vasculature. Fed Proc 31: 1226-1239
Altura BM (1973) Selective microvascular constrictor actions of
some neurohypophyseal peptides. Eur ] Pharmacol 24: 49-60
Appelgren KL (1972) Capillary transport in relation to perfusion
pressure and capillary flow in hyperemic dog skeletal muscle in
shock. Eur Surg Res 4: 211-220
Baez S, Kopman AF, Orkin LR (1967) Microvascular hypersensitivity subsequent to chemical denervation. Circ Res 20: 328-337
Bevington PR (1969) Data Reduction and Error Analysis for the
Physical Sciences. New York, McGraw-Hill, p 336
Bohlen HG, Gore RW, Hutchins PM (1977) Comparison of microvascular pressures in normal and spontaneously hypertensive
rats. Microvasc Res 13: 125-130
Chance B, Thurman RG, Gosalvez M (1969) Oxygen affinities of
cellular respiration. Forsvarsmedicin 5: 235-243
Duling BR (1972) Microvascular responses to alterations in oxygen
tension. Circ Res 31: 481-489
Duling BR (1973) Changes in microvascular diameter and oxygen
tension induced by carbon dioxide. Circ Res 32: 370-376
Duling BR, Gore RW, Dacey RG Jr, Damon DN (1981) Methods
for the isolation, cannulation and in vitro study of single microvessels. Am J Physiol 241: H108-H116
Emerson TE Jr, Meier PD, Daugherty RM Jr (1973) Dependence of
skeletal muscle vascular response to serotonin upon the level of
vascular resistance. Proc Soc Exp Biol Med 142: 1185-1188
Eriksson E, Myrhage R (1972) Microvascular dimensions and blood
flow in skeletal muscle. Acta Physiol Scand 86: 211-222
Fronek K, Zweifach BW (1975) Microvascular pressure distribution
in skeletal muscle and the effect of vasodilation. Am J Physiol
228: 791-796
Gorczynski RJ, Duling BR (1978) Role of oxygen in arteriolar
functional vasodilation in hamster striated muscle. Am J Physiol
235: H505-H515
Gorczynski RJ, Klitzman B, Duling BR (1978) Interrelations between contracting striated muscle and precapillary microvessels.
Am J Physiol 235: H494-H504
Gore RW (1972) Wall stress: A determinant of regional differences
in response of frog microvessels to norepinephrine. Am J Physiol
222: 82-91
Granger HJ, Shepherd AP (1979) Dynamics and control of the
Klitzman et a/./Capillary-Arteriolar Interactions
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
microcirculation. Adv Biomed Eng 7: 1-63
Granger HJ, Goodman AH, Granger DN (1976) Role of resistance
and exchange vessels in local microvascular control of skeletal
muscle oxygenation in the dog. Circ Res 38: 379-385
Gray SD, Carlsson E, Staub NC (1967) Site of increased vascular
resistance during isometric muscle contraction. Am J Physiol
213: 683-689
Herlihy JT, Murphy RA (1973) Length-tension relationship of
smooth muscle of the hog carotid artery. Circ Res 33: 275-283
Hirst GDS (1977) Neuromuscular transmission in arterioles of
guinea pig submucosa. J Physiol (Lond) 273: 263-275
Honig CR, Odoroff CL, Frierson JL (1980) Capillary recruitment in
exercise: rate, extent, uniformity, and relation to blood flow. Am
J Physiol 238: H31-H42
Hoppeler H, Mathieu O, Krauer R, Classen H, Armstrong RB,
Weibel FR (1981) Design of the mammalian respiratory system.
VI. Distribution of mitochondria and capillaries in various muscles. Respiration Physiol 44: 87-111
Hudlicka O, Zweifach BW, Tyler KR (1982) Capillary recruitment
and flow velocity in skeletal muscle after contractions. Microvasc
Res 23: 201-213
Jacobs MH (1967) Diffusion Processes. New York, Springer-Verlag
Jarhult J (1971) Comparative effects of angiotensin and noradrenaline on resistance, capacitance, and precapillary sphincter vessels in cat skeletal muscle. Acta Physiol Scand 81: 315-324
Jobsis FF (1972) Oxidative metabolism at low PO 2 . Fed Proc 31:
1404-1413
Johnson PC (1968) Autoregulatory responses of cat mesenteric
arterioles measured in vivo. Circ Res 22: 199-212
Jones RD, Berne RM (1965) Evidence for a metabolic mechanism
in autoregulation of blood flow in skeletal muscle. Circ Res 17:
540-554
Katz MA (1978) Dissociation of acetylcholine-induced responses
of arteriolar resistance vessels and precapillary sphincter-like
arterioles. Microvasc Res 16: 133-140
Kjellmer I (1965) Studies on exercise hyperemia. Acta Physiol
Scand 64 (suppl 244): 1-27
Klitzman B (1979) Microvascular Determinants of Oxygen Supply
in Resting and Contracting Striated Muscle. Dissertation, University of Virginia. University Microfilms #8002544, Ann Arbor,
Michigan
Klitzman B, Johnson PC (1982) Capillary network geometry and
red cell distribution in hamster cremaster muscle. Am J Physiol
242: H211-H219
Klitzman B, Poppel AS, Duling BR (in press) Oxygen transport in
resting and contracting hamster cremaster muscle: Experimental
and theoretical microvascular studies. Microvasc Res
Krogh A (1918-1919) The number and distribution of capillaries in
muscles with calculations of the oxygen pressure head necessary
for supplying the tissue. J Physiol (Lond) 52: 409-415
Lewis DH, Mellander S (1962) Competitive effects of sympathetic
control and tissue metabolites on resistance and capacitance
vessels and capillary filtration in skeletal muscle. Acta Physiol
Scand 56: 162-188
Lindbom L, Tuma RF, Arfors KE (1980) Influence of oxygen on
perfused capillary density and capillary red cell velocity in rabbit
skeletal muscle. Microvasc Res 19: 197-208
Lipowsky HH, Usami S, Chien S (1980) In vivo measurements of
721
"apparent viscosity" and microvessel hematocrit in the mesentery of the cat. Microvasc Res 19: 297-319
Longmuir IS, Knopp JA, Lee T-P, Benson D, Tang A (1978) The
intracellular heterogeneity of oxygen concentrations as measured
by ultraviolet television microscopy of PBA fluorescence quenching by oxygen. Adv Exp Med Biol 94: 93-97
Mayrovitz HN, Tuma RF, Wiedeman MP (1977) Relationship
between microvascular blood velocity and pressure distribution.
