Abstract 490: Constrictor Mechanism of the Afferent Arteriole Initiated by Na/H Exchanger in the Nephron

Abstract

The afferent arteriole (Af-Art) accounts for most of renal vasculature resistance, thus controlling glomerular filtration rate and renal function. Two mechanisms have been described in which the nephron helps control Af-Art resistance, namely tubuloglomerular feedback (TGF), and connecting tubule glomerular feedback (CTGF). TGF is a constrictor mechanism initiated at the macula densa by the Na/K/2Cl cotransporter (NKCC2), while CTGF is a dilator mechanism initiated at the connecting tubule by the epithelial Na channel (ENaC). However, when NKCC2 is blocked by furosemide, CTGF-induced Af-Art dilation is not evident, thus we hypothesize that there is a constrictor mechanism that is furosemide-insensitive and counters CTGF. To test this, we used in vivo micropuncture of single nephrons. Stop-flow pressure (P SF ) was measured as an index of glomerular capillary pressure (which decreases when the Af-Art constricts). Two consecutive P SF curves were generated by raising nephron perfusion from 0 to 40 nL/min while adding drugs to the tubular perfusate. The decrease in P SF induced by increasing nephron perfusion was blocked by furosemide 10 -4 M (control: 7.9±0.2 mmHg, furosemide: 0.4±0.2 mmHg, P <0.001, n=6), but was partially restored when blocking CTGF with the ENaC inhibitor benzamil 10 -6 M (furosemide: 0.2±0.1 mmHg, furosemide+benzamil: 4.3±0.3 mmHg, P <0.001, n=6). A possible explanation for these observations is that TGF may be activated to some degree by Na/H exchanger (NHE)-mediated Na entry at the macula densa when NKCC2 is blocked. When we added the NHE blocker dimethylamiloride (DMA, 10 -4 M) in the presence of furosemide and benzamil, the decrease in PSF was significantly prevented (furosemide+benzamil: 4.6±0.3 mmHg, furosemide+benzamil+DMA: 1.1±0.2 mmHg, P <0.001, n=6). Therefore, we conclude that a constrictor mechanism of the Af-Art is initiated by NHE in the nephron and is observable when NKCC2-mediated TGF and ENaC-mediated CTGF have been blocked.