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Relaxin Treatment Alters the Kinetic Properties of Myosin Light Chain Kinase Activity in Rat Myometrial Cells in Culture^*

C. J. HSU and B. M. SANBORN

Department of Obstetrics, Gynecology and Reproductive Sciences, University of Texas Medical School and Graduate School Biomedical Sciences Houston, Texas 77030
Department of Biochemistry and Molecular Biology, University of Texas Medical School and Graduate School Biomedical Sciences Houston, Texas 77030

Address all correspondence and reprint requests to: Dr. B. Sanborn, Department of Biochemistry and Molecular Biology, University of Texas Medical School, P.O. Box 20708, Houston, Texas 77225.

Abstract

Relaxin treatment altered the kinetic properties of rat myometrial cell myosin light chain kinase (MLCK) by increasing the K₅₀ of the enzyme for calmodulin (CaM) from 1.1 ± 0.1 to 38 ± 14 nM. When MLCK was assayed in the presence of 7 nM CaM to maximize the effect of the decreased affinity for CaM between control and relaxin-treated groups, a rapid concentration-dependent effect of the hormone was observed. Relaxin decreased MLCK activity significantly within 1 min. The ED₅₀ of the effect was 0.4 µml. In addition to its effect on Ca²⁺-CaM-dependent activity, relaxin also decreased Ca²⁺-CaM-independent MLCK activity. This decrease was not attributable to a decrease in the affinity of the enzyme for myosin.

There was a temporal association between the effects of relaxin on mean cell length, elevation of cAMP levels in the presence of 0.4 µM forskolin previously shown in other studies, and the alteration of MLCK activity. All three parameters were changed significantly within 1 min after exposure to relaxin. The ED₅₀ of relaxin for cell shape changes, cAMP elevation, and effects on MLCK activity were all approximately 0.4 µg/ml. Relaxin may act in part by a cAMP-mediated phosphorylation of MLCK, thereby decreasing its affinity for CaM. The effect on MLCK may be linked to a decrease in the phosphorylation of myosin light chains and the promotion of uterine relaxation. (Endocrinology 118: 499–505, 1986)

Footnotes

^* This work was supported in part by NIH Grant HD-09618 (to B.M.S.).

This work was performed by C. J. Hsu in partial fulfillment of the requirements for the Ph.D. degree of the Program in Reproductive Biology and Endocrinology, Graduate School of Biomedical Sciences, University of Texas Health Science Center. Present address: Department of Medicine, Division of Endocrinology, Pennsylvania State University, Milton S. Hershey Medical Center, Hershey, Pennsylvania 17033.

Received February 15, 1985.

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