Influence of Mitochondrial Inhibition on Global and Local [Ca 2+ ] i in Rat Tail Artery

Abstract

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 α₁-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 α₁ 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 InsP₃ 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.