The research on intracellular signal transduction and the smooth muscle contraction have made much progress since 1970. Effects of relaxin, a peptide hormone mainly produced and released from ovary, are reviewed in the above point of view. Cyclic AMP content in myometrial cells is increased by relaxin. One site of relaxin action can be the suppression of myosin light chain kinase activity. Another effect of relaxin is the suppression of action potential, whereby the Ca-influx is reduced. However, relaxin up to 300 mU does not hyperpolarize the myometrial membrane of estrogen-primed rat, which differs from the adrenergic β-action. Unrealed signal transduction seems involved in the relaxin action.
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