Endothelin increases cytoplasmic calcium and myosin phosphorylation in human myometrium

Endothelin, a recently discovered sarafotoxin-like peptide secreted by endothelial cells, is a potent stimulator of vascular smooth muscle contraction. We found that the action of endothelin is not restricted to the vasculature; we demonstrated that endothelin causes an increase in the concentration of intracellular Ca⁺⁺ and phosphorylation of the 20 kd light chain of myosin in human uterine smooth muscle cells in culture. In the absence of Ca⁺⁺ in the buffer medium of myometrial cells, the effects of endothelin on intracellular Ca⁺⁺ and myosin light chain phosphorylation are attenuated but not abolished. Endothelin also increases the frequency of contraction of human uterine smooth muscle (longitudinal and circular). The contractile effects of endothelin on myometrial strips are diminished in the presence of nifedipine. We conclude that (1) human myometrium is responsive to endothelin, (2) endothelin promotes contraction in myometrium by effecting an increase in intracellular Ca⁺⁺ and thus an increase in myosin light chain phosphorylation, and (3) endothelin acts in myometrium by stimulating Ca⁺⁺ influx as well as Ca⁺⁺ release from intracellular stores.