The effect of captopril on autonomic reflexes in human hypertension.

The effects of acute administration of captopril on heart rate, blood pressure and physiological responses to head-up tilt, hand-grip, exercise and Valsalva manoeuvre were studied in 25 hypertensive subjects (10 untreated, 15 previously treated). After chronic administration for 8-12 weeks the effect on heart rate, blood pressure and the response to head-up tilt were studied again in 15 of these subjects. After the first dose of captopril (0.5 mg/kg), both systolic and diastolic blood pressures fell significantly, with no change in heart rate. Head-up tilting produced a significant increase in heart rate without any alteration in the blood pressure; hand-grip produced a significant increase in heart rate and blood pressure. These responses and exercise-induced tachycardia were unaltered by captopril. There was a significant reduction in the Valsalva ratio. After chronic administration the hypotensive efficacy was maintained for 10-12 h after the last dose, suggesting that twice daily dosage may be sufficient. There was a significant reduction in the basal supine heart rate by captopril. Head-up tilt produced the same physiological changes as in the acute study. These findings suggest that captopril has a vagotonic effect, possibly associated with resetting of baroceptor mechanisms. This could be one explanation for the hypotensive effect of captopril even in low-renin states.