G(o) protein does not regulate ATP-stimulated [Ca2+]i elevation or noradrenaline release in PC12 cells

The roles of G(o), a heterotrimeric GTP binding (G) protein with a 40-kDa alpha subunit and which is localized predominantly in neuronal cells, in exocytosis have been discussed recently. PC12 pheochromocytoma cell line is a convenient model in which to study the Ca(2+)-dependent mechanisms of the neurosecretory process. The stimulation of ATP receptors or addition of KCl stimulated an elevation of cytosolic free Ca2+ concentration ([Ca2+]i) and [3H]noradrenaline (NA) release in PC12 cells. In this study, we investigated the roles of G(o) in ATP- and KCl-stimulated reactions. Nerve growth factor treatment for 2 days and transfection of PC12 cells with cDNA of subunit of (G(o alpha) had no effect on ATP-stimulated [3H]NA release, although both treatments increased levels of the G(o alpha) and its trimeric form by about twofold over those in unstimulated cells. The [Ca2+]i rise induced by ATP in NGF-treated cells was similar to that in control cells, although the maximal response was slightly smaller. Cholera toxin treatment for 2 days inhibited ATP- and KCl-stimulated NA release, although this treatment caused an approximately twofold increase in the level of G(o). Pertussis toxin treatment, which ADP ribosylated over 90% of endogenous G proteins such as G(o), had no effect on ATP-stimulated reactions. These findings show that G(o) does not directly regulate ATP-stimulated Ca2+ channels or the NA release process in PC12 cells. Cholera toxin-sensitive protein(s) may regulate exocytosis.