Lithium Substitution and the Distribution of Sodium in the Rat Tail
Artery
By Sydney M. Friedman
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ABSTRACT
The exchange of lithium (Li) with sodium (Na) was explored in an effort to quantify
cellular Na in the rat tail artery. Two phases of cellular Na were demonstrated by the
kinetics of exchange at 37°C. Cellular Na, was rapidly lost following cell disruption and
increased in proportion to potassium (K) loss during conventional Na enrichment in a Kfree medium. Cooling to 2°C almost abolished the transmembrane movement of Li;
therefore, a simple 30-minute incubation in cold Li medium removed extracellular but
not cellular Na. To prove that the residual Na after such a wash in cold Li was cellular, we
demonstrated that Na exchanges readily with Li even at 2°C after cells are disrupted, increases slowly in proportion to K loss during prolonged cooling, and increases rapidly in a
precise one-to-one ratio with K loss when active Na transport is stopped by incubation in
a K-free medium. Cellular Na in the normal artery was about 20-25 mmoles/kg dry
weight when cellular K was about 225 mmolesAg dry weight. In arteries from rats with
deoxycorticosterone acetate-induced hypertension of 8 weeks duration, a 13% fall in
cellular K was balanced one to one by an increase in cellular Na.
KEY WORDS
vascular smooth muscle
potassium
deoxycorticosterone
vascular cations
hypertension
• An understanding of the role of sodium (Na) in
the regulation of vascular smooth muscle tension
depends largely on reasonable measurements of intracellular Na. Although reciprocal movements of
Na and potassium (K) have been observed in association with acute vasoconstriction (1,2), it is difficult
to make equally definite statements about the
steady-state conditions of sustained hypertension.
Accurate estimates of cellular Na are also basic to
the interpretation of electrophysiological measurements.
There is, as yet, no simple way to differentiate
cellular from extracellular phases of Na in vascular
tissue. All of the methods applied to the problem so
far have been indirect; they are based on the use of
a marker to measure free extracellular fluid volume
so that the Na which it contains can be subtracted
from the measured total in the tissue (3). An analysis of isotope washout curves subdivides the remaining Na into extracellular and intracellular
components. This method is capable of considerable precision but depends, in the last analysis, on
the acceptability of the extracellular fluid volume
measurements (4, 5).
From the Department of Anatomy, The University of British
Columbia, Vancouver, Canada.
This work was supported by grants from the Medical
Research Council of Canada and the British Columbia Heart
Foundation.
Received May 25, 1973. Accepted for publication November
14, 1973.
168
blood vessels
Ion exchange procedures offer, at least
theoretically, the possibility of a direct approach to
the definition of Na distribution that is independent
of the vagaries of the free fluid volume. Palaty et al.
(6) used this principle in a limited way to examine
Na binding in the arterial wall. It is possible, however, to differentiate cellular Na from all noncellular components, provided a substitute monovalent
ion acceptable to the paracellular matrix as an exchanger and relatively restricted in its entrance to
the cell is available. Lithium (Li) is an obvious
choice, because the affinity of chondroitin sulfate, a
reasonable representative of the polyanionic gel of
blood vessels (7), is slightly greater for Li than it is
for Na throughout the physiological range (8).
Therefore, we studied Li substitution in the rat tail
artery. A simple method for measuring cellular Na
and the basic measurements in the tail artery of
normal and hypertensive rats are presented.
Methods
The physiological salt solutions listed in Table 1 were
based on normal Krebs solution as it has been modified
by Palaty (9). No difficulties developed when Li was
substituted for Na in these solutions except when the
removal of K necessitated the reintroduction of a trace
amount of Na to preserve buffering capacity.
Adult male albino rats weighing 300-400 g were
anesthetized with sodium pentobarbital (50 mg/kg, ip).
