Sodium-coupled chloride transport
by epithelial tissues
Cl absorption;
Na gradient
tion; model for electrogenic
; co-transport
Cl secretion
ERA
OF THE
STUDY
of ion transport by
epithelia,
in vitro, was ushered in by the brilliant
studies of Ussing and his collaborators (65) on frog skin
and, later, those of Leaf (39) and his collaborators on
toad urinary bladder. For some years thereafter
Na
occupied center stage in this unfolding drama, with Cl
relegated to the unglamorous role of a passive partner.
Recently, however, there has been an upsurge in interest in transepithelial
Cl transport,
sparked by the
recognition
that active Cl transport
is a far more
widespread phenomenon than was previously appreciated and that it may be the primary ionic movement
responsible for a number of secretory processes stimulated by elevations in intracellular
levels of cyclic 3 ’ ,5’adenosine monophosphate
(CAMP) and/or calcium.
In vitro studies have fairly well established three
modes of transepithelial
Cl transport.
The first is a
strictly passive mode driven by transepithelial
differences in concentration
and electrical potential as exemplified by frog skin (65) and toad urinary bladder (39).
The second involves an electrically
silent Cl-HCO,
exchange found, for example, in turtle urinary bladder
(40), in some species of mammalian
colon (2’7, 35), and,
perhaps, in pancreatic ducts (35, 62). Finally, in recent
years it has become apparent that Cl absorption by a
variety of epithelia is coupled in an electrically neutral
fashion to Na transport and there is strongly suggestive
THE MODERN
0363-6127/79/0000-0000$01.25
Copyright
0 1979 the American
Physiological
processes; electrogenic
Cl secre-
evidence that a number of Cl secretory processes1 also
involve a Na-dependent
movement across one of the
limiting cell membranes.
Currently there is no compelling evidence for “primary active” transport of Cl
directly linked to a source of metabolic energy in animal
epithelia.
This Editorial Review will focus on Na-coupled transepithelial Cl transport.
We will first summarize the
rather compelling evidence for Na-coupled Cl absorption and then turn our attention to the more speculative
issue of Cl secretion.
Electrically
Neutral
NaCl Absorption
The existence of a mechanism capable of mediating
electrically neutral (one-for-one) NaCl absorption was
first strongly suggested by the findings of Diamond (17,
181, Wheeler (68), and Dietschy (19) that Na and Cl are
both actively absorbed by fish and rabbit gallbladder at
near equal rates, and that a) these tissues are characterized by negligibly
small transepithelial
electrical
potential differences (PD), b) replacement of Na with
nonabsorbed cations abolishes active Cl absorption with
l Absorption
hollow
organ
direction.
Society
is defined
and secretion
as a net movement
from
refers to a net movement
the lumen
of a
in the opposite
Fl
Downloaded from http://ajprenal.physiology.org/ by 10.220.32.246 on June 16, 2017
FRIZZELL, RAYMOND A., MICHAEL FIELD, AND STANLEY C. SCHULTZ.
Sodium-coupled
chZoride transport
by epithelial
tissues. Am. J. Physiol.
236(l): Fl-F8, 1979 or Am. J. Physiol.: Renal Fluid Electrolyte
Physiol.
5(l): Fl-FS, 1979. -There
is compelling
evidence that active Cl absorption
by a variety
of epithelia,
widely
distributed
throughout
the animal
kingdom,
is the result of an electrically
neutral
Na-coupled
transport
process at the luminal membrane
and that the energy for transcellular
Cl
movement is derived from the Na gradient across that barrier. These cotransport
processes are found predominantly
in “leaky”
or “moderately
leaky” epithelia
and permit these tissues to absorb Na and Cl with high
degrees of efficacy. In addition,
there is a growing body of evidence that
cyclic AMP and Ca-induced
electrogenic
Cl secretion by a wide variety of
epithelia
may involve electrically
neutral, Na-coupled Cl entry across the
contraluminal
membrane and that the energy for these secretory processes
is derived from the Na-gradient
across that barrier. A model for electrogenie Cl secretion that accounts for the available data is presented.
F2
FRoIZZELL,
FIELD,
AND
SCHULTZ
‘LThe cell interior is 50-60 mV negative with respect to the
mucosal solution and is not significantly affected by replacement of
Cl with nontransported anions (e.g., sulfate, isethionate). Therefore,
electrical coupling between Na and Cl entry is precluded.
FIG.
1.
Model for NaCl co-transport
by rabbit gallbladder
(26).
Downloaded from http://ajprenal.physiology.org/ by 10.220.32.246 on June 16, 2017
no change in the PD, and c) replacement of Cl with
cell even if the efficiency of energy transfer is relatively
nonabsorbed anions abolishes active Na absorption with
low.
no change in PD. These findings differ strikingly from
Martin and Diamond (42) found that approximately
those in other epithelia (65) where transepithelial
Cl 25 mol of NaCl were absorbed actively by rabbit galltransport is passively (electrically) coupled to Na ab- bladder for every mole of O2 consumed above the basal
level and, a&r comparing their data with those resorption. These studies, however, provided no insight
into the possible site(s) of interaction between Na and ported for other epithelia, concluded: “Thus, the gallCl. Subsequent studies of the unidirectional
influxes of bladder, in which both Na and Cl transport are active,
Na and Cl from the mucosal solution into rabbit gall- pumps approximately
twice as many ions activeZy per
bladder epithelial cells by Frizzell et al. (26) and Cre- extra oxygen consumed as do epithelia which transport
only Na actively.” This conclusion, drawn well before
maschi and Henin (14) disclosed the presence of one-forone coupling between the movements of Na and Cl the- notion that &tive Cl transport by gallbladder may
remains a most
across the mucosal membrane. Electrophysiologic
stud- be energized by the Na gradient,
ies employing intracellular
microelectrodes ’ (26, 32) ex- articulate expressi .on of the energy transducing function
cluded the possibility that this interaction could be the of Na-coupled transport process?The teleologic significance of coupled NaCl transport will be considered
result of electrical coupling and provided compelling
evidence for the mediated entry of NaCl into the cell in below.
