( ¹Institute of Aquatic Economic Animals, School of Life Sciences, Zhongshan University, Guangzhou 510275, China; ²Department of Zoology, the University of Hong Kong, Hong Kong, China )

Abstract

In order to study signal transduction in PACAP-stimulated GH releas e , effects of ovine PACAPs (oPACAPs) on cAMP synthesis and intracellular free Ca²⁺([Ca²⁺]i) levels in primarily cultured common carp (Cyprinus carpio) pituitary cells were investigated using radioimmunoassay (RIA) for cAMP and [Ca²⁺]i Indo-1/AM loading techniques. The main results obtained could be briefly summarized as follows: parallel with their GH-releasing effects, oPACAP-38 (0.01 nmol/L to 1 μmol/L) and oPACAP-27 (0.01 nmol/L to 1 μmol/L) were also effective in increasing cAMP release, cellular cAMP content, and total cAMP production in a dose-dependent manner in common carp pituitary cells. In addition to their cAMP synthesis , 0.5 μmol/L oPACAP-38 and 0.5 μmol/L oPACAP-27 were also effective in elevating [Ca²⁺]i levels in common carp pituitary cells preloaded with the Ca²⁺-sensitive dye Indo-1; the stimulatory actions were dependent on the presence of extracel lular Ca²⁺ ([Ca²⁺]e), as removal of [Ca²⁺]e by adding the Ca²⁺ chelator 4 mmol/L E GTA blocked the rise of [Ca²⁺]i induced by oPACAP-38 and oPACAP-27, respectively. Besides, the rise in [Ca²⁺]i induced by 0.5 μmol/L oPACAP-38 was blocked by treatment with 10 μmol/L nifedipine, an inhibitor of "L-type" voltage-sensitive Ca²⁺ channels (VSCC); but 0.5 μmol/L oPACAP-38 was not effective in elevating [Ca²⁺]i levels after the cells treated with Ca²⁺-sensitive dye Indo-1 were challeng ed with 10 μmol/L nifedipine. These results, taken as a whole, strongly suggest the involvement of the cAMP- and Ca²⁺-dependent pathways in PACAP-stimulated GH release in common carp.