Biochemical markers of contraction in human myometrial smooth muscle cells in culture.

Phosphorylation of a light chain subunit of myosin by Ca2+ and calmodulin-dependent myosin light chain kinase is believed to be essential for smooth muscle contraction. The biochemical properties of the myosin phosphorylation system in human myometrial smooth muscle cells in monolayer culture were compared with those of human myometrial tissue and nonmuscle cells in culture. Native myosin was isolated from other cellular proteins of crude homogenates by polyacrylamide gel electrophoresis (in the presence of pyrophosphate) and quantified by densitometry. The myosin content of myometrial smooth muscle cells in culture and that of myometrial tissue were similar and four- to five-fold greater than that of human endometrial stromal cells or skin fibroblasts in culture. The specific activities of myosin light chain kinase in homogenates of myometrial smooth muscle cells that were maintained in culture and in myometrial tissue were similar (2.05 +/- 0.18 and 1.60 +/- 0.37 nmol phosphate incorporated per min per mg protein, respectively). On the other hand, enzyme activity in skin fibroblasts was only 5% of that in myometrial smooth muscle cells. Myosin light chain kinase activity in myometrial smooth muscle cells was dependent upon Ca2+ and was inhibited reversibly by the calmodulin antagonist, calmidazolium. The intracellular Ca2+ concentration measured by quin2 fluorescence was 0.12 microM in resting cells and increased in a concentration-dependent manner with KCl to a maximal value of 0.47 microM. These results indicate that biochemical processes important for smooth muscle contraction are retained in human myometrial smooth muscle cells in culture.