Plasma renin activity and metabolic clearance rate of angiotensin II in the unstressed aging rat.

We conducted studies in conscious chronically catheterized, trained young (3-5 months) and old (18-20 months) rats to assess the impact of aging on baseline renin activity (PRA) and metabolic clearance rate (MCR) of angiotensin II (ANG II). We observed that under unstressed conditions the baseline values of PRA and plasma ANG II were no different in young versus old rats (1.8 +/- 0.2 versus 1.5 +/- 0.2 ng Al/ml/h and 18 +/- 3 versus 15 +/- 2 fmol/ml, respectively). Values of PRA in the present study were similar to those reported by others for old rats, but our young rat values were lower than usually reported. This probably reflects our use of an unstressed preparation. We also observed a blunted increase in PRA in old rats in response to acute converting enzyme inhibition. Overall, our observations suggest that old rats may lose their ability to increase PRA in response to acute stimuli, including perhaps, the stress of blood drawing in emotionally or surgically stressed preparations. We also observed that the MCR of ANG II increased with age, despite similar baseline plasma ANG II concentrations in young and old. This suggests that with aging, an increase occurs in the rate of synthesis of ANG II. These results emphasize the importance of establishing true baseline values for indices of the renin-ANG II system in aging.