Differential modulation of L-type calcium channel subunits by oleate

Nonesterified fatty acids such as oleate and palmitate acutely potentiate insulin secretion from pancreatic islets in a glucose-dependent manner. In addition, recent studies show that fatty acids elevate intracellular free Ca(2+) and increase voltage-gated Ca(2+) current in mouse beta-cells, although the mechanisms involved are poorly understood. Here we utilized a heterologous system to express subunit-defined voltage-dependent L-type Ca(2+) channels (LTCC) and demonstrate that beta-cell calcium may increase in part from an interaction between fatty acid and specific calcium channel subunits. Distinct functional LTCC were assembled in both COS-7 and HEK-293 cells by expressing either one of the EYFP-tagged L-type alpha(1)-subunits (beta-cell Cav1.3 or lung Cav1.2) and ERFP-tagged islet beta-subunits (ibeta(2a) or ibeta(3)). In COS-7 cells, elevations in intracellular Ca(2+) mediated by LTCC were enhanced by an oleate-BSA complex. To extend these findings, Ca(2+) current was measured in LTCC-expressing HEK-293 cells that revealed an increase in peak Ca(2+) current within 2 min after addition of the oleate complex, with maximal potentiation occurring at voltages <0 mV. Both Cav1.3 and Cav1.2 were modulated by oleate, and the presence of different auxiliary beta-subunits resulted in differential augmentation. The potentiating effect of oleate on Cav1.2 was abolished by the pretreatment of cells with triacsin C, suggesting that long-chain CoA synthesis is necessary for Ca(2+) channel modulation. These results show for the first time that two L-type Ca(2+) channels expressed in beta-cells (Cav1.3 and Cav1.2) appear to be targeted by nonesterified fatty acids. This effect may account in part for the acute potentiation of glucose-dependent insulin secretion by fatty acids.