The ventral tail artery, weighing 20-30 mg, was excised
and halved; each half was separately incubated in aerated physiological salt solution for 45-60 minutes. Any
Circulation Research, VoL XXXIV, February 1974
169
Li EXCHANGE FOR Na AND K IN SMALL ARTERY
TABLE 1
Millimolar Composition of Physiological Salt Solutions
Normal physiological
salt solution
K-free solution
Li-substituted solution
K-free, Li-substituted
solution
Na
K
Ca
Mg
Cl
HCO 3
HPO4
141.2
5
1.7
1.2
123.4
25
1.2
1.7
1.7
1.7
1.2
1.2
1.2
123.4
123.4
123.4
25
5
1.2
1.2
1.2
146.2
1.2
Li
CO 3
Glucose
11
141.2
141.2
25
25
11
11
11
All solutions were aerated with 95% O2-5% CO2, and they had a pH of 7.4 ± 0.1 at 37° C.
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traces of blood remaining in the lumen of the artery
were removed at the end of this period before incubation was continued for another hour in fresh medium.
Samples were then transferred to stoppered test tubes
filled with K-free medium and refrigerated overnight at
3°C. The next morning, the arteries were transferred to
continuously aerated physiological salt solution at 37°C,
and the preparatory phase was completed with incubation for 3 hours at 37°C with a change to fresh medium
after 1.5 hours. With this procedure, recommended by
Jones and his associates (5, 10), the smooth muscle cells
consistently reaccumulated high levels of K. The artery
was then incubated in the experimental bathing solution
as specified.
At the end of the experimental incubation, the artery
was blotted gently on filter paper, weighed in a stoppered glass weighing bottle, dried to a constant weight,
and extracted for 7 days with 0.75M HNO3. Na, K, and
Li were determined in the extract by an atomic absorption spectrophotometer with the usual precautions (11).
Results are expressed as means ± SE in terms of mmoles/
kg dry weight unless noted otherwise.
a continuous slow gain which was apparently inverse to the K loss, because its rate of approach to
its limit was described by the same rate constant.
The Li values were more variable than the K
values, however, because they were greatly affected by small variations in the amount of water
left clinging to the tissue after incubation. This
finding suggests that Li enters cells continuously
throughout the incubation, at first in exchange for
Na and then, when active transport stops, mainly in
exchange for K. If so, then the zero intercept of 263
mmoles/kg dry weight obtained for K loss could
represent the initial value of cellular Na plus K.
According to this interpretation, most of the extracellular Na, both free and site bound, is removed
within a few minutes of exposure of the tissue to
the Li-substituted medium; this process occurred
too quickly at 37°C to allow subdivision of the ex-
Results
Na. K. AND Li EXCHANGES DURING LI SUBSTITUTION AT 37'C
The changes in tissue cation composition observed when arteries were transferred from a normal medium to a medium in which Li replaced Na
are shown in Figure 1. No loss of K was observed
during the first 20 minutes of incubation, suggesting
that active Na extrusion was maintained to this
point. This observation was confirmed by the immediate K loss in artery samples incubated with 1
mM ouabain beginning in the last 15 minutes of
preliminary incubation in normal medium and continuing for a subsequent 30 minutes after Li
substitution. The loss of K can be described by a
single exponential with a rate constant of
-0.007 ± 0.0002/min and a zero intercept of 263
± 7 mmoles/kg dry weight.
Contrary to the observation with K, there was no
apparent initial pause in the continuity of the curve
describing Li uptake after the first 6 minutes of exposure. A very rapid initial uptake was followed by
Circulation Researdi. Vol. XXXIV, February 1974
•As.
ISO 540
FIGURE 1
Whole artery Na, K, and Li during incubation in Li medium
(solid symbols) or in Li medium with 1 mM ouabain (open symbols) at 37°C. Each point is the average of six samples. The
curve for K was fitted to a single exponential by the leastsquares method. The curve for Li was drawn as a mirror copy of
that for K. SE is indicated by vertical lines when it exceeds symbol size.
