Involvement of a pertussis toxin-sensitive G protein in the action of gastrin on gastric parietal cells

The mechanism whereby gastrin triggers phosphoinositide breakdown was investigated in an enriched preparation of isolated rabbit parietal cells (approx. 75%). In a permeabilized preparation of myo-3H]inositol-labelled cells, GTPS], a non-hydrolysable GTP analogue, enhanced 3H]inositol trisphosphate (3H]InsP3 accumulation in a dose-dependent manner; submaximal concentrations of GTPS] (less than 10 microM), potentiated gastrin-induced 3H]InsP3 release; preincubation for 5 min with GDPS], a non-hydrolysable GDP analogue, dose-dependently reduced 3H]InsP3 accumulation stimulated by gastrin even in presence of GTPS]. Exposure of intact parietal cells for 3 h to pertussis toxin (PTx) (200 ng/ml) led to a 15-50% reduction in gastrin-induced 14C]aminopyrine (14C]AP) uptake (an index of in vitro acid secretion) and 3H]inositol phosphate (3H]InsP) accumulation. A decrease in the accumulation of the different 3H]inositol phosphate occurred in gastrin-stimulated parietal cells treated with PTx. A rightward shift of gastrin dose-response curves in the presence of PTx was observed for 14C]AP uptake (EC50 values: 0.125 +/- 0.045 nM without PTx and 1.05 +/- 0.63 nM with PTx), for 3H]InsP accumulation (EC50 values: 0.16 +/- 0.08 nM without PTx and 1.56 +/- 0.58 nM with PTx) and 125I]gastrin binding (IC50 values: 0.247 +/- 0.03 nM without PTx and 2.38 +/- 0.56 nM with PTx). In contrast, cholera toxin (CTx) treatment (100 ng/ml) for 3 h was without effect on gastrin-induced 3H]InsP accumulation. CTx induced a pronounced potentiation of gastrin-stimulated 14C]AP uptake; this effect can be mimicked by IBMX (a phosphodiesterase inhibitor) and by forskolin (an activator of adenylyl cyclase). We conclude that: (i) one or more than one G protein appeared to be involved in gastrin receptor coupling to phospholipase C (PL-C); (ii) these G proteins are not substrates for CTx; (iii) one of these appeared to be a PTx-sensitive 'Gi-like' protein which could be involved in hormone-induced acid secretion, (iiii) the potentiating effect of CTx observed on AP uptake stimulated by gastrin suggests the existence of a cooperative effect between cAMP pathway (CTx) and the gastrin-induced phosphoinositide breakdown in acid secretory activity of parietal cells.