Nitric oxide mediates anomalous tubuloglomerular feedback in rats fed high-NaCl diet after subtotal nephrectomy

Abstract

Tubuloglomerular feedback (TGF) responses become anomalous in rats fed high-NaCl diet after subtotal nephrectomy (STN), such that stimulating TGF causes single nephron GFR (SNGFR) to increase rather than decrease. Micropuncture experiments were performed to determine whether this anomaly results from heightened nitric oxide response to distal delivery, which is a known mechanism for resetting TGF, or from connecting tubule TGF (cTGF), which is a novel amiloride-inhibitable system for offsetting TGF responses. Micropuncture was done in Wistar Froemter rats fed high-NaCl diet (HS) for 8-10 days after STN or sham nephrectomy. TGF was manipulated by orthograde microperfusion of Henle's loop with artificial tubular fluid with or without NOS inhibitor, LNMMA, or the cell-impermeant amiloride analog, benzamil. SNGFR was measured by inulin clearance in tubular fluid collections from the late proximal tubule. TGF responses were quantified as the increase in SNGFR that occurred when the perfusion rate was reduced from 50 to 8 nl/min in STN or 40 to 8 nl/min in sham animals. The baseline TGF response was anomalous in STN HS (-4 ± 3 vs 14 ± 3 nl/min, P < 0.001). TGF response was normalized by perfusing STN nephron with LNMMA (14 ± 3 nl/min, P < 0.005 for ANOVA cross term) but not with benzamil (-3 ± 4 nl/min, P = 0.4 for ANOVA cross term). Anomalous TGF occurs in STN HS due to heightened effect of tubular flow on nitric oxide signaling, which increases to the point of overriding the normal TGF response. There is no role for cTGF in this phenomenon.

Keywords: glomerulotubular balance; hyperfiltration; micropuncture; proximal tubular reabsorption; remnant kidney; salt balance.

Fig. 1.

Line graphs depict tubuloglomerular feedback (TGF) responses for individual nephrons when Henle's loop is perfused with or without LNMMA. TGF(+) denotes maximum TGF stimulus. TGF(−) denotes minimal TGF stimulus. Tukey box plots show how TGF responses distribute within each group. Change in single-nephron glomerular filtration rate (ΔSNGFR) is the difference in SNGFR between TGF(−) and TGF(+). A TGF response is anomalous if ΔSNGFR < 0.

Fig. 2.

Line graphs depict changes in single-nephron glomerular filtration rate (SNGFR) for individual nephrons. A: SNGFR during maximum (+) or minimum (−) stimulation of tubuloglomerular feedback (TGF) using artificial tubular fluid (ATF) or ATF containing benzamil. B: TGF responses (ΔSNGFR) when perfusing Henleʼs loop with control ATF or ATF+benzamil. C: mean ± SE for ΔSNGFR. The effect of benzamil was not statistically significant between groups or within groups.

Fig. 3.

Unadjusted raw measure of net proximal reabsorption (Jprox) shown for each of 667 late proximal tubular fluid collections pooled from the current experiments and previously published experiments in this model (13). SNGFR, single-nephron glomerular filtration rate. Lines depict results of linear regression with 95% confidence intervals.

Fig. 4.

Unadjusted raw measure of fractional proximal reabsorption (FRprox) shown for each of 667 late proximal tubular fluid collections pooled from the current experiments and previously published experiments in this model (13). Lines show results of linear regression with 95% confidence intervals. FRprox varies reciprocally with single-nephron glomerular filtration rate (SNGFR), which can make raw FRProx a misleading indicator of primary effect on the tubule. The regression slopes are shallower in subtotal nephrectomy, which fails to mount high FRprox at low SNGFR. HS, high salt; LS, low salt.