A model of the NaCl co-transport process in gallbladthe form of a neutral complex.* Nellans et al. (44) had
der is illustrated
in Fig. 1. Although
it seems quite
described a similar process in the brush border of rabbit
ileum somewhat earlier and, in both ileum and gall- certain that the coupled entry step brings about the
bladder, this coupled influx process appears to be in- uphill movement of Cl into the cell, energized by the
hibited by procedures that lead to an elevation of downhill flow of Na, the mechanisms responsible for Na
and Cl exit from the cell are unclear. As is the case for
intracellular
CAMP (26,44).
activity,
In addition to providing a reasonable explanation for all epithelia studied to date, Na-K-ATPase
the earlier findings of Diamond (17, 18) and others (19, ubiqu tously associated w ith the Na-K active exchange
68), the localization of the coupled mechanism to the PWP has been identified in the basolateral m .embranes
epithelial
cells (43, 63), and indirect
mucosal membrane raised the attractive possibility that a of gallbladder
evidence has been presented which implicates
this
the energy necessary for the active transcellular
transpump mechanism in transcellular
Na transport
(47).
port of Cl might be derived from coupling to the electroHowever, this evidence is circumstantial
and, at the
chemical potential difference for Na across that barrier,
analogous to Na-coupled sugar and amino acid transport by small intestine and renal proximal tubule (54, TABLE
1. Intracellular
chloride activities
55). The findings that the intracellular
concentration of in some epithelia
Cl exceeds that expected for a passive distribution
(14,
26) and declines toward the equilibrium
value when the
Na-Ringer
Na-Free Ringer
tissue is bathed in a Na-free medium (26) lent encouragement to this possibility; but, given the uncertainties
-42
that becloud the measurement
and interpretation
of Rabbit gallbladder
-49
35
2.4
19 1
20
Necturus proximal
-52
25
2.3
-61
10 1.4 59
intracellular
concentrations,
these observations cannot
renal tubule
be considered conclusive.
Amphiuma small
-33
1.4
Recently, Dtiey et al. (20) have determined intracelintestine
lular Cl activities in rabbit gallbladder in the presence
Bullfrog small
71 2.3
3
intestine
and absence of extracellular
Na using Cl-selective (liquid ion exchanger) microelectrodes;
their results are
+ is the electrical potential difference across the luminal memgiven in Table 1. In the presence of Na, the thermodybran? with respect to the luminal solution, hcl is the intracellular
namic activity of cell Cl is 2-3 times that predicted by Cl activity, and R is the ratio of the measured activity to that
predicted for a passive distribution
of an anion.
the Nernst equation for a passively distributed anion,
whereas in the absence of Na there is excellent agreement between the predicted and the observed activities.
Furthermore,
as discussed by Duffey et al. (20), al- MUCOSAL
SEROSAL
though the intracellular
Na activity is not known, it is SOLUTION
CELL
SOLUTION
likely to be well below the estimated intracellular
Na
concentrations
of 66-80 mM (14, 26). Accordingly, the
electrochemical
potential difference for Na across the
mucosal membrane is almost certainly far more than
that required to energize uphill Cl movement into the
EDITORIAL
F3
REVIEW
even if the entire tissue conductance
passively driven ion .” Furthermore,
3 Rabbit
gallbladder
also absorbs
HCO,
and some other anions
in
what
appears
to be an electrically
silent
manner.
Thus,
the cotransport
system
appears
to have a high specificity
for Na but is less
demanding
with respect
to the co-transported
anion (70).
is assigned
to the
it can be readily
shown that the data of Diamond and Martin (42) on the
energetics of NaCl transport by rabbit gallbladder and
those of Frizzell et al. (26) on the electrical properties of
that tissue absolutely preclude the possibility of electri-
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present time, a direct relation between transcellular Na
TABLE
2. Epithelial
tissues that appear to possess
coupled NaCl absorptive mechanisms
transport and K uptake across the ba solateral membranes has not been demonstra ted for gallbladder or,
Mammals
for that matter, for any other Na-transporting
epitheHuman
ileum
(64)
lium (46, 52). Furthermore,
although it seems certain
Rabbit
ileum (44)
that Cl exit from the cell is a downhill process directed
Rabbit
gallbladder
(26)
along a favorable electrochemical
potential gradient, it
Rat colon (5)
is not clear wheth .er the Cl conductan .ce of the basolatBovine
rumen
(13)
era1 membrane is sufficiently high to permit a strictly
Amphibia
diffusional outflow. The available data suggest that this
Necturus
gallbladder
(31)
barrier is relativelv impermeable
to Cl and raise the
Necturus
proximal
renal tubule
(59)
possibility that theWexit bf this anion is nonconductive,
Bullfrog
small intestine
(48)
Frog skin (66, 67)
i.e., coupled to the co-transport
of a cation or the
countertransport
of another anion ( 1% 20). Clearly,
Fish
additional studies aimed at defining the active and
Flounder
intestine
(24)
passive properties of the basolateral
membrane
are
Sculpin
intestine
(34)
necessary.
Marine
eel intestine
(58)
Flounder
(seawater
acclimated)
urinary
bladder
(49)
Rabbit gallbladder provided an ideal preparation for
Trout
(fresh water)
urinary
bladder
(38)
the demopstration
and characterization
of NaCl cotransport and certainly remains the tissue of choice for
Molluscs
the further study of this process inasmuch as the
Aplysia
intestine
(30)
epithelium is comprised of a single layer of what apArthropods
pears to be, histologically,
one cell type (61, and NaCl
Prawn
intestine
(1)
(or more generally, Na anion)” co-transport is the only
transcellular
transport
process present. The results
cited above, therefore, are not complicated by the presFinally, it is of some interest to note that with the
ence of heterogeneous cell types or multiple ion transexception
of a few reported instances (e.g., 49, 66, 67)
port processes, and the interpretation
of these results in
NaCl
co-transport
processes appear to be largely reterms of the model illustrated in Fig. 1 seems eminently
reasonable. The conclusive demonstration
of NaCl co- stricted to that class of epithelia referred to as “leaky”
or “moderately leaky” because of the presence of relatransport becomes somewhat more difficult when tistively low-resistance paracellular
shunt pathways (53).
sues are characterized
by heterogeneous
cell populaIn
general,
these
epithelia
are
characterized
by relations and several mechanisms
for transcellular
ion
tively high rates of Na and Cl absorption accompanied
transport that may reside in different cell types [e.g.,
by the isotonic equivalent of water. The biologic “advanrabbit ileum, which in addition to NaCl co-transport
tage” conferred on these epithelia by the possession of
possesses mechanisms
for Na-coupled nonelectrolyte
transport and for transcellular
Na transport that is NaCl co-transport processes can be appreciated from the
following considerations.
apparently uncoupled to the movement of other solutes
In “tight” epithelia such as frog skin and toad urinary
(55)]. Nonetheless, there is now a considerable body of
bladder,
Na is actively absorbed and Cl is “dragged
-evidence for the presence of NaCl co-transport processes
along,” passively driven by the transepithelial
PD esin epithelia from a wide variety of species ranging down
tablished by the movement of Na. Therefore, one might
the phylogenetic
scale from man through arthropods.
loosely
refer to this phenomenon as NaCl “co-transport”
Some examples are listed in Table 2. This listing should
in
which
the coupling is electrical.
