Myosin light chain phosphorylation and Mn2+ -dependent norepinephrine-induced contractions in guinea-pig vas deferens

We have reported that norepinephrine but not K+ induced a sustained and dose-dependent contraction without extracellular Ca2+ and Mn2+ in Ca2+-depleted Mn2+-loaded vas deferens from the guinea-pig. In the present study, we determined the phosphorylation of the 20-kDa myosin light chain and examined the effects of inhibitors of calmodulin and myosin light chain kinase on the Mn2+-dependent norepinephrine-induced contraction in order to evaluate the contribution of phosphorylation to this contraction. W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide], ML-9 [1-(5-chloronaphthalene-1-sulfonyl)-homopiperazine] and wortmannin inhibited this contraction. However, the Mn2+-dependent norepinephrine-induced contraction developed without a significant increase in the phosphorylation of the 20-kDa myosin light chain. In beta-escin-permeabilized preparations, Mn2+ induced contractions that were inhibited by ML-9. These results suggest that the activation of myosin light chain kinase is essential for the development of Mn2+-dependent norepinephrine-induced contractions and that the phosphorylation of myosin light chain may act as a trigger for these contractions.