Effect of Propranolol on Blood Pressure and Plasma Renin Activity in the Spontaneously Hypertensive Rat

Spontaneously hypertensive VVistar rats have normal basal levels of plasma renin activity but do not respond to sodium depletion or the stress of ether anesthesia plus laparotomy. Both age and rat strain influence the development of this hyporesponsive renin system: normal Wistar and spontaneously hypertensive rats but not Sprague-Dawley rats develop the hyporesponsive system as they age. Therefore, these rat strains may serve as useful models for studying the effects of age and hypertension on plasma renin activity. In the present study, propranolol administration up to 18 mg/kg day" for 15 days did not lower plasma renin activity but did cause a paradoxical rise in blood pressure in spontaneously hypertensive rats. This paradoxical rise was prevented by placing the rats on a low-sodium diet prior to propranolol administration. The mechanism of the paradoxical rise is unknown, but we suggest that it could be the result of increased alpha-adrenergic activity in response to a fall in cardiac output.