GnRH-associated peptide decreases cyclic AMP accumulation in the GH3 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 production in a dose-dependent manner. A 20-min incubation with GAP at concentrations greater than 0.1 nM resulted in a significant decrease in cAMP production. 100 nM GAP decreased cAMP production to 65% of that in control. The kinetics of the GAP effect were also studied. We show that cAMP levels fall to 68% of control levels within 2 min following administration of 10 nM GAP. Maximal inhibition was obtained after 3 min. The effect was only transient since GAP had no significant residual effect after 40 min. Conversely, up to 100 nM GAP failed to affect cGMP levels. We have also studied the effect of GAP on [Ca2+]i levels and showed 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+ transients was abolished by pertussis toxin treatment. These observations are consistent with GAP inhibiting PRL secretion through a cAMP, Ca(2+)-dependent process.