The effect of aldosterone on the energetics of sodium transport in the frog skin

Dexamethasone enhances expression of mitochondrial oxidative phosphorylation genes in rat distal colon

Dexamethasone and aldosterone are major activators of Na+ reabsorption in tight epithelia. The genes whose expression mediates the steroid actions are mostly unknown. To identify such genes, we performed differential screening of a rat colon cDNA library with total 32P-labeled cDNA probes reverse transcribed from steroid-stimulated and steroid-depleted poly(A)+ RNA. Several cDNAs whose corresponding mRNA is enhanced two- to threefold after dexamethasone injection were identified. Partial sequencing indicated that four of them code for subunits of cytochrome-c oxidase and 16S mitochondrial mRNA. The dexamethasone-induced increase in mitochondrial RNA abundance could not be mimicked by a low-salt diet, found to increase plasma aldosterone from 1.0 +/- 0.1 to 12.8 +/- 1.4 nM. Induction of mitochondrial genes by adrenal steroids may serve to prevent limitation of transport by the ATP supply to the Na(+)-K+ pump under conditions of maximal stimulation of Na+ transport.

Metabolic regulation of apical sodium permeability in toad urinary bladder in the presence and absence of aldosterone

In the present study, further evidence was adduced for energy-dependent regulation of passive apical transport of Na in toad bladder epithelium. In potassium-depolarized preparations studied by current-voltage analysis, additions of pyruvate or glucose to the media of substrate-depleted bladders evoked proportionate increases in the transepithelial Na current and in apical Na permeability. These responses were large in aldosterone pretreated hemibladders and almost absent in the aldosterone-depleted preparations or when hormonal action was blocked by spironolactone or cycloheximide. The substrate-induced increases in apical Na permeability were fully reversed by appropriate metabolic inhibitors, i.e. 2-deoxyglucose and oxythiamine. Moreover, the inhibitory effect of 2-deoxyglucose was bypassed by the addition of pyruvate to the serosal medium. Thus apical Na permeability is clearly sensitive to the supply of cellular energy. The possibility that changes in intracellular free Na activity may mediate metabolic regulation of apical Na permeability was evaluated by prolonged exposure to Na-free mucosal and serosal media, with and without inhibition of the Na/K-pump by ouabain. The stimulatory and inhibitory effects of pyruvate, 2-deoxyglucose and oxythiamine on Na currents and Na conductances were preserved under these circumstances. Furthermore, reduction of serosal Ca to a minimal level of 3 microM, was without effect on the response to metabolic inhibition. These experiments demonstrate the existence of Na-independent metabolic regulation of apical Na transport and imply that neither basal-lateral nor mitochondrial Na/Ca exchange is required for this regulatory process under the imposed conditions. The possibility that a Na-independent, Ca transport mechanism in mitochondria or endoplasmic reticulum may be involved in metabolic regulation of apical Na transport, however, remains to be evaluated.

The significance of changes in thermodynamic affinity induced by aldosterone in sodium-transporting epithelia

The energetics of sodium transport were examined in toad (and occasionally frog) skin, with particular emphasis on the effect of aldosterone. Thermodynamic affinity was computed according to Essig and Caplan. Following treatment with antidiuretic hormone or drugs believed to affect only the apical membrane barrier, no change in thermodynamic affinity was observed either acutely (after one to two hours) or chronically (after 18-odd hours hours). By contrast, following treatment with aldosterone overnight, thermodynamic affinity was considerably increased, whether or not incubation was conducted in the presence of sodium in the outer solution; addition of glucose at the end of incubation, whereby sodium transport was stimulated further, failed to influence affinity as measured. The stoichiometry between sodium transport and oxygen consumption was, however, unchanged by aldosterone treatment in short-circuit conditions, neither was that fraction of aerobic metabolism unrelated to sodium transport influenced. It is concluded that the change observed with aldosterone can be directly ascribed to the hormone, as it is independent of glucose availability and of sodium transport. Aldosterone action, at least following prolonged incubation, therefore does not involve only an increase in apical conductance for sodium.

