Role of nucleotide P2 receptors in calcium signaling and prolactin release in pituitary lactotrophs

Anterior pituitary cells express nucleotide-gated G protein-coupled P2 receptors (P2YRs) and cation-conducting channels (P2XRs). However, the identification of P2 receptors subtypes and their native ligands, and the distribution and function of these receptors within the secretory and non-secretory pituitary cells has been incompletely characterized. The focus in this study was on lactotroph subpopulation of cells. ATP and ADP, but not UTP and UDP, triggered calcium signaling in a majority (85%) of lactotrophs and prolactin release in mixed pituitary cells. Consistent with the role of P2 receptors in signaling and secretion, the actions of ATP and ADP were abolished in the presence of apyrase, an ectonucleotidase. Transcripts for G(q)-coupled calcium-mobilizing P2Y(1)R, P2Y(2)R, P2Y(4)R, and P2Y(6)R, as well as G(i)-coupled P2Y(12)R, were identified in mixed anterior pituitary cells. The ligand-selectivity profile of calcium mobilization-dependent signaling and prolactin secretion and the blockade of these responses by pyridoxal 5-phosphate 6-azophenyl-2', 4'-disulphonic acid indicated that P2Y(1)R mediates the stimulatory action of ATP and ADP. Within the channels expressed in anterior pituitary (P2X(2)R, P2X(3)R, P2X(4)R, and P2X(7)R), the P2X(4)R subtype provides a major pathway for calcium influx-dependent signaling and prolactin secretion. This conclusion was based on comparison of native to recombinant channels with respect to their ligand preference, sensitivity to pyridoxal 5-phosphate 6-azophenyl-2', 4'-disulphonic acid, and the rates of calcium signal desensitization.