PLASMA-RENIN ACTIVITY AND METABOLIC-CLEARANCE RATE OF ANGIOTENSIN-II IN THE UNSTRESSED AGING FAT

We conducted studies in conscious chronically catheterized, trained yo ung (3-5 months) and old (18-20 months) rats to assess the impact of a ging on baseline plasma renin activity (PRA) and metabolic clearance r ate (MCR) of angiotensin II (ANG II). We observed that under unstresse d conditions the baseline values of PRA and plasma ANG II were no diff erent in young versus old rats (1.8 +/- 0.2 versus 1.5 +/- 0.2ng 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 ra ts, but our young rat values were lower than usually reported. This pr obably 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 ma y lose their ability to increase PRA in response to acute stimuli, inc luding perhaps, the stress of blood drawing in emotionally or surgical ly stressed preparations. We also observed that the MCR of ANG II incr eased 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 importan ce of establishing true baseline values for the indices of the renin-A NG II system in aging. (C) 1997 Elsevier Science Ireland Ltd.