MECHANISM OF THE ANTINATRIURETIC ACTION OF ALDOSTERONE

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Action of compounds with effective in vivo mineralocorticoid activity on ion transport in leucocytes

We have studied the in vitro short-term effects of aldosterone (1.0-1000 nmol l-1), cortisol (0.5-5.0 mumol l-1), fludrocortisone (1.0-10 nmol l-1) and carbenoxolone (0.5-3 mmol l-1) on 86rubidium influx (a model for potassium), 22sodium efflux, and [3H]-ouabain binding capacity in intact human leucocytes. No effect of aldosterone (at concentrations present in Conn's syndrome) or fludrocortisone could be demonstrated on cation fluxes or [3H]-ouabain binding compared to controls. No significant effect of cortisol, at concentrations either physiological or present in Cushing's syndrome, could be demonstrated on cation fluxes or [3H]-ouabain binding compared to controls. Carbenoxolone significantly increased 86Rb influx and 22Na efflux at concentrations known to cause hypokalaemia in man. The effect was not blocked by propranolol. No effect could be demonstrated for [3H]-ouabain binding.

Effect of aldosterone antagonists on aldosterone-induced activation of Mg2+ -HCO3- -ATPase and carbonic anhydrase in rat intestinal mucosa

In previous studies, Mg2+ -dependent, HCO3- -activated ATPase in the brush border and carbonic anhydrase in the cytoplasm of rat duodenal and jejunal mucosa decreased after adrenalectomy. Both enzyme activities increased to near normal levels 4 h after i.p. injection of aldosterone (40 micrograms/kg). These results suggest the possibility that both enzymes in the small intestinal mucosa may be mediators of the action of aldosterone. In the present studies, therefore, the effects of actinomycin D (500 micrograms/kg, i.p.), spironolactone (50 mg/kg, s.c.) and potassium canrenoate (50 mg/kg, s.c.) on aldosterone-induced activation of both enzymes in the upper small intestinal mucosa from adrenalectomized rats were examined to clarify the mechanism of action of aldosterone in enzyme levels. Actinomycin D inhibited carbonic anhydrase activity in small intestinal mucosa from normal rats 4 h after i.p. injection but had no effect on ATPase activity, while two other drugs had no effect on either enzyme activity in normal rats up to 4 h later. Pretreatment with these 3 drugs 1 h before aldosterone administration (40 micrograms/kg, i.p.) to adrenalectomized rats blocked the aldosterone-induced activation of ATPase and carbonic anhydrase in the upper small intestine. On the other hand, adrenalectomy and administration of aldosterone and its antagonists, alone or in combination, had no effect on kidney enzyme activities. These results confirm that Mg2+ -HCO3- -ATPase and carbonic anhydrase are mediators of the action of aldosterone in the upper small intestinal mucosa.

Effects of aldosterone and corticosterone on cloacal water and electrolyte excretion of constantly-loaded intact and colostomized ducks (Anas platyrhynchos)

1. Doses of aldosterone (50, 100 and 200 micrograms per kg body wt) evoked similar changes in Na+ and K+ excretion by intact and colostomized ducks loaded with either distilled water or 0.5 isotonic saline (70 mM NaCl, 1.5 mM KCl); both antinatriuretic and antikaliuretic responses were observed. 2. The lowest dose of aldosterone had no effect on electrlyte excretion in intact and colostomized ducks loaded with a solution containing more K+ than Na+ (74 mM KCl, 36 mM NaCl) but the higher doses caused an antinatriuretic response in both groups; a retention of K+ occurred only in intact birds given this solution. 3. The lower dose of corticosterone (1.25 mg per kg body weight) caused both antidiuresis and antinatriuresis in intact birds, but in colostomized birds the decrease in Na+ excretion was not accompanied by an antidiuresis. 4. The higher dose of corticosterone (2.50 mg per kg body wt) caused a significant increase in K+ excretion in colostomized birds, whereas no kaliuresis was ever observed in intact birds. 5. Intact and colostomized birds loaded with 0.5 isotonic saline showed no responses to the lower dose of corticosterone, whereas the higher dose had an antikaliuretic effect in intact birds and an antinatriuretic effect in colostomized birds. 6. Corticosterone had no effect on cloacal water and electrolyte excretion by intact and colostomized birds given loads containing more K+ than Na+.

Mineralocorticoid-induced membrane proteins in MDCK cells

The Madin-Darby canine kidney (MDCK) cell line, which exhibits properties indicative of a distal tubule origin, evidently binds and responds to mineralocorticoid hormones. We investigated the effects of aldosterone and deoxycorticosterone on protein synthesis in MDCK cells grown either in medium supplemented with serum or in a hormonally defined, serum-free medium. Aldosterone induced the synthesis of at least 2 membrane proteins with molecular weights of 35000 and 14000. The MDCK line may prove a useful model system for examining the mechanism of mineralocorticoid-regulated sodium transport and, in particular, the identification and study of hormone-induced proteins in a homogeneous cell population.