Obviously,
the
not be considered exhaustive, but is meant to illustrate
energy invested in the mechanism responsible for active
the widespread distribution
of these processes throughNa absorption also indirectly energizes Cl absorption.
out the animal kingdom. In most instances the evidence
However, electrical coupling is not a very efficacious
for co-transport
is that both Na and Cl are actively
way
to ensure rapid rates-of NaCl absorption by leaky
absorbed, that replacement
of Na with nonabsorbed
epithelia in which low-resistance shunt pathways precations reduces or abolishes Cl absorption, and that
of significant
transepithelial
replacement of Cl with nonabsorbed anions reduces the clude the development
PDs.
Accordingly,
as
pointed
out
by
Frizzell
et al. (26),
rate of active Na absorption to the same degree. In tt
in spite of the fact that total tissue conductance [of
several instances, evidence for co-transport has been
is very large, it is nonetheless
supplemented
by measurements
of intracellular
Cl ac- rabbit gallbladder]
grossly
insufficient
to
permit pure electrical coupling of
tivities (a$‘) which have uniformly
indicated active
passive
Cl
transport
to
active Na transport or passive
accumulation by the cells; these studies are summarized
Na transport to active Cl transport at the observed rates
in Table 1.
F4
FRIZZELL,
“Nonneutral”
Na-Coupled
MUCOSAL
SOLUTION
AND
SCHULTZ
SEROSAL
SOLUTION
A
Cl Absorption
Although generally ignored because of its small magnitude (~1 mV), there is usually an electrical potential
difference across rabbit gallbladder oriented such that
the serosal solution is electrically negative with respect
to the mucosal solution (26, 41). An ingenious explanation for this finding was suggested by Machen and
Diamond (41), namely, that the PD is the result of a
NaCl diffusion potential between the lateral intercellular spaces and the mucosal solution across cation-selective tight junctions. In accordance with the “standing
gradient” hypothesis advanced earlier, they argued that
Na and Cl are accumulated in the lateral spaces where
they achieve a concentration
somewhat greater than
that in the mucosal solution. This concentration difference provides the driving force for the backdiffusion of
NaCl into the mucosal solution. Since the junctions are
more permeable to Na than to Cl, this recycling process
generates a PD oriented in the observed direction. This
notion is illustrated in Fig. 2A.
Recent studies (24) of Na and Cl transport by the
isolated intestinal mucosa of the winter flounder, Pseudopleuronectes
americanus,
strongly suggest a similar
model, but in this instance the transepithelial
PD is as
much as 5 mV, serosa negative; although this value is
small by comparison with tight epithelia it is by no
means negligible. The central findings consistent with
the model illustrated in Fig. 2A are the following.
a) As shown in Table 3, under short-circuit conditions
both Na and Cl are actively absorbed but the rate of Cl
absorption exceeds that of Na; the short-circuit current
J From measurements
of the Cl conductance
of rabbit
gallbladder,
Frizzell
et al. (26) have estimated
that the transepithelial
electrical
potential
difference
(&,s> would have to be 34 mV, serosa positive,
in
order to drive
net Cl absorption
at the observed
rate of 14 peq/cm*
per h. Since the conductance
of the paracellular
shunt
pathway
accounts
for at least
95% of the entire
tissue
conductance,
the
“electromotive
force” of the Na pump (ENa) would have to be (34/0.05)
= 680 mV in order
to generate
this ems. The energy
that can be
obtained
from
the hydrolysis
of 1 mol of ATP is probably
in the
range of 400-600
mV (i.e., 8,000-12,000
callmol).
Therefore,
even if 1
Na were actively
pumped
per ATP there would
be scarcely
enough
energy
to generate
the required
E,,; if 3 Na were pumped
per ATP
hydrolyzed,
the required
E,, is energetically
impossible.
FIG. 2. A : model
proposed
by Machen
and Diamond
(41) to account
for small
serosa-negative
PD across
rabbit
gallbladder.
B:
model proposed
by Field et al. (24) to account
for the fact that under
short-circuit
conditions,
the rate of active
Cl absorption
by flounder
small intestine
exceeds
that of active
Na absorption.
Clearly,
when
the model
illustrated
in A is short
circuited,
a current
must
flow
across the intercellular
space comprised
of cations
(Na) flowing
from
serosal
to mucosal
solution
and/or
anions
(Cl) flowing
in opposite
direction.
If the permeability
(transference
number)
of the tight
junctions
for Na is greater
than
that for Cl, most of the current
passing
through
the junctions
will be due to recycling
of Na.
TABLE
flounder
Control
3. Na and Cl transport
small intestine
13.6
11.6
2.0
Na-free
Cl-free
13.8
13.9
-O.l*
by short-circuited
8.4
3.0
5.4
-3.4
-4.0
6.3*
6.2*
O.l*
-0.2”
0
o.o*
*
-0.2”
Data from Field
et al. (24). J&-.,
is the unidirectional
flux of i
from
the mucosal
solution
to the serosal
solution;
J&
is the
unidirectional
flux in the opposite
direction;
Jiet = Jk*
- Jk,.
All
fluxes
and the short-circuit
current
(I,,) are in peq/cm2
per h. $m is
the transepithelial
electrical
potential
difference
with respect
to the
mucosal
solution
in mV.
* Differ
from control
byP < 0.05.
(Zsc>is in excellent agreement with the algebraic sum of
the Na and Cl currents. However, in the absence of Na,
active Cl absorption ceases and the Zsc disappears; and
in the absence of Cl, active Na absorption ceases and
the Zsc again disappears.
b) Frizzell et al. (28) have recently demonstrated
that
the unidirectional
influx of Na into the transporting
cells of winter flounder is abolished when Cl in the
mucosal solution is replaced with sulfate, and that the
unidirectional
influx of Cl is abolished when Na is
replaced with choline. The absolute decreases observed
in these experiments were similar in magnitude
and
large enough to account for the net flux changes shown
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tally coupled Cl absorption
on purely
energetic
grounds?
In short, the biological function of many leaky epithelia is to absorb Na and Cl at relatively rapid rates
against minimal transepithelial
electrochemical potential differences. In these systems the efficiency of energy
conversion (i.e., the ratio of osmotic work performed to
the associated investment of metabolic energy) may be
vanishingly
small. However, a far more important
biological parameter, in the case of many epithelia, is
the rate of transport per unit of energy invested, or the
efficacy of the transport process (22), rather than the
steepness of the gradients that can be generated or
sustained. The evolution of mediated, neutral NaCl cotransport processes in leaky epithelia assures the attainment of a high level of efficacy which could not
possibly be achieved by electrical coupling between the
flows of these ions.