Molecular action of aldosterone

Aldosterone stimulates the reabsorption of sodium across epithelial cells of various target tissues. The initial events in the molecular action of the mineralocorticoid are the following: (1) Diffusion of the steroid across the cellular (baso-lateral, serosal) plasma membrane into the cytoplasmic compartment. (2) Binding of the steroid to a receptor protein specific for the class of steroid and activation of this steroid-receptor-complex. (3) Translocation of the activated aldosterone-receptor complex to the nucleus and stimulation of RNA synthesis (including the synthesis of messenger RNA and ribosomal RNA). (4) Translation of the steroid-induced messenger RNAs at the ribosomal level into the aldosterone-induced proteins (AIP) within the cytoplasmic compartment. Whereas these induction steps are uniformly accepted, the mechanisms by which the AIPs increase the activity of a rate-limiting step in the sodium transport process are still object of debate. In this paper we discuss the initial events in the mode of action of aldosterone and the biochemical and physiological approaches to the aldosterone-induced proteins with special reference to the "sodium permease", the "energy", and the "sodium pump" theory. Our analysis shows that despite serious efforts by multiple laboratories, the first AIP with an established relationship to the mineralocorticoid actions of aldosterone is yet to be identified.

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.

Effects of antidiuretic hormone on kinetic and energetic determinants of active sodium transport in frog skin

The effects of antidiuretic hormone (ADH) on the rate of transepithelial active Na transport JaNa and the rate of suprabasal O2 consumption of Jsbr were studied in paired hemiskins of frog. Within some 30 min following administration of ADH both JaNa and Jsbr increased to near-maximal levels and then remained stable for at least an hour. On symmetric perturbation of the transepithelial electrical potential delta psi at 6-min intervals, the dependence of JaNa and Jsbr on delta psi was near-linear, both in control and experimental hemi-skins. The stability and near-linearity of the system permitted systematic analysis of the parameters of linear non-equilibrium thermodynamic (NET) and electrical equivalent circuit (EC) formulations. ADH (100 mU/ml) stimulated two of the three NET phenomenological L coefficients, as well as A, the affinity (negative Gibbs free energy) of a metabolic reaction driving transport. Observations at partially depressed levels of transport indicated that the effects of kinetic and energetic factors are to some extent discrete. EC analysis showed stimulation of the amiloride-sensitive conductance Ka, but not of the apparent electromitive force of Na transport 'ENa'. Similar effects were produced by 10 mU/ml of ADH or by 10 mM dibutyryl cyclic AMP, although less marked effects on the L coefficients were noted with the lower concentration of hormone. It is suggested that, in contrast to EC analysis, the NET formulation distinguishes between kinetic and energetic determinants of transport, supporting a dual mechanism of action of ADH.

Time course of active Na transport and oxidative metabolism following transepithelial potential perturbation in toad urinary bladder

The use of an Ussing chamber with well-defined mixing characteristics coupled to a mass spectrometer permits the concurrent evaluation of transepithelial current and oxidative metabolism with improved temporal resolution. The time-course of the amiloride-sensitive current Ia and the rate of suprabasal CO2 production JsbCO2 were observed in 10 toad urinary bladders at short-circuit and after clamping delta psi at 100 mV, serosa positive. Following perturbation of delta psi (0 leads to 100 mV), Ia declined sharply within 1/2 min, remaining near constant approximately 15 min, and then increased slightly. JsbCO2 declined more gradually, remained near constant at approximately 4-7 min, and then declined further. Detailed analysis revealed an early quasi-steady state with near constancy of JsbCO2 starting at 2.9 +/- 1.1 (SD) min and lasting 4.7 +/- 1.8 (SD) min, followed by relaxation to a later steady state at about 15 min. During the early quasi-steady state, Ia was also nearly constant. Considering that in steady states Ia/F approximately or equal to JaNa, the rate of transepithelial active Na transport, during the early quasi-steady state mean values +/- SE of JaNa, JsbCO2 and (JaNa/JsbCO2) were, respectively, 29.9 +/- 1.7%, 59.4 +/- 3.2%, and 56.4 +/- 5.7% of values at short-circuit. Corresponding values during the late steady state were 41.4 +/- 6.0%, 38.2 +/- 6.1%, and 111.3 +/- 8.6%. Thus the flow ratio JaNa/JsbCO2 was depressed significantly during the early quasi-steady state, but returned later to the original value. The results of measurements of Ia and JsbO2 in three hemibladders were qualitatively similar. In terms of a phenomenological "black-box" treatment the findings are consistent with earlier studies indicating incomplete coupling between transport and metabolism. Further studies will be required to clarify the molecular basis for these observations.

