EFFECTS OF A NOVEL HYPOTHALAMIC PEPTIDE, PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE, ON PITUITARY-HORMONE RELEASE IN RATS

We have demonstrated that the novel hypothalamic peptide pituitary adenylate cyclase-activating polypeptide (PACAP-38; 0.1-100 nmol/l) caused an increase in the release of GH, ACTH, LH and alpha-subunit and accumulation of intracellular cyclic AMP from dispersed rat anterior pituitary cells in static culture for 24 h. There were no significant effects on TSH or prolactin release over the same time-period. PACAP-38 (10 nmol/l) increased the release of GH by 1.3-fold (P<0.05), ACTH by 1.9-fold (P<0.05), LH by 3.5-fold (P<0.001) and alpha-subunit by 2.0-fold (P<0.005) and the accumulation of intracellular cyclic AMP by >2-fold (P<0.001) after 24 h. However, the time-course for the effect of PACAP-38 (1 mmol/l) on hormone release and intracellular cyclic AMP levels showed a temporal dissociation. The effect of PACAP-38 on GH and ACTH levels did not reach significance until 24 h whereas the effect of PACAP-38 on LH and alpha-subunit release reached significance after 4 h implying a different mechanism of action for their release. To investigate the PACAP-induced secretion of LH and alpha-subunit further, we examined the effects of PACAP after down-regulation of protein kinase C (PKC). PACAP-38 at a dose maximal for the stimulation of LH and alpha-subunit release (10 nmol/l) added together with the PKC activator, 12-O-tetradecanoylphorbol-13-acetate (TPA; 0.1-mu-mol/l) had no greater effect on LH and alpha-subunit release than TPA alone over a 4 h incubation period. Increasing the pretreatment time with TPA (0-5 h) at a dose (0.1-mu-mol/l) known to deplete PKC activity substantially, reduced the ability of PACAP-38 to stimulate LH and alpha-subunit release and intracellular cyclic AMP levels significantly. We conclude that the stimulatory actions of PACAP on LH and alpha-subunit relies in part on PKC activity.