This project addresses the cellular signaling cascade in endocrine and neuroendocrine cells and the interactions between plasma membrane electrical events and receptor-mediated calcium signaling. Current emphasis is on the characterization of ATP-gated receptor-channels (P2XR) in these cells. When expressed in excitable GT1 cells, P2XR subtypes differed in the peak amplitudes of their calcium responses independently of the level of receptor expression, with the order: P2X1R < P2X3R < P2X4R < P2X2bR < P2X2aR < P2X7R. During prolonged agonist stimulation, calcium signals desensitized with different rates: P2X3R > P2X1R > P2X2bR > P2X4R >> P2X2aR >> P2X7R. A slow desensitizing pattern of P2X2aR was mimicked partially by P2X3R and fully by P2X4R mutants in which six amino acid (6-aa) sequences in their C-termini were substituted with the corresponding arginine 371 to proline 376 sequence of P2X2aR. Changing the total net charge in the 6-aa sequence of P2X4R to a more positive direction also slowed receptor desensitization. On the other hand, substitution of the arginine 371 - proline 376 sequence of P2X2aR with the corresponding sequences of P2X1R, P2X3R, and P2X4R increased the rate of receptor desensitization. Furthermore, heterologous polymerization of wild-type P2X2aR and mutant P2X3R having the C-terminal 6-aa of P2X2aR at its analogous position resulted in a functional channel, with significantly delayed desensitization. The P2X1R, P2X3R, and P2X4R-derived calcium signals were predominantly dependent on activation of voltage-sensitive calcium influx, whereas voltage-sensitive and -insensitive entry pathways equally contributed to cytosolic calcium responses in P2X2aR-, P2X2bR- and P2X7R-expressing cells. The transcripts and functional P2X7R, P2X3R, and P2X4R were identified in lactotrophs and GH3 cells, but not in somatotrophs and gonadotrophs. Functional P2X7R were identified in 74% of lactotrophs, whereas 50% of these cells expressed P2X3R and 33% expressed P2X4R. Co-expression of these receptor subtypes in single lactotrophs was frequently observed. Purified somatotrophs expressed transcripts for P2X2aR and P2X2bR, and functional receptors were identified in somatotrophs and gonadotrophs, but not in lactotrophs. These results indicate that calcium signaling by homomeric P2XRs expressed in an excitable cell is subtype-specific. This provides an effective mechanism for generating variable calcium patterns in response to a common agonist.
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