EFFECTS OF PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE IN THE PITUITARY - ACTIVATION OF 2 SIGNAL-TRANSDUCTION PATHWAYS IN THE GONADOTROPE-DERIVED ALPHA-T3-1 CELL-LINE

Pituitary adenylate cyclase-activating polypeptide (PACAP) is thought to pray a hypophysiotropic role, but little is known of the identity o f PACAP-stimulated cells in the pituitary, the nature of the PACAP rec eptors on specific cell types, and the effector systems for these rece ptors. Here we describe the effects of PACAP in alpha T3-1 cells, a go nadotrope-derived cell line. In these cells, PACAP38 causes concentrat ion-dependent increases in cAMP accumulation (EC(50), 3 nM), [H-3]inos itol phosphate ([H-3]IP) production (EC(50), 20 nM), and the cytosolic Ca2+ concentration. The Ca2+ response is biphasic and is sustained on ly in Ca2+-containing medium. Intact alpha T3-1 cells possess a single class of [I-125]PACAP27-binding sites (K-d, 3.3 nM; binding capacity, 35 fmol/10(6) cells). The rank orders of potencies for stimulation of cAMP and [3H]IP production and for inhibition of [I-125] PACAP27 bind ing by three related peptides are identical (PACAP38 = PACAP27 >> vaso active intestinal peptide). In addition to stimulation of LH release f rom primary cultures of rat pituitary cells and [H-3] IP accumulation in alpha T3-1 cells, PACAP38 synergizes with low GnRH concentrations i n the production of these effects. Moreover, long term exposure to PAC AP38 stimulates [H-3]thymidine incorporation and increases steady stat e levels of the gonadotropin alpha-subunit in alpha T3-1 cells. We con clude that alpha T3-1 cells possess type I PACAP receptors which media te the observed effector system responses, and demonstration of the ef fects of PACAP on this gonadotrope-derived cell line provides further evidence that gonadotropes are direct targets for PACAP action. The da ta imply that stimulation of phospholipase-C by PACAP is responsible ( at least in part) for the observed increase in cytosolic Ca2+, which, in turn, probably mediates the effects of PACAP on LH release. We sugg est, however, that in gonadotropes, the effects of PACAP on cell repli cation and gonadotropin synthesis may prove more important than the pe ptide's modest effects on LH release.