The guanine nucleotide-binding protein Gs activates a novel calcium transporter in Xenopus oocytes

Calcium influx is an important aspect of receptor-mediated signal transduction, yet limited information is available regarding the pathways of calcium influx into nonexcitable cells. We show that treatment of oocytes from Xenopus laevis with cholera toxin, a potent activator of the guanine nucleotide-binding protein Gs, specifically stimulates a sustained inward whole cell flux of calcium through a novel membrane transporter. The calcium is distributed into a mobilizable pool. The flux is voltage-independent and is completely and specifically blocked by microinjection of oocytes with an antiserum directed against Gs alpha. The flux is not activated by treatment of the cells with forskolin or 8-bromo-cyclic adenosine monophosphate indicating that the effect of Gs alpha on the transporter occurs independently of adenylylcyclase activation. Transporter activity is insensitive to benzyl amiloride, does not require a sodium gradient, and is not stimulated by external calcium, indicating that it is not a sodium-calcium exchanger. The Gs-activated flux is dramatically potentiated by lanthanum ion and other trivalent cations but not by any of six divalent cations that were tested; all other known calcium channels and exchangers are, in contrast, potently blocked by lanthanum. The divalent cation cadmium inhibited transporter activity in a concentration-dependent manner. This novel calcium transporter may be important for receptor-mediated calcium influx in the oocyte and perhaps other cell types.