Dissociation of cellular K+ accumulation from net Na+ transport by toad urinary bladder

Apical Na+ permeability of frog skin during serosal Cl- replacement

Gluconate substitution for serosal Cl- reduces the transepithelial short-circuit current (Isc) and depolarizes short-circuited frog skins. These effects could result either from inhibition of basolateral K+ conductance, or from two actions to inhibit both apical Na+ permeability (PapNa) and basolateral pump activity. We have addressed this question by studying whole-and split-thickness frog skins. Intracellular Na+ concentration (CcNa) and PapNa have been monitored by measuring the current-voltage relationship for apical Na+ entry. This analysis was conducted by applying trains of voltage pulses, with pulse durations of 16 to 32 msec. Estimates of PapNa and CcNa were not detectably dependent on pulse duration over the range 16 to 80 msec. Serosal Cl- replacement uniformly depolarized short-circuited tissues. The depolarization was associated with inhibition of Isc across each split skin, but only occasionally across the whole-thickness preparations. This difference may reflect the better ionic exchange between the bulk medium and the extracellular fluid in contact with the basolateral membranes, following removal of the underlying dermis in the split-skin preparations. PapNa was either unchanged or increased, and CcNa either unchanged or reduced after the anionic replacement. These data are incompatible with the concept that serosal Cl- replacement inhibits PapNa and Na,K-pump activity. Gluconate substitution likely reduces cell volume, triggering inhibition of the basolateral K+ channels, consistent with the data and conclusions of S.A. Lewis, A.G. Butt, M.J. Bowler, J.P. Leader and A.D.C. Macknight (J. Membrane Biol. 83:119-137, 1985) for toad bladder. The resulting depolarization reduces the electrical force favoring apical Na+ entry. The volume-conductance coupling serves to conserve volume by reducing K+ solute loss. Its molecular basis remains to be identified.

Diacylglycerols stimulate short-circuit current across frog skin by increasing apical Na+ permeability

The phorbol ester TPA (12-O-tetradecanoylphorbol-13-acetate) stimulates baseline Na+ transport across frog skin epithelium and partially inhibits the natriferic response to vasopressin. The effects are produced largely or solely when TPA is added to the mucosal surface of the tissue. Although TPA activates protein kinase C, it has other effects, as well. Thus, the biochemical basis for the effects and the ionic events involved have been unclear. Furthermore, the physiologic implications have been obscure because of the sidedness of TPA's actions. We now report that two synthetic diacylglycerols (DAG) replicate the stimulatory and inhibitory effects of TPA on frog skin. DAG is the physiologic activator of PKC. In this tissue, it produces half-maximal stimulation at a concentration of less than or equal to 19 microM. In contrast to TPA, DAG is about equally effective from either tissue surface. In a series of eight experiments, DAG was found to depolarize the apical membrane. Diacylglycerol also increases the paracellular conductance of frog skins bathed with mucosal Cl- Ringer's solution. The latter effect can be minimized by replacing NO3- for Cl- in the mucosal solution. Under these conditions, combined intracellular and transepithelial measurements indicated that DAG increased both the apical Na+ permeability and intracellular Na+ concentration. These results are qualitatively similar to the effects of cyclic 3',5'-AMP on this tissue, suggesting that activation of PKC by DAG causes phosphorylation of the same or nearby gating sites phosphorylated by cAMP. We propose that apical Na+ entry is regulated in part by activation of PKC, and that insulin may be a physiologic trigger of this activation.

Apical membrane K conductance in the toad urinary bladder

The conductance of the apical membrane of the toad urinary bladder was studied under voltage-clamp conditions at hyperpolarizing potentials (mucosa negative to serosa). The serosal medium contained high KCl concentrations to reduce the voltage and electrical resistance across the basal-lateral membrane, and the mucosal solution was Na free, or contained amiloride, to eliminate the conductance of the apical Na channels. As the mucosal potential (Vm) was made more negative the slope conductance of the epithelium increased, reaching a maximum at Vm = -100 mV. This rectifying conductance activated with a time constant of 2 msec when Vm was changed abruptly from 0 to -100 mV, and remained elevated for at least 10 min, although some decrease of current was observed. Returning Vm to +100 mV deactivated the conductance within 1 msec. Ion substitution experiments showed that the rectified current was carried mostly by cations moving from cell to mucosa. Measurement of K flux showed that the current could be accounted for by net movement of K across the apical membrane, implying a voltage-dependent conductance to K (GK). Mucosal addition of the K channel blockers TEA and Cs had no effect on GK, while 29 mM Ba diminished it slightly. Mucosal Mg (29 mM) also reduced GK, while Ca (29 mM) stimulated it. GK was blocked by lowering the mucosal pH with an apparent pKI of 4.5. Quinidine (0.5 mM in the serosal bath) reduced GK by 80%. GK was stimulated by ADH (20 mU/ml), 8-Br-cAMP (1 mM), carbachol (100 microM), aldosterone (5 X 10(-7) M for 18 hr), intracellular Li and extracellular CO2.

