PILOCARPINE AND CARBACHOL EXHIBIT MARKEDLY DIFFERENT PATTERNS OF CA2+ SIGNALING IN RAT PANCREATIC ACINAR-CELLS

The effects of the partial muscarinic agonist pilocarpine on physiological responses were investigated in rat pancreatic acinar cells and compared with carbachol, a full muscarinic agonist, together with previous results using JMV-180, a partial agonist of CCK-A receptors. Pilocarpine was found to stimulate amylase release from isolated pancreatic acini in a concentration-dependent manner. At a maximal concentration (10 muM), pilocarpine was only capable of stimulating 63% of the secretion stimulated by a maximal concentration of carbachol. Moreover pilocarpine did not induce a decrease in secretion at supramaximal concentrations as does carbachol. In acini loaded with fura-2, superfusion of pilocarpine resulted exclusively in generation of intracellular Ca2+ concentration ([Ca2+]i) oscillations at all concentrations tested (0.3 muM-1 mM), in marked contrast to high concentrations of full agonists, which result in a biphasic sustained increase in [Ca2+]i. In common with low concentrations of other secretagogues that stimulate [Ca2+]i oscillations, pilocarpine at all concentrations was only able to stimulate a very small increase in phosphoinositide (PI) hydrolysis. In acini previously incubated with [H-3]inositol, pilocarpine was shown to stimulate PI hydrolysis 27% above basal, compared with 872% for carbachol. To ascertain if this small degree of PI hydrolysis seen with pilocarpine is responsible for the generation of [Ca2+]i oscillations, an inhibitor of phospholipase C-linked processes, U-73122, which has been shown to inhibit Ca2+ oscillations induced by carbachol and CCK but not JMV-180 was tested. This agent rapidly inhibited pilocarpine-stimulated oscillations, indicating that in contrast to JMV-180, oscillations induced by pilocarpine are the result of PI hydrolysis.