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.

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.

Induction of citrate synthase by aldosterone in the rat kidney

The possible induction of renal citrate synthase (E.C. 4.1.3.7) by aldosterone was evaluated in the adrenalectomized rat. Three hours after administration of aldosterone (0.8 microgram/100 g body wt), renal cortical and medullary citrate synthase activity was significantly increased as reported previously by Kinne and Kirsten (Kinne, R., Kirsten, R. 1968. Pfleugers Arch. 300:244). In contrast, no change in this activity was detected in the renal papilla or the liver, under the same conditions. Kinetic analysis revealed that injection of aldosterone had no effect on the KmS for acetyl-CoA and oxalacetate but augmented Vmax of renal medullary citrate synthase activity by 40%. The aldosterone-dependent increase in medullary citrate synthase activity was proportionate to the associated increase in the quantity of antiserum (specific for citrate synthase) required for half-maximal immuno-precipitation. The possibility that aldosterone induced the synthesis of citrate synthase was evaluated in two sets of experiments. In the first set, adrenalectomized rats were injected intraperitoneally with either aldosterone (0.8 microgram/100 g body wt) or the diluent, and simultaneously with 3H or 35S methionine (500 muCi/rat). The isotopes were reversed in about half of the experiments. Three hours after the injection, renal citrate synthase was isolated by ATP-sepharose column chromatography and immuno-precipitation with the specific antiserum. Aldosterone augmented methionine incorporation into renal citrate synthase by 55% but had no effect on incorporation into the hepatic enzyme. In the second set, adrenalectomized rats were injected with either aldosterone (0.8 microcram/100 g body wt) or the diluent, the kidneys were removed 1 hr later and medullary slices were incubated in either 3H- or 35S-methionine at 20 degrees for 2 hr. Mitochondrial citrate synthase was isolated either by ATP-sepharose column chromatography and immuno-precipitation, or by polyacrylamide gel electrophoresis. Aldosterone increased methionine incorporation into the immuno-precipitates by 30% and into the enzyme peak resolved by polyacrylamide gel electrophoresis by 43%. The latter increase was eliminated by prior administration of either actinomycin D (70--80 microgram/100 g body wt) or spirolactone (SC-26304) (80 microgram/100 g body wt). An equimolar dose of dexamethasone (0.8 microgram/100 g body wt) had no effect on the isotope ratio associated with citrate synthase activity in the polyacrylamide gels.

Transients in toad skin: short circuit current and ionic fluxes related to inner sodium substitution by monovalent cations

