EFFECTS OF DIVALENT-CATIONS ON EXOCYTOSIS AND ENDOCYTOSIS FROM SINGLE-MOUSE PANCREATIC BETA-CELLS

1. The effects of the divalent cations Ca2+, Ba2+ and Sr2+ On exocytos is and endocytosis from single isolated mouse pancreatic beta-cells we re investigated by monitoring changes in cell capacitance. 2. The imme diate 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 capacitanc e (to 4.3 +/- 1.1 fF pC(-1)). This second phase of exocytosis was unaf fected by preincubation with colcemid (20 mu M, 45 min) or cytochalasi n D (10 mu M, 15 min), suggesting that interaction of secretory granul es with microtubules or microfilaments is not involved. 4. An increase in cell capacitance was elicited by depolarization in Ba2+ solutions when intracellular Ca2+ tvas 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 mu M free Ba2+) than for Ca2+ (39 +/- 5 fF s(-1); 2 mu M 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 t o the fact that this cation does not activate calmodulin-dependent pro cesses and the slow second phase of secretion may result from this ion being removed only slowly from the cytoplasm. 6. Endocytosis was fast er in Sr2+ than in Ca2+ or Ba2+ solution, and the speed increased when the external concentration of all three divalent cation species was r aised. The ability of Ba2+ to support endocytosis suggests calmodulin- dependent processes are not involved. These data suggest membrane retr ieval is regulated differently from exocytosis in beta-cells.