Dependence of serosal membrane potential on mucosal membrane potential in toad urinary bladder

Interaction between the basolateral K+ and apical Na+ conductances in Necturus urinary bladder

Experimental modulation of the apical membrane Na+ conductance or basolateral membrane Na+-K+ pump activity has been shown to result in parallel changes in the basolateral K+ conductance in a number of epithelia. To determine whether modulation of the basolateral K+ conductance would result in parallel changes in apical Na+ conductance and basolateral pump activity, Necturus urinary bladders stripped of serosal muscle and connective tissue were impaled through their basolateral membranes with microelectrodes in experiments that allowed rapid serosal solution changes. Exposure of the basolateral membrane to the K+ channel blockers Ba2+ (0.5 mM/liter), Cs+ (10 mM/liter), or Rb+ (10 mM/liter) increased the basolateral resistance (Rb) by greater than 75% in each case. The increases in Rb were accompanied simultaneously by significant increases in apical resistance (Ra) of greater than 20% and decreases in transepithelial Na+ transport. The increases in Ra, measured as slope resistances, cannot be attributed to nonlinearity of the I-V relationship of the apical membrane, since the measured cell membrane potentials with the K+ channel blockers present were not significantly different from those resulting from increasing serosal K+, a maneuver that did not affect Ra. Thus, blocking the K+ conductance causes a reduction in net Na+ transport by reducing K+ exit from the cell and simultaneously reducing Na+ entry into the cell. Close correlations between the calculated short-circuit current and the apical and basolateral conductances were preserved after the basolateral K+ conductance pathways had been blocked. Thus, the interaction between the basolateral and apical conductances revealed by blocking the basolateral K+ channels is part of a network of feedback relationships that normally serves to maintain cellular homeostasis during changes in the rate of transepithelial Na+ transport.

Effect of chronic hyperaldosteronism on the electrophysiology of rat distal colon

Microelectrodes have been used to study the effects of aldosterone on the barriers and forces controlling sodium and potassium transport in rat distal colon. Compared to control tissues, hyperaldosteronism induced by dietary sodium depletion resulted in a 7-fold increase in transepithelial voltage (VT) and a 52% decrease in total resistance (RT). Increased VT reflected both a rise in the basolateral membrane voltage (Vbl) and a fall in the apical membrane voltage (VA). RT was resolved into its separate membrane components using nystatin (585 U X ml-1), and the decrease in RT produced by aldosterone was found to be due entirely to a 66% decrease in the apical membrane resistance (RA). Amiloride had no effect on the control tissues, but restored VT, Vbl and VA in tissues from sodium deprived animals to control values. Amiloride also increased RT in the experimental tissue, but the post-amiloride values remained significantly lower than those in controls. These results indicate, therefore, that hyperaldosteronism results in an increase in VT by hyperpolarizing the basolateral membrane, as well as depolarizing apical membrane in rat distal colon. The fall in RT, however, is due only to a fall in RA since Rbl and junctional resistance (Rj) were unaffected. The data are consistent with the concept that aldosterone acts to stimulate sodium absorption by increasing the rate of cell entry of sodium, through the induction of amiloride-sensitive sodium channels in the apical membrane, and enhances the rate of potassium secretion by increasing the electrical driving force towards the mucosal solution.

Calcium reduces the sodium permeability of luminal membrane vesicles from toad bladder. Studies using a fast-reaction apparatus

Regulation of the sodium permeability of the luminal membrane is the major mechanism by which the net rate of sodium transport across tight epithelia is varied. Previous evidence has suggested that the permeability of the luminal membrane might be regulated by changes in intracellular sodium or calcium activities. To test this directly, we isolated a fraction of the plasma membrane from the toad urinary bladder, which contains a fast, amiloride-sensitive sodium flux with characteristics similar to those of the native luminal membrane. Using a flow-quench apparatus to measure the initial rate of sodium efflux from these vesicles in the millisecond time range, we have demonstrated that the isotope exchange permeability of these vesicles is very sensitive to calcium. Calcium reduces the sodium permeability, and the half-maximal inhibitory concentration is 0.5 microM, well within the range of calcium activity found in cells. Also, the permeability of the luminal membrane vesicles is little affected by the ambient sodium concentration. These results, when taken together with studies on whole tissue, suggest that cell calcium may be an important regulator of transepithelial sodium transport by its effect on luminal sodium permeability. The effect of cell sodium on permeability may be mediated by calcium rather than by sodium itself.

