Abstract
Propranolol produces a sustained rise in blood pressure in the rat, but little change in the guinea pig. In order to elucidate this distinct difference, the following experiments were carried out: (a) The pressor effect of propranolol was studied while the vascular tone was changed by infusion of several vasoactive substances in the guinea pig; (b) The effect of propranolol on the isolated atria of rats and guinea pigs was also compared. Propranolol always produced a pressor action during the infusion of adrenergic β-stimulating agents, but not during vasopressin infusion. In spontaneously beating or electrically driven guinea pig atria, propranolol reduced the contractile force to a greater extent than in rat atria. It is concluded that the difference in action of propranolol on the blood pressure of rat and guinea pig may be explained by the following two reasons: (a) the β-adrenoceptive vasodilator tone in skeletal muscle is stronger in the rat than in the guinea pig; (b) the heart of the rat is less sensitive to propranolol than that of the guinea pig.