FIELD,
EDITORIAL
F5
REVIEW
Electrogenic
Chloride
Secretion
The first clear-cut example of electrogenic Cl secretion was provided in 1955 by Hogben (33) who demonstrated that the transepithelial
electrical potential difference and short-circuit
current across frog gastric
mucosa, in vitro, can be accounted for by active Cl
transport from the serosal (“nutrient”)
solution to the
mucosal (“secretory”) solution. For a while thereafter,
this appeared to be a peculiarity of gastric mucosa, but
during the past lo-15 years other compelling examples
of electrogenic Cl secretion have been reported and a
pattern of common characteristics appears to be emerging. Among these “shared” properties are the following.
Electrogenic Cl secretion is dependent on the presence
of Na in the solution bathing the basolateral or contraluminal membrane of the secretory cells and is inhibited
by the presence of ouabain in that solution.
Cl secretion is rapidly elicited or enhanced by procedures that result in an elevation of intracellular
levels
of CAMP (see, for example, Table 4). In many instances,
physiological “primary messengers” such as hormones
and/or neurotransmitters
have been identified. In other
instances, CAMP remains a second messenger in search
of a first.
Cl secretion may be elicited by Ca ionophores (e.g.,
A23187) which, in the presence of extracellular
Ca,
bring about an increase in cytoplasmic free Ca activity
(Table 4). In some instances, procedures that result in
an increase in cell CAMP also increase the rate of Ca
exchange by the tissue, suggesting that they lead to an
increase in cytoplasmic Ca activity by bringing about
the release of sequestered Ca from intracellular
structures.
In several cases, Cl secretion is inhibited
by the
presence of furosemide in the solution bathing the
contraluminal
surface of the secretory cells.
Table 5 lists examples of tissues that possess electrogenie Cl secretory processes and indicates the extent to
which they conform to the pattern described above.
These observations suggest the model for cAMPstimulated electrogenic Cl secretion illustrated
in Fig. 3.
According to this model:
a) At the basolateral membrane,
a neutral NaCl
entry mechanism mediates the movement of Cl into the
cell against an electrochemical
potential difference by
coupling its flow to the flow of Na down its electrochemical potential difference. This coupled uptake is inhibited by furosemide, as is the case for the coupled
NaCl absorptive process in flounder small intestine (24,
28) .
TABLE
4. Effects of CAMP and A23187 on sodium
and chloride fluxes across rabbit colon
\a
J m-+s
J::,,,
J,‘,al
Pm
Control
+ CAMP
3.1
4.2
1.5
2.3
1.6
1.9
5.5
5.0
4.3
6.4*
1.2
-1.4
1.6
3.8*
4.5
6.3*
Control
+ A23187
3.4
3.4
1.5
1.6
1.9
1.8
6.3
6.1
4.9
7.5*
1.4
-1.4”
2.2
4.6*
4.6
6.5*
J“l
+\
J“'
\ -111
I SC
1lCl
Gl
the unidirectional
flux
of i from
the mucosal
JL+s designates
solution
to the serosal
solution;
JL+,,, is the unidirectional
flux in the
opposite
direction;
and Ji,, = Jb-,
- Jj,-,, . All values
are in peq/cm’
per h with
the exception
of G, (the tissue
conductance)
which
is
expressed
in mmho/cm
T Data on the effects of CAMP are from Ref.
27 and those
dealing
with the effects
of A23187
are from
Ref. 25.
Standard
errors
have
been omitted
for clarity.
* Differ
from
control
by P < 0.05.
TABLE
5. Possible examples of Na-coupled
electrogenic Cl secretion
Tissue
Rabbit
ileum
Rabbit
colon
Frog
stomach
Dogfish
rectal
gland
Frog cornea
Killifish
operculum
Canine
tracheai
epithelium
Sodium
Dependent
Inh&~~,b”
Stimu%tIP
+ (45)
+m
+ (51)
+(*)
+w
+(15,
+ (57)
+ (57)
+ (57)
+(ll,
71, 72)
+ (16)
+tll)
+ (16)
+ (73)
+(16)
+ La
+ca
+(2)
* M. Field, unpublished
lished observations.
observations.
$ F. Al-Bazzaz,
51)
+ (23)
+ (27)
+(51)
Stimulated by
Ca-Ionophore
Inhibited
.
bZ3t$Z-
+(7)
+ (25)
+(*)
+(-u
+ (57)
+(12)
* R. A. Frizzell,
personal
communication.
+ (74)
+(16)
+c#3
unpub-
Downloaded from http://ajprenal.physiology.org/ by 10.220.32.246 on June 16, 2017
in Table 3, suggesting that there is a one-for-one neutral
coupled NaCl entry mechanism at the mucosal membrane and that this mechanism provides most, if not all,
of the Cl transported across the epithelium.
Frizzell et
al. (28) also demonstrated that furosemide, when added
to the mucosal solution, inhibits the coupled entry of
NaCl, thereby reducing the transepithelial
net Cl flux
and PD.
c) M. E. Dtiey,
R. A. Frizzell, and S. G. Schultz
(unpublished
o b servations) have measured intracellular Cl activities in flounder intestinal cells using Clselective microelectrodes.
They found that a$l is approximately 3 times that predicted by the Nernst equation for a passively distributed monovalent anion; however, in the absence of Na, acC1does not differ from the
value predicted for a passive distribution.
The data obtained from studies on flounder small
intestine are in strict qualitative
agreement with those
described above for rabbit gallbladder, the only exception being that JEA1 exceeds Ji$ under short-circuit
conditions. However, as pointed out by Field et al. (24)
this observation can be readily accommodated by the
model illustrated
in Fig. 2B, since under short-circuit
conditions (i.e., in the presence of an external current),
the requirement
for electroneutral
flow across the epithelium per se is relieved.
Ion transport in flounder small intestine, therefore,
appears to be an instance in which the principal mechanism responsible for active Na and Cl transport is a
tightly coupled, neutral NaCl entry mechanism but in
which, at the same time, there is a significant serosanegative PD. It is tempting to speculate that such a
mechanism may be responsible for active Cl absorption
by the ascending thick limb of the loop of Henle (9, 10,
50) and studies explicitly
designed to examine this
possibility are clearly indicated.
FRIZZELL,
F6
MUCOSAL
SOLUTION
CELL
SEROSAL
SOLUTION
rosemide
FIG. 3. A working
model for electrogenic
CAMP. See text for discussion.
