Effect of Acute Administration of Prazosin on Blood Pressure, Heart Rate and Plasma Renin Level in the Conscious Normotensive Rat

This study investigated whether the specific alpha-antagonist, prazosin, stimulated basal plasma renin levels and heart rate. Furthermore the beta-adrenergic nervous system was also investigated to ascertain whether it was involved in this effect. Prazosin (0.1 or 1 mg/kg) was injected subcutaneously (s.c.) to conscious normotensive rats, either alone or in combination with the beta-adrenoceptor antagonist, DL-propranolol (1 or 3 mg/kg). Rats bore chronically implanted dorsal aorta cannula for measurement of blood pressure and heart rate and blood sampling for renin determinations. Acute administration of prazosin (1 mg/kg, s.c.) produced a fall in mean arterial pressure accompanied by renin release and tachycardia. A tenfold lower dose of prazosin did not alter blood pressure or heart rate but did stimulate renin release. Acute administration of DL-propranolol, (1 or 3 mg/kg, s.c.) produced falls in blood pressure and heart rate but did not affect plasma renin level. Combinations of prazosin with propranolol gave falls in blood pressure similar to those predicted on the basis of a simple addition of the effects of the two drugs given separately. Prazosin-induced tachycardia and renin release were attenuated by propranolol. It appears that prazosin produces renin release and tachycardia via stimulation of the beta-adrenergic adrenoceptor.