Effects of catecholamines on circular and longitudinal uterine muscle of the rat.

Responses of circular and longitudinal myometrial strips to norepinephrine and isoproterenol were compared in pregnant and non-pregnant rats. Norepinephrine stimulated spontaneous contractions in circular muscle and inhibited contractions in longitudinal muscle from mid-pregnancy until term. The sensitivity of circular muscle to norepinephrine declined over this period and at term norepinephrine 10(-6) M inhibited contractions. Isoproterenol inhibited contractions in both circular and longitudinal muscles during pregnancy. The sensitivity of circular muscle to isoproterenol increases at term but did not change in the longitudinal muscle. The effects of norepinephrine in circular muscle were not duplicated by treatment of non-pregnant rats with ovarian steroids. The results imply a relative dominance of alpha-adrenoceptors in circular muscle and beta-receptors in longitudinal muscle during pregnancy. The reversal of the effects of norepinephrine at term in the circular muscle is related to an increase in sensitivity and/or number of beta-receptors in this muscle.