Effect of aldosterone on active and passive conductance and ENA in the toad bladder

Increased chloride permeability of amphibian epithelia treated with aldosterone

The transepithelial flux of chloride was increased by aldosterone treatment of amphibian skin and bladder and this was reflected by increased "shunt" conductance. The hormonal effect depended on the presence of chloride on the epithelial side of the preparation. These changes in tissue conductance and chloride permeability appear to be a direct effect of aldosterone as they did not occur when sodium transport was stimulated with vasopressin or hypotonicity. Chloride efflux was reduced in magnitude by indacrinone and DIDS, as well as after removal of chloride from the solution on the epithelial side of the preparations. These results suggest that, rather than merely diffusing along (a) paracellular pathway(s), chloride flows through (a) cellular structure(s), notably mitochondria-rich cells. These cells can therefore be considered as targets for aldosterone.

Aldosterone control of the density of sodium channels in the toad urinary bladder

Near-instantaneous current-voltage relationships and shot-noise analysis of amiloride-induced current fluctuations were used to estimate apical membrane permeability to Na (PNa), intraepithelial Na activity (Nac), single-channel Na currents (i) and the number of open (conducting) apical Na channels (N0), in the urinary bladder of the toad (Bufo marinus). To facilitate voltage-clamping of the apical membrane, the serosal plasma membranes were depolarized by substitution of a high KCl (85 mM) sucrose (50 mM) medium for the conventional Na-Ringer's solution on the serosal side. Aldosterone (5 X 10(-7) M, serosal side only) elicited proportionate increases in the Na-specific current (INa and in PNa, with no significant change in the dependence of PNa on mucosal Na (Nao). PNa and the control of PNa by aldosterone were substrate-dependent: In substrate-depleted bladders, pretreatment with aldosterone markedly augmented the response to pyruvate (7.5 X 10(-3) M) which evoked coordinate and equivalent increases in INa and PNa. The aldosterone-dependent increase in PNa was a result of an equivalent increase in the area density of conducting apical Na channels. The computed single-channel current did not change. We propose that, following aldosterone-induced protein synthesis, there is a reversible metabolically-dependent recruitment of preexisting Na channels from a reservoir of electrically undetectable channels. The results do not exclude the possibility of a complementary induction of Na-channel synthesis.

Protocol-dependence of equivalent circuit parameters of toad urinary bladder

Determinations o- current-voltage relationships are widely employed in the characterization of epithelial sodium transport. In order to determine the protocol dependence of transport parameters in the toad urinary bladder, studies were carried out in the presence and absence of amiloride, an inhibitor of active sodium transport. With symmetric positive and negative perturbations of the transepithelial electrical potential difference delta psi (0 leads to +/- 100 mV) for 30 sec, the amiloride-sensitive current-voltage (ia-delta psi) relationship was near linear over the range -75 leads to +100 mV, indicating constancy of the conductance ka and the apparent electromotive force "ENa", lumped parameters of the standard electrical equivalent circuit model of the active transport system. With a reverse protocol (+/- 100 leads to 0 mV) or 15 min perturbations the ia-delta psi relationships were highly nonlinear. Nonlinearity reflected voltage dependence of parameters: perturbations that increased active transport decreased "ENa" and increased ka, as evaluated from 10 sec perturbations of delta psi; slowing of active transport produced the converse changes. These effects are usefully analyzed in both quasi-steady states and true steady states by means of a detailed equivalent circuit incorporating the significant ionic currents across each plasma membrane. Precise understanding of the significance of ka and "ENa" will require characterization of the partial ionic conductances on perturbation of delta psi.

Mechanism of action of aldosterone on active sodium transport across toad skin

Epithelium of the abdominal skin of the toad, Bufo marinus, has been studied by microelectrode impalement. Using an electrical equivalent circuit model, effective EMF's and specific conductances of the apical and basolateral membrane could be calculated. The skin was divided into 2 fragments for incubation in the presence, or not, of aldosterone (greater than or equal to 0.1 microM). After incubation overnight, sodium transport by the hormone-treated piece was increased 2.7-fold on average, compared to the untreated control. Concomitantly, conductance of the apical border increased more than 3-fold. Furthermore, mean conductance and electromotive force at the basolateral border increased by 80% and by 10%, respectively. Whether the latter changes merely represent delayed adaptation to increased apical conductance, cannot be settled from the data available.

