Proceedings of the Society for Experimental Biology and Medicine

The interrelationships among plasma renin activity (PRA, ng AJ/ml plasma/hr), aldosterone concentration (ng%), and renal Na+-K+-ATPase activity (J.lmole P04/mg protein/ hr) were studied in 9 weanling normotensive spontaneously hypertensive rats (SHR), 9 adult hypertensive SHR, and 9 weanling and 9 adult normotensive Wistar-Kyoto rats (WKY). All groups were placed on a normal (0.4% sodium) diet. PRA and plasma aldosterone, measured in samples drawn from the ether-anesthetized rat, were higher in weanling SHR (15.2 ± 2.0, 37 ± 4.2) than in WKY. PRA measured in samples collected from a separate group of unanesthetized weanling SHR was also greater than in age-matched WKY. In adult SHR, PRA (6.1 ± 0.9) and plasma aldosterone (20.0 ± 2.7) were decreased. During the weanling period Na+-K+-ATPase activity in SHR was not only greater than in age-matched WKY but was also increased compared to adult nonnotensive and hypertensive rats (137 ± 9 weanling SHR, 89 ± 7 weanling WKY, 73 ± II adult SHR, 84 ± 17 adult WKY). Thus, during the weanling period the renin-angiotensin-aldosterone (R-A-A) system and renal Na+-K+-ATPase activity are activated in SHR. The elevation of Na+-K+-ATPase activity may be due to increased aldosterone levels. It was noted, however, that plasma aldosterone was similar in adult WKY and weanling SHR, while Na+-K+-ATPase activity was higher in SHR. These findings involving R-A-A and renal Na+-K+-ATPase activity prior to the elevation of blood pressure suggest that the kidneys may playa role in the initiation of hypertension in SHR. © 1984 Socicty for ExpcrimcnlOl Biology and Medicine. Longitudinal studies of the renin-angio­ tensin system in spontaneously hypertensive rats (SHR) have revealed elevated plasma renin activity (PRA) in the weanling rat (1). PRA decreases gradually with increasing age and is lower in mature SHR than in age­ matched Wistar-Kyoto rats (WKY) (1-3). Activation of the renin-angiotensin system in weanling SHR may result in transient elevation of aldosterone and other mineralo­ corticoids. Although conflicting results have been reported (4-7), high plasma aldosterone concentration has been described in SHR from 5 to 6 weeks of age (4). In addition, others have found that the level of this steroid hormone appears to decline with in­ creasing age (5) although in these studies plasma aldosterone levels were only slightly and not significantly increased in weanling SHR. Increased activity of adrenocortical steroids during the initial stage of the hyperI To whom reprint requests should be addressed: Chief, Hypertension Section Veterans Administration Hospital, G.P.O. Box 4867 San Juan, Puerto Rico 00936.