Different Responses to β-Adrenoceptor Blocking Drugs of the Blood Pressure and Heart Rate in the Urethane-Anesthetized Dog and Rat

Abstract

I.v. propranolol (Prop) produced sustained pressor responses in rats but not in dogs under urethane anesthesia. In the dogs there was a progressive reduction in systolic blood pressure (SBP) in accordance with a significant heart rate (HR) reduction. Even at the i.v. dose (5 mg/kg) where vasoconstrictor response to i.v. norepinephrine (NE) is changed to the vasodilator one in rats, phenoxybenzamine could not completely suppress the NE-induced vasoconstriction in dogs. In pithed dogs, unlike pithed rats, i.v. Prop augmented neither sympathetic nerve stimulation- nor i.v. epinephrine (Epi)-induced pressor responses. Urethane anesthesia brought much higher concentrations in plasma Epi and NE in the dog than in the rat. In comparison with their respective values under the conscious state, SBP and HR of the dog were higher and those of the rat were lower under urethane anesthesia. As in urethane-anesthetized dogs, i.v. Prop elicited remarkable HR reduction accompanied by progressive hypotension in coronary-ligated rats with high sympathetic activity. Thus the reason for the absence of pressor response to β-receptor blockage in the dog may be due to less functional significance of β2-adrenoceptor-mediated vasodilation in determining the peripheral vascular tone; and also, the possibility that there may be the involvement of an inverse influence of urethane on the cardiovascular regulatory system in terms of enhancing sympathetic nervous activity in the dog and a decreasing one in the rat can not be ruled out.