Characterization of voltage-dependent sodium and calcium channels in mouse pancreatic A- and B-cells

Insulin and glucagon are the major hormones of the islets of Langerhans that are stored and released from the B- and A-cells, respectively. Both hormones are secreted when the intracellular cytosolic Ca2+ concentration ([Ca2+](i)) increases. The [Ca2+](i) is modulated by mutual inhibition and activation of different voltage-gated ion channels. The precise interplay of these ion channels in either glucagon or insulin release is unknown, owing in part to the difficulties in distinguishing A- from B-cells in electrophysiological experiments. We have established a single-cell RT-PCR method to identify A- and B-cells from the mouse. A combination of PCR, RT-PCR, electrophysiology and pharmacology enabled us to characterize the different sodium and calcium channels in mouse islet cells. In both A- and B-cells, 60% of the inward calcium current (I-Ca) is carried by L-type calcium channels. In B-cells, the predominant calcium channel is Ca(V)1.2, whereas Ca(V)1.2 and Ca(V)1.3 were identified in A-cells. These results were confirmed by using mice carrying A- or B-cell-specific inactivation of the Ca(V)1.2 gene. In B-cells, the remaining I-Ca flows in equal amounts through Ca(V)2.1, Ca(V)2.2 and CaCa(V)2.3. In A-cells, 30 and 15% Of ICa is due to Ca(V)2.3 and Ca(V)2.1 activity, respectively, whereas CaV2.2 current was not found in these cells. Low-voltage-activated T-type calcium channels could not be identified in A- and B-cells. Instead, two TTX-sensitive sodium currents were found: an early inactivating and a residual current. The residual current was only recovered in a subpopulation of B-cells. A putative genetic background for these currents is Na(V)1. 7.