Sodium transport inhibition by amiloride reduces basolateral membrane potassium conductance in tight epithelia

In toad and frog urinary bladder, electrophysiological data suggest that inhibition of transepithelial sodium transport by mucosal amiloride results in a decrease in basolateral membrane conductance. These results were confirmed by showing that amiloride addition caused a decrease in basolateral membrane potassium permeability.

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.

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.

Effect of amiloride on conductance of toad urinary bladder

The transepithelial conductance of toad bladder epithelia and the amplitude of the fluctuations of this conductance caused by the action of the underlying smooth muscle have been further investigated. In particular, amiloride was found to reduce both tissue conductance and its fluctuating component to the same extent. Analysis suggests that the steady-state conductance of the toad urinary bladder may be associated only with the paracellular pathway for ions.

Effect of amiloride on the apical cell membrane cation channels of a sodium-absorbing, potassium-secreting renal epithelium

The effect of the K-sparing diuretic amiloride was assessed electrophysiologically in the isolated cortical collecting tubule of the rabbit, a segment which absorbs Na and secretes K. Low concentrations of amiloride in the perfusate caused a rapid, reversible, decrease in the magnitude of the lumen negative transepithelial potential difference, Vte, transepithelial conductance Gte, and equivalent short-circuit current, Isc, with an apparent K1/2 of approximately 7 X 10(-8) M. The effects of a maximum inhibitory concentration of amiloride (10(-5) M) were identical to those observed upon Na removal from lumen and bath (Na removal from the bath alone has no effect). Removal of Na in the presence of 10(-5) M amiloride had no affect on Vte, Gte, or Isc, and is consistent with the view that amiloride blocks the Na conductive pathways of the apical cell membrane. Further, in the absence of Na, the subsequent addition of amiloride had no influence. In tubules where active Na absorption was either spontaneously low, or abolished by removal of Na from lumen and bath, the elevation of K from 5 to 155 meq/liter in the perfusate caused a marked change of the Vte in the negative direction and an increase in the Gte. These effects could be attributed to a high K permeability of the apical cell membrane and not of the tight junctions. Amiloride (10(-5) M) had no effect on these responses to K. It is concluded that amiloride selectively blocks the apical cell membrane Na channels but has no effect on the K conductive pathway(s). This selective nature of amiloride may indicate that Na and K are transported across the apical cell membrane via separate conductive pathways.

Saturable K+ pathway across the outer border of frog skin (rana temporaria): kinetics and inhibition by Cs+ and other cations

The reaction of abdominal skins of the frog species Rana temporaria on mucosal K+-containing solutions was studied in an Ussing-type chamber by recording transepithelial potential difference (PD), short-circuit current (SCC) and conductance (G). With Na-Ringer's as serosal medium, a linear correlation between PD and the logarithm of the mucosal K+-concentration ([K]o) was obtained. The K+-dependent SCC saturated with increasing [K]o, and could quickly and reversibly be depressed by addition of Rb+, Cs+, and H+. Li+, Na+, and NH4+ did not influence K+ current. A large scatter was obtained for kinetic parameters like the slope of the PD-log[K]o-line (18--36.5 mV/decade), the apparent Michaelis constant (13--200 mM), and the maximal current of the saturable SCC (6--50 microa . cm-2), as well as for the degree of inhibition by Cs+ ions. This seemed to be caused by a time-dependent change during long time exposure to high [K]o (more than 30 sec), thereby inducing a selectivity loss of K+-transporting structures, together with an increase in SCC and G and a decrease in PD. Short time exposure to K+-containing solutions showed a competitive inhibition of K+ current by Cs+ ions, and a Michaelis constant of 6.6 mM for the inhibitory action of Cs+. Proton titration resulted in a decrease of K+ current at pH less than 3. An acidic membrane component (apparent dissociation constant 2.5 x 10(-3) M) is virtually controlling K+ transfer. Reducing the transepithelial K+-concentration gradient by raising the serosal potassium concentration was accompanied by the disappearance of SCC and PD.

