GNRH-ASSOCIATED PEPTIDE DECREASES CYCLIC-AMP ACCUMULATION IN THE GH(3) PITUITARY CELL-LINE

We have studied the effects of GnRH-associated peptide (GAP) on cAMP, cGMP production and intracellular calcium (Ca2+) levels in the clonal rat GH3 pituitary cell line. We show that GAP decreases cAMP productio n in a dose-dependent manner. A 20-min incubation with GAP at concentr ations greater than 0.1 nM resulted in a significant decrease in cAMP production. 100 nM GAP decreased cAMP production to 65% of that in con trol. The kinetics of the GAP effect were also studied. We show that c AMP levels fall to 68% of control levels within 2 min following admini stration of 10 nM GAP. Maximal inhibition was obtained after 3 min. Th e effect was only transient since GAP had no significant residual effe ct after 40 min. Conversely, up to 100 nM GAP failed to affect cGMP le vels. We have also studied the effect of GAP on [Ca2+]i levels and sho wed that GAP decreases [Ca2+]i by inhibiting Ca2+ transients. GAP also reduced VIP-stimulated cAMP production and inhibited Ca2+ transients induced by VIP. The effect of GAP on cAMP production and Ca2+ transien ts was abolished by pertussis toxin treatment. These observations are consistent with GAP inhibiting PRL secretion through a cAMP, Ca2+-depe ndent process.