Preferential blockade of presynaptic alpha-adrenoceptors by yohimbine.

Strips of the rabbit main pulmonary artery were preincubated with 3H-noradrenaline. 3 X 10(-8) -- 10(6) M yohimbine enhanced the overflow of tritium and the smooth muscle contraction induced by transmural sympathetic nerve stimulation. The increase of the stimulation-evoked overflow of tritium was prevented by a high concentration of oxymetazoline. The results indicate that yohimbine is more potent in blocking the presynaptic than the postsynaptic alpha-adrenoceptors of the artery.