Converting enzyme inhibitor-induced changes of plasma angiotensinogen concentration in the rat

Chronic treatment with converting enzyme inhibitors induces a fall of plasma angiotensinogen concentration, which is thought to result from increased consumption by renin. As this explanation cannot account for all the observations, we reexamined this effect. Rats received captopril (50 mg/kg per day), enalapril (10 mg/kg per day) or Hoe 498 (1 mg/kg per day) for 7 days. Plasma angiotensinogen (by indirect assay) fell to 34, 37 and 43% of its initial values, respectively. Total immunoreactive angiotensinogen (by direct radioimmunoassay, which also measures des-AI-angiotensinogen) was 76, 70 and 95% of the initial values, respectively. This suggests that a major part of the fall of intact angiotensinogen can be attributed to increased cleavage by renin, but an additional factor is likely to be involved. Experiments with isolated hepatocytes indicated that converting enzyme inhibitors in high concentrations may have a direct effect on angiotensinogen secretion. Whether this can account for the fall in total immunoreactive angiotensinogen in vivo remain unclear.