Effects of a novel hypothalamic peptide, pituitary adenylate cyclase-activating polypeptide, on pituitary hormone release in rats

ABSTRACT We have demonstrated that the novel hypothalamic peptide pituitary adenylate cyclase-activating poly-peptide (PACAP-38; 0·1–100 nmol/l) caused an increase in the release of GH, ACTH, LH and α-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 α-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 α-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 α-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 α-subunit release (10 nmol/l) added together with the PKC activator, 12-0-tetradecanoyl-phorbol-13-acetate (TPA; 0·1 μmol/l) had no greater effect on LH and α-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 μmol/l) known to deplete PKC activity substantially, reduced the ability of PACAP-38 to stimulate LH and α-subunit release and intracellular cyclic AMP levels significantly. We conclude that the stimulatory actions of PACAP on LH and α-subunit relies in part on PKC activity. Journal of Endocrinology (1992) 134, 33–41