C-type natriuretic peptide (CNP) effects on intracellular calcium [Ca2+]i in mouse gonadotrope-derived alphaT3-1 cell line.

C-type natriuretic peptide (CNP), the third member of the atrial natriuretic peptide family, acts via guanylyl cyclase containing GC-B receptors to stimulate cyclic guanosine 3',5' monophosphate (cGMP) accumulation in the gonadotrope-derived alphaT3-1 cell line and rat pituitary cells. This effect is inhibited by concomitant activation of the phospholipase C (PLC)-coupled gonadotrophin hormone-releasing hormone (GnRH) receptors in these cells. Since GnRH stimulates gonadotrophin secretion from gonadotropes by increasing the cytosolic Ca2+ concentration ([Ca2+]i) and natriuretic peptides have been found to influence PLC/Ca2+ signalling in other systems, we have investigated whether CNP can alter basal or GnRH-stimulated changes in [Ca2+]i in alphaT3-1 cells. In Ca 2+-containing medium, 10(-7) M CNP modestly, but significantly increased [Ca2+]i over several min, but subsequently inhibited the elevation of [Ca2+]i in response to 10(-7) M GnRH in both Ca2+-containing and Ca2+-free medium. This inhibitory effect was mimicked by 10(-6) M 8-Br-cGMP, but not by ANP, indicating mediation by cyclic GMP and the CNP-specific GC-B receptor. However, basal and GnRH-stimulated inositol (1,4,5) trisphosphate (Ins(1,4,5)P3) generation were not measurably affected by CNP, and CNP failed to affect thapsigargin-induced capacitative Ca2+ entry. Thus, it appears that the cross-talk between CNP and GnRH in these cells is reciprocal in that GnRH modulates CNP effects on cGMP generation, whereas, CNP modulates GnRH effects on Ca2+ mobilisation.