Evidence against a role for a pertussis toxin-sensitive G protein in Ca2+ mobilization in rat parotid acinar cells

Hormone-induced Ca2+ mobilization in rat parotid acinar cells is reportedly mediated via an as yet uncharacterized G protein. We have studied the sensitivity to pertussis toxin (PTx) of this signal transduction mechanism. When rats were treated with Ptx (1.3-1.5 micrograms per animal) for 72 h, a 41 kDa membrane protein was ADP-ribosylated. This PTx treatment regimen, also, resulted in a more than 80% block of the ability of the muscarinic agonist carbachol to inhibit beta-adrenergic receptor-stimulated parotid adenylyl cyclase activity. However, cytosolic Ca2+ levels, in response to either carbachol or AIF-4, were comparable in cells prepared from both untreated or PTx-treated rats, when incubated either in the absence or presence of extracellular Ca2+. Further, both the sensitivity of the Ca2+ response to carbachol and the ability of the agonist-sensitive intracellular Ca2+ stores to be refilled by extracellular Ca2+ were unaffected by PTx treatment. Parotid membranes also contained three low-molecular-weight GTP-binding proteins (25, 22 and 18 kDa) which were unaffected by PTx. These results show that there is only one detectable substrate in parotid membranes for a PTx-catalyzed ADP-ribosylation and that hormone-induced Ca2+ mobilization events in parotid acinar cells are not mediated via PTx-sensitive components.