Intracellular calcium involvement in pituitary adenylate cyclase-activating polypeptide and gonadotrophin secretion stimulation of growth hormone in goldfish pituitary cells

The involvement of intracellular Ca2+ stores and their regulatory mechanisms in mediating pituitary adenylate cyclase-activating polypeptide (PACAP) stimulation of growth hormone (GH) and maturational gonadotrophin (GTH-II) secretion from goldfish pituitary cells was investigated using a cell column perifusion system. Pretreatment with caffeine abolished the GH and GTH-II responses to PACAP. Dantrolene attenuated PACAP-elicited GTH-II release but did not affect the GH response, whereas ryanodine and 8-bromo-cADP ribose did not alter PACAP-induced GH and GTH-II release. Two endoplasmic/sarcoplasmic reticulum Ca2+ ATPase (SERCA) inhibitors, thapsigargin and cyclopiazonic acid, augmented PACAP-induced GTH-II release; similarly, thapsigargin elevated GH responses to PACAP. Treatment with carbonyl cyanide m-chlorophenylhydrazone, a mitochondrial uncoupler, reduced PACAP-stimulated GH release; however, inhibition of the mitochondrial Ca2+ uniport by Ru360 did not affect GH and GTH-II responses. The phosphatidl inositol (PI)-specific phospholipase C (PLC) inhibitor ET-18-OCH3 inhibited, whereas the phosphatidyl-choline (PC)-specific PLC inhibitor D609 enhanced, PACAP-stimulated GH and GTH-II responses. On the other hand, the IP3 receptor blocker xestospongin D had no effect on PACAP-induced GTH-II response and potentiated the GH response. These results suggest that, despite some differences between GH and GTH-II cells, PACAP actions in both cell types generally rely on a caffeine-sensitive, but a largely ryanodine receptor-independent, mechanism. PC-PLC and some SERCA negatively modulate PACAP actions but mitochondrial Ca2+ stores per se are not important. A novel PI-PLC mechanism, which does not involve the traditional IP3/Ca2+ pathway, is also suggested.