The reactivity of the isolated rat uterus to isoprenaline, adrenaline, noradrenaline and phenylephrine was investigated during the different phases of the oestrous cycle, after oestradiol administration, during pregnancy, and after parturition.
There was no significant difference between the reactivity to any one of the four sympathomimetic agents during the different phases of the oestrous cycle. Oestradiol administration significantly decreased the sensitivity of the uterus to noradrenaline and phenylephrine when compared with natural oestrus. The decrease in the case of adrenaline was not significant, and reactivity to isoprenaline did not change following oestrogen administration.
Reactivity to the four agents decreased markedly during pregnancy and returned to normal between the 4th and the 7th day after parturition. The decrease in the reactivity of the uterus to the inhibitory action of some sympathomimetic agents is oestrogen-linked, and most probably due to the appearance of α-adrenergic excitatory receptors. Other factors which may have contributed to changes in reactivity to such agents are discussed.