Relationships of doses of α- and β-adrenergic receptor blockers to arterial pressure, cardiac output, total peripheral resistance, and heart rate were observed in SHR (spontaneously hypertensive rat) and NCR (normotensive control rat) under pentobarbital anesthesia and positive pressure respiration. The decreases in arterial pressure and total peripheral resistance by phenoxybenzamine were greater in SHR than in NCR. By increasing dose of this drug, the differences in these parameters between SHR and NCR were gradually diminished and finally disappeared. Propranolol induced a similar equalization of heart rate between SHR and NCR. These findings are consistent with that the hypertensive state in SHR is maintained by an elevation of the sympathetic tone. Sympathetic vasoconstrictors seem to play a major role over cardiac sympathetics in the hypertension of SHR, since propranolol alone could not markedly diminish the arterial pressure in SHR.
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Numao et al. (1974) studied this question.