MOLECULAR DIVERSITY AND FUNCTIONAL-CHARACTERIZATION OF VOLTAGE-DEPENDENT CALCIUM CHANNELS (CACN4) EXPRESSED IN PANCREATIC BETA-CELLS

Dihydropyridine-sensitive voltage-dependent calcium channels (VDCC) play a crucial role in insulin secretion. We recently have cloned a human alpha(1)-subunit of the VDCC expressed in pancreatic beta-cells, designated CACN4. In this study we have isolated complementary DNAs encoding two forms of rat CACN4 (rCACN4A and rCACN4B) from a rat insulinoma RINm5F complementary DNA library. Rat CACN4A is a protein of 2203 amino acids and is the rat homolog of human CACN4, whereas rCACN4B lacks 535 amino acids in the carboxyl-terminal region, probably due to alternative splicing. We have found two additional variations, one in the intracellular loop between repeats I and II and the other in the extracellular region between the third and fourth segments of repeat IV. Reverse transcriptase-polymerase chain reaction analysis of rat pancreatic islet messenger RNA reveals that these variants are present in pancreatic islets. In addition, whole-cell voltage-damp recordings of Chinese hamster ovary cells stably expressing the alpha(1)-subunit (rCACN4A or rCACN4B) with or without the calcium channel beta(2)-subunit show that coexpression of rCACN4A with the beta(2)-subunit or rCACN4B with the beta(2)-subunit elicits L-type VDCC currents, whereas expression of the alpha(1)-subunit alone does not, indicating that CACN4 can associate functionally with the beta(2)-subunit and that the beta-subunit is essential for functional expression of CACN4. These results suggest that there are various subtypes of CACN4 expressed in pancreatic beta-cells, and that both rCACN4A and rCACN4B can function as VDCC. Furthermore, the present study suggests that the expression of the beta-subunit as well as the alpha(1)-subunit may participate in the regulation of insulin secretion.