170
FRIEDMAN
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tracellular phases. The small amount of Na remaining after the initial rapid exchange with Li and tentatively identified as cellular is considered in
greater detail in Figure 2. Two components could
be differentiated: a very slow component that left
at — 0.002/min from an initial value of 6 ± 1 mmoles/kg dry weight and a larger, fast component
with a rate constant of — 0.044/min and a zero intercept of 15 ± 2 mmoles/kg dry weight. Although no
great reliance can be placed on the absolute values
obtained by such curve peeling, there can be little
doubt that there were two phases which differed
by more than a full order of magnitude. If both
these phases are indeed cellular, then the faster
phase might reflect a withdrawal of Na from the
cytoplasm, and the slower phase might belong to
membrane-limited intracellular structures such as
the nucleus and the mitochondria. In total, about 21
mmoles/kg dry weight is involved.
If this analysis is correct, then disruption of cell
integrity should remove constraints to diffusion and
permit the rapid substitution of all tissue Na with
Li. Based on the experience of Friedman et al. (12)
with the cell shattering that can be produced by ice
crystals, alternate freezing and thawing was
selected to achieve cell disruption. The residual Na
in intact arteries after 7 and 10 minutes of incubation in the Li medium was compared with that in
arteries dipped six times alternately into liquid nitrogen and into Li medium at 37°C during the first
2 minutes of these incubations. As shown in Figure
2, Na was almost wholly removed from the disrupted tissue even in the shorter interval.
Na- EXCHANGE IN Na-ENRICHED ARTERIES DURING Li
SUBSTITUTION AT 37#C
Proof of the cellular location of two components
of Na thus far rests on the demonstration that Li
enters cells and exchanges for both, that the cell
does not begin to lose K until about half of this Na is
lost, and that most of both components is liberated
following cell disruption. It still must be shown that
the value of cellular Na so defined increases in proportion to K loss following conventional Na enrichment of cells by interruption of Na transport (13).
Arteries were enriched with Na at the expense
of cellular K by overnight incubation in K-free
solution at 3°C following the standard procedure
only to this point. The arteries were then transferred to fresh K-free medium at 37°C for a further
90-minute incubation before the start of Li
substitution. They were then incubated in K-free
Li-substituted medium at 37°C for varying times.
To allow sufficient time to wash out extracellular
Na, no samples were taken before 10 minutes.
As shown in Figure 3, the dominant rate constant
for Na loss was -0.051 ± 0.002/min; this value was
similar to that derived earlier for the fast component of cellular Na. (The rate constant is not ap300
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a
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20
10
20
40
60
80
100
120
T i m e (min)
~~teo M O
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FIGURE 2
Enlarged logarithmic plot of the changes in Na during incuba
tion in Li medium shown in Figure 1 before ( • ) and after (a
cell disruption. Each point is the average of six samples, SE is in
dicated by vertical lines.
FIGURE 3
Whole artery Na during incubation in K-free Li-substituted
medium at 37°C following overnight Na enrichment in K-free
medium at 3°C. The curve was fitted to a single exponential by
the least-squares method. Each point is the average of six samples, SE is indicated by vertical lines.
Circulation Research, Vol. XXXIV, February 1974
Li EXCHANGE FOR Na AND K IN SMALL ARTERY
preciably affected by introducing weighting for
variance into the analysis.) A slow component,
almost twice as large as that previously defined,
was also observed. The initial value given by the
zero intercept was 287 ± 16 mmoles/kg dry
weight; this value was not significantly different
from the original estimate of 263 ± 7 mmoles/kg
dry weight for the sum of Na plus K in cells.
Therefore, it must be concluded that these phases
of Na are cellular. Also, the Na loss in this experiment was exactly balanced by the Li uptake.
H«, K, AND LI EXCHANGES DURING U SUBSTITUTION AT 2*C
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In these experiments, the exchange of extracellular Na with Li was too fast to permit identification of more than a single phase, although there
must be at least two. Even the cellular exchange
was too fast to permit accurate measurement of
cellular Na without multiple data points. A study of
Li exchanges at low temperature was accordingly
undertaken in the expectation of reducing the
transmembrane exchange rates for Na and Li (5,
14, 15).
