Activation of myosin light chain phosphatase in intact arterial smooth muscle during nitric oxide-induced relaxation

We investigated whether myosin light chain phosphatase activity changes dur ing nitric oxide-induced relaxation of contracted intact carotid media and how changes in phosphatase activity mediate this relaxation. We also invest igated one mechanism for regulating this phosphatase. Myosin phosphatase ac tivity, myosin light chain phosphorylation, guanosine 3',5'-cyclic monophos phate (cGMP) concentration, and phosphorylation of the inhibitory protein C PI-17 were all assayed in homogenates of one carotid media ring at each tim e point during nitric oxide-induced relaxation. The application of sodium n itroprusside to histamine-contracted media caused rapid declines in light c hain phosphorylation and force. These were temporally correlated with a rap id elevation of cGMP and a large transient increase in myosin phosphatase a ctivity. During the early response to nitroprusside, when force declined, i ncreases in myosin phosphatase activity, concurrent with cGMP-mediated decr eases in calcium and myosin light chain kinase activity, could accelerate l ight chain dephosphorylation. CPI-17 was dephosphorylated upon application of nitroprusside at the same time that myosin phosphatase activity increase d, suggesting that the removal of inhibition by phospho-CPI-17 contributed to the increase in myosin phosphatase activity. After 20 min of nitroprussi de, myosin phosphatase activity had declined to basal levels, however low f orce was sustained. Additional light chain phosphorylation-independent mech anisms may be involved in sustaining the relaxation.