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We have studied the contribution of neurohumoral and structural factors to pressor responsiveness and peripheral resistance in mild/moderate hypertension. Pressor responses to intravenous infusions of phenylephrine (an alpha1 agonist) and angiotensin II were studied in groups of patients with essential hypertension before and after treatment, for 6 weeks with either nifedipine (20 mg bid), enalapril (20 mg daily) or doxazosin (2 mg daily). All drugs lowered blood pressure to a similar extent. Pressor responsiveness to both phenylephrine and angiotensin II showed wide intersubject variation when expressed as the dose of agonist required to raise mean arterial pressure by 20 mmHg (PD20). A group of age-matched normotensive controls showed a similar PD20 for phenylephrine to hypertensives. Angiotensin 11 sensitivity was greater in hypertensives. Drug treatment had different effects in hypertensive patients. Doxazosin, an alpha blocker, reduced the responsiveness to phenylephrine but had no effect on responses to angiotensin II. Nifedipine attenuated responses to both agonists while treatment with enalapril increased responsiveness to both phenylephrine and angiotensin II. We have not found evidence of systematic differences in alpha 1 receptor responses in hypertensives and different "vasodilator" drugs can lower blood pressure with widely different effects on adrenergic and non-adrenergic vascular responses.
Reid et al. (1989) studied this question.