Elevation of plasma aldosterone levels of tadpoles at metamorphic climax

Plasma aldosterone concentrations in Rana catesbeiana tadpoles during metamorphosis were determined by radioimmunoassay. Aldosterone levels were relatively low (0.7-1.4 ng/ml) during pre- and prometamorphosis. At the onset of climax, aldosterone concentration increased markedly and reached a peak (10.9 ng/ml) at stage XXII. Subsequently, aldosterone levels declined and at the end of metamorphosis (stage XXV), the average concentration was 3.9 ng/ml plasma. Administration of ACTH elevated plasma aldosterone levels in hypophysectomized tadpoles. When hypophysectomized animals were kept in thyroxine (T4), plasma aldosterone concentrations also increased. Combined ACTH and T4 treatment caused a marked increase in plasma aldosterone concentration. The possible involvement of thyroid hormone in the elevation of plasma aldosterone during metamorphosis is discussed.