All of the monovalent cation exchanges were
greatly slowed at 2°C (Fig. 4). Although the bulk
exchange of Li for Na was sufficiently slowed from
that observed at 37CC to indicate more than one
phase, it was still substantially complete at 30
50
i
1
t
W
,11
i
Li
•
K
j
v
1
— k'-OOeShr"1
.11
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Na
* "~~-~-_ k- -0.042 hr"1
20
40
60
Time (mm)
1080
FIGURE 4
Whole artery Na, K, and Li during incubation in Li medium at
2°C. Each point is the average of six samples. S£ is indicated by
vertical lines when it exceeds symbol size.
Circulation Remrch, VoL XXXIV, February 1974
minutes. By this time, 370 mmoles/kg dry weight of
Na had been lost from the tissue. This value compares well with the value for extracellular Na previously calculated as 360 mmoles/kg dry weight at
37CC. The measured residual Na of 25 mmoles/kg
dry weight is close to the 21 mmoles/kg dry weight
previously identified as cellular Na at 37°C. No significant further loss of Na or K or gain of Li was
recorded in the first hour of Li incubation, showing
that only a negligibly small amount of Li could have
crossed the cell membrane in this time.
The continuing slow rate of cellular K loss during
immersion in the Li medium at 2°C calculated as a
single exponential for the interval between 1 and
18 hours was -0.065/hour, and that for Na was
—0.042/hour. The Li gain occurring over this very
long period and corrected for the variation in total
water was almost exactly equal to the loss of Na
plusK.
Therefore, it seems that Li exchanges only exceedingly slowly with cellular Na at low temperature so that a wash of the tissue for about 30
minutes in Li medium at 2°C removes extracellular
Na and leaves behind cellular Na. If this
phenomenon can be proved, low-temperature Li
substitution provides a direct, simple way to evaluate cellular Na. Two methods were used to establish that the residual Na was indeed cellular: cell
disruption and cellular Na enrichment.
In the first experiment, tissues were cooled to
2° C for 1-4 hours in normal medium. At the end of
the cooling period, each sample was rapidly
transferred to the Li medium at 2°C for 30 minutes.
Prolonged cooling in normal medium was expected
to produce a slow loss of cellular K and a slow gain
of cellular Na which would then be measured by
washing out extracellular Na with the cold Li
medium. To examine the possibility that only extracellular Na is exchanged in 30 minutes of incubation at 2°C, a group of tissues was disrupted by
alternate freezing and thawing in the first 2 minutes
of exposure to the Li medium. The total time during which the tissue was thawed in this procedure
was well under 30 seconds.
As shown in Figure 5, plotted simply rather than
logarithmically because of the small changes involved, cellular Na increased at the expense of K
over the 4-hour period. The ratio of exchange was
less than one to one, but the variability of the K
values rather than that of the Na values precluded
a decision on this point. A significant increase in
cellular Na was measured when only a 10% K loss
had been reached. When the cells were disrupted
172
FRIEDMAN
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240
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Na
160
^
Na.
140
120
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100
I20
ISO
240
60
Time (min)
20
60
FIGURE 5
Cellular Na and K during incubation in normal medium at 2°C
as measured after a 30-minute wash in Li-substituted medium
to remove extracellular Na (solid symbols) and after cell disruption (open symbols). Each point is the average of eleven samples, SE is indicated by vertical lines when it exceeds symbol
size.
120
180
240
120
180
240
240
200
160
by the freeze-thaw technique, both cellular Na and
K were readily washed out of the tissue even at
2°C.
This experiment showed, then, the lower limit of
usefulness of the method. To examine the upper
limit, the time course of Na enrichment of cells in
K-free medium was followed. Tissues were incubated for 1-4 hours in K-free medium at 37°C, and
immediately following this incubation they were
rapidly transferred to Li medium at 2°C for 30
minutes.
