Renin Release Is Blunted in Old Rats

Plasma renin activity (PRA) was similar in young versus old male Sprague Dawley rats under unstressed condi- tions (1 .3 ± 0.2 versus I.8 ± 0.3 ng angiotensin Llml per mm). Airjet stress increases PRA in young but not old rats (I 3.9 ± 3.8 versus 2.9 ± 0.8 ng angiotensin I/mb per mm), respectively. This response is ablated in young rats by �-adrenoceptor blockade, suggesting that the increased PRA is mediated by /3-adrenoceptors, and this response was blunted in old rats. It is generally accepted that plasma renin activity (PRA) de- dines with advancing age in several species, including humans and rats (1-5). However, we recently showed that in the conscious chronically catheterized rat, basal PRA was similar in young adult (3 to 5 mo old) and old (18 to 20 mo old) rats (6). Although values in old rats were similar to those published previously, PRA in young rats was lower in our conscious, unstressed preparation versus others (1-5,7). Elevated values of PRA indicate volume depletion and/on anesthetic, surgical, or emotional stress. It is possible that the higher values of PRA reported in young rats reflect stimulation due to one or more stressors. If so, the blunting or absence of a similar stress response in old animals could explain the apparent reduction in PRA with aging. To investigate this possibility, the present study was conducted in young and old, conscious, chronically catheterized trained rats to determine whether stress-induced, f3-adrenoceptor-dependent nenin release diminishes with ad- vancing age.