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The effects of catecholamines on the circular muscle of myometria in pregnant rats at term (21st and 22nd days) were investigated by recording electrical and mechanical responses. Slow potentials were found to be the dominant activity in the morning on the 21st day of pregnancy, and spike potentials were manifested on the 22nd day. The alpha-excitation of catecholamines in the circular muscle was represented by mechanical potentiation, prolongation of the slow potential and depolarization of the membrane. In contrast, the beta-inhibition was mechanical inhibition, depression of the slow potential and hyperpolarization. Noradrenaline at a concentration of 6 X 10(-6M caused excitatory action in the circular muscle on the 21st day of pregnancy, while the effect became inhibitory on the 22nd day of pregnancy. Results obtained by the use of adrenergic agonists and antagonists led to the conclusion that the reversal of the effect of noradrenaline could be ascribed largely to the enhancement of the beta-action, the mechanism of which was brought about probably through the endogenous change in the steroid hormone secretion at the very end of pregnancy.

Effects of Catecholamines on the Circular Muscle of Rat Myometria at Term during Pregnancy