Adrenergic sensitivity of circular and longitudinal muscles of pregnant rat myometrium

The effect of the adrenergic agonists noradrenaline (NA), isoproterenol (ISO) and salbutamol (SA) on spontaneous contractions of the circular and longitudinal muscles of rat uterus has been investigated in vitro in the middle and at the end of pregnancy. NA and ISO inhibited the contractions of longitudinal muscle during both stages of pregnancy. NA exerted a biphasic effect on circular muscle contractions (excitatory at lower concentrations and inhibitory at higher concentrations). Both the excitatory and the inhibitory effect of NA on the circular muscle contractions were more pronounced at the end of pregnancy. NA and ISO failed to inhibit completely the circular muscle contractions. SA inhibited the contractions of both muscles and the effect was more pronounced on the longitudinal muscle at the end of pregnancy. Analysis of the effect of the adrenergic antagonists indicated the presence of alpha- and beta-adrenoceptors in both uterine muscles. The results suggest that the difference of the contractile response to NA of pregnant uterine muscles may be related to a difference in number or sensitivity of their beta-adrenoceptors.