Overexpression of the Na/Ca Exchanger Shapes Stimulus-Induced Cytosolic Ca²⁺ Oscillations in Insulin-Producing BRIN-BD11 Cells

Abstract

In response to glucose, mouse β-cells display slow oscillations of the membrane potential and cytosolic free Ca²⁺ concentration ([Ca²⁺]_i), whereas rat β-cells display a staircase increase in these parameters. Mouse and rat islet cells differ also by their level of Na/Ca exchanger (NCX) activity. The view that the inward current generated by Na/Ca exchange shapes stimulus-induced electrical activity and [Ca²⁺]_i oscillations in pancreatic β-cells was examined in insulin-producing BRIN-BD11 cells overexpressing the Na/Ca exchanger. BRIN-BD11 cells were stably transfected with NCX1.7, one of the exchanger isoforms identified in the β-cell. Overexpression could be assessed at the mRNA and protein level. Appropriate targeting to the plasma membrane could be assessed by microfluorescence and the increase in Na/Ca exchange activity. In response to K⁺, overexpressing cells showed a more rapid increase in [Ca²⁺]_i on membrane depolarization as well as a more rapid decrease of [Ca²⁺]_i on membrane repolarization. In response to glucose and tolbutamide, control BRIN cells showed large amplitude [Ca²⁺]_i oscillations. In contrast, overexpressing cells showed a staircase increase in [Ca²⁺]_i without such large oscillations. Diazoxide-induced membrane hyperpolarization restored large amplitude [Ca²⁺]_i oscillations in overexpressing cells. The present data confirm that Na/Ca exchange plays a significant role in the rat β-cell [Ca²⁺]_i homeostasis, the exchanger being a versatile system allowing both Ca²⁺ entry and outflow. Our data suggest that the current generated by the exchanger shapes stimulus-induced membrane potential and [Ca²⁺]_i oscillations in insulin-secreting cells, with the difference in electrical activity and [Ca²⁺]_i behavior seen in mouse and rat β-cells resulting in part from a difference in Na/Ca exchange activity between these two cells.