Length-dependent myosin phosphorylation and contraction of arterial smooth muscle

Ca2+, calmodulin-dependent myosin light chain phosphorylation is generally considered to be an important regulatory mechanism of smooth muscle contraction. We investigated the length dependence of myosin phosphorylation and active stress induced by K+ depolarization in arterial smooth muscle by measuring the two variables in the swine carotid media held at three steady-state tissue lengths-optimal length for contraction (Lo), 1.5 Lo, and slack length. We found that the length dependence of peak and steady-state myosin phosphorylation with respect to tissue length was different. Peak myosin phosphorylation was highest at Lo but lower at both slack length and 1.5 Lo, whereas steady-state myosin phosphorylation was similar at both Lo and 1.5 Lo, but lower at slack length. Stretching tissues to 1.5 Lo did not significantly change the steady-state myosin phosphorylation induced by K+ depolarization, but releasing tissues to slack length was associated with a 42% decrease in the steady-state myosin phosphorylation induced by K+ depolarization. These data indicated that one or more steps coupling membrane depolarization and Ca(2+)-dependent myosin phosphorylation were length sensitive. Additional data from skinned tissue experiments indicated that the length-sensitive step was not the coupling between Ca2+ and myosin phosphorylation. Therefore, these data together suggest that one or more steps coupling membrane depolarization and the increase in cytosolic Ca2+ concentration are length sensitive.