Prejunctional alpha 2-adrenoreceptors modulate the stimulated release of noradrenaline in isolated follicle strips from bovine ovaries

1. Strips from the follicle wall of bovine ovaries were incubated in Krebs-Ringer solution containing 3H-noradrenaline for measurement of transmitter liberation during electrical field stimulation (5 Hz frequency, 1 ms pulse duration, 10 V between the electrodes). The effects of noradrenaline as well as selective alpha-adrenoreceptor agonists and antagonists were studied on the electrically induced efflux of radioactivity. 2. Noradrenaline (1 microM) inhibited the stimulated release of radioactivity. The alpha 2-adrenoreceptor agonist, oxymetazoline, significantly reduced the release of radioactivity in concentrations as low as 0.01 microM. The alpha 1-adrenoreceptor agonist, phenylephrine (0.01-1 microM), was without significant effect. 3. Phentolamine (0.01-1 microM) and the selective alpha 2-adrenoreceptor antagonist, idazoxan (0.01-1 microM) significantly enhanced the electrically evoked release. The alpha 1-adrenoreceptor antagonist, prazosin (0.01-1 microM), was without effect. Idazoxan (0.1 microM) reversed the inhibitory effect of oxymetazoline (0.1 microM). 4. It is concluded that administration of noradrenaline or the alpha 2-adrenoreceptor agonists reduces the release of labelled noradrenaline by acting on prejunctional alpha 2-adrenoreceptors in the noradrenergic nerves distributed in the wall of the bovine ovarian follicle. This is one of several prejunctional receptor mechanisms that modulate the activity of the sympathetic nerves innervating the smooth musculature of the follicle wall.