Relationship of transient electrical properties to active sodium transport by toad urinary bladder

Application of voltage pulses of 10 mV for periods of 9 sec across toad urinary bladder elicits a rapid deflection in transepithelial current. Frequently, the current decays back towards its baseline value during the course of the polarizing pulse. This transient phenomenon can be induced, or its magnitude increased, by raising the mucosal or serosal Na+ concentration. The transient can be abolished by sufficiently hyperpolarizing the tissue (rendering serosa positive to mucosa), by inhibiting transcellular Na+ transport with amiloride or ouabain, and by increasing the serosal K+ concentration. Vasopressin increases net Na+ movement across toad bladder but does not elicit these transients. It is proposed as a working hypothesis for further study that the transient behavior characterized in this study reflects: (1) the partition of Na+ between the apical plasma membrane and contiguous fluid layers, (2) the partition of K+ between the basolateral plasma membrane and adjacent submucosal fluid layer, and (3) the negative feedback interaction between intracellular Na+ activity and Na+ permeability of the apical plasma membrane of the transporting cells.

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".

A thermodynamic analysis of the correlation between active Na+ transport and the rate of oxygen consumption in epithelia

Active transport in epithelia is discussed in terms of the relationships between oxygen consumption and sodium flux as affected by each of the two corresponding thermodynamic forces. Analysis is presented of the use of nonequilibrium thermodynamics as a total in elucidating coupling and stoichiometry, and in evaluating drug action in the system. The analysis leads to the quantitative characterization of active transport in "two-flow" systems in terms of two plots: oxygen consumption nad sodium flow, each as a function of electrical potential difference, at constant affinity and constant concentrations. The relevant characteristic parameters are then shown to be represented by the slopes and intercepts of the two plots, the ratios of the slopes and of the intercepts, and by the difference--as well as the ratio--of the ratios. Distinction is made between experimental conditions in which the phenomenological coefficients remain constant and those in which these coefficients undergo appreciable changes. In terms of the above analysis, and examination is made of the effect of commonly used drugs. It is shown that while drugs may effect both the affinity and the phenomenological coefficients, they invariably affect the latter--at least in the cases hitherto reported.

Evaluation of kinetic and energetic parameters of active sodium transport

Active sodium transport is classically analyzed in terms of an equivalent circuit, comprising an active conductance Ka and an electromotive force of sodium transport ENa. Although ENa is commonly considered the driving force of transport, model experiments have suggested that ENa is a composite parameter, incorporating both kinetic and energetic factors. An alternative approach considers both transport and the associated oxidative metabolism in terms of a nonequilibrium thermodynamic (NET) formulation, involving phenomenological coefficients and the affinity A, presumed to represent kinetic and energetic factors, respectively. Model experiments testing the NET formulation suggest that the affinity is indeed an energetic parameter. Calculated values of A in untreated frog skins and toad bladders range from about 20 to 80 kcal per mole of O2 consumption. Assuming a P/O ratio of 3, this range corresponds to about 3--13 kcal per mole of ATP utilization, values compatible with reported direct measurements. Although brief perturbations of transepithelial electrical potential deltapsi resulted in linear current-voltage relationship, indicating constancy of ENa and Ka, 15-min perturbations of deltapsi resulted in nonlinearity, indicating changes in ENa and Ka; perturbations of deltapsi enhancing active transport were associated with decrease of ENa and increase of Ka; slowing of active transport produced the converse effects.

