The cardiovascular effects of trimazosin

Trimazosin, a quinazoline related to the antihypertensive agent prazosin, was studied in anesthetized animals and isolated tissue preparations for effects related to cardiovascular activity. In cats, there was no evidence for ganglion-, adrenergic neurons-, or beta -adrenoceptor blockade, but the pressor effect of epinephrine was reversed. In dogs, the hypotensive effect to trimazosin was due to selective blockade of vascular alpha 1-adrenoceptors. Trimazosin competitively antagonized norepinephrine-induced contraction of rabbit aorta, and in rabbit pulmonary artery it selectively blocked postsynaptic alpha 1-adrenoceptors. In spinal-pithed dogs and rats trimazosin lowered blood pressure, in contrast to the lack of such activity reported for prazosin in pithed rats. It is concluded that trimazosin lowers blood pressure by selective blockade of alpha 1-adrenoceptors, and has, in addition, a hypotensive effect in pithed animals which is not due to alpha -adrenoceptor blockade.