The relationship between acetylcholine-evoked Ca2+-dependent current and the Ca2+ concentrations in the cytosol and the lumen of the endoplasmic reticulum in pancreatic acinar cells

In a study of isolated mouse pancreatic acinar cells, we used the patch-clamp whole-cell recording configuration to monitor the Ca2+-dependent inward ionic current and simultaneously measured the Ca2+ concentration in either the cytosol ([Ca2+](i)) or the lumen of the endoplasmic reticulum ([Ca2+](Lu)), using appropriate Ca2+-sensitive fluorescent probes. A high concentration of acetylcholine (ACh, 10 mu M) evoked an increase in [Ca2+](i), which resulted in the activation of Ca2+-dependent inward current. Continued ACh application for several minutes led to a marked reduction in both the current and the [Ca2+](i) response and after about 4-10 min of sustained ACh stimulation, the inward current response had disappeared and [Ca2+](i) was back to the pre-stimulation level. Repeated stimulation with shorter pulses of ACh (10 mu M) resulted in responses of declining magnitude both in terms of inward current and [Ca2+](i) rises. The ACh-activated inward current was entirely dependent on the elevation of [Ca2+](i) but at a relatively high [Ca2+](i) the current: was saturated. ACh caused a rapid release of Ca2+ from the lumen of the endoplasmic reticulum and after discontinuation of stimulation, [Ca2+](Lu) was only very slowly (10-15 min) fully restored to the pre-stimulation level. Repeated applications of ACh did not change the relationships between the Ca2+-dependent current and [Ca2+](i) or the current and [Ca2+](Lu). When [Ca2+](Lu) was greater than 100 mu M, the ACh-evoked Ca2+ release from the store was so large that the current response was initially saturated. We conclude that the ACh-evoked current response essentially depends on the release of stored Ca2+. Desensitization is mainly due to the relatively slow reloading of the intracellular stores with Ca2+.