Expression of purinergic P2X(2) receptor-channels and their role in calcium signaling in pituitary cells

Pituitary cells express purinergic receptor-channels (P2XR), the activation of which by ATP is associated with the facilitation of Ca2+ influx. Pharma cological, RT-PCR, and nucleotide sequence analyses confirm the presence of a wild type P2X(2a)R and a spliced isoform P2X(2b)R, which lacks a portion of carboxyl terminal amino acids. Wild type and spliced isoform receptors have a similar EC50 for ATP and time-course for activation, but the spliced isoform exhibits rapid and complete desensitization, whereas the wild type channel desensitizes slowly and incompletely. Deletion and insertion studi es have revealed that a 6 residue sequence located in carboxyl tail (Arg(37 1)-Pro(376)) is required for sustained Ca2+ influx through wild type recept ors. When co-expressed, the wild type and spliced channels form functional heteropolymeric channels. The patterns of Ca2+ signaling in the majority of pituitary cells expressing ATP-gated receptor-channels are highly comparab le to those observed in cells co-transfected with P2X(2a)R and P2XR. ATP-in duced [Ca2+](i) response in pituitary cells is partially inhibited by nifed ipine, a blocker of voltage-gated L-type Ca2+ channels, suggesting that P2X (2)R not only drive Ca2+ into the cell, but also activate voltage-gated Ca2 + entry. Our results indicate that ATP represents a paracrine and (or) auto crine factor in the regulation of Ca2+ signaling, and that its actions are mediated in part by heteropolymeric P2X(2)R.