Sarcoplasmic reticulum Ca2+ uptake is impaired in coronary smooth muscle distal to coronary occlusion

Abstract

After chronic occlusion, collateral-dependent coronary arteries exhibit alterations in both vasomotor reactivity and associated myoplasmic free Ca2+ levels that are prevented by chronic exercise training. We tested the hypotheses that coronary occlusion diminishes Ca2+ uptake by the sarcoplasmic reticulum (SR) and that exercise training would prevent impaired SR Ca2+ uptake. Ameroid constrictors were surgically placed around the proximal left circumflex (LCx) artery of female swine 8 wk before initiating 16-wk sedentary (pen confined) or exercise-training (treadmill run) protocols. Twenty-four weeks after Ameroid placement, smooth muscles cells were enzymatically dissociated from both the LCx and nonoccluded left anterior descending (LAD) arteries of sedentary and exercise-trained pigs, and myoplasmic free Ca2+ was studied using fura 2 microfluorometry. After the SR Ca2+ store was partially depleted with caffeine (5 mM), KCl-induced membrane depolarization produced a significant decrease in the time to half-maximal ( t ½) myoplasmic free Ca2+ accumulation in LCx versus LAD cells of sedentary pigs. Furthermore, inhibition of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA; 10 μM cyclopiazonic acid) significantly reduced t ½ in cells isolated from the LAD but not from the LCx. Exercise training did not prevent the differences in t ½ myoplasmic free Ca2+ accumulation observed between LCx and LAD cells. Occlusion or exercise training did not alter SERCA protein levels. These results support our hypothesis of impaired SR Ca2+ uptake in coronary smooth muscle cells isolated distal to chronic occlusion. Impaired SR Ca2+ uptake was independent of SERCA protein levels and was not prevented by exercise training.