[Role of guanine nucleotide regulatory protein in stimulation of exocrine pancreatic secretion by cholecystokinin in isolated rat pancreatic acini].

To clarify the possible role of a guanine nucleotide regulatory protein in the signal transducing system activated by cholecystokinin (CCK), the actions of CCK on rat pancreatic acini were compared with those of NaF, which is a well-known activator of stimulatory and inhibitory guanine nucleotide regulatory proteins. Guanine nucleotides reduced the binding of 125I-CCK-octapeptide (CCK8) to acinar cell membranes, with the rank order of potency being guanyl-5'-yl imidodiphosphate (Gpp(NH)p) greater than GTP greater than GDP greater than GMP. Scatchard analysis of labeled CCK binding revealed that the decrease in CCK binding caused by Gpp(NH)p was due to the decrease in an affinity constant of CCK for its receptors with no significant change in the maximal binding capacity. When acini were incubated with increasing concentrations of either CCK8 or NaF, maximal stimulation of amylase release occurred at 100 pM CCK8 or 10 mM NaF, respectively, and supramaximal concentrations of CCK8 or NaF caused its submaximal stimulation. Further, CCK and NaF similarly stimulated the hydrolysis of polyphosphoinositide in acini and the release of Ca2+ from the intracellular Ca2+ store into the cytoplasm although there was a lag period prior to any detectable stimulation by NaF. The doses of NaF necessary for Ca2+ mobilization and inositol phosphate generation were nearly the same, with a maximal stimulation at 20 mM NaF. NaF, at concentrations up to 20 mM, a supramaximal concentration for amylase release, produced no significant change in the cellular cyclic AMP level. In addition, 10 mM NaF potentiated the amylase release stimulated by VIP, a well-known secretagogue which functions via cyclic AMP, suggesting that the stimulatory effects of NaF are independent of cellular cyclic AMP. Gpp(NH)p also activated the hydrolysis of polyphosphoinositide in a cell-free pancreatic acinar cell membrane preparation, with a half-maximal and a maximal stimulation at 1 microM and 10 microM, respectively. Furthermore, the effects of submaximal concentrations of CCK8 on polyphosphoinositide hydrolysis were markedly potentiated in the presence of 100 microM GTP, which alone was ineffective. These results, therefore, strongly suggest that pancreatic CCK receptors may be coupled to the activation of polyphosphoinositide breakdown by a guanine nucleotide regulatory protein.