Octanoate increases cytosolic Ca2+ concentration and membrane conductance in ovine pancreatic acinar cells

In order to investigate the cellular mechanisms involved in amylase release in response to stimulation with short-chain fatty acids, changes in intracellular calcium concentration ([Ca2+]i), membrane current and amylase release were measured in pancreatic acinar cells of sheep. Both octanoate and acetylcholine raised [Ca2+]i in acinar cells in a concentration-dependent manner. The rise in [Ca2+]i in response to the stimulation with octanoate (10 mmol.l-1) was reduced in a medium without CaCl2, but was markedly enhanced by reintroduction of CaCl2 into the medium up to 2.56 mmol.l-1. Perfusion of the cells with a medium containing octanoate (5 mmol.l-1) or acetylcholine (0.5 mumol.l-1) immediately raised inward current across the cell membrane at a holding-membrane potential of -30 mV. The inward current became greater as the holding potential became more negative. The equilibrium potential was 1.8 mV and 3.9 mV for octanoate and acetylcholine, respectively, being consistent with that for Cl-. Although intracellular application of octanoate through a patch-clamp pipette also raised inward current after several minutes in some cells (4 out of 12), this possibility was significantly smaller than that for extracellular application. In other cells, even though the intracellular application of octanoate did not cause an increase in current, it always caused responses immediately after introduction of the fatty acid into the medium. Stimulation with fatty acid as well as acetylcholine raised amylase release in a concentration-dependent manner in cells dispersed from tissue segments with crude collagenase and trypsin inhibitor. Without trypsin inhibitor, crude collagenase significantly and selectively reduced the octanoate (10 mmol.l-1)-induced amylase release. Dispersion with crude collagenase and trypsin significantly reduced both responses induced by octanoate and acetylcholine (5.5 mumol.l-1). We conclude that fatty acids and acetylcholine increase [Ca2+]i, which consequently evokes a rise in transmembrane ion (Cl-) conductance and amylase release, and that trypsin-sensitive protein(s) in the cell membrane are involved in secretory processes activated by stimulation with fatty acids in ovine pancreatic acinar cells.