Effects of estradiol on norepinephrine-induced contraction, alpha adrenoceptor number and norepinephrine content in the female rabbit urethra.

Several studies have revealed that estrogen treatment increases the reactivity to catecholamines of various tissues. In the present study it was found that estrogen treatment caused an increased sensitivity to norepinephrine (NE) of the isolated female rabbit urethra. There was a 3-fold shift to the left of the concentration-response curve for longitudinal tension in perfused urethras and for tension in urethral ring preparations. Possible mechanisms for this increase in sensitivity were investigated by studying radioligand binding to the alpha adrenoceptors and by measuring the NE content of the urethras. Using [3H]dihydro-alpha-ergocryptine as a marker for alpha adrenoceptors, a more than 2-fold increase in the receptor number was found after estrogen treatment. This is suggested to be attributable to a selective increase in the number of alpha-2 adrenoceptors. No significant change was found in the affinity of [3H]dihydro-alpha-ergocryptine to the receptor. The total NE content in the urethra was not changed by estrogen treatment. However, if calculated per milligram of wet weight, the NE content was reduced to half in estrogen treated animals. It is suggested that in the rabbit urethra the estrogen-induced increased sensitivity to the contractant effects of NE, at least in part, is attributable to an increase in the number of postjunctional alpha-2 adrenoceptors.