On the mechanism of action of aldosterone

Energetics of sodium transport in the urinary bladder of the toad. Effect of aldosterone and sodium cyanide

Experiments were designed to determine whether the stimulatory effect of aldosterone on sodium transport involves an increase in tissue ATP. Urinary bladders that were removed from toads presoaked in 0.6% saline for 48-72 h, mounted as sacs, and maintained in open circuit except for brief observation of short circuit current every 30 min responded to 100 nM aldosterone added to the serosal bath with an increase in short circuit current to 170% of control hemibladders, which plateaus at 2-3 h. Tissue (ATP)/(ADP) X (Pi) measured in perchloric acid extracts increased to a maximum of 208% of controls (P less than 0.001) and ATP increased to 116% of controls (P less than 0.01) at 180 min. The short circuit current response to aldosterone paralleled the increase in ATP and (ATP)/(ADP) X (Pi) measured at 75, 120, 180, and 240 min. In bladders clamped at -150 mV, the short circuit current response to aldosterone was greater: 280% of controls (P less than 0.001) and tissue (ATP)/(ADP) X (Pi) increased to 191% of controls (P less than 0.001). In continuously short circuited bladders and bladders clamped at +75 mV, the short circuit current response to aldosterone and the change in ATP, ADP, or Pi were markedly diminished. 100 microM amiloride added to mucosal bath decreased the short circuit current to zero and inhibited the short circuit current response to aldosterone, whereas tissue ATP increased to 141% (P less than 0.05). 100, 250, and 500 microM NaCN dropped the short circuit current to 59, 35, and 24% of control values, respectively. Concurrently, tissue ATP measured at 60 min after the addition of NaCN dropped to 79, 66, and 56% of control values, respectively, and tissue ATP/ADP dropped to 68, 50, and 40%, respectively. The data revealed significant correlation between the change in the rate of sodium transport produced by aldosterone or NaCN as measured by the short circuit current and the concentration of ATP (r = 0.96, P less than 0.001), as well as ATP/ADP (r = 0.95, P less than 0.001). In conclusion, these results support the view that the stimulatory effects of aldosterone on sodium transport involve an increase in ATP or (ATP)/(ADP) X (Pi).

Short-term effect of aldosterone on renal sodium transport and tubular Na-K-ATPase in the rat

The short-term effect of one single injection of aldosterone on the renal sodium transport on one hand, and the Na-K-ATPase activity on the other hand, was studied in chronic adrenalectomized rats. Sodium transport was estimated by clearances, and Na-K-ATPase was measured in microdissected fragments of the nephron, according to our microtechnique previously described. Five to eight days after adrenalectomy, only 30% of the initial enzyme activity was recovered in the cortical collecting tubule (CCT). Administration of aldosterone completely restored the ATP-ase activity within three hours. Adrenalectomy also curtailed by 20-45% the activity of other nephron segments but aldosterone had no stimulatory effect on them. Sodium-reabsorption also increased after the hormone injection, following the same time (0.5 less than t1/2 less than 1 h) and dose dependencies (0.8 less than K1/2 less than 0.9 micrograms/kg) as those observed for the enzyme activity in the CCT. It is concluded that the short-term stimulation of Na-K-ATPase in the collecting tubule, after an acute administration of aldosterone, may be responsible for the simultaneous increase in sodium transport.

The role of the kidney in sodium homeostasis during maturation

Evidence is presented that the retention of sodium observed during development is consequent primarily to enhanced tubular reabsorption rather than to low rates of glomerular filtration. The enhanced transport of sodium occurs in nephron segments located beyond the proximal tubule, apparently under the stimulation of the high plasma concentration of aldosterone. This adaptive mechanism may account for the fact that the infant thrives on a rather low intake of sodium, as prevails during the period of breast-feeding. The renin-angiotensin-aldosterone system cannot be fully inhibited even by intravascular volume expansion and this may account for the blunted natriuretic response of the developing animal and human to the acute infusion of saline or albumin solutions. Conversely, the renal sodium loss and the hyponatremia often encountered in premature babies appear to be due to an insufficient rise in aldosterone secretion or to a limited responsiveness of the distal tubule to aldosterone stimulation.

Oxidative response to aldosterone of pyridine nucleotide in rat kidney in situ

In an attempt to elucidate early biochemical events in the action of aldosterone on rat kidney in situ, the response of the pyridine nucleotide oxidation-reduction state of surface cells was directly and continuously recorded with an organ-fluorometer. Intravenous administration of aldosterone induced a rapid (maximum response in 10 min) and dose-related (2.5-25 microng/100 g rat) oxidative response that was specific to aldosterone and to the kidney. The oxidative response was 1) detectable with minute dise (approximately 0.2 microng/100 g rat) of hormone: 2) reproduced by other mineralocorticoids; 3) enhanced by a maneuver for expanding the extracellular fluid compartment or by adrenalectomy; and 4) prevented by spironolactone, progesterone, actinomycin D, and cycloheximide. All of these data argue that the redox response is related to the subsequent changes in aldosteronemediated ion transport. Experiments with an uncoupler and with redox substrates showed that mitochondrial NADH was the major nucleotide pool responding to hormone. The oxidation was not accompanied by changes in the adenylate energy charge level of the whole organ. These observations support the view that aldosterone acts on energy metabolism of tubular cells before developing apparent cation transport effects.

Aldosterone-Induced Membrane Phospholipid Fatty Acid Metabolism in the Toad Urinary Bladder

Aldosterone action in the isolated toad urinary bladder has been studied by incubation of the tissue with several specifically labeled lipogenic precursors. Within 30 min after hormone addition phospholipid synthesis is stimulated; the metabolism of oleic acid is particularly enhanced. Additionally, during this time interval a phospholipid deacylation-reacylation cycle is stimulated by aldosterone.

Effects of cytochalasin B on the response of toad urinary bladder to vasopressin

A combined physiological and morphological study of the effects of cytochalasin B (CB) on the toad urinary bladder has been carried out. CB inhibits the hydro-osmotic response to vasopressin without altering basal water permeability or diffusion, or the increase in (3)H(2)O diffusion observed after hormone addition. Although CB increases [(22)Na]-, [(36)Cl]-, and [(14)C]urea fluxes, and decreases transepithelial potential, no alteration in basal short-circuit current, the vasopressin-induced increase in this parameter, or [(14)C]inulin permeability occurs. In the absence of hormone, CB does not markedly alter the structure of the toad bladder. However, in the presence of vasopressin, CB induces the formation of large intracellular vacuoles. These results suggest a possible coupling of solute and water movement across the tissue.

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.