Adaptation of a corticosterone ELISA to demonstrate sequence-specific effects of angiotensin II peptides and C-type natriuretic peptide on 1α-hydroxycorticosterone synthesis and steroidogenic mRNAs in the elasmobranch interrenal gland

It is thought that a single corticosteroid, 1α-hydroxycorticosterone (1α-B), is both a glucocorticoid and mineralocorticoid in the elasmobranch fishes. We investigated the putative mineralocorticoid role of 1α-B by examining regulation of interrenal 1α-B synthesis by osmoregulatory hormones in the euryhaline stingray Dasyatis sabina. Using synthesized steroid, a commercial enzyme-linked immunoassay was validated for the quantification of 1α-B. In interrenal cultures, the antinatriuretic peptide angiotensin II (ANG II) was potently steroidogenic, whereas C-type natriuretic peptide had no effect on 1α-B titers. However, both peptides significantly decreased abundance of rate-limiting steroidogenic mRNAs (steroidogenic acute regulatory protein, StAR; cholesterol side-chain cleavage, P450scc). We also isolated cDNAs encoding ANG II from three species of elasmobranch, verifying heterogeneity among elasmobranch peptides at the first amino acid position. Potential implications of this heterogeneity were investigated by examining the effects of homologous and heterologous ANG II on interrenal steroid production and steroidogenic mRNAs. Changes at amino acid position three, but not position one, of ANG II significantly affected steroidogenic potency. Conversely, changes at position one, but not position three, significantly affected the potency of ANG II to alter levels of steroidogenic mRNAs. This study is the first to demonstrate regulation of elasmobranch steroidogenic mRNAs by osmoregulatory peptides.