Chloride secretion by canine tracheal epithelium: IV. Basolateral membrane K permeability parallels secretion rate

We evaluated the K conductance properties of the basolateral membranes of the surface cells of canine tracheal epithelium using microelectrode techniques. Studies were conducted under basal conditions (indomethacin, 10(-6) M, mucosal solution) and after stimulation of electrogenic Cl secretion with epinephrine (10(-6) M, serosal solution). Elevated serosal solution [K] depolarized the electrical potential differences across the apical (psi a) and basolateral (psi b) membranes in both the presence and absence of epinephrine. Serosal barium (0.5 mM) also depolarized psi a and psi b and selectively increased basolateral membrane resistance threefold. We also used K-selective microelectrodes to determine cell K activity (acK) and the driving force for K transport across the limiting membranes under basal and stimulated conditions. Stimulation of Cl secretion was not associated with significant changes in psi b or acK so that the driving force for K exit from cell to serosal solution (ca. 20 mV) was not altered. There was close agreement between the basolateral membrane electromotive force (Eb) determined from prior studies (M.J. Welsh, P.L. Smith and R.A. Frizzell, J. Membrane Biol. 71:209-218, 1983) and the chemical potential difference for K across this barrier (EbK) in the presence and absence of epinephrine. These findings support the notion that the basolateral membrane is characterized by a high conductance to K under both secreting and nonsecreting conditions and indicate that the decrease in basolateral membrane resistance that accompanies stimulation of Cl secretion results from an increase in its K conductance. This obviates changes in acK that would otherwise accompany increased Na/K pump activity and, by hyperpolarizing psi a, establishes the electrical driving force for Cl secretion across the apical membrane.

Intracellular ion activities in frog skin in relation to external sodium and effects of amiloride and/or ouabain

Intracellular activities of sodium, potassium and chloride ions, aiNa, aiK, and aiCl were measured with ion-selective single-, double- and triple-barrelled micro-electrodes in skin and isolated epithelia of Rana temporaria bathed on both sides with normal or modified physiological saline. Apical and basolateral membrane potentials, psi ac and psi cs and resistance Ra and Rb respectively were also measured and from the latter the fractional resistance of the apical membrane, F(Ra) and voltage divider ratio, delta psi ac/delta psi cs were measured as criteria of satisfactory membrane penetration by the micro-electrodes. Under control conditions, aiNa was 12.3 +/- 0.8 mM, aiK was 70.3 +/- 22 mM and aiCl was 20.3 +/- 1.6 mM with psi ac averaging -38.0 +/- 3.2 mV. When 10(-4) M-amiloride was added to the apical bathing fluid aiNa fell within 10 min to 1.18 +/- 0.1 mM and aiCl to 5.2 +/- 0.9 mM, while aiK increased to 86.2 +/- 3.8 mM as measured from the basolateral border of isolated epithelia. The sodium transport pool of the skin was measured from the fall in aiNa in the presence of amiloride and could be expressed as 33 X 10(-9) mol cm-2 of epithelium. The mean rate of fall of aiNa under these conditions corresponded to an efflux rate at the basolateral border of 30.1 X 10(-9) mol cm-2 min-1 (48 microA cm-2) giving a half-time for turnover of the sodium transport pool of 33 s. Reduction of sodium concentration in the apical fluid from the normal 79 mM-Na to 10, 1 and 0.1 mM caused aiNa to fall in stages to 2 mM. Because psi ac increased in negativity to -101 mV in the process, this driving force for passive sodium accumulation, more than offset the increased sodium gradient opposing sodium influx across the apical border.

