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Cholera toxin; Pertussis toxin; Inositol & phosphate; ADP-ribosylation; GTPγS
Abstract:
We have studied the involvement of GTP‐binding proteins in the stimulation of phospholipase C from rat pancreatic acinar cells. Pretreatment of permeabilized cells with activated cholera toxin inhibited both cholecystokinin‐octapeptide (CCK‐OP) and GTPγS but not carbachol (CCh)‐induced production of inositol trisphosphate. Pertussis toxin had no effect. Neither vasoactive intestinal polypeptide, a stimulator of adenylyl cyclase, nor the cAMP‐analogue, 8‐bromo cAMP, mimicked the inhibitory effect of cholera toxin on agonist‐induced phospholipase C activation. This indicates that inhibition by cholera toxin could not be attributed to a direct interaction of cholera toxin activated Gs with phospholipase C or to an elevation of cAMP. In isolated rat pancreatic plasma membranes cholera toxin ADP‐ribosylated a 40 kDa protein, which was inhibited by CCK‐OP but not by CCh. We conclude from these data that both CCK‐ and muscarinic acetylcholine receptors functionally couple to phospholipase C by two different GTP‐binding proteins.