Am J Physiol 232: H400-H405
Myers HA, Shenk EA, Honig CR (1975) Ganglion cells in arterioles
of skeletal muscle: Role in sympathetic vasodilation. Am J Physiol 229: 126-138
Proctor KG, Damon DN, Duling BR (1981) Microvascular adaptations during maturation of striated muscle. Am J Physiol 241:
H325-H331
Renkin EM, Rosell S (1962) Independence of sympathetic vasoconstrictor innervation of arterioles and precapillary sphincters. Acta
Physiol Scand 54: 381-384
Renkin EM, Hudlicka O, Sheehan RM (1966) Influence of metabolic vasodilation on blood-tissue diffusion in skeletal muscle.
Am J Physiol 211: 87-98
Sarelius IH, Damon DN, Duling BR (1981) Microvascular adaptations during maturation in striated muscle. Am J Physiol 241:
H317-H324
Schmidt-Schoenbein GW, Engler RL (1982) Leukocyte capillary
plugging in myocardial ischemia during reperfusion in the dog.
Microvasc Res 23: 273
Slaaf DW, Wiederhielm CA (1982) Cessation and onset of muscle
capillary flow at reduced arterial pressure. Microvasc Res 23: 274
Smaje L, Zweifach BW, Intaglietta M (1970) Micropressures and
capillary filtration coefficients in single vessels of the cremaster
muscle of the rat. Microvasc Res 2: 96-110
Stainsby WN (1962) Autoregulation of blood flow in skeletal
muscle during increased metabolic activity. Am J Physiol 202:
273-276
Sullivan SM, Johnson PC (1981) Effect of oxygen on blood flow
autoregulation in cat sartorius muscle. Am J Physiol 241:
H807-H815
Thews G (1960) Die Sauerstoffdiffusion im Gehirn: Ein Beitrag zur
Frage der Sauerstoffversorgung der Organe. Pfluegers Arch 271:
197-225
Vetterlein F, Schmidt G (1975) Effects of vasodilating agents on the
microcirculation in marginal parts of the skeletal muscle. Arch
Int Pharmacodyn 213: 4-16
Weigelt H, Fujii T, Lubbers DW, Hauck G (1981) Specialized
endothelial cells in the frog mesentery—attempt at an electrophysiological characterization. Biblioth Anat 20: 89-93
Whalen WJ, Riley J, Nair P (1967) A microelectrode for measuring
intracellular PO 2 . J Appl Physiol 23: 798-801
Wiedeman MP, Tuma RF, Mayrovitz HN (1976) Defining the
precapillary sphincter. Microvasc Res 12: 71-75
Wilson DF, Erecinska M, Drown C, Silver IA (1979) The oxygen
dependence of cellular energy metabolism. Arch Biochem Biophys 195: 485-493
INDEX TERMS: Arteriole • Capillary density • Hyperemia • Microcirculation • Vascular smooth muscle • Set point
Augmented tissue oxygen supply during striated muscle contraction in the hamster. Relative
contributions of capillary recruitment, functional dilation, and reduced tissue PO2.
B Klitzman, D N Damon, R J Gorczynski and B R Duling
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
Circ Res. 1982;51:711-721
doi: 10.1161/01.RES.51.6.711
Circulation Research is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 1982 American Heart Association, Inc. All rights reserved.
Print ISSN: 0009-7330. Online ISSN: 1524-4571
The online version of this article, along with updated information and services, is located on the
World Wide Web at:
http://circres.ahajournals.org/content/51/6/711
Permissions: Requests for permissions to reproduce figures, tables, or portions of articles originally published in
Circulation Research can be obtained via RightsLink, a service of the Copyright Clearance Center, not the
Editorial Office. Once the online version of the published article for which permission is being requested is
located, click Request Permissions in the middle column of the Web page under Services. Further information
about this process is available in the Permissions and Rights Question and Answer document.
Reprints: Information about reprints can be found online at:
http://www.lww.com/reprints
Subscriptions: Information about subscribing to Circulation Research is online at:
http://circres.ahajournals.org//subscriptions/