Apical sodium entry in split frog skin: current-voltage relationship

Apical Na+ entry into frog skin epithelium is widely presumed to be electrodiffusive in nature, as for other tight epithelia. However, in contrast to rabbit descending colon and Necturus urinary bladder, the constant field equation has been reported to fit the apical sodium current (INa)-membrane potential (psi mc) relationship over only a narrow range of apical membrane potentials or to be inapplicable altogether. We have re-examined this issue by impaling split frog skins across the basolateral membrane and examining the current-voltage relationships at extremely early endpoints in time after initiating pulses of constant transepithelial voltage. In this study, the rapid transient responses in psi mc were completed within 0.5 to 3.5 msec. Using endpoints to 1 to 25 msec, the Goldman equation provided excellent fits of the data over large ranges in apical potential of 300 to 420 mV, from approximately -200 to about +145 mV (cell relative to mucosa). Split skins were also studied when superfused with high serosal K+ in order to determine whether the INapsi mc relationship could be generated purely by transepithelial measurements. Under these conditions, the basolateral membrane potential was found to be -10 +/- 3 mV (cell relative to serosa, mean +/- SE), the basolateral fractional resistance was greater than zero, and the transepithelial current was markedly and reversibly reduced. For these reasons, use of high serosal K+ is considered inadvisable for determining the INa-psi mc relationship, at least in those tissues (such as frog skin) where more direct measurements are technically feasible. Analysis of the INa-psi mc relationships under baseline conditions provided estimates of intracellular Na+ concentration and of apical Na+ permeability of 9 to 14 mM and of approximately 3 X 10(-7) cm X sec-1, respectively, in reasonable agreement with estimates obtained by different techniques.

Microelectrode study of K+ accumulation by tight epithelia: I. Baseline values of split frog skin and toad urinary bladder

Toad bladder and split frog skin were impaled with fine-tipped single- and double-barrelled K+-selective microelectrodes. In order to circumvent membrane damage induced by impaling toad bladder, a null point method was developed, involving elevations of mucosal potassium concentration. The results suggest that intracellular potassium activity of short-circuited toad bladder is approximately 82 mM, twice as large as earlier estimates. Far more stable and rigorously defined intracellular measurements were recorded from short-circuited split frog skins. The intracellular positions of the micropipette and microelectrode tips were verified by transient hyperpolarizations of the membrane potential with mucosal amiloride or by transient depolarizations with serosal barium or strophanthidin. Simultaneous impalement of distant cells with separate micropipettes demonstrated that both the baseline membrane potentials and the responses to depolarizing agents were similar, further documenting that frog skin is a functional syncytium. Measurements with double-barrelled microelectrodes and simultaneous single-barrelled microelectrodes and reference micropipettes suggest that the intracellular potassium activity is about 104 mM, lower than previously reported. Taken together with measurements of intracellular potassium concentration, this datum suggests that potassium is uniformly distributed within the epithelial cells.

Microelectrode study of K+ accumulation by tight epithelia: II. Effect of inhibiting transepithelial Na+ transport on reaccumulation following depletion

The effects of restoring serosal potassium to potassium-depleted toad urinary bladders have been re-examined using double-barrelled microelectrodes. The data confirm the existence of a time-lag phenomenon, a dissociation between potassium reaccumulation and restoration of short-circuit current. Returning serosal potassium stimulates an increase in intracellular potassium activity 21-26 min before any increase can be detected in short-circuit current. The reaccumulation of potassium has been further studied using split frog skin, a far more suitable preparation for electrophysiologic study than toad bladder. Under baseline short-circuited conditions, potassium is accumulated against an electrochemical gradient of 22 +/- 4 mV. Reaccumulation of potassium by potassium-depleted tissues can be blocked by inhibiting the Na,K-exchange pump with high concentrations of ouabain. On the other hand, blocking apical sodium entry by the addition of 10(-4) M amiloride to the outer bathing medium does not interfere with reaccumulation of potassium. The data support the concept that the time-lag phenomenon of toad bladder reflects stimulation of potassium reaccumulation by the sodium pump in exchange for the extrusion of excess cell sodium collected during the period of potassium depletion. This reaccumulation of potassium can proceed before the entry of significant added amounts of sodium across the apical plasma membrane.

