Delay in granular fusion evoked by repetitive cytosolic Ca2+ spikes in mouse pancreatic acinar cells.

Patch-clamp whole-cell recording in combination with a phase-sensitive detection method was applied to single, enzymatically isolated, mouse pancreatic acinar cells. Either muscarinic stimulation with a low concentration of ACh (50 nM) or cell infusion of inositol 1,4,5-trisphosphate (InsP3) induced repetitive spike-like increases of membrane capacitance (delta C), membrane conductance (delta G) and membrane current (delta I). Cellular perfusion of InsP3, 10 microM in patch-pipettes, induced baseline spikes in delta C and delta G, resembling those evoked by ACh. The result indicates that exocytotic granular fusion is primarily triggered by the InsP3-induced repetitive rise of [Ca2+]i. The ACh-induced delta C took off almost synchronously with delta G with an apparent delay of less than 40 ms in the initial spike response. This delay of delta C, however, becomes longer by a factor of 7-12 during repetitive Ca2+ spike cycles. Concomitantly a faster decrease in delta C spikes than delta G spikes was observed during the cycles. Two explanations are proposed. First, the Ca2+ sensitivity of granular fusion decreases during the repetitive Ca2+ spikes. This might be due to gradual washout of low molecular components responsible for exocytosis under the whole-cell recording condition. Second, the pool of immediately releasable or of primed zymogen granules is easily exhausted or desensitized during the Ca2+ spike cycles, and has to be supplied from newly primed or sensitized resources. The progressive delay in delta C during the spike cycle is interpreted as a delay in the process of supplying fusible granules.