In short-term batch-incubation or perifusion experiments, we studied insulin release and associated 65Zn efflux from rat pancreatic islets loaded with 65Zn by 24-h tissue culture in low-glucose medium. The fractional basal insulin release and 65Zn efflux were 0.4% and 3% of total content/h/islet, respectively. Thus, basal 65Zn efflux was much greater than that to be accounted for if zinc was released proportionally with insulin release only; extragranular zinc flux was suggested. Two millimolar glucose, with or without 1 mM 3-isobutyl-1-methylxanthine (IBMX), affected neither insulin release nor associated 65Zn efflux. Twenty-five millimolar glucose produced a significant threefold increase in insulin release above baseline, but somewhat decreased 65Zn efflux at marginal significance. Glucose (25 mM) plus 1 mM IBMX provoked a high increase in insulin release and an associated 30% increase in fractional 65Zn efflux over basal. Calculations based on previous estimations of 65Zn distribution and equilibrium with islet zinc indicated that molar zinc efflux was more than sufficient to account for a 2-zinc-insulin hexamer. L-leucine (2 or 20 mM) plus 1 mM IBMX caused far greater 65Zn efflux for the amount of insulin released, indicating additional 65Zn mobilization not directly related to insulin secretion. To evaluate 65Zn efflux during inhibited insulin secretion, batch incubations were performed in 100% D2O or at 27°C, conditions that inhibited insulin release stimulated by high glucose plus IBMX. These agents decreased the 65Zn efflux far below the basal value (35% and 50%, respectively) and greater than could be accounted for by the attendent inhibition of insulin secretion. Perifusion experiments showed that the temporal kinetics for insulin release and associated 65Zn efflux stimulated by high glucose plus IBMX were parallel. We conclude that sufficient zinc is released from islets to permit insulin storage and secretion as a 2-zinc-insulin hexamer. In addition, 65Zn flux (presumably extragranular) is highly sensitive to secretagogues and cellularly active agents independent of insulin secretion.

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