Effects of external calcium on acetylcholine-evoked increases in membrane capacitance in rat pancreatic acinar cells

Effects of external calcium on acetylcholine-induced increases in membrane capacitance and conductance were investigated with the patch-clamp technique in combination with the phase-sensitive detection method, in single dialysed pancreatic acinar cells of rats. Both increases depended on an increase in [Ca²⁺]_i, and a high concentration of EGTA in the cell-dialysing solution made ACh ineffective. In acinar cells exposed to a bathing solution containing the normal concentration of Ca²⁺ (2.5 mM CaCl₂), the increase in membrane capacitance was transient and synchronous with that in membrane conductance (current) in response to 0.2 μM acetylcholine. However, in a bathing solution without CaCl₂ and with EGTA (0.2 mM), the increase in membrane capacitance was sustained after the membrane conductance recovered to the original level during the ACh-stimulation. The evidence suggests that external calcium facilitates either the resealing of the fusion- or fission-pores formed at the contact between the secretory granule and the luminal cell membrane, or the membrane retrieval (endocytosis) in Ca²⁺-dependent exocytosis.