The Zn2+-transporting pathways in pancreatic beta-cells: a role for the L-type voltage-gated Ca2+ channel.
In pancreatic beta-cells Zn(2+) is crucial for insulin biosynthesis and exocytosis. Despite this, little is known about mechanisms of Zn(2+) transport into beta-cells or the regulation and compartmentalization of Zn(2+) within this cell type. Evidence suggests that Zn(2+) in part enters neurons and myocytes through specific voltage-gated calcium channels (VGCC). Using a Zn(2+)-selective fluorescent dye with high affinity and quantum yield, FluoZin-3 AM and the plasma membrane potential dye DiBAC(4)(3) we applied fluorescent microscopy techniques for analysis of Zn(2+)-accumulating pathways in mouse islets, dispersed islet cells, and beta-cell lines (MIN6 and beta-TC6f7 cells). Because the stimulation of insulin secretion is associated with cell depolarization, Zn(2+) (5-10 mum) 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, beta-cells were capable of Zn(2+) uptake, and switching from basal to depolarizing conditions resulted in a marked increase in the rate of Zn(2+) accumulation. Importantly, L-type VGCC (L-VGCC) blockers (verapamil, nitrendipine, and nifedipine) as well as nonspecific inhibitors of Ca(2+) channels, Gd(3+) and La(3+), inhibited Zn(2+) uptake in beta-cells under stimulatory conditions with little or no change in Zn(2+) accumulation under low glucose conditions. To determine the mechanism of VGCC-independent Zn(2+) uptake the expression of a number of ZIP family Zn(2+) transporter mRNAs in islets and beta-cells was investigated. In conclusion, we demonstrate for the first time that, in part, Zn(2+) transport into beta-cells takes place through the L-VGCC. Our investigation demonstrates direct Zn(2+) accumulation in insulin-secreting cells by two pathways and suggests that the rate of Zn(2+) transport across the plasma membrane is dependent upon the metabolic status of the cell.
Gyulkhandanyan
Armen V
AV
Departments of Physiology and Laboratory Medicine, University of Toronto, Toronto, Ontario, M5S 1A8 Canada.
Lee
Simon C
SC
Bikopoulos
George
G
Dai
Feihan
F
Wheeler
Michael B
MB
D020746:Calcium Channels, L-Type / Q000502:physiology*; D027682:Cation Transport Proteins / Q000096:biosynthesis*; D002462:Cell Membrane; D020869:Gene Expression Profiling; D006801:Humans; D050417:Insulin-Secreting Cells / Q000378:metabolism*; D007340:Insulinoma / Q000473:pathology; D007477:Ions; D010190:Pancreatic Neoplasms / Q000473:pathology; D016133:Polymerase Chain Reaction; D012333:RNA, Messenger / Q000096:biosynthesis; D014407:Tumor Cells, Cultured; D015032:Zinc / Q000493:pharmacokinetics*
D020746:Calcium Channels, L-Type; D027682:Cation Transport Proteins; D007477:Ions; D012333:RNA, Messenger; D015032:Zinc