Voltage-dependent anion channel 2 modulates resting Ca2+ sparks, but not action potential-induced Ca2+ signaling in cardiac myocytes

Voltage-dependent anion channels (VDACs) are pore forming proteins predominantly found in the outer mitochondrial membrane and are thought to transport Ca2+. In this study, we have investigated the possible role of type 2 VDAC (VDAC2) in cardiac Ca2+ signaling and Ca2+ sparks using a lentiviral knock-down (KD) technique and two-dimensional confocal Ca2+ imaging in immortalized autorhythmic adult atrial cells, HL-1. We confirmed high expression of VDAC2 protein in ventricular, atrial, and HL-1 cells using Western blot analysis. Infection of HL-1 cells with VDAC2-targeting lentivirus reduced the level of VDAC2 protein to similar to 10%. Comparisons of autorhythmic Ca2+ transients between wild-type (WT) and VDAC2 KD cells showed no significant change in the magnitude, decay, and beating rate of the Ca2+. transients. Caffeine (10 mM)-induced Ca2+ release, which indicates sarcoplasmic reticulum (SR) Ca2+ content, was not altered by VDAC2 KD. Interestingly, however, the intensity, width, and duration of the individual Ca2+ sparks were significantly increased by VDAC2 KD in resting conditions, with no change in the frequency of sparks. VDAC2 KD significantly delayed mitochondrial Ca2+ uptake during artificial Ca2+ pulses in permeabilized HL-1 cells. These results suggest that VDAC2 may facilitate mitochondrial Ca2+ uptake and restrict Ca2+ spark expansion without regulating activations of sparks under resting conditions, thereby providing evidence on the functional role of VDAC2 in cardiac local Ca2+. signaling. (C) 2010 Elsevier Ltd. All rights reserved.