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The muscarinic agonist, carbachol (CCh), was shown to stimulate the production of inositol phosphates (IP) in isolated cells from rabbit fundic mucosa. This stimulatory effect was time- and dose-dependent: EC 50values for IP 1, IP 2and IP 3accumulation were not statistically different. The mean value was 30 ± 8 μM ( n= 6). The corresponding maximal stimulation (% of basal value) observed after 20 min incubation in the presence of 100 μM CCh was 160 ± 15%. CCh-induced IP accumulation was abolished by atropine ( Ki= 0.32 ± 0.18 nM ( n=3)). The CCh concentrations leading to half-maximal inhibition of N-[ 3H]methylscopolamine binding and half-maximal IP accumulation were similar. The half-maximal value for CCh-induced aminopyrine accumulation was 8-times lower. These results indicate that IP 3-mediated mobilization of intracellular Ca 2+might be involved in CCh-induced acid secretion by parietal cells.

Vasopressin stimulates the liberation of labelled inositol phosphate in partially purified plasma membranes prepared from myo‐[3H]inositol prelabelled WRK1 cells. This stimulatory effect was very rapid (165% stimulation of inositol trisphosphate accumulation after a 10 s incubation period in the presence of 1 μM vasopressin), concentration dependent (EC50= 12 nM) and was abolished by an antagonist of the vasopressor response to vasopressin. GTP, even at high concentrations (0.1 mM), did not increase inositol phosphate release: it was found to be absolutely necessary for hormonal stimulation of phospholipase C activity. Non‐hydrolysable analogues of GTP may also stimulate this enzyme activity.

Partially purified plasma membranes prepared from myo-[ 3H]inositol-prelabeled WRK1 cells exhibit a phosphatidylinositol 4,5-bisphosphate (PIP 2) phospholipase C activity sensitive to NaF. NaF increased the production of IP 2and IP 3in a time- and concentration-dependent manner. The maximal increase in IP 2and IP 3production rates represented 400 ± 18 and 360 ± 40% of the basal production rate, respectively. Half-maximum stimulation was reached with 2–4 mM NaF. The observed effect was specific for F −. Aluminium potentiated fluoride-induced IP 3and IP 2, accumulation in a concentration-dependent manner. The effect of fluoride on the PIP 2phospholipase C from WRK1 cell membranes appears to be similar to the well-documented effect of F −on the well-characterized N s. N iand transducin GTP-binding proteins. This observation constitutes an additional argument to suggest that a GTP-binding protein is involved in the process of receptor-mediated activation of PIP 2phospholipase C.