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

Abstract

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, GTP[S], a non-hydrolysable GTP analogue, enhanced [ 3H]inositol trisphosphate ([ 3H]InsP 3) accumulation in a dose-dependent manner; submaximal concentrations of GTP[S] (< 10 μM), potentiated gastrin-induced [ 3H]InsP 3 release; preincubation for 5 min with GDP[S], a non-hydrolysable GDP analogue, dose-dependently reduced [ 3H]InsP 3 accumulation stimulated by gastrin even in presence of GTP[S]. 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 ( EC 50 values: 0.125 ± 0.045 nM without PTx and 1.05 ± 0.63 nM with PTx), for [ 3H]InsP accumulation ( EC 50 values: 0.16 ± 0.08 nM without PTx and 1.56 ± 0.58 nM with PTx) and [ 125I]gastrin binding ( IC 50 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 ‘G i-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.