ACUTE ACTIONS OF 2‐(2,6‐DICHLOROPHENYLAMINO)‐ 2‐IMIDAZOLINE HYDROCHLORIDE (ST 155) IN THE RAT

Abstract

SummaryDoses of 2‐(2,6‐dichlorophenylamino)‐2‐imidazoline hydrochloride (ST 155) administered to rats under pentobarbitone anaesthesia produced graded reductions in blood pressure and heart rate which were preceded by a small pressor response. The pressor action of ST 155 is attributed to direct stimulation of of‐receptors, since it is affected in a similar manner to noradrenaline by small doses of an α‐blocking agent and is unaffected by pre‐treatment of the animals with reserpine or guanethidine. The depressor response and brudycardia, which were not seen in pithed or ganglion‐blocked animals, could not be attributed to depression of vascular smooth muscle, α‐bloekade, adrenergic neurone blockade or ganglion blockade. Involvement of the cholinergic system or the chemo‐ and baro‐ receptors was excluded, since the hypotensive response and slowing of the heart were still seen in atropinised and/or vagotomised animals and in animals in which the nerves to the carotid and aortic chemo‐ and baro‐ receptors had been sectioned. Since administration of ST 155 in such a way as to induce a higher concentration in the central nervous system than would be encountered after administration into the general circulation potentiated the actions, it is suggested that ST 155 has a predominantly central action, mediated by a reduction in sympathetic nervous activity.