The Zn2+-transporting Pathways in Pancreatic β-Cells A ROLE FOR THE L-TYPE VOLTAGE-GATED Ca2+ CHANNEL

Abstract In pancreatic β-cells Zn2+ is crucial for insulin biosynthesis and exocytosis. Despite this, little is known about mechanisms of Zn2+ transport into β-cells or the regulation and compartmentalization of Zn2+ within this cell type. Evidence suggests that Zn2+ in part enters neurons and myocytes through specific voltage-gated calcium channels (VGCC). Using a Zn2+-selective fluorescent dye with high affinity and quantum yield, FluoZin-3 AM and the plasma membrane potential dye DiBAC4(3) we applied fluorescent microscopy techniques for analysis of Zn2+-accumulating pathways in mouse islets, dispersed islet cells, and β-cell lines (MIN6 and β-TC6f7 cells). Because the stimulation of insulin secretion is associated with cell depolarization, Zn2+ (5-10 μm) uptake was analyzed under basal (1 mm glucose) and stimulatory (10-20 mm glucose, tolbutamide, tetraethylammonium, and high K+) conditions. Under both basal and depolarized states, β-cells were capable of Zn2+ uptake, and switching from basal to depolarizing conditions resulted in a marked increase in the rate of Zn2+ accumulation. Importantly, L-type VGCC (L-VGCC) blockers (verapamil, nitrendipine, and nifedipine) as well as nonspecific inhibitors of Ca2+ channels, Gd3+ and La3+, inhibited Zn2+ uptake in β-cells under stimulatory conditions with little or no change in Zn2+ accumulation under low glucose conditions. To determine the mechanism of VGCC-independent Zn2+ uptake the expression of a number of ZIP family Zn2+ transporter mRNAs in islets and β-cells was investigated. In conclusion, we demonstrate for the first time that, in part, Zn2+ transport into β-cells takes place through the L-VGCC. Our investigation demonstrates direct Zn2+ accumulation in insulin-secreting cells by two pathways and suggests that the rate of Zn2+ transport across the plasma membrane is dependent upon the metabolic status of the cell.