Dynamics of calcium clearance in mouse pancreatic β-cells

Pancreatic beta-cells maintain glucose homeostasis by their regulated Ca2+-dependent secretion of insulin. Several cellular mechanisms control intracellular Ca2+ levels, but their relative significance in mouse beta-cells is not fully known. We used photometry to measure the dynamics of cytosolic Ca2+ ([Ca2+](i)) clearance after brief, depolarization-induced Ca2+ entry. Treatment with thapsigargin or cyclopiazonic acid, inhibitors of the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) pumps, nearly doubled the peak and slowed the decay of the depolarization-induced Ca2+ transients. The remaining thapsigargin-insensitive decay was slowed further by inhibition of the plasma membrane Ca2+-ATPase (PMCA) and plasma membrane Na+/Ca2+ exchanger (NCX) via alkalization of the bath solution, by adding lanthanum, or by substitution of Na+ with Li+. Mitochondrial Ca2+ uptake contributed little to clearance in thapsigargin-pretreated cells. Together, the SERCA, PMCA, and NCX transport mechanisms accounted for 89 to 97% of clearance in normal solutions. We developed a quantitative model for the dynamic role of removal mechanisms over a wide range of [Ca2+](i). According to our model, 50 to 64% of initial Ca2+ removal is via the SERCA pump, whereas the NCX contributes 21-30% of the extrusion at high [Ca2+](i), and the PMCA contributes 21-27% at low [Ca2+](i).