Inhibition of turkey gizzard myosin light chain kinase activity by BAY K 8644

BAY K 8644, a dihydropyridine with positive inotropic effects in smooth muscle, was found to inhibit calmodulin-dependent turkey gizzard myosin light chain kinase activity with an IC50 of 80 microM. In contrast, when myosin light chain kinase was rendered calmodulin-independent by limited proteolysis, drug inhibition was markedly diminished. BAY K 8644 inhibition was additive with that of the negative-inotropic dihydropyridine felodipine. These results indicate that the inotropic effects of dihydropyridines in smooth muscle are unrelated to their calmodulin-antagonistic properties.