Age-related decrease in serum angiotensin converting enzyme activity: the role of thyroidal status and food intake.

The serum ACE activity was determined in male Fischer 344 rats at 2, 6, 13, and 25 months of age to determine whether serum angiotensin converting enzyme (ACE) activity is a potential biomarker of tissue hypothyroidism in aged rats. Since rodent serum contains an ACE activity inhibitor, the measurements were done in both undiluted and 1:8 diluted sera. The highest serum inhibitor activity was found in the 2-month-old animals. The serum ACE activity measured in the diluted serum of the aged rats (77.6 +/- 2.9 units/ml) was significantly reduced compared to 2-month-old (178.2 +/- 6.4 units/ml), 6-month-old (101.5 +/- 6.1 units/ml) and 13-month-old rats (84.9 +/- 8.6 units/ml); (p less than 0.001). Hyperthyroidism induced by injecting L-triiodothyronine (T3) 15 micrograms/100 gm body weight intraperitoneally for 10 days increased the serum ACE activity in the older rats, but reduced the levels in 2-month-old rats. There was no significant change in 6-month-old rats. The levels of serum ACE activity in hypothyroid 6-month-old rats (95.5 +/- 3.5 units/ml) and in 2-month-old-rats (94.2 +/- 4.0 units/ml) were similar to the level seen in hypothyroid old rats (88.9 +/- 5.8 units/ml). Pair feeding of young rats (8 months old) with old did not alter the baseline ACE level (117.4 +/- 3.7 units/ml) or the T3 stimulated (105.2 +/- 10.2 units/ml) serum ACE activity. It is concluded that the reduced serum ACE activity in aged rats cannot be accounted for by the reduced caloric intake or reduced serum thyroid hormone levels.(ABSTRACT TRUNCATED AT 250 WORDS)