Cl secretion induced by
AND SCHULTZ
for a secretory stimulus that “triggers” an increase in
the Cl permeability
of the apical membrane.
An increase in cytosolic Ca could conceivably bring about an
increase in the Cl permeability of the apical membrane
simply by interacting with membrane components that
bear fixed negative charges.
Although the model illustrated in Fig. 3 can accommodate all of the available data on CAMP-induced
active Cl secretory processes by a variety of epithelia,
there are a number of ttmissing links” that await direct
experimental confrontation.
Thus:
a) Although Cl secretion is Na dependent, there is no
direct evidence for Na-coupled Cl transport across the
basolateral membranes. The importance of distinguishing between Na dependence and Na coupling has been
stressed and illustrated previously (56).
b) There are no direct measurements
of Cl activities
in single secretory cells under “resting” and “secreting”
conditions, and it may be very difficult to obtain such
data in tissues characterized by a heterogeneous
cell
population. The data reported by Klyce and Wong (37)
suggest that the activity of Cl in rabbit cornea is greater
than that in the “aqueous” and “tear” sides but the
interpretation
of these experimental
results is complicated by the complex multicellular
structure of this
tissue.5
c) Needless to say, further studies are needed in order
to establish, unequivocally,
the role of intracellular
CAMP and/or Ca in the activation
of the secretory
process.
In short, this model is presented as a working hypothesis in the hope that it will serve to stimulate future
investigations
whose results will critically test its basic
tenets; nothing more can be asked of any model.
Some Speculations on “Active”
Transepithelial
Chloride Transport
It should be apparent from this brief review that what
is often referred to as “active” Cl absorption or secretion
by a variety of epithelia, widely distributed throughout
the animal kingdom, is in fact the result of NaCl cotransport across one of the limiting membranes where
the energy for Cl transport against an electrochemical
potential difference is derived from the Na gradient
across that barrier. This Na gradient, in turn, is the
result of active extrusion of Na from the cell mediated
by a well-defined enzymatic reaction. Accordingly,
Cl
transport is not directly linked to a source of metabolic
energy and should be referred to as “secondary active
transport.”
The only other well-defined mode of transepithelial
Cl transport in which Cl is propelled against an electrochemical potential difference appears to involve a onefor-one exchange of Cl for HCO, (cf. Ref. 40). Although
this mechanism requires further study, it is not unreasonable to suggest, on the basis of available data, that
the energy for uphill Cl transport may be derived from
coupling to the countertransport
of HCO:{ out of the cell
5 Preliminary
studies by M. E. Duffey,
Schultz on the Cl-secreting rectal gland
accznthias (see Table 5), indicate that the
6-8 times that expected for an equilibrium
R. A. Frizzell, and S. G.
of the dogfish, Sgualus
intracellular
Cl activity is
distribution.
Downloaded from http://ajprenal.physiology.org/ by 10.220.32.246 on June 16, 2017
b) Na carried into the cell by means of the coupled
mechanism is extruded by the ouabain-sensitive
Na-K
exchange pump which has been identified in the basolateral membranes of a wide variety of epithelial cells
(43, 60, 61, 63) [’inc 1u d ing secretory cells (22, 37)].
Therefofe, Na simply recycles across the basolateral
membrane and the Na gradient across that membrane
is maintained
by the energy-dependent
pump mechanism.
c) Active Cl secretion is the result of passive Cl exit
from the cell across the luminal or mucosal membranes
down a favorable electrochemical
potential difference.
d) An increase in cell CAMP and/or cytosolic free Ca
could stimulate
Cl secretion either by activating
a
latent, coupled NaCl entry mechanism at the basolatera1 membrane or by increasing the permeability of the
apical membrane to Cl, and, thereby, ease its diffusional outflow from the cell. The latter possibility seems
more attractive
for the following reasons. First, as
shown in Table 4, Cl secretion induced in rabbit colon
with either CAMP or A23187 is accompanied by a large
increase in tissue conductance (Gt ). Since the bidirectional Na fluxes and the unidirectional
flux of Cl from
mucosa to serosa are not affected, it seems reasonable
to conclude tha .t the conductance of the pa .racel lular
shunt pathway is no t significant] .y affected and that
most, if not all, of the increase originates in the transcellular pathway (27). Second, the electrophysiologic
studies of Klyce and Wong (37) demonstrate,
quite
clearly, that active Cl secretion by rabbit cornea, induced by epinephrine
acting via CAMP, is associated
with a marked decrease in the resistance of the outer
barrier (“tear side”) which would correspond to the
apical or luminal membrane of “hollow” organs. This
decrease in resistance was not observed in Cl-free mediums, suggesting that it represents a rather specific
increase in the Cl permeabi .lity of that barrier. Berridge
et al. (4) have demonstrated
a calcium-dependent
decrease in the apical resistance of insect salivary gland
cells in response to serotonin and CAMP, and similar
findings have been reported for pancreatic exocrine cells
in response to secretagogues and Ca-ionophores (cf. Ref.
29)
Accordingly, at present there does not seem to be any
cogent reason to speculate that increa .ses in cell CAMP
and/or cytosol .ic free Ca activ ,ate a latent NaCl cotransport process. Instead, it seems far more reasonable
to postulate that this mechanism 1s active in the absence of secretagogues and serves to “prime” the cells
FIELD,
EDITORIAL
F7
REVIEW
down its electrochemical potential gradient?
Finally, although a Na-activated plasma membrane
ATPase -has bee6 well characterized
in a number of
epithelia, there is at present no compelling evidence for
a- Cl-activated
ATPase in any of the Cl-transporting
epithelia. Earlier findings of an anion-sensitive ATPase
in plasma membrane preparations appear to be due to
contamination
with fragments of mitochondrial
membranes (8).
It may not be too presumptuous,
therefore, to speculate that there are no Cl transport
mechanisms in
animal epithelia that are energized by direct coupling
V
to the flow of a metabolic reaction (i.e., “primary active
transport”).
Certainly,
every effort should be made to
rigorously establish or exclude the participation
of Cl in
co-transport or countertransport
processes in those tissues which are capable of t&w@rting
Cl against an
electrochemical potential difference.
These
studies
were supported
by research
grants
from
the National
Institute
of Arthritis,
Metabolism,
and Digestive
Diseases
(AM-16275,
AM-18199,
and AM-21345).
R. A. Frizzell
is the recipient
of a Research
Career
Development
Award
from the National
Institute
of Arthritis,
Metabolism,
and
Digestive
Diseases
(AM-00173).
REFERENCES
6 Short-circuited
in vitro
preparations
of rabbit
descending
colon
bathed
by a normal
Ringer
solution
containing
20 mM HCOs
and
equilibrated
with a mixture
of 95% O,-5%
CO, “actively”
absorb
Cl.