Ion selectivity of the apical membrane Na channel in the toad urinary bladder

The ion selectivity of the apical membrane Na channel in the toad urinary bladder was investigated. The electrical potential difference and resistance across the basal-lateral membrane were reduced using high concentrations of KCl in the serosal bathing medium, and gradients for various ions were imposed across the apical membrane by altering the composition of the mucosal bathing medium. Ion fluxes through the channel were measured as the transepithelial current inhibited by amiloride, a specific blocker of the channel's Na conductance. The selectivity sequence for alkali metal cations was H greater than Li greater than Na much greater than K. K permeability was barely detectable; the selectivity for Na over K was about 1000:1. Ammonium, hydroxyl ammonium and hydrazinium ions were, like K, virtually impermeant. The results suggest that the size of the unhydrated ion is an important factor in determining permeability in this channel.

Kinetic studies on the effects of ouabain on Na+ fluxes in frog skin

Among 48 pieces of paired frog skins of Rana pipiens in Ringer's solution, 10 pieces showed a strictly monotone decrease in the short circuit current (SCC) following ouabain treatment (10(-4) M). In 9 cases a transient attenuation, and in 27 cases a distinct wave in the ebb of the SCC, was seen. In 2 instances, two waves were seen. Associated with the not-monotone events was a transient rise in electrical skin conductance. The reasons for these mixed skin responses are unknown. One possible reason is considered here: Early during the ouabain action, some of the Na+ entering from the mucosal side is trapped in the skin by electroneutral processes, in keeping with the already known fact that ultimately cellular KCl is partly replaced by NaCl. Computer assisted model studies show how monotone, and not-monotone "transepithelial" net Na+ flux curves can be generated. Essential conditions for the generation of not-monotone Na+ flux curves are: 1. Presence of two distinct "cellular", active Na+ pools in the model. 2. Presence of a loop pathway in which a principal "transepithelial Na+ transport compartment", and a constituent "Na+/K+ maintenance compartment", are connected to each other and to the "extracellular" compartment. The model, then, predicts under which kinetic conditions monotone and not-monotone transepithelial Na+ flux curves will be seen.

Ouabain-sensitive 86Rb(K) influx is linked to transepithelial Na transport in pig kidney cell line

The pig kidney cell line, LLC-PK1, exhibits rheogenic D-glucose coupled transepithelial Na+ transport that is inhibited by phlorizin. By measuring the difference in initial rates of influx of 86Rb+ with and without coupled Na+ transport, we can demonstrate an 86Rb+ uptake linked to Na+ transport, The simultaneous determination of phlorizin-inhibited Na coupled D-[3H] glucose uptake and 86Rb+ influx allows calculation of an Na+/Rb+ stoichiometry that is consistent with an electrogenic Na+ for Rb+ exchange.

Transepithelial Na+ transport and the intracellular fluids: a computer study

Computer simulations of tight epithelia under three experimental conditions have been carried out, using the rheogenic nonlinear model of Lew, Ferreira and Moura (Proc. Roy. Soc. London. B 206:53-83, 1979) based largely on the formulation of Koefoed-Johnsen and Ussing (Acta Physiol. Scand. 42: 298-308. 1958). First, analysis of the transition between the short-circuited and open-circuited states has indicated that (i) apical Cl- permeability is a critical parameter requiring experimental definition in order to analyze cell volume regulation, and (ii) contrary to certain experimental reports, intracellular Na+ concentration (ccNa) is expected to be a strong function of transepithelial clamping voltage. Second, analysis of the effects of lowering serosal K+ concentration (csK) indicates that the basic model cannot simulate several well-documented observations; these defects can be overcome, at least qualitatively, by modifying the model to take account of the negative feedback interaction likely to exist between the apical Na+ permeability and ccNa. Third, analysis of the strongly supports the concept that osmotically induced permeability changes in the apical intercellular junctions play a physiological role in conserving the body's stores of NaCl. The analyses also demonstrate that the importance of Na+ entry across the basolateral membrane is strongly dependent upon transepithelial potential, cmNa and csK; under certain conditions, net Na+ entry could be appreciably greater across the basolateral than across the apical membrane.