Effect of aldosterone on ion transport by rabbit colon in vitro

Segments of descending colon obtained from rabbits, that had been maintained on drinking water containing 25 mM NaCl and an artificial diet which contains 1% Na and is nominally K-free, respond to aldosterone in vitro (after a 30 to 60-min lag period) with a marked increase in the short-circuit current (Isc), an equivalent increase in the rate of active Na absorption (JNa net) and a decline in tissue resistance (Rt). Aldosterone also brings about a marked increase in the unidirection influx of Na into the cells across the mucosal membrane ("zero-time" rate of uptake) which does not differ significantly from the inrease m Isc. Treatment of control tissues with amphotericin B brings about sustained increases in Isc and JNa net to levels observed in aldosterone-treated tissues. However, addition of amphotericin B to the mucosal solution of aldosterone-treated tissues does not result in a sustained increase in Isc or JNa net and these values do not differ markedly from those observed in control tissues treated with amphotericin B. These findings, together with other evidence that Na entry in the presence of amphotericin B is sufficiently rapid to saturate the active Na extrusion mechanism at the baso-lateral membrane, are consistent with the notion that the aldosterone-induced protein increases the permeability of the mucosal membrane to Na but does not increase the "saturation level" of the active Na "pump" within the time-frame of these studies (3 hr). Finally, aldosterone has no effect on the bidirectional or net transepithelial movements of K under short-circuit conditions, suggesting that the enhanced secretion of K observed in vivo is the result of increased diffusion of K from plasma to lumen via paracellular pathways in response to an increased transepithelial electrical potential difference (lumen negative).

Effects of anions on amiloride-sensitive, active sodium transport across rabbit colon, in vitro. Evidence for "trans-inhibition" of the Na entry mechanism

Replacement of Cl in the solutions bathing partial mucosal strips of rabbit descending colon with sulfate, isethionate, hydroxypropane-sulfonate and, to a lesser degree, ethanesulfonate stimulates active Na absorption (JNanet) when the baso-lateral pump mechanism is not saturated. These effects are rapid in onset and are readily reversible. Our findings indicate that these stimulatory anions decrease the resistance of the amiloridesensitive Na entry step at the mucosal membrane (RmNa). However, when the active Na pump mechanism at the baso-lateral membrane is saturated these stimulatory anions do not decrease the resistance of the Na entry process. These findings suggest the presence of a negative feedback between the activity of the pump mechanism and the resistance of the Na entry step which may be mediated by the size of the intracellular Na transport pool. In other words, it seems that when the baso-lateral pump is operating at its maximal rate the resistance to Na entry across the mucosal membrane through the amiloride-sensitive pathway is at a minimum and cannot be further decreased.

Thermodynamic analysis of active sodium transport and oxidative metabolism in toad urinary bladder

Measurements of electrical current and oxygen consumption were carried out concurrently under voltage clamp conditions in 11 toad hemibladders. Inhibition of active transport with amiloride then permitted evaluation of the passive conductance and the rate of basal oxygen consumption Jbr, allowing the simultaneous determination of the rates of active sodium transport JaNa and suprabasal oxygen consumption Jsbr-JaNa and Jabr were linear functions of the electrical potential difference over a range of +/- 80 mV. This allowed the comprehensive application of a linear nonequilibrium thermodynamic formalism, leading to the evaluation of the affinity A (negative free energy) of the metabolic reaction driving transport, all phenomenological coefficients, and the degree of coupling q relating transport to metabolism. Values of A determined by two techniques were A1=56.0 +/- 5.8 and A2=58.2 +/- 6.5 kcal per mole. Values of q determined by two techniques agreed well and were less than 1, indicating incompleteness of coupling, and hence lack of fixed stoichiometry between Na transort and O2 consumption. The affinity and the electromotive force of sodium transport ENa are not closely correlated, reflecting the fact that ENa comprises both kinetic and energetic factors.

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.

