Novel action of pituitary adenylate cyclase-activating polypeptide: Stimulation of extracellular acidification in rat pituitary GH4C1 cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a member of t he vasoactive intestinal peptide/secretin family. Using microphysiometry, w e have found that PACAP acutely (1 min) increased the extracellular acidifi cation rate (ECAR) in GH4C1 cells approximately 40% above basal in a concen tration-dependent manner. ECAR, maximally induced by PACAP, can be increase d further by thyrotropin-releasing hormone (TRH), indicating that the signa lling pathways for these two neuropeptides are not identical. In studies on the mechanism of PACAP-enhanced ECAR, we found that maximum stimulation of the cAMP/ PKA pathway by treatment with FSK, or the PKC pathway with PMA, did not inhibit the ECAR response to PACAP. The PKC inhibitor calphostin C and the MAP kinase inhibitor PD98059 had no effect on the ECAR response to PACAP. Furthermore, PACAP induced little or no change in cytosolic Ca2+ ([C a2+](i)), while TRH induced a large increase in [Ca2+](i). However, the tyr osine kinase inhibitor genistein completely blocked PACAP-induced ECAR, sug gesting involvement of tyrosine kinase(s). We conclude that PACAP causes an increase in ECAR in GH4C1 rat pituitary cells, which is not dependent on t he PKA, PKC, MAP kinase or Ca2+ signalling pathways, but does require tyros ine kinase activity. (C) 2000 Elsevier Science Inc. All rights reserved.