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Abstract

The involvement of guanosine triphosphate (GTP)-binding proteins in the receptor-mediated activation of phospholipase C in isolated, permeabilized acinar cells of rat pancreas was studied. Stimulation of phospholipase C (PLC) by agonists such as cholecystokinin (CCK), carbachol (Cch) or GTP-gamma-S, a weakly hydrolysable GTP-analog, induced production of inositol-1,4,5-trisphosphate (IP₃) by hydrolysis of its precursor phosphatidylinositol-4,5-bisphosphate (PIP₂). Preincubation of permeabilized cells with activated cholera toxin (CT) inhibited cholecystokinin-octapeptide (CCK-OP) and GTP-gamma-S--but not Cch-induced production of IP₃. Pertussis toxin had no effect on PLC activity. Neither cyclic adenosine monophosphate (cAMP) nor hormones which activate adenylyl cyclase, inhibited activation of PLC. This indicates that the inhibitory effect of CT is not mediated by stimulation of adenylyl cyclase activity. In isolated plasma membranes of pancreatic acinar cells a 40 kDa protein was adenosine diphosphate (ADP)-ribosylated by CT, which was inhibited by CCK-OP but not by Cch. A 40 kDa protein was also labelled by the photosensitive affinity marker GTP [alpha ³²P]-gamma-azidoanilide. Binding of this GTP-analog was enhanced by CCK-OP but not by Cch. It is concluded that CCK- and muscarinic acetylcholine-receptors are functionally coupled by two different G-proteins to phospholipase C. IP₃, which is produced by activation of phospholipase C leads to release of Ca²⁺ from a nonmitochondrial Ca²⁺ pool, which is likely the endoplasmatic reticulum (ER). Reuptake of Ca²⁺ by Ca²⁺ pumps into ER compartments was studied in isolated permeabilized pancreas- and parotid cells as well as in isolated ER vesicles