The two stage nature of the aldosterone response

Aldosterone-induced glycoproteins: further characterization

Aldosterone induces the synthesis of a group of glycoproteins (GP65,70) in toad urinary bladders which are potential effectors of the natriferic action of this hormone. The GP65,70 complex is composed of two molecular weight classes of proteins (Mr 65 and 70 kDa), each class being composed of several discrete proteins of varying isoelectric points (5.8-6.2). These proteins can be partially enriched (approximately 20-fold) using wheat germ agglutinin-sepharose affinity chromatography, are neuraminidase-resistant, and can be N-deglycosylated by endoglycosidase-H and N-glycanase. Treatment with N-glycanase leads to the appearance of a microheterogeneous group of proteins, all having the same Mr (approximately 40 kDa). From these studies it can be concluded that these particular aldosterone-induced proteins: (1) are heavily glycosylated, (2) contain multiple high mannose and hybrid oligosaccharides side chains, and (3) contain similar (if not identical) peptide backbones. Post-translational N-glycosylation accounts, at least in part, for their electrophoretic polymorphism (variation in Mr) but not for their electrophoretic microheterogeneity (variation in pI). The latter may reflect other types of post-translational modification (e.g. O-glycosylation, phosphorylation) or may be due to subtle differences in amino acid composition. The partial purification and biochemical characterization of GP65,70 should ultimately lead to a better understanding of the function of these putative "effectors" of aldosterone-stimulated Na+ transport.

Aldosterone-induced glycoproteins: electrophysiological-biochemical correlation

Aldosterone induces the synthesis of a group of glycoproteins (GP65,70) in toad urinary bladders which are potential effectors of the natriferic action of this hormone. In the present study we have confirmed that aldosterone produces a two-phase electrophysiological response. During the early phase (less than 3 h) short-circuit current and transepithelial conductance increase in parallel, while during the late phase (greater than 3 h) short-circuit current continues to increase without any further change in conductance. By biosynthetically labeling aldosterone-treated toad bladders with [35S]methionine either during the early (h 0-2 or 1-3) or the late (h 4-6 or 7-9) phases of the natriferic response, we have demonstrated that GP65,70 is synthesized as a late effect of aldosterone. Since synthesis of GP65,70 occurs at a time when the electromotive force of the Na+ pump is increasing, and since GP65,70 biochemically resembles the beta subunit of Na+/K+-ATPase, studies were undertaken to examine whether GP65,70 is the beta subunit. Purified amphibian renal beta subunit was analyzed by two-dimensional polyacrylamide gel electrophoresis and was found to have an isoelectric point and Mr value similar to those of GP65,70. However, when nitrocellulose blots containing wheat germ agglutinin-purified proteins from aldosterone-treated bladders were stained with monospecific polyclonal antibodies developed against the beta subunit, GP65,70 was not recognized, whereas a group of slightly more acidic proteins of similar Mr were recognized. Thus, GP65,70 is not the beta subunit of Na+/Ka+-ATPase. Further studies are needed to determine the cellular function of GP65,70.

Receptor occupancy vs. induction of Na+-K+-ATPase and Na+ transport by aldosterone

In the urinary bladder of the toad Bufo marinus aldosterone (between 0.8 and 100 nM) stimulates Na+ transport [half-maximal induction concentration (K1/2) = 6.5 nM]. At low hormone concentrations (0.8-8 nM), the increase of Na+ transport between 0.75 and 2.5 h is accompanied by a fall in transepithelial resistance (R). Higher hormone concentrations (30-800 nM) induce an additional resistance-independent fraction of Na+ transport within 2.5-8 h. From 6 h on, aldosterone (between 0.2 and 20 nM) stimulates in the same tissue the biosynthesis rate of the alpha- and beta-subunits of Na+-K+-ATPase (K1/2 = 3 and 1.5 nM, respectively). New pump synthesis is thus not a prerequisite for the early mineralocorticoid response but might be linked to the late transport event. The mineralocorticoid response is usually ascribed to interaction with the higher affinity type 1 receptor. In the present study we show, however, that at least 55% of the overall Na+ transport response is linked to nuclear occupation of the lower affinity type 2 receptors [dissociation constant (Kd) = 50 nM, maximum number of binding sites (Nmax) = 315 fmol/mg protein]. Distinct aldosterone effects, such as the fall in R and the increase in Na+-K+-ATPase synthesis, are more closely related to occupation of type 1 receptors (Kd = 0.3 nM, Nmax = 23 fmol/mg protein). At maximal induction of these latter parameters, only about 20% of type 2 receptors are occupied. These results suggest that both types of aldosterone receptors are involved in the mediation of the full mineralocorticoid response: type 1 in the early and late and type 2 particularly in the late tissue response.

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.

Effect of steroid depletion on the response of toad bladder to vasopressin

1. We have investigated the water transport and short-circuit current (s.c.c.) response to vasopressin (1 mu./ml. and 100 mu./ml.) in isolated toad urinary bladders (Bufo marinus) following overnight incubation in the presence or absence of steroid-containing Ringer solution. 2. The water transport response to the lower dose of vasopressin (1 mu./ml.) was considerably reduced in 'steroid depleted' conditions, wheras the response to the higher dose of vasopressin (100 mu./ml.) was not similarly affected. 3. Aldosterone 3. Aldosterone 3. Aldosterone (10(-7)M) potentiated the water transport response to the lower dose of vasopressin (1 mu./ml.) but had no effect on the response to the higher dose (100 mu./ml.). 4. There was no effect of 'steroid depletion' or aldosterone treatment on the vasopressin s.c.c. response when measured as a percentage increase above basal levels. 5. In 'steroid depleted' conditions vasopressin (1 mu./ml.) maximally stimulated Na+ transport (s.c.c.) but a higher dose of vasopressin (100 mu./ml.) was required for maximum water transport. 6. We have failed to obtain any potentiation effect of corticosterone (10(-7)M) on the water transport or s.c.c. response to vasopressin (1 mu./ml.).

Aldosterone action and sodium- and potassium-activated adenosine triphosphatase in toad bladder

Urinary hemibladders obtained from toads soaked in water or saline were treated with aldosterone, 10(-6) M, either 1(1/2) or 16 h after mounting. After 2(1/2) h exposure to the hormone, short-circuit current was increased by 110-192% and open-circuit potential by 20-44% as compared with untreated paired hemibladders. Mucosal cells were then assayed for sodium-potassium-stimulated adenosine triphosphatase (ATPase). No increase occurred in activity per milligram protein or in the portion of total activity dependent on sodium. Activity at low sodium concentrations was also measured and analyzed by means of the Hill equation in terms of K, the apparent dissociation constant of the enzyme-sodium complex, and n, a number that expresses the degree of interaction between binding sites. Neither K nor n was significantly altered by aldosterone. A few experiments were also carried out at low ATP concentrations (0.3 mM); again no change in sodium-dependent activity was noted. The results indicate that aldosterone does not stimulate sodium transport by increasing the quantity of sodium-potassium adenosine triphosphatase in mucosal cells or the dependence of this activity on sodium or ATP concentrations.