Evidence for an enhanced transmembrane sodium (Na+) gradient induced by aldosterone in the incubated rat tail artery

Aldosterone is known to stimulate Na+ transport as measured in terms of current-carrying capacity of epithelial sheets or of Na+ - K+ ATPase activity in cells. The possibility that this is reflected in an altered steady-state transmembrane Na+ distribution in vascular smooth muscle was here examined directly. Transmembrane Na+ and K+ gradients were first dissipated by overnight incubation in K-free physiological salt solution (PSS) at 10 degrees C and then reestablished by 3 hours in normal PSS at 37 degrees C. The addition of d-aldosterone (but not corticosterone) to these media significantly reduced cell Na. This involved only free cell Na which was reduced by about 20% of 3 mmole/kg dry wt. No significant change in membrane permeability measured in terms of net Li uptake at 3 degrees C or at 37 degrees C was observed. The lowest effective aldosterone concentration was 2.8 x 10(-9) M. These results are consistent with the observed enhancement of net Na+ transport in incubated arteries in DOCA-induced hypertension and in the SHR but do not account for the increased Na+ permeability observed in these states.

Effect of insulinopenia and adrenal hormone deficiency on acute potassium tolerance

The ability to dispose of an acute intravenous potassium load was examined in glucocorticoid-replaced adrenalectomized rats and in rats made insulinopenic with somatostatin. Adrenalectomy resulted in a significantly greater rise in plasma potassium concentration compared with controls (1.46 +/- 0.11 vs. 0.92 +/- 0.05 mEq/liter, P less than 0.001) despite the excretion of an identical percentage (47%) of the administered potassium load in 2 hours. Somatostatin-induced insulinopenia (insulin levels decreased from 37 +/- 5 to 20 +/- 3 microU/ml) was also associated with a significantly greater increment in plasma potassium controls, despite the excretion of a similar amount (39%) of the administered potassium load. In animals with combined adrenal and insulin deficiency, the rise in plasma potassium concentration occurred earlier and remained elevated for a more prolonged period of time compared with animals with either adrenalectomy or insulinopenia alone. Conclusion. During acute potassium loading in the rat, insulin and adrenal hormones play an important role in maintaining normal potassium homeostasis, primarily by enhancing potassium uptake by external tissues.

Role of RNA in the action of aldosterone on Na+ transport

Recent data describing the effects of aldosterone on the induction of messenger RNA (= mRNA) and ribosomal RNA (= rRNA) are reviewed. In the urinary bladder of the toad, aldosterone induces a few specific polyadenylated mRNAs (= poly(A)(+)mRNA) during the latent period, i.e., 30 to 60 min after hormone addition. Later i.e., 90 to 240 min after aldosterone addition, 18S and 28S cytoplasmic rRNA subunits are also induced. The effect of poly(A)(+)mRNA is mineralocorticoid-specific and correlates well with the aldosterone-dependent Na+ transport. Actinomycin D which inhibits both poly(A)(+)mRNA and nonpolyadenylated mRNA (= POLY(A)(-)mRNA) totally abolishes the response to aldosterone on Na+ transport. 3'deoxyadenosine (cordycepin), which inhibits poly(A)(+)mRNA but not poly(A)(-)mRNA, only inhibits 50 to 60% of the physiological response. These differential effects suggest that an intact poly(A)(-)mRNA pathway is also an important factor in mediating the action of aldosterone. In contrast, 3'deoxycytidine, which inhibits rRNA but not mRNA, does not impair the mineralocorticoid response, at least during the first 3 hr of aldosterone action.

Comparison of toad bladder aldosterone-induced proteins and proteins synthesized in vitro using aldosterone-induced messenger RNA as template

Using double-labeled isotope techniques, it can be shown that aldosterone induces the synthesis of several proteins in the mitochondria-rich (MR) cells of the toad's urinary bladder. Induced proteins have been identified both in the plasma membrane (mol wt = 170,000, 85,000 and 12,000) and the cytosol (mol wt = 36,000, 12,000 and 6,000) fractions of these mucosal cells. We have also shown that aldosterone (Aldo) induces the synthesis of a class of RNA having the properties of messenger RNA (mRNA). mRNA isolated from Aldo-treated mucosal cells was used as template in a cell-free protein-synthesis system prepared from rabbit reticulocytes. Preparations charged with mRNA from Aldo-treated cells synthesized two proteins that were not labeled when mRNA from control tissues was used as template. The electrophoretic mobility of one of these proteins was similar to an Aldo-induced membrane protein (mol wt = 70,000) found in the intact tissue.

