Synergistic action of postsynaptic alpha-adrenergic receptor stimulation on vasoactive intestinal polypeptide-induced increases in pineal N-acetyltransferase activity.

The alpha-adrenergic agonists phenylephrine and methoxamine, at concentrations that have little effect on pineal N-acetyltransferase activity, markedly enhance stimulation of this enzyme by vasoactive intestinal polypeptide (VIP). This augmentation can be blocked by the alpha 1-adrenergic antagonists phenoxybenzamine and prazosin and, at 10 but not 1 microM, by the alpha 2-antagonist yohimbine. The time course for VIP stimulation is not altered by concomitant alpha-adrenergic stimulation. Augmented activity does not require concomitant alpha-adrenergic stimulation, but alpha-adrenergic agonists must be present for augmentation to be maintained. Phorbol 12,13-diacetate or -dibutyrate but not 4 alpha-phorbol can substitute for phenylephrine, a finding suggesting that protein kinase C is involved in the augmentation. These results are, in general, analogous to alpha-adrenergic magnification of N-acetyltransferase induction by beta-adrenergic agonists.