Gestational Changes in L-Arginine--Induced Relaxation of Pregnant Rat and Human Myometrial Smooth Muscle

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OBJECTIVEWe intended to demonstrate the presence of an L-arginine--nitric oxide system in human myometrium and to clarify the mechanisms of action of nitric oxide on rat myometrium during gestation.STUDY DESIGNBy examining very small myometrial muscle strips (approximately 750 muscle cells), characteristic features of contraction of rat longitudinal muscle at the midstage of gestation (day 16) and during delivery at term were determined.RESULTSSpontaneous contractions were significantly different during delivery compared with the midstage of gestation of rat myometrium. L-Arginine relaxed spontaneous and carbachol-induced, but not potassium chloride--evoked, contractions at both stages. However, much higher concentrations of L-arginine were required during delivery, 8-Bromo-cyclic guanosine monophosphate inhibited spontaneous contractions from concentrations of 1 nmol/L in the midstage of gestation and from 0.1 mmol/L during delivery. In human myometrial tissues L-arginine also inhibited contractions during the late stages of gestation.CONCLUSION(1) The experimental model is sufficient to compare properties of longitudinal myometrial strips during gestation. (2) In rat and human myometrium an L-arginine--nitric oxide system has an important role in inhibiting uterine contractility and possibly maintaining pregnancy. (3) The relaxing effect of the nitric oxide system is largely because of the voltage-independent action of cyclic guanosine monophosphate systems. (AM J OBSTET GYNECOL 1993;169:1327-37.)

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