Pressor Effect of Beta Blocking Agents in Rats

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Abstract
In rats anesthetized with urethane, intravenous injection of oxprenolol, propranolol, and d-propranolol increases arterial pressure, while practolol is ineffective. These changes of arterial pressure occur in rats previously treated with phentolamine or phenoxybenzamine, indicating that the pharmacological block of alpha receptors does not prevent vasoconstriction by oxprenolol and propranolol. On the contrary, beta receptor blocking agents have little effect on the arterial pressure of rats treated with a ganglion blocker (pentolinium).The pressor action of adrenaline and angiotensin is significantly depressed by alpha blockers, but small doses of oxprenolol restore completely the action of angiotensin and, in part, that of adrenaline. Adrenaline produces a biphasic effect on the blood pressure of anesthetized rats. Both phases of the effect are eliminated by alpha blockers while ganglion blocker abolishes the hypotensive phase only. Administration of small doses of oxprenolol and propranolol (1–10 μg/kg) in untreated rats or in animals treated with alpha blockers increases significantly the blood pressure, but does not modify the biphasic effect of adrenaline.The results indicate that the pressor effect of small doses of beta blocker may not be entirely related to the elimination of the beta inhibitory effect of exogenous adrenaline on the vascular system. No changes of blood pressure occur when beta blockers are administered to animals treated with ganglion blockers, suggesting that the vasoconstriction produced by propranolol and oxprenolol is not due to a direct effect on the vascular smooth muscles.

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