Adrenoceptors and cholinoceptors controlling noradrenaline release from adrenergic nerves in the urethra of rabbit and man.

The possible influence of adreno- and cholinoceptors on noradrenaline release from adrenergic nerve endings in urethral smooth muscle of rabbit and man was investigated in preparations preincubated with 3H-noradrenaline. The preparations were continuously perfused in a temperature controlled ( 37C ) chamber and the amount of electrically induced efflux of 3H calculated as a fraction of the total amount of 3H in the preparation at the time of stimulation. In preparations from both rabbit and man, the fractional release of 3H induced by electrical field stimulation was reproducible, and significantly increased in the presence of the alpha 2-receptor antagonist rauwolscine and the muscarinic cholinoceptor antagonists scopolamine and atropine. There was a significant decrease of 3H-release in the presence of the alpha 2-receptor agonist clonidine and the cholinoceptor agonist carbachol. No evidence for the presence of prejunctionally located alpha 1 or nicotinic receptors was found. The results suggest the possibilities of 1) a negative feed-back regulation of the release of noradrenaline from adrenergic nerve endings in the urethra, mediated via prejunctionally located alpha 2-adrenoceptors, 2) an interaction at the nerve terminal level between adrenergic and cholinergic nerves mediated via muscarinic cholinoceptors on the adrenergic nerve terminals.