RELAXIN ALTERS RAT UTERINE MYOSIN LIGHT CHAIN PHOSPHORYLATION AND RELATED ENZYMATIC ACTIVITIES

Abstract

Uteri from estrogen-primed rats were suspended isometrically, contracted by 0.28 μ M prostaglandin F_2α, and exposed to 1 μg/ml purified porcine relaxin for 1 and 10 min. Relaxin treatment for 10 min, but not for 1 min, resulted in a significant decrease in the activity of myosin light chain kinase (MLCKase). Calcium-activated ATPase activity of a crude actomyosin fraction was also decreased by treatment with relaxin for 10 min. Relaxin treatment (10 min) also decreased the relative amount of phosphorylated rayosin 20,000 dalton light chains. These effects were consistent with the degree of inhibition of uterine contractions by relaxin. The data suggest that relaxin may inhibit uterine contractile activity by decreasing MLCKase activity and, in turn, myosin phosphorylation and ATPase activity.