This process
is .electrically
silent
and almost
certainly
involves
a
one-for-one
exchange
of Cl for HCO,
across
the apical
membrane
(27). The electrical
potential
difference
across the apical
membrane
under
short-circuit
conditions
is 46 mV, cell interior
negative
with
respect
to the mucosal
solution.
Therefore,
if HCO,
is at electrochemical
equilibrium
across this membrane,
the intracellular
HCO,
concentration
predicted
by the Nernst
equation
should
be approximately
3.6 mM.
Assuming
that the intracellular
Pco2 is equal to
that in the bathing
medium
(40 mmHg),
the intracellular
pH would
be 6.6 in the presence
of 3.6 mM HCO,.
This pH is considerably
lower than that estimated
for most cells.
Even if the cell pH were
approximately
7.0, as estimated
for a variety
of nonepithelial
cells,
the intracellular
HCO,
concentration
would be 9.5 mM, or 2.6 times
the predicted
equilibrium
value.
Therefore,
tight
coupling
between
HCO,
efflux
across
the apical
membrane
and Cl influx
(“countertransport”
or “antiport”)
could raise cell Cl to levels well above its
equilibrium
value,
and, in a manner
analogous
to NaCl co-transport,
energize
transcellular
Cl transport.
1057, 1972.
12. CANDIA,
0. A., R. MONTOREANO,
AND S. M. PODOS. Effect
of the
ionophore
A23187
on chloride
transport
across
isolated
frog
cornea.
Am. J. Physiol.
233: F94-FlOl,
1977 or Am. J. Physiol:
Renal FZuid Electrolyte
Physiol.
2: F94-F101,
1977.
13. CHIEN,
W., AND C. E. STEVENS. Coupled
active
transport
of Na
and Cl across forestomach
epithelium.
Am. J. Physiol.
223: 9971003, 1972.
14. CREMASCHI,
D., AND S. HENIN.
Na and Cl transepithelial
routes
in rabbit
gallbladder.
Tracer
analysis
of the transports.
Pfluegers Arch.
361: 33-41, 1975.
15. DAVENPORT,
H. W. Effect
of ouabain
on acid secretion
and
electrolyte
content
of frog gastric
mucosa.
Proc. Sot. Exp. BioZ.
Med. 110: 613-615,
1962.
16. DEGNAN,
K. J., K. J. KARNAKY,
AND J. A. ZADUNAISKY.
Active
chloride
transport
in the in vitro
opercular
skin of a teleost
(Fundulus
heteroclitus),
a gill-like
epithelium
rich in chloride
cells. J. Physiol.
London
271: 155-191,
1977.
17. DIAMOND,
J. M. Transport
of salt and water in rabbit
and guinea
pig gallbladder.
J. Gen. Physiol.
48: 1-14, 1964.
18. DIAMOND,
J. M. The mechanism
of solute
transport
by the
gallbladder.
J. Physiol.
London
161: 474-502,
1962.
19. DIETSCHY,
J. M. Water
and solute
movement
across
the wall of
the everted
rabbit
gallbladder.
Gastroenterology
47: 395-408,
1964.
20. DUFFEY,
M. E., K. TURNHEIM,
R. A. FRIZZELL,
AND S. G.
SCHULTZ.
Intracellular
chloride
activities
in rabbit
gallbladder:
direct
evidence
for the role of the sodium-gradient
in energizing
“uphill”
chloride
transport.
J. Membr.
BioZ. 42: 229-245,
1978.
21. ERNST, S. A. Transport
adenosine
triphosphatase
cytochemistry.
II. Cytochemical
localization
of ouabain-sensitive,
potassiumdependent
activity
in the secretory
epithelium
of the avian
salt
gland. J. Histochem.
Cytochem.
20: 23-38,
1972.
22. ESSIG, A., AND S. R. CAPLAN.
Energetics
of active
transport
processes.
J. Gen. PhysioZ.
8: 1434-1457,
1968.
23. FIELD,
M. Ion transport
in rabbit
ileal mucosa.
II. Effects
of
cyclic 3’,5’-AMP.
Am. J. Physiol.
221: 992-997,
1971.
24. FIELD, M., K. J. KARNAKY,
P. L. SMITH,
J. E. BOLT~N,
and W.
B. KINTER.
Ion transport
across
the isolated
intestinal
mucosa
of the winter
flounder,
PseudopZeuronectes
americanus.
I. Functional
and structural
properties
of cellular
and paracellular
pathways
for Na and Cl. J. Membr.
BioZ. In press.
25. FRIZZELL,
R. A. Active
chloride
secretion
by rabbit
colon:
calcium-dependent
stimulation
by ionophore
A23187.
J. Membr.
Biol. 35: 175-187,
1977.
26. FRIZ~ELL,
R. A., M. DUGAS, and S. G. SCHULTZ.
Sodium
chloride
transport
by rabbit
gallbladder:
direct
evidence
for a coupled
NaCl influx
processes.
J. Gen. Physiol.
65: 769-795,
1975.
27. FRIZZELL,
R. A., M. J. KOCH, AND S. G. SCHULTZ.
Ion transport
by rabbit
colon.
I. Active
and passive
components.
J. Membr.
BioZ. 27: 297-316,
1976.
28. FRIZZELL,
R. A., P. L. SMITH,
E. VOSBURGH,
AND M. FIELD.
Coupled
sodium-chloride
influx
across
brush
border
of flounder
intestine.
J. Membr.
BioZ. In press.
29. GARDINER,
J. D. Regulation
of pancreatic
exocrine
function
in
vitro:
initial
steps in the actions
of secretagogues.
Ann.
Rev.
Physiol.
In press.
30. GERENCSER,
G. A., S. K. HONG,
AND G. MALVIN.
Metabolic
dependence
of active
chloride
transport
in isolated
aplysia
intestine. Federation
Proc. 35: 464, 1976.
Downloaded from http://ajprenal.physiology.org/ by 10.220.32.246 on June 16, 2017
1. AHEARN,
G. A., L. A. MAGINNISS,
Y. K. SONG, AND A. TORNQUIST. Intestinal
water
and ion transport
in fresh water
malacostracan
prawns
(crustacea).
In: Water
Relations
in Membrane
Transport
in Plants
and Animals,
edited by A. M. Jungreis,
T.
K. Hodges,
A. Kleinzeller,
and S. G. Schultz.
New
York:
Academic,
1977, p. 129-142.
2. AL-BAZZAZ,
F., AND Q. AL-AWQATI.
Interaction
between
Na and
Cl transport
in canine
tracheal
mucosa.
