Rabbit myometrial adrenergic sensitivity is increased by estrogen but is independent of changes in alpha adrenoceptor concentration.

The ability of estrogen to increase the alpha-1 adrenergic sensitivity of rabbit uterine smooth muscle was compared with its effects on the concentration of alpha adrenoceptor subtypes in myometrium. Compared to ovariectomized controls, estrogen treatment was found to increase the adrenergic contractile sensitivity of rabbit uterus determined in vitro. Estrogen treatment increased uterine alpha-2 adrenoceptor concentration nearly 5-fold. Mature rabbits (endogenous estrogen) had the same uterine sensitivity and alpha-2 adrenoceptor concentration as estrogen-treated rabbits. Alpha-1 receptor concentration was increased only in the estrogen-treated group, and was accompanied by increased maximal contractile response. Thus, the increase in adrenergic sensitivity after estrogen was correlated closely with an increased concentration of alpha-2 adrenoceptors. However, enhanced adrenergic sensitivity persisted after alpha-1 and alpha-2 receptor concentration returned to pre-estrogen levels in rabbits pretreated with estrogen before ovariectomy. Studies utilizing subtype-selective competitive antagonists verified that contractile response is mediated primarily by alpha-1 adrenoceptors, with no apparent influence of alpha-2 adrenoceptors. Finally, we found that adrenergic sensitivity is altered by temperature and divalent cation concentration, but these effects do not prevent the expression of the regulatory action of estradiol. We conclude that estrogen increases the alpha-1 adrenergic sensitivity of rabbit uterus without changes in alpha-1 receptors, and thus may act on postreceptor response events.(ABSTRACT TRUNCATED AT 250 WORDS)