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University Laboratory of Physiology, Oxford, UK.

1. The effects of the divalent cations Ca2+, Ba2+ and Sr2+ on exocytosis and endocytosis from single isolated mouse pancreatic beta-cells were investigated by monitoring changes in cell capacitance. 2. The immediate increase in capacitance elicited by a single depolarization from -70 to +20 mV was dependent on the divalent cation species, with Ca2+ (8.2 +/- 1.1 fF pC-1) > Ba2+ (1.0 +/- 0.2 fF pC-1) > Sr2+ (0.7 +/- 0.2 fF pC-1) in perforated-patch recordings. 3. In Ba2+ solutions alone there was subsequently an additional slow increase in capacitance (to 4.3 +/- 1.1 fF pC-1). This second phase of exocytosis was unaffected by preincubation with colcemid (20 microM, 45 min) or cytochalasin D (10 microM, 15 min), suggesting that interaction of secretory granules with microtubules or microfilaments is not involved. 4. An increase in cell capacitance was elicited by depolarization in Ba2+ solutions when intracellular Ca2+ was buffered with 10 mM EGTA. Infusion of the beta-cell with Ba2+ also stimulated exocytosis although the rate was much slower (1.1 +/- 0.2 fF s-1; 8 microM free Ba2+) than for Ca2+ (39 +/- 5 fF s-1; 2 microM free Ca2+). These data indicate that Ba2+ does not evoke secretion by promoting Ca2+ release from internal stores. 5. The lower efficacy of Ba2+ in supporting exocytosis may be related to the fact that this cation does not activate calmodulin-dependent processes and the slow second phase of secretion may result from this ion being removed only slowly from the cytoplasm. 6. Endocytosis was faster in Sr2+ than in Ca2+ or Ba2+ solution, and the speed increased when the external concentration of all three divalent cation species was raised. The ability of Ba2+ to support endocytosis suggests calmodulin-dependent processes are not involved. These data suggest membrane retrieval is regulated differently from exocytosis in beta-cells.

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