J. AppZ. Physiol.
In
press.
3. ARMSTRONG,
W. McD.,
W. WOJTKOWSKI,
AND W. R. BIXENMAN.
A new solid-state
microelectrode
for measuring
intracellular
chloride
activities.
Biochim.
Biophys.
Actu 465: 165-170,
1977.
4. BERRIDGE,
M. J., B. D. LINDLEY,
AND W. T. PRINCE. Membrane
permeability
changes
during
stimulation
of isolated
salivary
glands
of CuZZiphoru
by 5-hydroxytryptamine.
J. PhysioZ.
London 244: 549-567,
1975.
5. BINDER,
H. J., AND C. L. RAWLINS.
Electrolyte
transport
across
isolated
large intestinal
mucosa.
Am. J. Physiol.
225: 1232-1239,
1973.
6. BLOM, H., AND H. F. HELANDER.
Quantitative
electronmicroscopical
studies
on in vitro incubated
rabbit
gallbladder
epithelium. J. Membr.
BioZ. 37: 45-61,
1977.
7. BOLTON,
J. E., AND M. FIELD.
Ca ionophore-stimulated
ion
secretion
in rabbit
ileal mucosa:
relation
to actions
of cyclic
3’,5’-AMP
and carbamylcholine.
J. Membr.
BioZ. 35: 159-173,
1977.
8. BONTING,
S. L., J. M. M. VAN AMELSVOORT,
AND J. J. H. H. M.
DE PONT. Is anion-sensitive
ATPase
a plasma
membrane
located
transport
system?
In: Gastric
Ion Transport,
edited
by K. J.
Obrink
and G. Flemstrom.
Uppsala:
Acta
Physiol.
Stand.,
Special
Suppl.,
1978.
9. BURG, M. B., AND N. GREEN.
Function
of the thick
ascending
limb of Henle’s
loop. Am. J. Physiol.
224: 659-668,
1973.
10. BURG, M. B., L. STONER, J. CARDINAL,
AND N. GREEN. Furosemide effect on isolated
perfused
tubules.
Am. J. Physiol.
225: 119124, 1973.
11. CANDIA,
0. A. Ouabain
and sodium
effects
on chloride
fluxes
across
the isolated
bullfrog
cornea.
Am. J. PhysioZ.
223: 1053-
FS
FRIZZELL,
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
69.
70.
71.
72.
73.
74.
AND
SCHULTZ
SCHULTZ,
S. G. The role of paracellular
pathways
in isotonic
fluid transport.
YaZe J. BioZ. Med. 50: 99-113,
1977.
SCHULTZ,
S. G. Sodium-coupled
solute
transport
by small intestine: a status
report.
Am. J. PhysioZ.
233: E249-E254,
1977 or
Am.
J. Physiol.:
EndocrinoZ.
Metab.
Gastrointest.
Physiol.
2:
E249-E254,
1977.
SCHULTZ,
S. G., AND P. F. CURRAN.
Coupled
transport
of sodium
and organic
solutes.
Physiol.
Rev. 50: 637-718,
1970.
SCHULTZ,
S. G., R. A. FRIZZELL,
AND H. N. NELLANS.
Ion
transport
by mammalian
small intestine.
Ann. Rev. Physiol.
36:
51-91, 1974.
SILVA, P., J. STOW, M. FIELD, L. FINE, J. N. FORREST, AND F. H.
EPSTEIN. Mechanism
of active
chloride
secretion
by shark
rectal
gland:
role of Na-K-ATPase
in chloride
transport.
Am. J. PhysioZ. 233: F298-F306,
1977 or Am.
J. PhysioZ.:
RenaZ
Fluid
Electrolyte
PhysioZ.
2: F298-F306,
1977.
SKADHAUGE,
E. Coupling
of transmural
flows of NaCl and water
in the intestine
of the eel (AnguiZZa
anguilla).
J. Exp. BioZ. 60:
535-546,
1974.
SPRING, K. A., AND G. KIMURA.
Chloride
reabsorption
by renal
proximal
tubules
ofNecturus.
J. Membr.
BioZ. 38: 233-254,
1978.
STIRLING,
C. E. Radioautographic
localization
of sodium
pump
sites in rabbit
intestine.
J. CeZZ BioZ. 53: 704-714,
1972.
STIRLING,
C. E . High-resolution
autoradiography
of “H-ouabain
binding
in salt transporting
epithelia.
J. Microsc.
106: 145-157,
1976.
SWANSON,
C. H., AND A. K. SOLOMON.
A micropuncture
investigation
of the whole tissue
mechanism
of electrolyte
secretion
by
the in vitro
rabbit
pancreas.
J. Gen. PhysioZ.
62: 407-429,
1973.
TORMEY,
J. M. Anatomical
methods
for studying
transport
across epithelia.
In: Water ReZations
in Membrane
Transport
in
Plants
and Animals,
edited by A. M. Jungreis,
T. K. Hodges,
A.
Kleinzeller,
and S. G. Schultz.
New York:
Academic,
1977, p.
233-248.
TURNBERG,
L. A., F. A. BIEBERDORF,
S. G. MORAWSKI,
AND J. S.
FORDTRAN.
Interrelationships
of chloride,
bicarbonate,
sodium
and hydrogen
transport
in the human
ileum. J. CZin. Invest.
49:
557-567,
1970.
USSING,
H. H. The AZkaZi Metal Ions in Biology.
Berlin:
Springer-Verlag,
1960.
WATLINGTON,
C. O., AND F. JESSEE, JR. Net Cl flux in shortcircuit
skin
of Rana
pipiens:
ouabain
sensitivity
and Na+K
dependence.
Biochim.
Biophys.
Acta 382: 204-212,
1975.
WATLINGTON,
C. O., S. D. JESSEE, AND G. BALDWIN.
Ouabain,
acetazolamide,
and Cl- flux in isolated
frog skin:
evidence
for
two distinct
active
Cl- transport
mechanisms.
Am. J. PhysioZ.
232: F550-F558,
1977 or Am. J. Physiol.:
Renal Fluid
Electrolyte
Physiol.
1: F550-F558,
1977.
WHEELER,
H. 0. Transport
of electrolytes
and water
across wall
of rabbit
gallbladder.
Am. J. Physiol.
205: 427-438,
1963.
WHITE,
J. F. Activity
of chloride
in absorptive
cells of Amphiuma
small intestine.
Am. J. PhysioZ.
232: E553-E559,
1977 or
Am.
J. PhysioZ.:
EndocrinoZ.
Metab.
Gastrointest.
Physiol.
1:
E553-E559,
1977.
