Vasoactive-intestinal-polypeptide-stimulated adenosine 3',5'-cyclic monophosphate accumulation in GH3 pituitary tumour cells. Reversal of desensitization by forskolin.

The interaction between forskolin and vasoactive intestinal polypeptide (VIP) in the regulation of cyclic AMP production in GH3 pituitary tumour cells was investigated. Both forskolin (10nM-10 microns) and VIP (10pM-10nM) increased the cyclic AMP content of GH3 cells. Forskolin (50-100nM) was additive with VIP in stimulating cyclic AMP accumulation when low concentrations (less than 1 nM) of the peptide were used, but exhibited a synergistic interaction with higher VIP concentrations (10-100 nM). These effects on cyclic AMP accumulation were reflected in a leftward shift in the concentration-response curve for VIP-stimulated prolactin release from GH3 cells, a process known to be regulated by intracellular cyclic AMP concentrations. The synergy observed did not appear to be related to changes in cyclic nucleotide phosphodiesterase activity, since it was even more marked in the presence of isobutylmethylxanthine, a phosphodiesterase inhibitor. Studies of the time-course of VIP-induced changes in GH3-cell cyclic AMP content revealed that, with high concentrations of VIP, production ceased within 2 min of addition. This attenuation of cyclic AMP synthesis was still observed in the presence of isobutylmethylxanthine, but was markedly inhibited by low concentrations of forskolin (50-100nM). The results suggest that VIP-induced cyclic AMP production rapidly becomes desensitized. This process, which is prevented by forskolin, may be related to changes in the ability of the guanine nucleotide regulatory protein to couple receptor occupancy to activation of adenylate cyclase.