Myosin light chain phosphorylation in human myometrial smooth muscle cells

Ca²⁺/calmodulin-dependent phosphorylation of the 20-kDa regulatory light chain of myosin is of signal importance in the initiation of contraction in a number of smooth muscle tissues. In this investigation, we evaluated the relationship between intracellular free Ca²⁺ concentration ([Ca²⁺](i)) and the extent of myosin light chain phosphorylation in cultured human myometrial smooth muscle cells. Treatment of myometrial cells with ionomycin caused a concentration- and time-dependent increase in [Ca²⁺](i) and phosphorylation of myosin light chain. Temporally, the increases in light chain phosphorylation and [Ca²⁺](i) in response to ionomycin were similar. In myometrial cells treated with ionomycin (10^-5 M) for 10 s, [Ca²⁺](i) increased from 138 to 800 nM; in these same cells, myosin light chain phosphorylation increased from 5% to a maximum value of 54%. Half-maximal phosphorylation of myosin light chain was attained at 300 nM [Ca²⁺](i). Treatment of myometrial smooth muscle cells with prostaglandin (PG) F(2α) (10^-8 M) and PGE₂ (10^-8 M) caused a proportionate increase in [Ca²⁺](i) and myosin light chain phosphorylation. In addition, [Ca²⁺](i) and myosin light chain phosphorylation increased in response to oxytocin and angiotensin II. These findings indicate that a number of uterotonic agents effect an increase in [Ca²⁺](i), which in turn causes phosphorylation of myosin light chain. Furthermore, the concentration of Ca²⁺ in the cytoplasm is a primary determinant for myosin light chain phosphorylation in human myometrial smooth muscle cells.