When the Na electrochemical potential difference across the skin (delta muNa) is altered by perturbing the transmembrane electrical potential difference or the external Na concentration, effects on transport and associated oxygen consumption can be described by the formalism of linear nonequilibrium thermodynamics (Vieira, Caplan & Essig, 1972, J. Gen. Physiol. 59:77; Danisi & Lacaz-Vieira, 1974, J. Gen. Physiol. 64:372; Procópio and Lacaz-Vieira, 1977, J. Membrane Biol. 35:219). We now show that with modifications of delta muNa by substitution of Li or choline for Na in the inner bathing solution, this formalism is no longer applicable. Inner Na by K substitution ((Na X K)i) causes profound alterations in short-circuit current (SCC), JinNa, K efflux (JeffK) and PD. SCC drops transiently after (Na X K)i in Cl and in SO4 media, increasing subsequently. In Cl medium, following the initial transient, there is a late decline in SCC toward a steady state. The rate of SCC decline in Cl medium is more pronounced than that observed in SO4 medium. (Na X K)i causes a transient increase in JinNa with a peak synchronous to the minimum in SCC, both in Cl and in SO4 media. This was interpreted as due to depolarization of the inner membrane. In SO4 medium, following the peak observed after (Na X K)i, JimNa drops, to increase again toward a steady state in which SCC and JinNa are not statistically different, resembling the control condition before (Na X K)i. In Cl medium, however, the JinNa steady state is approximately 100% higher than SCC. This difference is due to an important K efflux (JeffK), which builds up progressively after the substitution. The apparent K permeability [JeffK/(Ki)] is of comparable magnitude in Cl and in SO4 media before (Na X K)i and also in SO4 medium after (Na X K)i. However, in Cl medium, after (Na X K)i, the apparent K permeability increases one order of magnitude as compared to the control condition before the ionic substitution. In Cl medium, the high levels of JinNa and of Jeff(K) observed in the steady state after (Na X K)i were interpreted as being a consequence of cell swelling. SCC and PD follow very different temporal patterns after (Na X K)i which are characterized by transients in SCC and a simple fall in PD. Reasons for these differences are discussed.

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).

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.

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.

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.

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.

A comparison of the effects of ouabain and 2-deoxy-D-glucose on the thermodynamic variables of the frog skin

Previous studies support the validity of a linear thermodynamic formalism relating the rates of active Na-+ transport and oxygen consumption Jr to the electrical potential difference delta-psi and the affinity A (negative free energy) of the metabolic driving reaction. The formulation was further tested in paired control and experimental hemiskins by the use of two inhibitors of Na-+ transport. Ouabain, a specific inhibitor of the Na-+ pump, might be expected to diminish the dependence of Jr on delta-psi without affecting A, whereas 2-deoxy-D-glucose, a competitive inhibitor of glucose metabolism should be expected to diminish A. Both inhibitors were used at concentrations adequate to depress Na-+ transport (i.e. short-circuit current Io) to some 50% of control level. Measurements were made of Io and dJr/d(delat-psi), and the apparent value of the affinity Aapp was calculated according to the thermodynamic formulation. Ouabain depressed minus dJr/d(delta-psi) without affecting Aapp whereas 2-deoxy-D-glucose depressed Aapp without affecting minus dJr/d(delta-psi). The demonstration of these effects indicated the utility of the formalism.

Effects of vasopressin and aldosterone on amiloride binding in toad bladder epithelial cells

Methods have been devised to measure the binding of [ 14 C]amiloride to isolated cells from bladders of toads, Bufo marinus . This agent blocks transepithelial sodium transport across bladders by preventing sodium entry to the transporting mechanism. A saturable binding component has been found with an affinity of 5.6 x 10 7 m -1 in the presence of 1.1 mM Na + , which corresponds to the affinity of amiloride when used as a transport inhibitor at the same sodium concentration. In freshly isolated cells the capacity of the binding sites is 3.6 x 10 5 sites/cell, but this value falls to about one third in aged suspensions. When cells are treated with vasopressin (100 mU/ml) somewhat less specific binding is measured at an amiloride concentration giving 50 % occupancy. The results are consistent with the view that vasopressin moves the binding curve to the right along the concentration axis, reducing the affinity of amiloride by a factor of approximately two, while leaving the total capacity unaffected. The affinity of amiloride when used as an inhibitor of transport is also found to be reduced by a factor of two by vasopressin, and complete inhibition of transport can still be achieved. d -Aldosterone in vitro increases the number of amiloride binding sites in isolated cells by approximately 115%, and results from transport studies indicate that there is no significant change in the affinity of amiloride after d -aldosterone treatment. Inhibitors of transcription and translation (actinomycin D and cycloheximide) prevent the increase in amiloride binding caused by d -aldosterone. In vivo the effects of d -aldosterone are more complex, but it is shown that the steroid increases the transport capacity of the tissue, when this is expressed as the number of amiloride binding sites per unit mass of tissue. The results are discussed in terms of the ways in which the two hormones may alter the entry of sodium into the epithelial cells, and so in turn affect transepithelial sodium transport.