The influence of repeated administration of prazosin on its hypotensive effect and on renin release in conscious dogs. A comparison with urapidil.

The effects of prazosin (0.1, 0.5 and 2.5 mg/kg, orally) on blood pressure, heart rate and PRA were investigated after acute and chronic administration in conscious, normotensive dogs. The acute administration of prazosin caused a reduction in both systolic and diastolic blood pressure and an increase in heart rate and PRA. After chronic administration, twice daily for four days, tolerance developed, which was not due to persistent counterregulation. On comparison with urapidil, another alpha-adrenoceptor antagonist, the hypotensive and counterregulatory response to acute administration were similar with both substances; the development of tolerance to prazosin after chronic administration contrasted with the results with urapidil.