THE STIMULATORY EFFECT OF PITUITARY ADENYLATE-CYCLASE ACTIVATING POLYPEPTIDE (PACAP) ON LH-RELEASE FROM RAT PITUITARY-CELLS IN-VITRO DOES NOT INVOLVE CALCIUM MOBILIZATION

The hypothalamic peptide ''pituitary adenylate cyclase activating poly peptide (PACAP)'' stimulates cAMP production in cultured rat pituitary cells and enhances LH release. It has been suggested that the stimula tion of LH release by PACAP comprises two distinct mechanisms: a direc t stimulatory action on LH secretion and a potentiation of the respons e of the gonadotrophes to LHRH. Thus the possibility exists that PACAP may enhance LH secretion not only by increased cAMP production but al so by increasing cytosolic Ca2+ concentrations ([Ca2+]). In the presen t study we examined whether PACAP affects cytosolic [Ca2+] in identifi ed rat gonadotrophes (as determined by the fura-method) and whether th e suggested potentiating effect of PACAP on LHRH induced LH release is dependent on Ca2+. PACAP (1 nM) and 0.1 nM LHRH significantly increas ed LH concentrations in the culture medium after 5 hrs of incubation. Coincubation of cells with both peptides resulted in an additive incre ase of LH release. While the stimulatory effect LHRH was blunted in Ca 2+-free medium, PACAP remained stimulatory to LH release. PACAP stimul ated cAMP formation regardless whether the culture medium contained Ca 2+ or not. Gonadotrophes were selected by their response to LHRH (1 mu M) and were subsequently challenged with PACAP (1 muM). About 75% of g onadotrophes responded also to PACAP with an increase of cytosolic [Ca 2+] which was blunted by removal of extracellular Ca2+. We suggest tha t in the rat pituitary the majority of the gonadotrophes are PACAP res ponsive as determined by an increase of cytosolic [Ca2+]. However, it appears that the stimulatory action of PACAP on LH release is primaril y due to enhanced cAMP production since LH secretion and cAMP formatio n were elevated by an amount of PACAP ineffective to modulate cytosoli c [Ca2+]. Our data do not support the assumption that PACAP potentiate s the action of LHRH as we found an additive effect of both peptides o n LH secretion.