Localization, identification, and action of calcitonin gene-related peptide in the frog adrenal gland

The localization of calcitonin gene-related peptide (CGRP)-like immunoreactivity in the adrenal gland of the frog, Rana ridibunda, was examined by the indirect immunofluorescence technique. Using an antiserum directed against rat α-CGRP, the presence of a network of positive fibers was observed in the adrenal parenchyma. The immunoreactive material has been characterized by HPLC analysis combined with RIA quantification. The elution profile revealed the existence of a single form of CGRP exhibiting the same retention time as synthetic frog CGRP. The possible involvement of CGRP in the regulation of corticosteroid secretion was studied in vitro using a perifusion system for frog adrenal slices. Graded doses of frog CGRP (from 3 x 10^-9 to 3 x 10^-6 M) increased corticosterone and aldosterone secretion in a dose-dependent manner (ED₅₀, 4.1 x 10^-8 M). Several other forms of CGRP, i.e. rat α-CGRP and β-CGRP, and human α-CGRP and β-CGRP, were also capable of enhancing steroid output, but frog CGRP was the most effective stimulator of steroidogenesis. Repeated administration of rat αCGRP induced a reproducible stimulation of corticosteroid secretion without any tachyphylaxis. Prolonged infusion of the peptide (3 h) caused a rapid increase in corticosteroid release, followed by a gradual decline of steroid secretion, suggesting the occurrence of a desensitization phenomenon. Rat α- CGRP also gave rise to a significant increase in corticosteroid release from acutely dispersed adrenal cells. These results show the presence of CGRP in fibers innervating the frog adrenal gland. The data also demonstrate that synthetic CGRP exerts a direct stimulatory effect on corticosteroid secretion. Taken together, these findings suggest that CGRP, released by nerve fibers in the adrenal tissue, can locally regulate corticosteroid secretion.