Intra- and transepithelial analytical techniques

Epithelia transport a variety of solutes and water. Study of such transport requires a determination of the driving forces responsible for transport, of the pathways through which transport occurs, and of the factors controlling such transport. Transepithelial driving forces are readily determined where the composition of the bathing media can be altered and electrical forces negated. Where substances move only through a paracellular pathway such manipulations may be adequate to define the permeability and selectivity of the pathways. For substances utilizing a cellular pathway, driving forces and permeabilities across the two dissimilar apical and basolateral cellular membranes must be determined. Where a substance can be shown to move across a membrane against its electrochemical potential gradient, the source of the energy for such movement must be assessed. This review focuses on the applicability and validity of a variety of techniques utilized for the study of epithelial transport to answer these questions. These include microelectrode techniques, chemical analyses, microprobe analysis, microscopy, and techniques for assessing the coupling of metabolism to transport.

Intracellular ionic activities in frog skin

Intracellular Na+, K+, and Cl- activities (aiNa, aiK, aiCl) and transapical membrane potentials (V0) were measured with liquid ion-exchanger and open-tip microelectrodes in isolated short-circuited frog skins (R. pipiens) incubated at 23 degrees C in normal amphibian Ringer's solution. Under control conditions aiNa = 14 +/- 3 mM, aiK = 132 +/- 10 mM and aiCl = 18 +/- 3 mM (SD). The value of aiCl is 4.4 times the value corresponding to electrochemical equilibrium for this ion. Thus, Cl- is actively accumulated by epithelial cells of the frog skin. Shortly after addition of amiloride (2--5 microM) to the apical bathing medium, aiK, aiNa, and aiCl were essentially unchanged although V0 had hyperpolarized by about 30--40 mV. During long-term exposure to amiloride aiK and aiCl did not change significantly, V0 depolarized by about 16 mV from the maximal value and aiNa decreased to 8 +/- 3 mM. Immediately after exposure to amiloride the transmembrane driving force for Na+ increased from 124 to 154 mV. During further exposure to amiloride, despite changes in both V0 and aiNa, this driving force remained virtually constant. Since Isc during this period was close to zero, it is suggested that the observed driving force for Na+ under these condition approximates the maximal driving force generated by the Na+--K+ ATP-ase pump in the basolateral cell membrane.

Regulation of the sodium permeability of the luminal border of toad bladder by intracellular sodium and calcium: role of sodium-calcium exchange in the basolateral membrane

Sodium movement across the luminal membrane of the toad bladder is the rate-limiting step for active transepithelial transport. Recent studies suggest that changes in intracellular sodium regulate the Na permeability of the luminal border, either directly or indirectly via increases in cell calcium induced by the high intracellular sodium. To test these proposals, we measured Na movement across the luminal membrane (th Na influx) and found that it is reduced when intracellular Na is increased by ouabain or by removal of external potassium. Removal of serosal sodium also reduced the influx, suggesting that the Na gradient across the serosal border rather than the cell Na concentration is the critical factor. Because in tissues such as muscle and nerve a steep transmembrane sodium gradient is necessary to maintain low cytosolic calcium, it is possible that a reduction in the sodium gradient in the toad bladder reduces luminal permeability by increasing the cell calcium activity. We found that the inhibition of the influx by ouabain or low serosal Na was prevented, in part, by removal of serosal calcium. To test for the existence of a sodium-calcium exchanger, we studied calcium transport in isolated basolateral membrane vesicles and found that calcium uptake was proportional to the outward directed sodium gradient. Uptake was not the result of a sodium diffusion potential. Calcium efflux from preloaded vesicles was accelerated by an inward directed sodium gradient. Preliminary kinetic analysis showed that the sodium gradient changes the Vmax but not the Km of calcium transport. These results suggest that the effect of intracellular sodium on the luminal sodium permeability is due to changes in intracellular calcium.

Electrophysiologic changes associated with potassium depletion of frog skin

Skins from the frog Rana pipiens pipiens were studied under short-circuited conditions during the course of removing and replacing potassium in the inner bathing media in 14 experiments. The intracellular potential (Vsc), fractional resistance (FR), short-circuit current (Isc) and total tissue conductance (gr) were constantly monitored during impalements of the epithelial cells. The mean value (+/- SE) for Vsc was --79 (+/- 3) mV under baseline conditions. Removal of potassium from the inner bathing solution transiently stimulated the short-circuit current and hyperpolarized the basolateral membrane; with sufficiently long incubations, the basolateral membrane was eventually depolarized. Restoration of potassium to the inner solution within 43 min after initiating the perfusion with K+-free solution depolarized the basolateral membrane. However, restoration of potassium after at least 1 1/2 hr of incubation hyperpolarized the membrane. Ouabain consistently depolarized the basolateral membrane, even after extended periods of potassium depletion as long as 320 min. In the presence of ouabain, restoration of potassium depolarized the basolateral membrane. The data provide further evidence for the concept that the Na--K exchange pump of frog skin is rheogenic. Furthermore, the results suggest that the pump continues to be active even during prolonged periods of potassium depletion, reaccumulating potassium which has leaked out of the epithelial cells.