Limitation of resistance as a parameter by which to characterize epithelia that actively transport ions

It is theoretically inappropriate to characterize an actively transporting epithelim by "resistance" alone which is correctly applied only to passive circuit elements. Rather, such epithelia (if they actively transport sodium) required, as a minimum, characterization by an active circuit element parameter such as voltage (E Na), current (I Na) or power (WNa) in some configuration with resistances. Recent experimental studies of epithelia which actively transport sodium have omitted consideration of the active circuit element and attributed all measured changes observed to "resistance" changes in the epithelium as the transepithelial sodium gradient is altered. It is suggested that the observed changes in voltage/current ratio could be consequences of changes in the electrical behavior of the active circuit element of such epithelia. It may be biologically impossible to suppress all electromotive forces in epithelia to measure the truly passive characteristics of the epithelim without destroying tissue viability. The actual methods used to date for measurement of "resistance" in epithelia consist of perturbing signals which might alter the electrical behavior of an active element such as an "ion pump"; the observed changes in voltage to current ratio observed in such experiments can be better explained by a change in the active ion pump rather than by changes in passive epithelial "resistance".

Basolateral membrane potential of a tight epithelium: ionic diffusion and electrogenic pumps

The contribution of specific ions to the conductance and potential of the basolateral membrane of the rabbit urinary bladder has been studied with both conventional and ion-specific microelectrode techniques. In addition, the possibility of an electrogenic active transport process located at the basolateral membrane was studied using the polyene antibiotic nystatin. The effect of ion-specific microelectrode impalement damage on intracellular ion activities was examined and a criterion set for acceptance or rejection of intracellular activity measurements. Using this criterion, we found (K+) = 72 mM and (Cl-) = 15.8 mM. Cl- but not K+ was in electrochemical equilibrium across the basolateral membrane. The selective permeability of the basolateral membrane was measured using microelectrodes, and the data analyzed using the Goldman, Hodgkin-Katz equation. The sodium to potassium permeability ratio (PNa/PK) was 0.044, and the chloride to potassium permeability ratio (PCl/PK) was 1.17. Since K+ was not in electrochemical equilibrium, intracellular (K+) is maintained by active metabolic processes, and the basolateral membrane potential is a diffusion potential with K+and C1- the most permeable ions. After depolarizing the basolateral membrane with high serosal potassium bathing solutions and eliminating the apical membrane as a rate limiting step for ion movement using the polyene antibiotic nystatin, we found that the addition of equal aliquots of NaCl to both solutions caused the basolateral membrane potential to hyperpolarize by up to 20mV (cell interior negative). This potential was reduced by 80% within 3 min of the addition of ouabain to the serosal solution. This hyperpolarization most probably represents a ouabain sensitive active transport process sensitive to intracellular Na+. An equivalent electrical circuit for Na+ transport across rabbit urinary bladder is derived, tested, and compared to previous results. This circuit is also used to predict the effects that microelectrode impalement damage will have on individual membrane potentials as well as time-dependent phenomena; e.g., effect of amiloride on apical and basolateral membrane potentials.

Active sodium transport and the electrophysiology of rabbit colon

The electrophysiologic properties of rabbit colonic epithelial cells were investigated employing microelectrode techniques. Under open-circuit conditions, the transepithelial electrical potential difference (PD) averaged 20 mV, serosa positive, and the intracellular electrical potential (psimc) averaged -32 mV, cell interior negative with respect to the mucosal solution; under short-circuit conditions, psimc averaged -46 mV. The addition of amiloride to the mucosal solution abolishes the transepithelial PD and active Na transport, and psimc is hyperpolarized to an average value of -53 mV. These results indicate that Na entry into the mucosal cell is a conductive process which, normally, depolarized psimc. The data obtained were interpreted using a double-membrane equivalent electrical circuit model of the "active Na transport pathway" involving two voltage-independent electromotive forces (emf's) and two voltage-independent resistances arrayed in series. Our observations are consistent with the notions that: (a) The emf's and resistances across the mucosal and baso-lateral membranes are determined predominantly by the emf (64 mV) and resistance of the Na entry process and the emf (53 mV) and resistance of the process responsible for active Na extrusion across the baso-lateral membranes: that is, the electrophysiological properties of the cell appear to be determined solely by the properties and processes responsible for transcellular active Na transport. The emf of the Na entry process is consistent with the notion that the Na activity in the intracellular transport pool is approximately one-tenth that in the mucosal solution or about 14 mM. (b) In the presence of amiloride, the transcellular conductance is essentially abolished and the total tissue conductance is the result of ionic diffusion through paracellular pathways. (c) The negative intracellular potential (with respect to the mucosal solution) is due primarily to the presence of a low resistance paracellular "shunt" pathway which permits electrical coupling between the emf at the baso-lateral membrane and the potential difference across the mucosal membrane; in the absence of this shunt, the "well-type" electrical potential profile characteristic of rabbit colonic cells would be 'converted' into a "staircase-type" profile similar to those reported for frog skin and toad urinary bladder by some investigators.

