Effect of captopril on the release of the components of the renin-angiotensin system into plasma and lymph.

Abstract

The effects of captopril on the intrarenal renin-angiotensin system were assessed from measurements in arterial plasma, renal venous plasma, and renal lymph from salt-depleted dogs. In the basal state, immunoreactive angiotensin II (Ang II) in renal venous plasma averaged only 60 +/- 12% (P less than 0.01) of arterial plasma, although the concentration of Ang II in renal lymph was 2.0 +/- 0.4-fold (P less than 0.05) greater. The Ang II concentration of renal lymph incubated ex vivo at 37 degrees C doubled in 10 to 15 min, which was the time taken to collect renal lymph samples. Compared with arterial plasma, renal lymph contained lower concentrations (P less than 0.01) of renin substrate and angiotensin-converting enzyme but higher concentrations of active (5.3 +/- 2.1-fold) and inactive (8.9 +/- 3.2-fold) renin. Although captopril increased the secretion of active renin into renal venous plasma by six-fold, the secretion of total renin was unchanged because of a reciprocal fall in the secretion of inactive renin. The percent reduction in renal vascular resistance with captopril correlated with the percent fall in Ang II in renal lymph (r = 0.70). In conclusion: (1) all components of the renin-angiotensin system are represented in the renal interstitium, as reflected in lymph; (2) Ang II concentrations in renal lymph in vivo approximate arterial levels; (3) increased secretion of active renin into plasma during intrarenal infusion of captopril into denervated kidneys is due predominantly to renin activation; and (4) renal vascular resistance may depend on the concentration of Ang II in the renal interstitium.