Influence of mitochondrial inhibition on global and local [Ca2+]i in rat tail artery

Inhibition of oxidative metabolism is often found to decrease contractility of systemic vascular smooth muscle, but not to reduce global [Ca2+](i). In the present study, we probe the hypothesis that it is associated with an altered pattern of intracellular Ca2+ oscillations (waves) influencing force development. In the rat tail artery, mitochondrial inhibitors (rotenone, antimycin A, and cyanide) reduced alpha(1)-adrenoceptor-stimulated force by 50% to 80%, but did not reduce global [Ca2+](i). Less relaxation (about 30%) was observed after inhibition of myosin phosphatase activity with calyculin A, suggesting that part of the metabolic sensitivity involves the regulation of myosin 20-kDa light chain phosphorylation, although no decrease in phosphorylation was found in freeze-clamped tissue. Confocal imaging revealed that the mitochondrial inhibitors increased the frequency but reduced the amplitude of asynchronous cellular Ca2+ waves elicited by alpha(1) stimulation. The altered wave pattern, in association with increased basal [Ca2+](i), accounted for the unchanged global [Ca2+](i). Inhibition of glycolytic ATP production by arsenate caused similar effects on Ca2+ waves and global [Ca2+](i), developing gradually in parallel with decreased contractility. Inhibition of wave activity by the InsP(3) receptor antagonist 2-APB correlated closely with relaxation. Furthermore, abolition of waves with thapsigargin in the presence of verapamil reduced force by about 50%, despite unaltered global [Ca2+](i), suggesting that contraction may at least partly depend on Ca2+ wave activity. This study therefore indicates that mitochondrial inhibition influences Ca2+ wave activity, possibly due to a close spatial relationship of mitochondria and the sarcoplasmic reticulum and that this contributes to metabolic vascular relaxation.