ROLE OF L-TYPE CA2-INDUCED ADRENAL CATECHOLAMINE RELEASE IN-VIVO( CHANNEL IN PACAP)

The aim of the present study was to investigate whether the dihydropyr idine-sensitive L-type Ca2+ channel is operative in adrenal catecholam ine (CA) secretion induced by a novel neuropeptide, pituitary adenylat e cyclase-activating polypeptide (PACAP), in anesthetized dogs. Plasma CA concentrations in adrenal venous and aortic blood were determined by a high-performance liquid chromatography method. All drugs tested w ere locally infused into the left adrenal gland via the left adrenolum bar artery. PACAP, with the isoform consisting of 27 (PACAP-27) and 38 (PACAP-38) amino acid residues, significantly increased CA output in a dose-dependent manner, with doses ranging from 5 to 500 ng and 7 to 700 ng, respectively. However, the amplitude of epinephrine response t o PACAP-27 was three times greater than that obtained with PACAP-38 at the highest dose tested. In a separate group, a single dose of PACAP- 27 (50 ng) induced highly reproducible CA responses when the same dose was repeated with an interval of 35 min. In dogs treated with nifedip ine (50 mu g), 5 min before the second administration of PACAP-27, the net CA response was significantly inhibited by similar to 50% compare d with that obtained in the presence of vehicle. A similar CA response to BAY K 8644 (5 mu g) was completely abolished by the same dose of n ifedipine. The present results indicate that both PACAP-27 and PACAP-3 8 have the direct local secretagogue effect on the adrenal medulla in vivo and that CA responses to PACAP-27 were greater than those observe d with PACAP-38 at equivalent mole doses. The study suggests that the dihydropyridine-sensitive L-type Ca2+ channel is functionally involved in PACAP-induced adrenal CA secretion in the canine adrenal medulla i n vivo.