Current-voltage analysis of apical sodium transport in toad urinary bladder: effects of inhibitors of transport and metabolism

The basal-lateral surface of the epithelium of the urinary bladder of the toad (Bufo marinus) was depolarized by exposure of the serosal surface to 85 mM KCL and 50 mM sucrose. The extent of depolarization appeared to be virtually complete, as evaluated by the invariance in the transepithelial electrical potential difference and conductance on addition of nystatin (a monovalent cation ionophore) to the serosal medium. The Na-specific current (INa) was defined as the current sensitive to the removal of Na from the mucosal medium or inhibitable by addition of amiloride to this medium. In the presence of the high K-sucrose serosal medium, rapid, serial, stepwise clamping of the transepithelial voltage (V) yielded a curvilinear dependence of INa on V; which is taken to represent the I--V curve of the apical Na channels. The constant field equation (Goldman, D.E. 1943; J. Gen. Physiol. 27:37) fits the I--V data points closely, allowing estimates to be made of the permeability to Na of the apical membrane (PNa) and of the intracellular Na activity (Nac). Exposure of the apical surface to amiloride (5 X 10(-7) M) decreased PNa in proportion to the decrease in INa (i.e., approximately 70%) but decreased Nac only 25%. In contrast, an equivalent reduction in INa elicited by exposure of the basal-lateral surface to ouabain was accompanied by only a 20% decrease in PNa and a sixfold increase in Nac. The effects of amiloride on PNa and ouabain on Nac are consistent with the primary pharmacological actions of these drugs. In addition, PNa appears to be under metabolic control, in that 2-deoxyglucose, a specific inhibitor of glycolysis, decreased INa and PNa proportionately, and lowered Nac marginally, effects indistinguishable from those obtained with amiloride.

Microprobe study of toad urinary bladder in absence of serosal K+

The bulk of the intracellular potassium in mucosal epithelial cells from toad urinary bladder has been previously reported to exchange very slowly with the serosal medium, with a half-time of some 9 hr. This observation, based on chemical analyses of mucosal cell scrapings, has been reexamined with stimultaneous diffractive and energy dispersive electron probe X-ray microanalysis. Fifty-three intracellular sites in hydrated sections and 286 sites in dehydrated sections were studied in bladders from eight toads under baseline conditions and after removal of serosal K+ for 83-133 min, with or without 10(-2) M ouabain. The baseline data confirm and extend previous examinations of the intracellular ionic composition, and provide the most direct measure of intracellular water thus far available for this tissue. Removal of serosal K+ reduced the intracellular K+ content by 20%, increased intracellular Na+ content threefold, and slightly reduced the intracellular Cl- and water contents, qualitatively consistent with published chemical analyses. The intracellular Na+ content of mucosal origin, measured by radioactive tracers and chemical analyses of cell scrapings, has been reported to be unchanged under these conditions. Simultaneous addition of ouabain and removal of external K+ produced a dramatic fall in intracellular K+ of more than 80% in a third of the cells and reduced the mean intracellular K+ content by 60%; 20% of the cells appeared to retain K+ more effectively than the bulk of the epithelial cell population. We conclude that: (i) the low rate of net exchange of intracellular K+ with the serosal bulk solution primarily reflects recycling of K+ across the basolateral membranes, (ii) radioactive tracer and chemical measurements of the intracellular Na+ pool of mucosal origin substantially understimate the total intracellular Na+ content under certain experimental conditions, and (iii) the epithelial cells display a functional heterogeneity of response to the effects of adding ouabain and withdrawing external K+.