Mode of action of amiloride in toad urinary bladder. An electrophysiological study of the drug action on sodium permeability of the mucosal border

The effect of amiloride on the sensitivity to Na of the mucosal border of toad urinary bladder was investigated by recording Na concentration-dependent transepithelial potential difference (Vt) and the intracellular potential. When mucosal Na concentration was normal, amiloride added to the mucosal solution at 10(-4) M markedly reduced the mucosal membrane potential (Vm) and altered the potential profile from a two-step type to a well type. Similar changes were observed when Na was totally eliminated from the mucosal medium. The serosal membrane potential was insensitive to amiloride and elimination of mucosal Na. In the absence of amiloride, the Vt could be described by the Goldman-Hodgkin-Katz equation in the range of mucosal Na concentration from 0 to 16 mM, and amiloride extended this concentration range. By using the Goldman-Hodgkin-Katz equation. Na permeability was calculated from the data of Vt's obtained in the allowed ranges of Na concentration and compared before and after the addition of amiloride. The results show that Na permeability decreases to 1/600 of control when the maximum dose of amiloride (10(-4) M) is applied. The relationship between Na permeability and amiloride concentration is well explained on the basis of assumptions that amiloride binds to the Na site of the mucosal border in one-to-one fashion and in a competitive manner with Na and that Na permeability reduces in proportion to increase in number of the sites bound with amiloride.

Site and mode of adrenaline action on chloride transport across the rabbit corneal epithelium

1. Membrane events accompanying adrenaline-stimulated Cl secretion by the isolated rabbit corneal epithelium were investigated with micro-electrodes. 2. Pulses of adrenaline (5 X 10(-10) M final concentration) delivered to either side of the epithelium produced a transient decrease in epithelial resistance occuring at the outer membrane of the squamous cell. This response was reversible and could be blocked completely by total Cl substitution with SO4. 3. Adrenaline generally produced a small transient increase in epithelial potential occuring also at the squamous cell outer membrane. Reversal potentials obtained for the adrenaline response were 45-1 mV for corneal potential and 22-8 mV for outer membrane. 4. Adrenaline always hyperpolarized epithelial potential when the tear side was bathed in Cl-free solution. Reversing the gradient (Cl-free on the stromal side) slowly and consistently changed the response to a depolarization which reached a steady level after 2 hr. 5. The reversal potential of the outer membrane for the adrenaline response was found to be a semilogarithmic function of the tear side Cl concentration over a broad range with a slope of 56 mV/decade. The reversal potential was zero at a tear side Cl concentration of 41-5 mM, which value may be taken to be representative of cell Cl concentration. 6. After abolishing the adrenaline response by perfusing both sides of the tissue with Cl-free solution, reintroduction of Cl to the stromal side led to a recovery of the epithelial potential response in the hyperpolarizing direction. The recovery of the response was inhibited by ouabain (10(-5) M). 7. The results supported the following model for the influence of adrenaline on anion transport in the epithelium: Cl is transported against an electrochemical potential gradient into the cells from the stromal side by an active process linked to Na-k activated ATPase. Normally a slight gradient exists from cells to tears favouring the passive outward diffusion of Cl. This latter process is enhanged by adrenaline, which increases cell cyclic AMP, in turn increasing the passive Cl permeability of the outer cellular membrane.

