Ras reduces L-type calcium channel current in cardiac myocytes: Corrective effects of L-channels and SERCA2 on [Ca²⁺]_i regulation and cell morphology

Heart failure is associated with dysregulation of intracellular calcium ([Ca²⁺]_i), reduction in myofibrils, and increased activation of Ras, a regulator of signal-transduction pathways. To evaluate the potential effects of Ras on [Ca²⁺]_i, we expressed constitutively active Ras (Ha-Ras^V12) in cardiac myocytes and monitored [Ca²⁺]_i via fluorescence and electrophysiological techniques. Ha-Ras^V12 reduced the magnitude of the contractile calcium transients. Unexpectedly, however, calcium loading of the sarcoplasmic reticulum was increased, suggesting that Ha-Ras^V12 introduces a defect in excitation-calcium release coupling. Consistent with this idea, L-channel calcium currents were reduced by Ha-Ras^V12, which also downregulated the activity of the L-channel gene promoter. Coexpression of L-channels and SERCA2 largely corrected Ha-Ras^V12-induced dysregulation of [Ca²⁺]_i. Furthermore, whereas Ha-Ras^V12 downregulated myofibrils, this effect was blocked by coexpression of L-channels. These results suggest that Ras downregulates L-channel expression, which may play a pathophysiological role in cardiac disease.