Micropuncture determination of nephron function in mice without tissue angiotensin-converting enzyme

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Abstract
To determine the role of the local renin-angiotensin system in renal function, micropuncture was performed on two lines of mice in which genetic changes to the angiotensin-converting enzyme (ACE) gene markedly reduced or eliminated the expression of renal tissue ACE. Whereas blood pressure is low in one line (ACE 2/2), it is normal in the other (ACE 1/3) due to ectopic hepatic ACE expression. When normalized for renal size, levels of glomerular filtration rate [GFR; μl·min−1·g kidney wt−1(KW)] and single-nephron GFR (SNGFR; nl·min−1·g KW−1) were similar between wild-type (WT) and ACE 1/3 mice, while both measures were significantly reduced in ACE 2/2 mice (WT: 500 ± 63 and 41.7 ± 3.5; ACE 1/3: 515.8 ± 71 and 44.3 ± 3.3; ACE 2/2: 131.4 ± 23 and 30.3 ± 3.5). Proximal fractional reabsorption was not significantly different between WT and ACE 1/3 mice (51 ± 3.5 and 49 ± 2.3%), and it was increased significantly in ACE 2/2 mice (74 ± 3.5%). Infusion of ANG II (50 ng·kg−1·min−1) increased mean arterial pressure by ∼7 mmHg in all groups of mice and reduced SNGFR in WT and ACE 1/3 mice (to 30.9 ± 2.8 and 31.9 ± 2.5 nl·min−1·g KW−1) while increasing it in ACE 2/2 mice (to 55.3 ± 5.3 nl·min−1·g KW−1) despite an increase in total renal vascular resistance. The tubuloglomerular feedback (TGF) response was markedly reduced in ACE 1/3 mice (stop-flow pressure change −2.5 ± 0.9 mmHg) compared with WT despite similar blood pressures (−8.3 ± 0.6 mmHg). In ACE 2/2 mice, TGF was absent (−0.7 ± 0.2 mmHg). We conclude that the chronic lack of ACE, and presumably ANG II generation, in the proximal tubule was not associated with sustained proximal fluid transport defects. However, renal tissue ACE is an important contributor to TGF.