Influence of converting-enzyme inhibition on rat des-angiotensin I-angiotensinogen

The effects of high plasma renin levels on plasma levels of both total immunoreactive angiotensinogen (direct radioimmunoassay) and intact angiotensinogen measured by angiotensin I released by renin (indirect assay) were studied in sodium-depleted rats both with and without captopril treatment and in adrenalectomized rats. The direct assay measures both intact angiotensinogen and des-angiotensin I-angiotensinogen, its residue cleaved by renin. The indirect assay measures only intact angiotensinogen. Neither sodium depletion, captopril treatment, nor adrenalectomy modified the circulating levels of total angiotensinogen. However these treatments produced a decrease in intact angiotensinogen that was in proportion to the elevation of renin levels. The difference between the two assays for angiotensin represents the level of des-angiotensin I-angiotensinogen and correlated satisfactorily with the plasma levels of renin. Identical correlations were observed in adrenalectomized rats and captopril-treated rats. We conclude that des-angiotensin I-angiotensinogen levels are an index of activation of the renin-angiotensin system dependent on the circulating level of renin.