Effects of aldosterone on Na+ transport in the toad bladder. II. The anaerobic response

The action of aldosterone on active Na+ transport was assessed under aerobic and anaerobic conditions in the isolated urinary bladder of the toad, Bufo marinus. Aldosterone augmented the short-circuit current (Isc) under rigorous anaerobiosis. Four lines of evidence indicate that the increase in anaerobic Isc does not represent an equivalent increase in active Na+ transport: 1. Net Na transport, determined by isotopic fluxes, was the same in the aldosterone-treated and control quarter-bladders, and significantly greater than the simultaneously measured Isc-2. Amiloride, an inhibitor of the apical entry of Na+, did not reduce the steroid-dependent increase in the anaerobic Isc-3. Substitution of choline for Na+ in the mucosal medium reduced the magnitude of the anaerobic Isc values but did not eliminate the effect of aldosterone. 4. Addition of ouabain, a potent inhibitor of the Na+ pump, partially inhibited the effect of aldosterone on the anerobic Isc but a significant hormonal increment remained. The source of the anaerobic Isc was not identified; an effort was made, however, to determine the dependence of this current on glycolysis. During anaerobiosis, aldosterone increased the integral Isc by 42% but did not alter lactate production. These results suggest that the steroid-dependent increase in the anaerobic Isc may involve effects on permeability properties of the epithelium rather than on active transport systems.

Na+ transport by rabbit urinary bladder, a tight epithelium

By in vitro experiments on rabbit bladder, we reassessed the traditional view that mammalian urinary bladder lacks ion transport mechanisms. Since the ratio of actual-to-nominal membrane area in folded epithelia is variable and hard to estimate, we normalized membrane properties to apical membrane capacitance rather than to nominal area (probably 1 muF approximately 1 cm2 actual area). A new mounting technique that virtually eliminates edge damage yielded resistances up to 78,000 omega muF for rabbit bladder, and resistances for amphibian skin and bladder much higher than those usually reported. This technique made it possible to observe a transport-related conductance pathway, and a close correlation between transepithelial conductance (G) and short-circuit current (Isc) in these tight epithelia. G and Isc were increased by mucosal (Na+) [Isc approximately 0 when (Na+) approximately 0], aldosterone, serosal (HCO-3) and high mucosal (H+); were decreased by amiloride, mucosal (Ca++), ouabain, metabolic inhibitors and serosal (H+); and were unaffected by (Cl-) and little affected by antidiuretic hormone (ADH). Physiological variation in the rabbits' dietary Na+ intake caused variations in bladder G and Isc similar to those caused by the expected in vivo changes in aldosterone levels. The relation between G and Isc was the same whether defined by diet changes, natural variation among individual rabbits, or most of the above agents. A method was developed for separately resolving conductances of junctions, basolateral cell membrane, and apical cell membrane from this G--Isc relation. Net Na+ flux equalled Isc. Net Cl- flux was zero on short circuit and equalled only 25% of net Na+ flux in open circuit. Bladder membrane fragments contained a Na+-K+-activated, ouabain-inhibited ATPase. The physiological significance of Na+ absorption against steep gradients in rabbit bladder may be to maintain kidney-generated ion gradients during bladder storage of urine, especially when the animal is Na+-depleted.

Effects of 2-deoxy-D-glucose, amiloride, vasopressin, and ouabain on active conductance and ENa in the toad bladder

The effects of various agents on active sodium transport were studied in the toad bladder in terms of the equivalent circuit comprising an active conductance Ka, an electromotive force ENa, and a parallel passive conductance Kp. For agents which affect Ka, but not ENa or Kp, the inverse slope of the plot of total conductance K against short-circuit current IO evaluates ENa, and the intercept represents Kp. Studies employing 5 X 10(-7) M amiloride to depress Ka indicate a changing ENa, invalidating the use of the slope technique with this agent. An alternative suitable technique employs 10(-5) M amiloride, which reduces IO reversibly to near zero without effect on Kp. Despite curvilinearity of the K-IO plot under these conditions, Kp may therefore be estimated fairly precisely from the residual conductance. It then becomes possible to follow the dynamic behavior of Ka and ENa (in the absence of 10(-5) M amiloride) by frequent measurements of K and IO, utilizing the relationships Ka=K-Kp, and ENa=IO/(K-Kp). 2-deoxy-D-glucose (7.5 X 10(-3)M) depressed both Ka and ENa. All of the above effects were noted promptly; Kp was unaffected. The "electromotive force of Na transport" ENa appears not to be a pure energetic parameter, but to relfect kinetic factors as well, in accordance with thermodynamic considerations.