Sodium transport and oxygen consumption in toad bladder. A thermodynamic approach

The relationship between active sodium transport and oxygen consumption was investigated in toad urinary bladder exposed to identical sodium-Ringer's solution at each surface, while controlling the transepithelial electrical potential difference delta phi. Rates of sodium transport and oxygen consumption were measured simultaneously, both in the short-circuited state (delta phi = 0) and when delta phi was varied. Under short-circuit conditions, when the rates of active sodium transport changed spontaneously or were depressed with amiloride, the ratio of active sodium transport to the estimated suprabasal oxygen consumption Na/O2 was constant for each tissue, but varied among different tissues. Only when delta phi was varied did the ratio Na+/O2 change with the rate of active sodium transport; under these circumstances dNa+/dO2 was constant but exceeded the ratio measured at short-circuit [(Na+/O2)delta phi = 0[. This suggests that coupling between transport and metabolism is incomplete. The results are analyzed according to the principles of nonequilibrium thermodynamics, and intepreted in terms of a simple model of the transepithelial sodium transport system.

Resolution of parameters in the equivalent electrical circuit of the sodium transport mechanism across toad skin

In amphibian epithelia, amiloride reduces net sodium transport by hindering the entry of sodium to the active transport mechanism, that is, by increasing the series resistance (Rser). Theoretically, therefore, analysis of amiloride-induced changes in potential differences and short-circuit current should yield numerical estimates of all the parameters in the equivalent electrical circuit of the sodium transport mechanism. The concept has been explored by analysis of such changes in toad skins (Xenopus laevis) bathed in hypotonic sulphate Ringer's, after exposure to varying doses of amiloride, or to amphotericin, dinitrophenol or Pitressin. The estimated values of Rser, of the electromotive force of the sodium pump (ENa), and of the shunt resistance (Rsh) were independent of the dose of amiloride employed. Skins bathed in hypotonic sulphate Ringer's exhibited a progressive rise in ENa. Amphotericin produced a fall in Rser, while dinitrophenol caused a fall in ENa; washout of the drugs reversed these effects. Pitressin produced a fall in both Rser and Rsh, with a rise in ENa. These results are in accord with earlier suggestions regarding the site(s) of action of these agents.

A new model for regulation of sodium transport in high resistance epithelia

A new three barrier, four compartment model for sodium transport in high resistance urinary epithelia is presented. This model provides a unified and simplified mechanistic explanation for sodium transport and its quantitative regulation. Sodium enters the epithelial cell by passive diffusion. Active extrusion occurs across the lateral cell membrane into the lateral intercellular space (LICS). Sodium movement from the LICS into the serosal compartment is not free and unobstructed as in the models for low resistance epithelia, but rather occurs through a regulatory channel of the LICS passing through desmosomes and the basilar slit. The exact configuration of this regulatory channel controls the rate of sodium movement from the LICS into the serosal compartment. Thus, the configuration of the regulatory channel controls the afterload on the sodium pump and thus ultimately controls the rate of transepithelial sodium transport. Antidiuretic hormone could act by increasing the effective width of this regulatory channel by contraction of intracellular microtubules or microfilaments. Present theories for regulation of transepithelial sodium transport in high resistance epithelia invoke a regulatory barrier at the apical cell membrane or at the active sodium pump located in the basolateral cell membrane. The hypothetical model presented here invokes a new alternative: regulation of the active pump rate by the sodium concentration in the LICS serving as an afterload on the pump; sodium escape from the LICS into the serosal compartment thus becomes the regulatory step for transepithelial transport.

Electrical properties of the cellular transepithelial pathway in Necturus gallbladder. I. Circuit analysis and steady-state effects of mucosal solution ionic substitutions

Microelectrode techniques were employed to measure the electrical resistance of the cell membranes and the shunt pathway, and the equivalent electromotive forces (EMF's) at both cell borders in Necturus gallbladder epithelium. The cell is, on the average, 57 mV negative to the mucosal solution and 59 mV negative to the serosal solution. The transepithelial potential (Vms) ranges from 0.5 to 5 mV, serosal solution positive. Assuming that the shunt EMF (Vs) is zero with standard Ringer's bathing oth sides of the tissue, both cell membrane EMF's are oriented with the negative pole toward the cell interior and are 39.9 +/- 3.6 mV (apical, Va), and 69.4 +/- 1.8 mV (basal-lateral, Vb)...

