Phosphoinositide synthesis in desensitized rat pancreatic acinar cells

To help understand the possible role of phosphoinositide turnover in the desensitization process, the availability of phosphatidylinositol 4,5-bisphosphate was investigated in normal and desensitized pancreatic acinar cells treated with carbamylcholine (Cch), caerulein (Cae), and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). In control acini, incorporation of [myo-3H]inositol into total phosphoinositides was maximal at 120 min, was Cch and Cae dose dependent, and was insensitive to TPA. Cch stimulation increased the proportion of [myo-3H]inositol incorporated into phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], whereas Cae specifically channeled [myo-3H]inositol incorporation into phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. In the desensitized cells, preexposure to Cch and Cae, but not to TPA, increased the subsequent basal rate of [myo-3H]inositol incorporation into total phosphoinositol (PI) by 66 and 50% above control values. There were no subsequent responses to increasing concentrations of Cch, Cae, and TPA during a second incubation. Desensitization of the pancreatic secretory responses to Cch, Cae, and TPA does not seem to result from a decrease either in total PI or in specific PtdIns(4,5)P2 synthesis, which is needed for inositol trisphosphate and diacylglycerol production.