Abstract

Small, intravenous doses of tyramine, bretylium and guanethidine caused reproducible increases of rat blood pressure, which were significantly increased following the intravenous administration of reserpine. When rats were pretreated with reserpine on each of 2 days before use, the blood pressure responses following these drugs were very much reduced, but could be significantly increased in magnitude by a norepinephrine infusion. The increases of blood pressure caused by the 3 drugs were abolished by tolazoline or phenoxybenzamine, which did not, however, influence the action of pitressin on blood pressure.

In contrast, bretylium and guanethidine had depressor actions on rabbit blood pressure, both of which were converted to pressor effects by intravenous reserpine or, in the case of guanethidine, by an infusion of norepinephrine. Similar blood pressure responses were obtained using rabbits which had received reserpine for 2 days before use.

It is concluded that the pressor effects of bretylium and guanethidine in the rat are due, at least partly, to the liberation of catecholamine from some source, probably in the heart and/or blood vessels. In the rabbit also, this mechanism may be operative but is normally overshadowed by a depressor action of unknown origin.