Microelectrode studies in toad urinary bladder epithelium. effects of Na concentration changes in the mucosal solution on equivalent electromotive forces

Microelectrode techniques were employed to measure membrane potentials, the electrical resistance of the cell membranes, and the shunt pathway, and to compute the equivalent electromotive forces (EMF) at both cell borders in toad urinary bladder epithelium before and after reductions in mucosal sodium concentration. Basal electrical parameters were not significantly different from those obtained with impalements from the serosal side, indicating that mucosal impalements do not produce significant leaks in the apical membrane. A decrease in mucosal Na concentration caused the cellular resistance to increase and both apical and basolateral EMF to depolarize. When Na was reduced from 112 to 2.4 mM in bladders with spontaneously different baseline values of transepithelial potential difference (Vms), a direct relationship was found between the change in Vms brought about by the Na reduction and the base-line Vms before the change. A direct relationship was also found by plotting the change in EMF at the apical or basolateral border caused by a mucosal Na reduction with the corresponding base-line EMF before the change. These results indicate that resting apical membrane EMF (and, therefore, resting apical membrane potential) is determined by the Na selectivity of the apical membrane, whereas basolateral EMF is at least in part the result of rheogenic Na transport. These results are consistent with data of others that suggested a link between the activity of the basolateral Na pump and apical Na conductance.

Intracellular potassium activity in epithelial cells of frog fundic gastric mucosa

Microelectrodes were used to measure membrane potential and intracellular potassium activity in surface epithelial cells (SEC) of frog (Rana esculenta) fundic gastric mucosa in vitro. Separate measurements were carried out by applying fine-tipped, single barrelled, KCl filled non-selective electrodes and liquid K+ -selective electrodes. Membrane potentials with respect to the mucosal and serosal surfaces, measured with non-selective electrodes, were -54.5 +/- 1.0 S.E mV (n = 59) and -73.0 +/- 1.1 S.E. mV (n = 59) respectively. The electrical potential difference referred to the mucosal surface, when measured with K+ -sensitive electrodes, was +21.2 +/- 0.8 S.E mV (n = 35), and intracellular K+ activity was 98.5 mmol/l. Assuming that intracellular and extracellular K+ activity coefficients are equal (gamma K = gamma K), the K+ concentration is 135.0 mmol/l. The K+ equilibrium potential, EK, was calculated as -90. mV i.e. more negative than both membrane potentials. This result indicates active potassium accumulation in the SEC and provides direct evidence of the presence of an active K+ pump in either both or in only one of the cell membranes.

Coupled transepithelial sodium and potassium transport across isolated frog skin: effect of ouabain, amiloride and the polyene antibiotic filipin

Addition of the polyene antibiotic filipin (50 microM) to the outside bathing solution (OBS) of the isolated frog skin resulted in a highly significant active outward transport of K+ because filipin per se increases the nonspecific Na+ and K+ permeability of the outward facing membrane. The K+ transport was calculated from the chemically determined changes in K+ concentrations in the solution bathing the two sides of the skin. The active transepithelial K+ transport required the presence of Na+ in the OBS, but not in the inside bathing solution (IBS), and it was inhibited by the Na+, K+-ATPase inhibitor ouabain. The addition of Ba++ to the IBS in the presence of filipin in the OBS resulted in an activation of the transepithelial K+ transport and in an inhibition of the active Na+ transport. This is in agreement with the notion that Ba++ decreases the passive K+ permeability of the inward facing membrane. In the presence of amiloride (which blocks the specific Na permeability of the outward facing membrane) and Ba++ there was a good correlation between the active Na+ and K+ transport. It is concluded that the active transepithelial K+ transport is carried out by a coupled electrogenic Na-K pump, and it is suggested that the pump ratio (Na/K) is 1.5.

Intracellular ionic activities and transmembrane electrochemical potential differences in gallbladder epithelium

Intracellular ion activities in Necturus gallbladder epithelium were measured with liquid ion-exchanger microelectrodes. Mean values for K, Cl and Na activities were 87, 35 and 22 mM, respectively. The intracellular activities of both K and Cl are above their respective equilibrium values, whereas the Na activity is far below. This indicates that K and Cl are transported uphill toward the cell interior, whereas Na is extruded against its electrochemical gradient. The epithelium transports NaCl from mucosa to serosa. From the data presented and the known Na and Cl conductances of the cell membranes, we conclude that neutral transport driven by the Na electrochemical potential difference can account for NaCl entry at the apical membrane. At the basolateral membrane, Na is actively transported. Because of the low Cl conductance of the membrane, only a small fraction of Cl transport can be explained by diffusion. These data suggest that Cl transport across the basolateral membrane is a coupled process which involves a neutral NaCl pump, downhill KCl transport, or a Cl-anion exchange system.