Three parameters of renin angiotensin activity were studied in rats with spontaneous hypertension. The juxtaglomerular index of these animals in the prehypertensive, early hypertensive, and chronic stage of the high blood pressure was significantly lower than in normal rats. Bioassays of renal vein blood for a pressor agent were consistently negative in hypertensive rats 6 weeks to 12 months of age, including tests performed in animals 6–8 weeks old when the blood pressure was rising to hypertensive levels. The renal vein plasma renin activity was within normal limits except for rats with longstanding high blood pressure which showed a significant reduction in activity. The findings indicate that neither the development nor maintenance of the high blood pressure in animals with spontaneous hypertension is due to increased activity of the renin-angiotensin system.
Koletsky et al. (1970) studied this question.