As the ionophore A23187 is believed to act by increasing cytosolic Ca2+ ([Ca]i), it offers a mechanism for experimentally controlling [Ca]i. Ca2+-selective microelectrodes were employed to examine the effect of A23187 on [Ca]i and the role of [Ca]i in acinar secretion. The mean [Ca]i in acinar cells of the mouse pancreas was determined to be 0.43 +/- 0.03 microM. When the ionophore was added to the saline bathing the acinar cells, 10(-6) M A23187 depolarized the membrane potential (Em) by 5.2 +/- 0.3 mM and the intracellular Ca-electrode potential (ECs) by 9.8 +/- 0.6 while 10(-5) M A23187 depolarized Em by 7.4 +/- 0.3 mV and ECs by 14.1 +/- 0.8. These changes in potentials reflect an increase in [Ca]i to 0.62 +/- 0.03 microM with 10(-6) M and 0.73 +/- 0.05 microM with 10(-5) M A23187. The increase in [Ca]i observed with 10(-6) M A23187 was similar to that found with concentrations of acetylcholine (Ach) that produced maximal enzyme secretion, whereas the increase in [Ca]i with 10(-5) M was similar in magnitude to that observed with ACh concentration that inhibited or reduced secretion. Measurements of amylase release during 30 min exposure of A23187 produced an 88.4% increase in amylase activity over basal levels with 10(-6) M and little or no change with (10(-5) M, indicating that the ionophore influences secretion through changes in [Ca]i in a manner analogous to the natural secretagogue ACh. This report establishes that acinar secretion occurs only within a narrow range of [Ca]i activities and suggests intracellular increases in both "bound" and "free" calcium may occur during cell activation.