LOCALIZATION, CHARACTERIZATION AND ACTIVITY OF PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE IN THE FROG ADRENAL-GLAND

Pituitary adenylate cyclase-activating polypeptide (PACAP) has recentl y been isolated from the frog brain and the sequence of the peptide ap pears to be strikingly similar to that of mammalian PACAP. In the pres ent study, we have investigated the localization of PACAP in the frog interrenal (adrenal) gland by immunocytochemistry using antisera direc ted against PACAP 38 or PACAP 27. Two types of PACAP-immunoreactive fi bres were observed: thick varicose fibres coursing between adrenal cel ls and thin processes located in the walls of blood vessels irrigating the gland. Bilateral transection of the splanchnic nerves did not aff ect the intensity and distribution of PACAP immunoreactivity. The mean +/- S.E.M. concentration of PACAP, measured by radioimmunoassay in cr ude adrenal extracts, was 0.65 +/- 0.16 nmol/g wet tissue. Two molecul ar forms of PACAP in the adrenal gland were characterized by reversed phase high-performance liquid chromatography combined with radioimmuno assay quantification. The elution profiles revealed the existence of t wo peaks exhibiting the same retention times as synthetic frog PACAP 3 8 (fPACAP 38) and PACAP 27, the predominant form being PACAP 38. The p ossible involvement of PACAP in the regulation of adrenal steroidogene sis was investigated in vitro using a perifusion system for frog adren al slices. Graded doses of fPACAP 38 (0.1-10 mu mol/l) increased the s ecretion of both corticosterone and aldosterone in a dose-dependent ma nner. Administration of repeated pulses of fPACAP 38 (1 mu mol/l), at 120-min intervals, led to a reproducible stimulation of corticosteroid secretion without any tachyphylaxis. Prolonged infusion (2 h) of the peptide induced a rapid increase in corticosterone and aldosterone out put, followed by a gradual decline in the secretion rate, suggesting t he occurrence of a desensitization phenomenon. Synthetic porcine vasoa ctive intestinal peptide, which is structurally related to PACAP, was about ten times less potent than fPACAP 38 in stimulating steroidogene sis while the [Des-His(1)]-fPACAP 38 analogue was 100 times less effec tive. These results demonstrate that a peptide closely related to fPAC AP 38 is present in fibres innervating the frog adrenal gland and coul d participate in the regulation of corticosteroid secretion, particula rly during neurogenic stress.