Aldosterone effects on renal metabolism

1. Manometric studies of the sodium dependent oxygen consumption in rat kidney slices have failed to reveal any significant effect of aldosterone treatment, adrenalectomy or sodium diet on sodium diet on sodium metabolism. 2. However, ammonia release from kidney slices was significantly reduced following adrenalectomy and this decrease was influenced by aldosterone treatment. 3. The dose-response characteristic obtained for this aldosterone-stimulated ammonia release has been determined. 4. The effect of a high Na+ diet on the ammonia release has been studied. An initial decrease after 2 days may be associated with decreased endogenous aldosterone secretion. However, aldosterone (2.5 microgram/100 g body weight)injections into these high Na+ treated animals fails to restore the normal ammonia release. 5. The effects of aldosterone (2.5 microgram/100 g body weight), dexamethasone (2.5 microgram/100 g body weight) and corticosterone (2.5 microgram/100 g body weight) injections, in adrenalectomized rats, on ammonia release and tissue tyrosine aminotransferase activities have been compared.

The effect of spironolactone on transport of Na+, K+ and H+. A microperfusion study in rat main submaxillary duct

The epithelium of the main excretory duct of the rat submaxillary gland was used as a target tissue for studies on the effect of a spironolactone on electrolyte transport. The spironolactone decreased net Na+ reabsorption by 27% and net K+ secretion by 23%. HCO-3 was found to be about 2-fold accumulated in the duct lumen, which was considered to result from decreased H+ion secretion. The results can be reconciled with an action of spironolactone on 1) the peritubular Na+-K+-exchange mechanism and 2) the functional coupling of Na+ entry from lumen to cell with K+ and H+ transfer from cell to lumen.

Stimulation by aldosterone of the sodium efflux in barnacle muscle fibres: effects of RNA inhibitors and spironolactone

1. Single muscle fibres from the barnacle Balanus nubilus have been studied to provide information about the mode of action of aldosterone on Na transport in a symmetric cell. 2. Neither internal nor external application of aldosterone modified the course of the Na efflux. However, fibres pre-exposed overnight to a dose of aldosterone showed a biphasic response to external aldosterone. The first phase was prompt in onset and consisted of a falling rate constant for Na efflux becoming a constant. This has been interpreted as indicating that aldosterone acts by releasing sequestered or bound Na. The second phase was delayed in onset with the average latent period being 68 min. It consisted of a step-up in the rate of Na efflux, followed by a transitory rise in the efflux rate constant. This second phase was dose-dependent, the minimum concentration being 10(-9) M. 3. Internal application of aldosterone in a concentration as low as 10(-10) M promptly stopped the rate constant for Na efflux from further falling but failed to cause delayed stimulation. 4. The response of the Na efflux into Li-ASW following external application of aldosterone was bi-phasic. The magnitude of the delayed stimulation was comparable to that found in controls. 5. No correlation between latent period or size of the internal Na bound fraction, and the magnitude of the delayed stimulation could be established. 6. Internal application of actinomycin-D, alpha-amanitin or cordycepin shortly before application of external aldosterone caused complete abolition of the delayed stimulation. 7. Overnight pre-exposure of the barnacle to actinomycin-D caused complete abolition of the falling rate constant for Na efflux, as well as the delayed stimulation caused by external aldosterone. 8. Internal application of spironolactone SC-14266 shortly before external application of aldosterone caused complete abolition of the biphasic response to the steroid. 9. It is concluded that (i) barnacle fibres can be made sensitive to aldosterone, (ii) the biphasic effects on the Na efflux depend on prior induction of RNA, (iii) the first effect caused by internal or external aldosterone involves mobilization of sequestered Na, (iv) the results obtained with spironolactone are in keeping with the current view that aldosterone interacts with a receptor system before causing de-repression. 10. The implications of the finding that aldosterone releases sequestered Na are briefly touched upon.

Studies on the mechanism of action of angiotensin on ion transport by kidney cortex slices

1. A study has been made of the effects of cyclic AMP, phosphodiesterase inhibitors and protein synthesis inhibitors on the response of rat kidney cortex slices to physiological doses of angiotensin.2. The additions of cyclic AMP, dibutyryl cyclic AMP and/or phosphodiesterase inhibitors to the incubation medium (conditions which would be expected to increase intracellular cyclic AMP levels) were without effect on sodium or potassium transport by kidney slices.3. Actinomycin D (an inhibitor of the transcription stage of protein synthesis), at concentrations which inhibit RNA synthesis by 75%, has no effect on either control or angiotensin stimulated sodium transport.4. Cycloheximide or puromycin (inhibitors of the translation stage of protein synthesis), at concentrations which inhibit protein synthesis by 70-80%, have no effect on control sodium and potassium transport by kidney slices, but completely block the angiotensin stimulation of these processes.5. These findings are discussed in relation to the possible involvement of cyclic AMP and protein synthesis in the mechanism of action of angiotensin on kidney sodium and potassium transport.