During the 4-hour period, cells gained Na in a
one to one relation to their loss of K (Fig. 6). The
rate of K loss could be accurately expressed as a
single exponential with a rate constant of
-0.0090 ± 0.0003/min and a zero intercept of
263 ± 3 mmoles/kg dry weight. The same function
described the Na gain almost as well. As shown in
Table 2, the one-to-one exchange ratio was apparent in simple terms, since the loss of K of 211 mmoles/kg dry weight in the 4-hour period was
almost exactly balanced by a gain of 196 mmoles/kg
dry weight of Na. Furthermore, the value of cellular Na plus K at 4 hours was not significantly
different from the value measured initially. To
determine that the rate of exchange of cell cations
5'20
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o 60
o
40
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* 20
10
60
T i m e (min)
FIGURE 6
A: Cellular Na during incubation in K-free medium at 37°C. B:
Tissue K taken as cellular K. Extracellular Na was removed by a
30-minute wash in Li-substituted medium at 2"C. Curves were
fitted to single exponentials by the least-squares method. Each
point is the average of eleven samples, SE is indicated by vertical
lines when it exceeds symbol size.
with Li at 2CC was not changed by the experiment,
ion values at 4 hours were measured after 30
Circulation Researdi, Vol. XXXIV, February 1974
173
Li EXCHANGE FOR Na AND K IN SMALL ARTERY
TABLE 2
Cell Monovalent Cation Balance in Rat Tail Artery Incubated in K-Free Physiological Salt Solution
Incubation
(minutes)
2°C Li wash
(minutes)
Na,
(mmoles/kg
drywt)
K,
(mmoles/kg
drywt)
(Na + K),
(mmoles/kg
drywt)
Lit
(mmoles/kg
drywt)
H2O
(liters/kg
drywt)
0
240
240
30
30
60
30
20.8 ± 0.6
217 ± 7
217 ± 4
228 ± 7
17.2 ± 0.5
20.3 ± 1.0
-211
249 ± 7
234 ± 7
237 ± 10
353 ± 6
348 ± 7
367 -± 4
3.32 ± 0.04
3.27 ±0.05
3.39 ±0.06
Change
196
Six rats were tested for each incubation period. Nai - residual Na after cold Li wash, K, = total K less 7 mmoles in extracellular
water, and Lit = total Li.
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pense of K was observed in the incubated arteries
from the hypertensive rats. The cellular location of
most, if not all, of this change was indicated not
only by the constancy of the sum of Na and K, but
also by the uniform uptake of Li during the wash at
2°C in both groups; therefore, the sum of
monovalent ions was the same in both groups.
As usual, the K values in fresh arteries were
moderately higher than those in incubated vessels
(5,13). An increase in tissue Na and a tendency to a
decrease in K were apparent in this experiment.
minutes of washing in cold Li-substituted physiological salt solution and again after 60 minutes. There
was no significant difference between the two sets
of measurements.
CELLULAR Na AND K IN THE TAIL ARTERY OF NORMAL AND
HYPERTENSIVE RATS
Because the Na remaining in the artery after a
simple wash in cold Li medium for 30 minutes had
been identified as cellular, a basic comparison of arteries from normal and hypertensive rats was undertaken. Hypertension was induced by five injections of deoxycorticosterone acetate (DOCA) (12.5
mg/kg, sc) administered at 3-day intervals and
reinforced by 1% saline as drinking water. The rats
had been under treatment for 8 weeks at the time
of study. The results are presented in Table 3 with
total Na and K values measured in fresh arteries
from two similar groups of rats in separate parallel
experiments. For these investigations, the tail artery was rapidly but gently excised (less than 30
seconds) and taken for processing without any
liquid immersion stage.