Tracer flow, permeability, and partial conductance

It is often not possible to evaluate a permeability coefficient for net flow P from the small flows produced by physiological gradients of concentration or electrical potential. The common use of a tracer permeability coefficient P-x for this purpose, under the assumption that P-x = P, requires that the species be transported passively, and that there be no significant coupling between its flow and that of other chemical species, and between the flows of its tracer and abundant isotopes (isotope interaction). These conditions are often not satisfied. However, for passive transport in the absence of coupling of flows of different chemical species the measurement of tracer flow at two values of electrical potential difference evaluates (P-x/P) and thus P. In the presence of coupling of flows of different chemical species, although these measurements no longer evaluate P, they evaluate the partial conductance G. A graphical method of evaluating (p-x/P), P, and G is presented.

Further studies on the effect of aldosterone on electrical resistance of toad bladder

The fall in transepithelial electrical resistance which accompanies aldosterone stimulation of short-circuit current (Isc) in toad urinary bladder has been studied further to evaluate the possible causal role of this response in hormonal stimulation of Na+ transport. A steady-state change in tissue conductance was found to depend upon both the simultaneous stimulation of transport by the steroid and the metabolic state of the tissue. Changes in metabolic state alone did not alter resistance. A sustained increase in Na+ transport, dependent on pretreatment with aldosterone and elicited by addition of glucose, could be obtained without a sustained decrease in resistance. Amiloride, an inhibitor of Na+ uptake, produced changes in Isc that were linearly correlated with its effects on tissue conductance. On the basis of the conductance-Isc relationship with amiloride, the Isc response to aldosterone was about two-fold higher than would be predicted from its effects on conductance alone. Despite the apparent lack of a simple quantitative dependence of the change in Isc on the change in conductance when the response is fully developed, the results suggest that conductance changes may mediate the initial or early stage of the response.

Action of aldosterone on frog skin in the presence and absence of in vitro molting effects

The molting which occurs in frog skin following exposure to high concentrations of aldosterone interferes with the interpretation of physiological measurements. Exposure of skins from frogs maintained in standard smooth tanks to 5 - 10(-7) M aldosterone caused within a few hours erratic responses in short-circuit current Io and conductance K followed by sustained stimulation of Io and K; 10(-8) M aldosterone caused only stimulation of Io and K. Storage of frogs in "rojgh tanks" eliminated in vitro molting on exposure to 5 - 10(-7) M aldosterone. IO and K were then superimposable for 3 h, after which Io increased far more rapidly than K. These results are consistent with an early effect on permeability of the active pathway and later effects on metabolism, either a direct effect on the pump or enhanced interaction between transport and metabolism.

Energetics of active transport processes

Active sodium transport across epithelial membranes has been analyzed by means of linear nonequilibirium thermodynamics. In this formulation the rates of active sodium transport JNa and the associated metabolic reaction Jr are postulated to be linear functions of both the electrochemical potential difference of sodium--XNa and the affinity A (negative free energy) of the metabolic reaction of driving transport. Experimental studies in various epithelia demonstrate that both JNa and Jr (oxygen consumption) are indeed linear functions of XNa. Theoretical considerations and experimental studies in other systems suggest that likelihood of linearity in A as well. If so, A may be evaluated. Several observations indicate that the quantity A evaluated from the thermodynamic formalism does in fact reflect the substrate-product ratio of the metabolic reaction which supports transport. This is in contrast to measurements of mean cellular concentrations, which may not reflect conditions at the site of transport. Associated studies of isotope kinetics permit the distinction between effects on the permeability of the active and passive transport pathways. With these combined approaches, it may prove possible to characterize both the energetic and permeability factors which regulate transport. The formulation has been applied to an analysis of the mechanism of action of the hormone aldosterone.