Résumé
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.