Decrease in Tetrahydrobiopterin Content Promotes L-Arginine-Induced Elevation of Cytosolic Calcium Concentration in Insulin-Secreting Cells

We examined the role of endogenous tetrahydrobiopterin (BH4) on L-arginine (L-Arg)-induced increase in cytosolic calcium concentration ([Ca]i) in the β-cell line, HIT-T15 cells. HIT-T15 cells contain a large amount of BH4 with a high expression of GTP cyclohydrolase I (GTPCH) mRNA compared with mouse brain microvascular endothelial cells and bovine aortic endothelial cells. 2,4-Diamino-6-hydroxypyrimidine (DAHP), a selective inhibitor of GTPCH, reduced the basal intracellular BH4 content to approximately 14%. The addition of LArg, but not D-Arg, to the cells increased [Ca]i in the presence of glucose in a concentration-dependent manner. Co-treatment with nifedipine, a blocker of L-type voltage-operated Ca channel, inhibited the increase in [Ca]i by L-Arg, suggesting that L-Arg stimulates Ca influx via L-type Ca channel in the plasma membrane. The LArg-induced [Ca]i elevation was stimulated by N-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthase, suggesting that NO negatively controls it. Moreover, the elevation of [Ca]i by L-Arg was also stimulated by DAHP treatment. These findings suggest that BH4 negatively regulates the L-Arg-induced increase in [Ca]i through the production of NO in β-cells.