Résumé
[Ca^2+ ]i and cyclic AMP both medicate the release of prolactin from pituitary cells. TRH and VIP, which each elevate one of these second messengers, stimulate prolactin release. Somatostatin and adenosine A1 receptors inhibit prolactin release. We had previously shown that both somatostatin and adenosine could inhibit the elevation of both second messengers through the mediation of apparently similar pertussis toxin-sensitive GTP regulatory proteins (Schlegel et al., 1984, Cell Calcium, 5, 223; Copper et al., 1989, Cellular Signaling 1, 85). GH_3 cells express at least three pertussis toxin substrates, viz. G_i2 , G_i3 and G_o , so that it is uncertain whether any of the effects of adenosine receptors are mediated selectively via one of these G-proteins. The previous studies of adenosine effects on [Ca^2+ ]i were performed on populations of GH_3 cells, which do not permit determination of the mechanisms involved. We have now gone on to examine in detail the mechanism whereby adenosine might inhibit the elevation of [Ca^2+ ]i in simultaneous measurements of [Ca^2+ ]i, using indo 1 with a dual-emission apparatus for microspectrofluorometry, and electrical activity, by whole cell patching of individual GH_3 cells. [Ca^2+ ]i in GH_3 cells undergoes spontaneous oscillations, the frequency and amplitude of which can be increased by TRH (Schlegel et al, 1987, Nature 329, 719). Somatostatin inhibits both the spontaneous and stimulated oscillations as a consequence of stimulating K^+ -channels and inhibiting voltage-gated Ca^2+ -channels (Mollard et al., 1988, Endocrinology 123, 721). Adenosine receptor agonists exert the same effect as somatostatin on [Ca^2+ ]i and electrical activity in parallel measurements. It therefore seems likely that similar channels will be regulated by A1 receptors, although the detailed electrophysiological characterization remains to be carried out. The inhibition by A1 receptors of the two second messenger systems, via analogous (though not necessarily the same) GTP regulatory proteins, provides a conservative mechanism for the autocrine regulation of prolactin release. A further device for the regulation of prolactin release also seems to have been elaborated in these cells; Ca^2+ , in concentrations that are achieved intracellularly, exerts a potent inhibition of the adenylyl cyclase. This inhibition, which depends on stimulation by VIP, is probably mediated at the catalytic site of cyclase enzyme. This additional device may provide an additional negative feedback loop on hormone secretion, whereby VIP through the activation of cAMP-dependent protein kinase, may inhibit K^+ -channels thereby leading to an increase in [Ca^2+ ]i, which facilitates hormone release; this elevated [Ca^2+ ]i may then directly inhibit the VIP-stimulated adenylyl cyclase activity leading to a decline in [Ca^2+ ]i and the subsequent hormone secretion.