LOCALIZATION, IDENTIFICATION, AND ACTION OF CALCITONIN-GENE-RELATED PEPTIDE IN THE FROG ADRENAL-GLAND

The localization of calcitonin gene-related peptide (CGRP)-like immuno reactivity in the adrenal gland of the frog, Rana ridibunda, was exami ned by the indirect immunofluorescence technique. Using an antiserum d irected against rat alpha-CGRP, the presence of a network of positive fibers was observed in the adrenal parenchyma. The immunoreactive mate rial has been characterized by HPLC analysis combined with RIA quantif ication. The elution profile revealed the existence of a single form o f CGRP exhibiting the same retention time as synthetic frog CGRP. The possible involvement of CGRP in the regulation of corticosteroid secre tion was studied in vitro using a perifusion system for frog adrenal s lices. Graded doses of frog CGRP (from 3 x 10(-9) to 3 x 10(-6) M) inc reased corticosterone and aldosterone secretion in a dose-dependent ma nner (ED(50), 4.1 x 10(-8) M). Several other forms of CGRP, i.e. rat a lpha-CGRP and beta-CGRP, and human alpha-CGRP and beta-CGRP, also capa ble of enhancing steroid output, but frog CGRP was the most effective stimulator of steroidogenesis. Repeated administration of rat alpha CG RP induced a reproducible stimulation of corticosteroid secretion with out any tachyphylaxis. Prolonged infusion of the peptide (3 h) caused a rapid increase in corticosteroid release, followed by a gradual decl ine of steroid secretion, suggesting the occurrence of a desensitizati on phenomenon. Rat alpha-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 adre nal gland. The data also demonstrate that synthetic CGRP exerts a dire ct stimulatory effect on corticosteroid secretion. Taken together, the se findings suggest that CGRP, released by nerve fibers in the adrenal tissue, can locally regulate corticosteroid secretion.