Effect of metabolic inhibition on intracellular Ca2+, phosphorylation of myosin regulatory light chain and force in rat smooth muscle.

1. The effect of the inhibition of oxidative phosphorylation on intracellular calcium concentration ([Ca2+]i), phosphorylation of the 20 kDa regulatory light chain of myosin (MLC20) and contractility was investigated in isolated longitudinal smooth muscle from rat uteri. 2. Cyanide (2 mM) application to normally polarized preparations resulted in an elevation of basal [Ca2+]i but an inhibition of [Ca2+]i transients and the accompanying contractions. 3. Depolarization with high-K+ solution (40 mM KCI) resulted in elevation of [Ca2+]i and maintained force production. Phosphorylation of MLC20 was transiently increased followed by a steady-state augmentation above resting levels. 4. Carbachol (100 microM) produced a transient elevation of [Ca2+]i and force of depolarized tissues followed by a steady-state augmentation of both parameters. PGF2 alpha (1 microM) did not significantly potentiate [Ca2+]i or force in depolarized preparations. Both carbachol and PGF2 alpha potentiated phosphorylation of MLC20 in depolarized tissues. 5. Addition of cyanide to depolarized preparations, in the presence or absence of carbachol or PGF2 alpha, resulted in significant attenuation of force under each condition. The magnitude and normalized rates of force inhibition by cyanide were not significantly different for each stimulus condition. MLC20 phosphorylation levels were unaltered by cyanide treatment. However, cyanide increased the maintained level of [Ca2+]i under each experimental protocol. 6. It is concluded that the inhibition of oxidative phosphorylation with cyanide results in dissociation of both the [Ca2+]i-force and MLC20 phosphorylation-force relationships in rat uterine smooth muscle.