A significant enrichment of cellular Na at the ex-
Discussion
In general, the identification of cellular Na in
these experiments depended on the demonstration
that the Na was constrained by a diffusion barrier
from free exchange with Li, that it shared this
phase with K, that the barrier could be removed by
cell disruption, and finally, that the amount of this
phase was metabolically regulated. On these principles, two components of cellular Na were identified by Li exchange at 37°C. At 2°C the rate of Li
exchange with cell cations was so slowed that it was
possible to choose a time of exposure sufficiently
TABLES
Comparison of Cellular Na and Whole Tissue Na in Arteries from Normal and Hypertensive Rats
Method
Group
2° C Li, physio- Normal (9)
logical salt
solution (30 Hypertenminutes)
sive (9)
Fresh
Normal (8)
Hypertensive (8)
Blood pressure
(mm Hg)
Na
K°
Na + K
(mmoles/kg (mmoles/kg
drywt)
drywt)
H2O
(liter/kg
drywt)
Na + K + LI
(mmoles/kg
drywt)
Systolic
Diastollc
142 ± 2
86 ± 2
20.5 ± 1.1
218 ± 5
238 ± 6
3.29 ± 0.04
605 ± 5
199.± 2t
124 ± 4
135 ± It
72 ± 4
52.1 ± 1.8t
189 ± 2t
241 ± 3
3.29 ± 0.05
602 ± 10
336 ± 6
242 ± 4
197 ± 7t
123 ± 6t
382 ± 5t
228 ± 6
°K = total tissue ion not corrected for extracellular fluid content.
tP<0.02.
Circulation Research, VoL XXXIV, February 1974
174
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long to complete the ion exchange process in the
unrestricted extracellular phase and yet sufficiently
short to preclude significant cell exchange.
Several important principles resulted from these
observations. First, simple incubation of the rat tail
artery for 30 minutes at 2°C in a physiological salt
solution in which Na is replaced by Li washes out
extracellular Na and provides a ready measure of
cellular Na. Second, because this process involves
ion substitution, the sum of cations in an experiment
must remain constant within the limits of experimental accuracy. Where Na, Li, and K are the bulk
cations, Mg and Ca can usually be neglected in
drawing up the cation balance sheet. Third, as long
as the cell membrane is not crossed by Li at 2°C,
the sum of cellular Na and K will remain constant;
conversely, a decrease in this sum coupled with an
increase in Li indicates that the cell membrane has
permitted the passage of Li despite the low
temperature. To the degree that significant
transmembrane Li exchange occurs at 2°C, cellular
Na will be undervalued; this variable must be
carefully controlled if absolute values are sought.
In the first application of the method to the
problems of hypertension, a decrease in cellular K
of about 13% matched one to one by a gain in
cellular Na was observed in established (8 week)
DOCA hypertension. This observation is the first
step toward an examination of the relation of
transmembrane steady-state distribution of Na and
K to sustained vasoconstrictive states. Before now
there has been no proof that intracellular Na is in
fact increased in any kind of hypertension (16).
There is general agreement that, with tracer
kinetics which describe the distribution of Na in arteries, at least three components can be distinguished (3). The first component exchanges very
rapidly (t1/4 30-50 seconds), accounts for about 95%
of the ion, is extracellular, and can be further subdivided (5,17, 18). The second component which is
rather fast (^2.5-5 minutes) accounts for about 3%
of the ion and is the subject of vigorous debate both
as to its quantity and location. Earlier workers considered all of this component to be intracellular,
although in the most recent detailed analysis it is all
extracellular. All investigators agree that the third
component which exchanges slowly (tiA 40-70
minutes) is cellular and accounts for about 2% of
the total.
The minimal values reported for cellular Na are
1.97 mmoles/kg wet weight and 2 mmoles/kg wet
weight (5, 19). These low values were obtained by
using strips of adventitia as the control or by using a
FRIEDMAN
superior marker for extracellular fluid and giving
rigorous attention to diffusion kinetics (20). This
value rises to 3.5 mmoles/kg wet weight in the
calculations of Rorive (21) and is fourfold higher
than this value in other studies (4). Some of the
differences depend on the choice of marker for the
extracellular water, but there are also differences
in the state of the artery as well. It has repeatedly
been pointed out that the transmembrane
monovalent cation gradients in arterial tissue run
downhill with very slight provocation; therefore, a
convenient test of normality is the arterial K content. Although incubated arteries do not contain
quite as much K as fresh vessels, they can certainly
accumulate some 90% of the normal K complement
(10, 13). If this K complement is not attained, it
may be anticipated that elevated values of cellular
Na will be observed.
