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. Prolactin (PRL) secretion is tonically inhibited by dopamine (DA), the escape from which triggers acute episodes of hormone secretion. The magnitude of these episodes is explained by a potentiation of the PRL-releasing action of secretagogues such as thyrotropin-releasing hormone (TRH). While the mechanisms of this potentiation are not fully understood, it is known to be mimicked by the dihydropyridine, L-type Ca2+ channel agonist Bay K 8644 and blocked by nifedipine and methoxyverapamil. The potentiation is also blocked by inhibitors of cyclic AMP-dependent protein kinase and protein kinase C. We recently described that the escape from tonic actions of DA results in increased macroscopic Ca2+ currents in GH4C1 lactotropic clonal cells transfected with a cDNA encoding the long form of the human D2-DA receptor. Here we show that the withdrawal from DA potentiates the PRL-releasing action of TRH in GH4C1/D2-DAR cells to the same extent as in enriched lactotropes in primary culture. In both experimental models a low density of dihydropyridine receptors was shown by (+)-[3H]PN200-110 binding. Photoaffinity labelling with the dihydropyridine [3H]azidopine revealed a protein consistent with the α1 subunit of L-type Ca2+ channels of 165–170 kDa. In both experimental models, the facilitation of TRH action triggered by the escape from DA was correlated with an enhanced rate of phosphorylation of this putative α1 subunit, the nature of which was further supported by immunoprecipitation with selective antibodies directed against the α1C and α1D subunit of voltage-gated calcium channels. We propose that PKA- and PKC-dependent phosphorylation of the α1 subunit of high voltage activated, L-type Ca2+ channels is responsible for the facilitation of Ca2+ currents in lactotropes, and hence for the potentiation of secretagogue-mediated PRL secretion. Thus, phosphorylation of Ca2+ channels in endocrine cells may be a mechanism for the regulation of various functions including amplification of hormone secretion.

Keywords:
Prolactin, Ca2+ channels, Catecholamines, Thyrotropin-releasing hormone, GH4C1 cells, Electrophysiology, Phosphorylation, Protein kinases
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