Tissue converting enzyme activity is low in the kidney of spontaneously hypertensive rats

Angiotensin-converting enzyme (ACE) activity was measured in the plasma, the kidney, and other organs of 5-, 8-, and 11-week-old spontaneously hypertensive male rats (SHR) of the Okamoto-Aoki strain and compared with that of age-matched Wistar-Kyoto (WKY, bred by Iffa-Credo) or normotensive Wistar rats. ACE activity was measured spectrofluorometrically, using the artificial substrate N-CBZ-L-phe-L-his-L-leu. ACE activity was constantly lower in the plasma and renal cortex of SHR from 5 weeks on than in WKY rats. This difference in the renal cortex ACE activity persisted after 1 h of open circuit perfusion of the isolated kidney with Krebs-Henseleit medium. On the other hand, there lung, the brain occipital cortex, or the abdominal aorta of hypertensive or normotensive rats. The percentage inhibition of ACE activity provoked by 7 days of oral administration of ramipril (Hoe 498, 1 mg/kg/day) was analogous in the kidneys and lungs of WKY rats and SHR. Enalapril (MK 421, 30 mg/kg/day) was equipotent to ramipril in the kidney but had lower inhibitory effects on the pulmonary ACE activities of WKY rats and SHR.