Shortening velocity, myosin light chain phosphorylation and Ca2+ dependence of force during metabolic inhibition in smooth muscle of rat portal vein

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Abstract
The concentration-response relation for Ca2+ (0.2-0.5 mM) of high-K contractures (40 mM) in the rat portal vein during respiratory inhibition by 0.2 mM cyanide was investigated. A reduction of force in the presence of cyanide to about 30% of control was associated with a leftward shift of the normalized concentration-response relation. When force at the plateau of high-K+ contractures (at about 2 min) was reduced to 65 .+-. 2% due to the addition of cyanide, the maximal shortening preparations giving short tetanic contractions, a reduction of active force to 58 .+-. 2% of control in the presence of cyanide was associated with a reduction of Vmax to 83 .+-. 5% (n = 7). Phosphorylation of the 20-kDa regulatory light chains (LC20) of the myosin molecule was studied in the relaxed state and at the plateau of high-K+ contractures for comparison with the mechanical data. Both control and cyanide-treated preparations showed 9% LC20 phosphorylation in nominally Ca2+-free solution (n = 6). After activation the level of phosphorylation increased to 30 .+-. 3% (n = 9) in the control veins. In cyanide-treated veins, where force was reduced to 42 .+-. 6% compared to a preceding control period, the phosphorylation level was 17 .+-. 2% (n = 7). The study suggests that the mechanical changes caused by inhibition of cellular respiration may involve the combined effect of several metabolic alterations, including decreased LC20 phosphorylation during contraction, but apparently not decreased intracellular Ca2+ concentration or sensitivity of the contractile system to Ca2+.