Presynaptic activity of the imidazolidine derivative ST 587, a highly selective alpha 1-adrenoceptor agonist

St 587 increased the blood pressure of pithed rats. When the antagonists prazosin (alpha 1) and rauwolscine (alpha 2) were used the drug was shown to be a more selective alpha 1-adrenoceptor agonist than methoxamine. St 587 inhibited the tachycardia elicited by sympathetic stimulation in pithed rats. This effect was presynaptic and due to stimulation of alpha 1-adrenoceptors, as revealed by the different antagonism exerted by prazosin and yohimbine (alpha 2), respectively. The existence of presynaptic alpha 1-adrenoceptors is discussed.