PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE REGULATES CYTOSOLIC CA2+ IN RAT GONADOTROPES AND SOMATOTROPES THROUGH DIFFERENT INTRACELLULAR MECHANISMS

The hypothalamic factor pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates an increase in the cytoplasmic free Ca2+ ion concentration ([Ca2+]i) in GH-secreting somatotropes and LH-secreting gonadotropes of the rat anterior pituitary gland. The dynamics of the PACAP-induced Ca2+ responses and their dependence on extracellular Ca2+ are markedly different in the two cell types, suggesting separate mechanisms of action of PACAP in somatotropes and gonadotropes. The present study reports a full characterization of the Ca2+ responses seen in the two cell types over a wide range of PACAP concentrations. In addition, the involvement of the cAMP-dependent protein kinase (PKA) system in the mediation of PACAP-stimulated Ca2+ was tested using the R-isomer of cAMP (RpcAMPs) as a specific inhibitor of PKA. In identified somatotropes, PACAP (10(-11)-10(-6) M) stimulated Ca2+ responses in 49% of the cells tested, with two types of Ca2+ response profile observed. The first was a slow rise in [Ca2+]i to a new level (Ca2+ step; 28% of somatotropes); the second was characterized by repetitive transient rises in [Ca2+]i (Ca2+ transients; 21% of somatotropes). The range of PACAP concentrations tested (10(-11)-10(-6) M) did not markedly alter the number of cells responding or the type of response observed. In some gonadotropes, PACAP stimulated Ca2+ step responses similar to those seen in somatotropes; however, the most common response observed was a rapid, high amplitude, but transient spike of [Ca2+]i, which was often accompanied by rapid oscillations in [Ca2+]i. This response profile was termed a Ca2+ spike-oscillations response, and the proportion of cells exhibiting this response increased from 25% at a PACAP concentration of 10(-11) M to 73% at 10(-6) M PACAP. PACAP-induced Ca2+ responses in somatotropes were blocked by pretreatment with the cAMP antagonist RpcAMPs (10(-3) M). In addition, other factors known to increase cAMP in somatotropes (GH-releasing factor and 8-bromo-cAMP) stimulated Ca2+ responses in these cells that were qualitatively similar to those induced by PACAP. In contrast, the Ca2+ responses in gonadotropes were insensitive to the cAMP antagonist RpcAMPs, and the membrane-permeable cAMP analog 8-bromo-cAMP failed to stimulate responses in this cell type. These results suggest that the intracellular mechanisms of PACAP action in the two cell types are markedly different. In somatotropes, the rise in [Ca2+]i Stimulated by PACAP is probably through the production of cAMP and the activation of protein kinase-A. However, surprisingly, the effects of PACAP on [Ca2+]i in gonadotropes appear to be independent of a rise in cAMP or activation of PKA.