The Effects of Sympatholytics, Angiotensin and Vasopressin on the Cardiovascular Response to Minoxidil or Hydralazine in Conscious Dogs

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Abstract
Oral administration of minoxidil, 1 mg/kg, or hydralazine, 12 mg/kg, decreased arterial blood pressure and peripheral vascular resistance, while cardiac output and rate increased in conscious dogs. These responses were not significantly altered by treatment with propranolol or phenoxybenzamine. Angiotensin infusions sufficient to increase arterial blood pressure to levels observed before treatment with the vasodilators did not obtund the heightened cardiac activity. In contrast, vasopressin infusions sufficient to normalize blood pressure markedly attenuated the increased cardiac output and rate associated with minoxidil or bydralazine. The results suggest that the increased cardiac output and tachycardia associated with administration of minoxidil or hydralazine are not dependent upon sympathetic nervous reflexes or cardiac afterload. The compensatory bradycardia normally associated with blood pressure increases appeared to be inhibited in the presence of minoxidil or hydralazine. In these experiments, vasopressin was a functional antagonist to the cardiovascular response associated with the administration of minoxidil or hydralazine.