Estrogen inhibition of noradrenaline release in the rabbit oviduct

In the present study, we investigated the influence of estrogen on 3H-noradrenaline (3H-NA) release induced in the oviductal isthmus by electrical stimulation, potassium and calcium. The fractional release of 3H-NA was measured in oviducts isolated from ovariectomized rabbits and from ovariectomized rabbits treated with estradiol cypionate, 70 micrograms/kg im 72 h before an experiment. Electrical field stimulation of the intramural nerves induced muscle contraction and augmented the release of labelled NA from the muscle. The 3H-NA release was reduced after estrogen treatment when reuptake of NA into the nerve terminals was blocked by desipramine, 10(-6) M. Estrogen also reduced the 3H-NA release evoked by exposure of the oviducts to 121 mM KC1 in the presence of calcium (2.5 mM) and in a high potassium, calcium-free medium upon the addition of 2.5 mM calcium. In the presence of desipramine a small fraction of 3H-NA was released in high potassium, calcium-free medium. This release was unaffected by estrogen. These results suggest that estrogen reduces the release of NA from the adrenergic nerves within the oviduct and that this action is exerted primarily on the calcium-dependent release. It therefore might be due to a reduction in the entry of calcium into the nerve terminal.