Pituitary adenylate cyclase-activating polypeptide induces somatolactin release from cultured goldfish pituitary cells

In the goldfish pituitary, nerve fibers containing pituitary adenylate cyclase-activating polypeptide (PACAP) are located in close proximity to somatolactin (SL)-producing cells, and PACAP enhances SL release from cultured pituitary cells. However, there is little information about the mechanism of PACAP-induced SL release. In order to elucidate this issue, we used the cell immunoblot method. Treatment with PACAP at 10−8 and 10−7 M, but not with vasoactive intestinal polypeptide (VIP) at the same concentrations, increased the immunoblot area for SL-like immunoreactivity from dispersed pituitary cells, and PACAP-induced SL release was blocked by treatment with the PACAP selective receptor (PAC1R) antagonist, PACAP(6–38), at 10−6 M, but not with the PACAP/VIP receptor antagonist, VIP(6–28). PACAP-induced SL release was also attenuated by treatment with the calmodulin inhibitor, calmidazolium at 10−6 M. This led us to explore the signal transduction mechanism up to SL release, and we examined whether PACAP-induced SL release is mediated by the adenylate cyclase (AC)/cAMP/protein kinase A (PKA)- or the phospholipase C (PLC)/inositol 1,4,5-trisphosphate (IP3)/protein kinase C (PKC)-signaling pathway. PACAP-induced SL release was attenuated by treatment with the AC inhibitor, MDL-12330A, at 10−5 M or with the PKA inhibitor, H-89, at 10−5 M. PACAP-induced SL release was suppressed by treatment with the PLC inhibitor, U-73122, at 3 × 10−6 M or with the PKC inhibitor, GF109203X, at 10−6 M. These results suggest that PACAP can potentially function as a hypophysiotropic factor mediating SL release via the PAC1R and subsequently through perhaps the AC/cAMP/PKA- and the PLC/IP3/PKC-signaling pathways in goldfish pituitary cells.