Inhibition of locally produced nitric oxide resets tubuloglomerular feedback mechanism

This study was designed to compare the effects of systemic and intratubular infusions of the nitric oxide (NO) synthase inhibitor N omega-nitro-L-arginine (L-NNA) on the tubuloglomerular feedback (TGF) mechanism in anesthetized rats. We recently showed that intravenous infusion of L-NNA led to increases in mean arterial blood pressure (Pa), proximal tubular stop-flow pressure (Psf), and enhanced TGF sensitivity and reactivity. To avoid major systemic effects, in this study TGF was studied after intratubular NO inhibition. Intratubular infusion of L-NNA (10(-3) M) yielded similar results as shown with intravenous infusion, without systemic effects. TGF sensitivity and reactivity were increased, indicated by decreased turning point (TP) from 19.8 +/- 1.0 to 15.2 +/- 0.7 nl/min and increased delta Psf from 10.0 +/- 0.8 to 23.9 +/- 1.9 mmHg (24.3 vs. 59.1%). L-NNA at a concentration of 10(-4) M showed significant changes in both TP (from 20.9 +/- 1.1 to 17.8 +/- 1.0 nl/min) and delta Psf (from 7.6 +/- 0.6 to 13.9 +/- 0.7 mmHg), whereas 10(-5) M only increased delta Psf (9.7 +/- 1.0 vs. 12.1 +/- 1.1 mmHg). However, at low tubular perfusion rates Psf was not influenced by L-NNA. The early proximal flow rate (EPFR) showed no change at low tubular perfusion rates with L-NNA. At maximal TGF activation (40 nl/min), delta EPFR was increased from 34% in control to 62%. Our results suggest that NO not only regulates glomerular capillary pressure but also decreases the sensitivity of the TGF mechanism.