Carbamoylcholine regulation of polyphosphoinositide synthesis and hydrolysis in cultured, dispersed, digitonin-permeabilized mouse pancreatic islet cells.

Continuing formation of inositol phosphates during stimulation of pancreatic beta-cells by hormones and neurotransmitters requires the continued synthesis of the polyphosphoinositides phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5 bisphosphate (PIP2) from phosphatidylinositol (PI). In the present study we have investigated how this pathway and the activity of phosphoinositide-specific phospholipase C (PI-PLC) are regulated by carbamoylcholine (CCh), Ca2+, the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), GTP gamma S and NaF in 44-h [3H]inositol-labelled, dispersed and digitonin-permeabilized mouse pancreatic islet cells. CCh stimulated not only PI-PLC (G-protein-mediated) but also, by an as yet unknown mechanism, significantly enhanced PI 4-kinase activity, estimated as the PIP:PI ratio, by 100%, and further increased the flux from PI to PIP and PIP2, GTP gamma S and NaF mimicked the effects of CCh on PI-PLC but had no effect on the levels of PIP and PIP2, TPA raised the PIP:PI ratio by 75%.

Role of glucose metabolism and phosphoinositide hydrolysis in glucose-induced sensitization/desensitization of insulin secretion from mouse pancreatic islets.

The role of glucose metabolism and phosphoinositide hydrolysis in glucose-induced sensitization/desensitization of insulin secretion was studied. A change in glucose concentration from 5.5 to 16.

Exogenous arachidonic acid inactivates protein kinase C in mouse pancreatic islets.

The effect of arachidonic acid on protein kinase C activity and insulin secretion in mouse islets was investigated. Arachidonic acid stimulated protein kinase C activity in islet cytosol and membrane fractions by substituting for phosphatidylserine. Stimulation by arachidonic acid was dependent on either Ca2+ or the phorbol ester 12-O-tetradecanoylphorbol 13-acetate, was potentiated by the combined addition of Ca(2+) + 12-O-tetradecanoylphorbol 13-acetate, and did not further increase protein kinase C activity in the presence of saturating concentrations of phosphatidylserine.