Force-velocity relation and myosin light chain phosphorylation in bovine coronary arterial smooth muscle

We have investigated in bovine left ventricular coronary arteries the relation between the biochemical regulatory event of myosin light chain phosphorylation and the mechanical events of isometric stress and isotonic shortening, under conditions of stimulation by depolarization (65 mM KCl) or receptor occupancy (2 μM 5-hydroxytryptamine [5-HT]). At rest, levels of light chain phosphorylation were 0.07 ± 0.01 mol phosphate/mol light chain. Maximal values were significantly different for KCl (0.42 ± 0.02 mol phosphate/mol light chain at 1 minute) and 5-HT stimulation (0.58 ± 0.01 mol phosphate/mol light chain at 30 seconds). Increases in light chain phosphorylation preceded isometric stress development, and values remained elevated at ~0.35 mol phosphate/mol light chain for up to 2 hours with both KCl and 5-HT. The sites of phosphorylation were identical for KCl and 5-HT at 2 hours. Maximal stresses for each stimulus were also maintained for 2 hours. Values of maximum velocity of shortening (V₀ in muscle lengths [ML]/sec), obtained from the force-velocity relation, did not change significantly between 1 minute and 2 hours with KCl (0.070 ± 0.008 ML/sec at 1 minute and 0.056 ± 0.007 ML/sec at 2 hours, p>0.2). However, during 5-HT stimulation, V₀ declined significantly (0.053 ± 0.006 ML/sec at 1 minute and 0.032 ± 0.003 ML/sec at 2 hours, p<0.025). The relation between V₀ and light chain phosphorylation was different for KCl and 5-HT, indicating that factors in addition to myosin light chain phosphorylation may modulate smooth muscle shortening velocity.