Acid-base-electrolyte balance responses of Bufo marinus to aminoglutethimide, corticosterone, and aldosterone during hypercapnia

Abstract

Experiments were conducted to test the hypothesis that one or more interrenal steroids are active in regulatory responses to respiratory acidosis in the toad, Bufo marinus. Toads were divided into four experimental groups. The first group received sham injections. The second group received 1-3 mg of aminoglutethimide (AG) every 8 hr. AG inhibits the conversion of cholesterol to pregnenolone, thus inhibiting all steroid hormone synthesis. The third group received AG + 5 micrograms of aldosterone on the same schedule. The fourth group received AG + 25 micrograms of corticosterone on the same schedule as the other groups. All four groups were subjected to hypercapnia using 5% CO2 to induce a respiratory acidosis. The sham-operated animals displayed the normal compensatory pattern of producing a metabolic alkalosis (elevated plasma HCO3-) after 24 hr. AG-treated toads failed to elevate plasma HCO3-. Administration of interrenal steroids produced compensation in varying degrees. Aldosterone produced a small compensation while corticosterone produced a compensation similar to that seen in sham-operated animals. Analysis of steroid titers in toad plasma during hypercapnia showed that Bufo marinus does not elevate aldosterone during respiratory acidosis, but that corticosterone is elevated. AG blocked the corticosterone elevation, however. AG also produced a hyponatremia that was corrected with aldosterone or corticosterone. Normocapnic controls showed that AG does not produce deleterious effects on pH or blood gases in toads in the absence of a respiratory acidosis. We conclude that corticosterone is important in acid-base regulatory responses to respiratory acidosis in this amphibian.