The problem of ion distribution in arterial tissue
has been approached by studying the exchanges of
one ion in bulk for another, either directly or indirectly (6, 22, 23). In an earlier work (6) two fractions of sodium in the arterial wall that can, by slow
titration, be selectively and reversibly exchanged
for other ions were quantitatively studied. These
fractions are believed to be bound to the acid mucopolysaccharide-protein component of the
paracellular matrix. The Li substitution procedure
used in the present paper replaces extracellular Na
quickly and does not permit a comparable subdivision into discrete phases. In other work (22, 23)
using the substitution of Na by lactose, cellular Na
was estimated to be less than 30 mmoles/kg dry
weight, i.e., about 7 mmoles/kg wet weight.
Although substitution of Na by a nonelectrolyte is,
of course, open to the criticism that the ionic
strength of the medium is grossly disturbed, the
results compare well with the values for cellular Na
of 20-25 mmoles/kg dry weight, i.e., 5-6 mmoles/kg
wet weight, obtained by Li substitution. The small
difference between this value and the lowest
values obtained with radioisotopes as cited above
probably reflects only the different arteries used
and, to some degree, a slightly less complete restoration of the transmembrane K gradient in these
small half arteries than that obtained in the whole
vessel.
One of the important reasons underlying the
quest for firm data concerning cellular Na, apart
from the study of hypertension, is the need to
relate chemical activity to functional and electrical
events. Therefore, the derived estimate of cellular
Na in arterial tissue is usually expressed in terms of
Circulation Research, Vol. XXXIV, februory 1974
175
Li EXCHANGE FOR Na AND K IN SMALL ARTERY
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concentration. Because there is no direct way to
measure cell water or make assumptions concerning its state, this procedure does not seem particularly useful. Based on our measurements with U Cinulin, cell water would be about 1.4 liters/kg dry
weight and the intracellular concentration of Na
would be about 14 mmoles/kg cell water. Inulin
probably overestimates cell water, and not all of
cellular Na can be in free solution; therefore, this
estimate is at best a rough approximation.
The Li substitution procedure evidently provides a very rapid, accurate estimation of cellular
Na. The rat tail artery is particularly suitable for
these studies since its wall thickness is only about
40/x, it is rich in smooth muscle cells, and it contains
few other cell types (24). There is, however, no bar
to other arteries, provided there is a reasonably
short diffusion path to the smooth muscle cells.
Acknowledgment
hypertension of coarctation of the aorta of short duration. JClin Invest 47:1221-1229,1968.
8. SIMARD, S.J., AND FRIEDMAN, S.M.: Binding of some sodium
substitutes to chondroitin sulfate. Experientia
26:834-836,1970.
9. PALATY. V.: Distribution of magnesium in the arterial wall.
J Physiol (Lond) 218:353-368,1971.
10.
JONES, A.W., AND KAHREMAN, G.: Ion exchange properties of
11.
PALATY, V., GUSTAFSON, B.K., AND FRIEDMAN, S.M.: Role of
the canine carotid artery. Biophys J 9:884-909,1969.
protein-polysaccharides in hydration of the arterial
wall. Can J Physiol Pharmacol 48:54-60,1970.
12.
morphology in hypertensive states in the rat. Anat Rec
171:529-544,1971.
13.
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Lithium Substitution and the Distribution of Sodium in the Rat Tail Artery
SYDNEY M. FRIEDMAN
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Circ Res. 1974;34:168-175
doi: 10.1161/01.RES.34.2.168
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