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 (InsP(3)) induced repetitive spike-like increases of membrane capacitance (Delta C), membrane conductance (Delta G) and membrane current (Delta I). Cellular perfusion of InsP(3), 10 mu M 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 [Ca2+](i). The ACh-induced Delta C took off almost synchronously by the InsP(3)-induced repetitive rise of [Ca2+](i). chronously 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.