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Isometrically suspended uteri from estrogen-primed rats were stimulated with prostaglandin F2 alpha and then exposed to relaxin. Relaxin-dependent decreases in the ratio of phosphorylated to total myosin light chains (MLC) and in MLC kinase activity, measured in the presence of 0.5 mg/ml of uterine myosin and the absence and presence of Ca2+-calmodulin (CaM), were observed. The time-course and concentration-response of these biochemical effects of relaxin paralleled the hormone-induced inhibition of uterine contractile activity. Relaxin treatment resulted in a change in the requirements of MLC kinase for Ca2+, CaM, and myosin. Titrations of MLC kinase activity showed a shift in K50 values for Ca2+ from 82 to 260 nM and for CaM from 2.2 to 25 nM in extracts from control and relaxin-treated tissues, respectively. The myosin Km values of MLC kinase from control and relaxin-treated tissues were 0.33 and 0.71 mg/ml, respectively. Under optimal assay conditions (100 microM Ca2+, 1 microM CaM, and 1.2 mg/ml of myosin) the activities of MLC kinase in both extracts were identical, regardless of hormone concentration or exposure time. These data suggest that relaxin-treatment results in a change in the affinity of MLC kinase for its substrate and modulator and that relaxin inhibits uterine contractile activity by a mechanism which involves a decrease in MLC kinase activity and, in turn, a decrease in phosphorylation of the 20,000-dalton light chains of myosin.
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