Decrease of acidity inside zymogen granules inhibits acetylcholine- or inositol trisphosphate-evoked cytosolic Ca2+...

In isolated pancreatic acinar cells application of the proton-potassium ionophore nigericin or the proton-sodium ionophore monensin led to a reduction of acidity inside the zymogen granules which could be visualized in an imaging system by a rapid reduction in the intragranular quinacrine fluorescence. Cytosolic Ca2+ spikes in response to acetylcholine stimulation or intracellular inositol trisphosphate application were assessed by recording Ca2+ -sensitive ionic currents in the patch clamp whole-cell recording configuration. Both nigericin and monensin evoked marked reductions in frequency and amplitude of spikes and in many experiments abolished spiking altogether. The Ca2+ -sensitive membrane currents could still be activated after nigericin or monensin treatment since subsequent application of the Ca2+ ionophore ionomycin evoked a large current response. The decrease in intragranular acidity would appear to inhibit intracellular Ca2+ release perhaps due to a reduction in the free intragranular Ca2+ concentration.