G protein in stimulation of PI hydrolysis by CCK in isolated rat pancreatic acinar cells.

To clarify the possible role of a guanine nucleotide-binding protein (G protein) in the signal transducing system activated by cholecystokinin (CCK), actions of CCK on rat pancreatic acini were compared with those of fluoride, a well-known activator of stimulatory (Gs) or inhibitory (Gi) G protein. When acini were incubated with increasing concentrations of either CCK-octapeptide (CCK8) or NaF, a maximal stimulation of amylase release from acini occurred at 100 pM CCK8 or 10 mM NaF, respectively; this secretory rate decreased as CCK8 or NaF concentration was increased. NaF caused an increased in cytoplasmic Ca2+ concentration from the internal Ca2+ store and stimulated accumulation of inositol phosphates in acini, as observed with CCK. However, NaF-stimulated Ca2+ mobilization had a lag period before detectable stimulation and was potentiated by AlCl3. These stimulatory effects of NaF appeared to be independent of cellular adenosine 3',5'-cyclic monophosphate (cAMP). Pretreatment with cholera toxin or pertussis toxin did not affect CCK8- or NaF-induced inositol phosphate accumulation or Ca2+ mobilization. 5'-Guanylimidodiphosphate activated the generation of inositol phosphates in the [3H]inositol-labeled pancreatic acinar cell membrane preparation, with half-maximal and maximal stimulation at 1 and 10 microM, respectively. Furthermore, the effects of submaximal CCK concentrations on inositol phosphate accumulation in membranes were markedly potentiated in the presence of 100 microM GTP, which alone was ineffective. Combined findings of the present study strongly suggest that pancreatic CCK receptors are probably coupled to the activation of polyphosphoinositide (PI) breakdown by a G protein, which appears to be fluoride sensitive but is other than Gs- or Gi-like protein.