WHITLOCK,
R. T., AND H. 0. WHEELER.
Anion
transport
by
isolated
rabbit
gallbladders.
Am.
J. Physiol.
213: 1199-1204,
1967.
ZADUNAISKY,
J. A. Active
transport
of chloride
in frog cornea.
Am. J. PhysioZ.
211: 506-512,
1966.
ZADUNAISKY,
J. A. Sodium
activation
of chloride
transport
in the
frog cornea.
Biochim.
Biophys.
Acta 282: 255-257,
1972.
ZADUNAISKY,
J. A., M. A. LANDE,
M. CHALFIE,
AND A. H.
NEUFELD.
Ion pumps
in the cornea
and their
stimulation
by
epinephrine
and cyclic AMP. Exp. Eye Res. 15: 577-584,
1973.
ZADUNAISKY,
J. A., AND B. SPINOWITZ.
Drugs
affecting
the
transport
and permeability
of the cornea1
epithelium.
In: Drugs
and OcuZar
Tissues,
edited by S. Dikstein.
Basel: Karger,
1977,
p. 57-78.
Raymond A. Frizzell, Michael Field, and Stanley G. Schultz
Department of Physiology, University of Pittsburgh School of Medicine,
Pittsburgh, Pennsylvania 15261; Department of Medicine, University
of Chicago, Chicago, Illinois 60637; and Mount Desert Island
Biological Laboratory, Salsbury Cove, Maine 04672
Downloaded from http://ajprenal.physiology.org/ by 10.220.32.246 on June 16, 2017
31. GRAF, J., AND G. GIEBISCH.
Intracellular
sodium
activity
and
sodium
transport
in Necturus
gallbladder
epithelium.
J. Membr.
BioZ. In press.
32. HENIN,
S., AND D. CREMASCHI.
Intracellular
ion route in rabbit
gallbladder.
Electric
properties
of the epithelial
cells. PfZuegers
Arch.
355: 125-139,
1975.
33. HOGBEN,
C. A. M. Active
transport
of chloride
by isolated
frog
gastric
mucosal
potential.
Am. J. PhysioZ.
180: 641-649,
1955.
34. HOUSE, C. R., AND K. GREEN. Ion and water transport
in isolated
intestine
of the marine
teleost,
Cottus
scorpius.
J. Exp. BioZ. 42:
177-189,
1965.
35. HUBEL,
K. A. The ins and outs of bicarbonate
in the alimentary
tract. Gustroenterology
56: 647-651,
1968.
36. KARNAKY,
K. J., JR., L. B. KINTER,
W. B. KINTER,
AND C. E.
STIRLING.
Teleost
chloride
cell. II. Autoradiographic
localization
of gill Na,K-ATPase
in killifish
FunduZus
heteroczitus
adapted
to low and high salinity
environments.
J. CeZZ BioZ. 70: 157-177,
1976.
37. KLYCE, S. D., AND R. K. S. WONG. Site and mode of adrenaline
action
on chloride
transport
across
the rabbit
cornea1
epithelium. J. PhysioZ.
London
266: 777-799,
1977.
38. LAHLOU,
B., AND B. FOSSAT. Mechanisme
du transport
de l’eau
et du se1 a travers
la vessie urinaire
d’un Poisson
teleosteen
en
eau deuce,
la truite
arc-en-ciel.
C. R. Acad.
Sci. 273: 2108, 1971.
39. LEAF, A. Transepithelial
transport
and its hormonal
control
in
toad bladder.
Ergeb.
Physiol.
BioZ. Chem. Exp. Pharmakol.
56:
216-263,
1965.
40. LESLIE, B. R., J. H. SCHWARTZ,
AND P. R. STEINMETZ.
Coupling
between
Cl absorption
and HCO,
secretion
in turtle
urinary
bladder.
Am. J. PhysioZ.
25: 610-617,
1973.
41. MACHEN,
T. E., AND J. M. DIAMOND.
An estimate
of the salt
concentration
in the lateral
intercellular
spaces of rabbit
gallbladder
during
maximal
fluid transport.
J. Membr.
BioZ. 1: 194213, 1969.
42. MARTIN,
D. W., AND J. M. DIAMOND.
Energetics
of coupled
active
transport
of sodium
and chloride.
J. Gen. Physiol.
50: 295315, 1966.
43 MILLS, J. W., AND D. R. DIBONA.
Distribution
of Na pump sites
in the frog gallbladder.
Nature
271: 273-275,
1978.
44 NELLANS,
H. N., R. A. FRIZZELL,
AND S. G. SCHULTZ.
Coupled
sodium-chloride
influx
across the brush
border
of rabbit
ileum.
Am. J. Physiol.
225: 467-475,
1973.
45 NELLANS,
H. N., R. A. FRIZZELL,
AND S. G. SCHULTZ.
Brush
border
processes
and transepithelial
Na and Cl transport
by
rabbit
ileum.
Am. J. PhysioZ.
226: 1131-1141,
1974.
46 NELLANS,
H. N., AND S. G. SCHULTZ.
Relations
among
transepithelial
sodium
transport,
potassium
exchange
and cell volume
in rabbit
ileum. J. Gen. Physiol.
68: 441-463,
1976.
47 OS, C. H., VAN, AND J. F. G. SLEEGERS.
Correlation
between
(Na-K)-activated
ATPase
activities
and the rate of isotonic
fluid
transport
of gallbladder
epithelium.
Biochim.
Biophys.
Acta 241:
89-96,
1971.
48. QUAY,
J. F., AND W. M. ARMSTRONG.
Sodium
and chloride
transport
by isolated
bullfrog
small
intestine.
Am. J. PhysioZ.
217: 694-702,
1969.
49. RENFRO, J. L. Interdependence
of active
Na and Cl transport
by
the isolated
urinary
bladder
of the teleost,
Pseudopleuronectes
americanus.
J. Exp. ZooZ. 199: 383-390,
1977.
50. MOCHA,
A. S., AND J. P. KOKKO.
Sodium,
chloride
and water
transport
in the medullary
thick
ascending
limb
of Henle.
Evidence
for active
chloride
transport.
J. CZin. Invest.
52: 612623, 1973.
51. SACHS, G., J. G. SPENNEY,
AND M. LEWIN.
H+ transport:
regulation and mechanism
in gastric
mucosa
and membrane
vesicles.
PhysioZ.
Reu. 58: 106-173,
1978.
52. SCHULTZ,
S. G. Is a coupled
Na-K
exchange
pump
involved
in
active
transepithelial
Na transport?
A status
report.
In: Membrane Transport
Processes,
edited
by J. F. Hoffman.
New York:
Raven Press,
1978, vol. 1, p. 213-227.
FIELD,