Potentiation of prolactin secretion following lactotrope escape from dopamine action - II. Phosphorylation of the alpha(1) subunit of L-type, voltage-dependent calcium channels

Modulation of Ca2+ channels has been shown to alter cellular functions. it can play an important role in the amplification of signals in the endocrine system, including the pleiotropically regulated pituitary lactotropes. Pro lactin (PRL) secretion is tonically inhibited by dopamine (DA), the escape from which triggers acute episodes of hormone secretion. The magnitude of t hese episodes is explained by a potentiation of the PRL-releasing action of secretagogues such as thyrotropin-releasing hormone (TRH). While the mecha nisms of this potentiation are not fully understood, it is known to be mimi cked by the dihydropyridine, L-type Ca2+ channel agonist Bay K 8644 and blo cked by nifedipine and methoxyverapamil. The potentiation is also blocked b y inhibitors of cyclic AMP-dependent protein kinase and protein kinase C. W e recently described that the escape from tonic actions of DA results in in creased macroscopic Ca2+ currents in GH(4)C(1) lactotropic clonal cells tra nsfected with a cDNA encoding the long form of the human D-2-DA receptor. H ere we show that the withdrawal from DA potentiates the PRL-releasing actio n of TRH in GH(4)C(1)/D-2-DAR cells to the same extent as in enriched lacto tropes in primary culture. In both experimental models a low density of dih ydropyridine receptors was shown by (+)-[H-3]PN200-110 binding. Photoaffini ty labelling with the dihydropyridine [H-3]azidopine revealed a protein con sistent with the al subunit of L-type Ca2+ channels of 165-170 kDa. In both experimental models, the facilitation of TRH action triggered by the escap e from DA was correlated with an enhanced rate of phosphorylation of this p utative al subunit, the nature of which was further supported by immunoprec ipitation with selective antibodies directed against the ate and all, subun it of voltage-gated calcium channels. We propose that PKA- and PKC-dependen t phosphorylation of the alpha(1) subunit of high voltage activated, L-type Ca2+ channels is responsible for the facilitation of Ca2+ currents in lact otropes, and hence for the potentiation of secretagogue-mediated PRL secret ion. Thus, phosphorylation of Ca2+ channels in endocrine cells may be a mec hanism for the regulation of various functions including amplification of h ormone secretion.