The electrical potential profile of gallbladder epithelium

In this study the relative ionic permeabilities of the cell membranes of Necturus gallbladder epithelium have been determined by means of simultaneous measurement of transmural and transmucosal membrane potential differences (PD) and by ionic substitution experiments with sodium, potassium and chloride ions. It is shown that the mucosal membrane is permeable to sodium and to potassium ions. The baso-lateral membrane PD is only sensitive to potassium ions. In both membranes chloride conductance is negligible or absent. The ratio of the resistances of the mucosal and baso-lateral membranes, RM/RS, increases upon reducing the sodium concentration in the mucosal solution. The same ratio decreases when sodium is replaced by potassium which implies a greater potassium than sodium conductance in the mucosal membrane. The relative permeability of the shunt for potassium, sodium and chloride ions is: PK/PNa/PCl=1.81:1.00:0.32. From the results obtained in this study a value for the PK/PNa ratio of the mucosal membrane could be evaluated. This ratio is 2.7. From the same data the magnitude of the electromotive forces generated across the cell membranes could be calculated. The EMF's are -15mV across the mucosal membrane and -81mV across the baso-lateral one. Due to the presence of the low resistance shunt the transmucosal membrane PD is -53.2mV (cell inside negative) and the transmural PD is +2.6mV (serosal side positive). The change in potential profile brought about by the low resistance shunt favors passive entry of Na ions into the cell across the mucosal membrane. Calculations show that this passive Na influx is maximally 64% of the net Na flux estimated from fluid transport measurements. The C-1 conductive of the baso-lateral membrane is too small to allow electrogenic coupling of C1 with Na transport across this membrane. Experiments with rabbit gallbladder epithelium indicate that the membrane properties in this tissue are qualitatively similar to those of Necturus gallbladder epithelium.

Effects of changes in the composition of the mucosal solution on the electrical properties of the toad urinary bladder epithelium

By the use of microelectrode techniques, the potential profile and the electrical resistances of the cellular and shunt pathways across the toad urinary bladder epithelium were measured under control conditions and after exposing the mucosal side to solutions of low and high NaCl concentrations and osmolalities. The resistance of the shunt pathway increases at low NaCl concentration (even if the osmolality is kept constant), and decreases at high NaCl concentration (by a nonspecific osmotic mechanism). The inverse relationship between mucosal NaCl concentration and shunt resistance suggests a regulatory mechanism of net sodium transport by reduction of the passive blood-to-urine sodium flux at low urinary sodium concentrations. In addition, the transepithelial potential and the potentials at both cell borders fall in both low and high mucosal NaCl, and the magnitude of these changes is such that they cannot be explained by changes in the shunt pathway alone.

Effects of changes in the composition of the serosal solution on the electrical properties of the toad urinary bladder epithelium

1. The potential profile and the cellular and paracellular transepithelial resistances of the toad urinary bladder were measured, by means of micro-electrode techniques, as functions of the osmolality of the serosal solution. 2. Reductions in serosal osmolality (that increase the rate of active sodium transport) produced proportional decreases in the electrical resistances of the apical and basal-lateral cell membranes, while the changes in resistance of the paracellular pathway were more complex. The apical membrane potential increased. 3. Increases in serosal osmolality (that decrease sodium transport) produced increases in the electrical resistances of both cell membranes, and moderate reduction in the paracellular resistance. The polarity of the apical membrane potential reversed. 4. These results indicate that reductions in serosal solution osmolality stimulate sodium transport by increasing both the sodium permeability of the luminal cell membrane (thus increasing sodium entry), and the electromotive force generated at the serosal border of the cell, thus enhancing the rate of sodium pumping. Conversely, increases in osmolality reduced sodium transport by reducing both the sodium permeability of the luminal membrane and the serosal membrane electromotive force.

Action of ouabain on sodium transport in toad urinary bladder, Evidence for two pathways for sodium entry

The cardiac glycoside ouabain inhibits transepithelial sodium transport in the toad urinary bladder. It is shown that this drug reduces the rate coefficient for sodium exit at the serosal pump site. In addition, ouabain inhibits entry across the mucosal border whenever the electrochemical potential gradient for sodium is made less favorable. The data are interpreted as indicating the existence of two separate pathways for sodium entry, one of which is ouabain inhibitable.