Structural characterization and effects on corticosteroid secretion of endothelin-1 and endothelin-3 from the frog Rana ridibunda

Despite the intensive study of endothelin (ET) in mammals, the primary structure and biological activity of the peptide is not known for any species of non-mammalian tetrapod. Extracts of the stomach and the liver of the European green frog Rana ridibunda contained ET-like immunoreactivity measured by RIA using an antiserum raised against human ET-1. The amino acid sequence of the peptide that was isolated in pure form from the stomach extract was identical to that of human ET-1 and the peptide purified from the liver extract was identical to human ET-3 except for a single amino acid substitution (Phe⁴→Tyr). These observations demonstrate that the amino acid sequences of ET family peptides have been very strongly conserved during evolution of tetrapods and suggest that the pathway of post-translational processing of preproendothelin in the frog is similar to that in mammals. Both frog/human ET-1, frog ET-3 and human ET-3 produced a concentration- dependent increase in the production of corticosteroids from perifused slices of the frog interrenal gland. The maximum responses produced by the peptides (approximately 2-fold increase over basal levels for both corticosterone and aldosterone production) were not significantly different. The potency of ET-1 (-log EC₅₀=9.81 ± 0.01 (S.E.M.) for corticosterone and 9.52 ± 0.29 for aldosterone production) was significantly (P<0.01) greater than that of frog ET-3 (-log EC₅₀=8.13 ± 1.6 for corticosterone and 8.15 ± 0.33 for aldosterone production) but the potencies of frog ET-3 and human ET-3 (-log EC₅₀=8.29 ± 0.34 and 7.87 ± 0.18) were not significantly different.