Action of propranolol on arterial pressure and on cardiovascular reflexes

In the rabbit, administration of 500 mug (+/-) propranolol into the lateral cerebral ventricle resulted in rapid leakage of drug into the blood. The drug produced a small reduction in blood pressure at 2 and 4 hours after injection but because of the rapid leakage, this could have been due to its peripheral actions. To investigate possible central mechanisms of action which were independent of peripheral effects, we tested the action of intravenously administered propranolol on the reflex rise in TPR evoked in rabbits by graded Valsalva-like manoeuvres. This was performed in unanaesthetised rabbits with Doppler flowmeters for measuring cardiac output. Graded expiratory pressures (EP) were applied to the inlet and outlet tubes of the tracheotomised animal's respiratory valve and to the cuff around their thorax and abdomen. Administration of propranolol at two levels (168 +/- 35 ng/ml, and 240 +/- 33 ng/ml) for 1 hour had no effect on slope and threshold of the EP-TPR relationship. By contrast to the minimal effects on the reflex with propranolol, the EP-related rise in TPR was attenuated by clonidine. We conclude that propranolol does not lower blood pressure in the rabbit by attenuating sympathetic constrictor activity.