Roles of NO and oxygen radicals in tubuloglomerular feedback in SHR

The spontaneously hypertensive rat (SHR) has enhanced tubuloglomerular feedback (TGF) responses and diminished buffering by juxtaglomerular apparatus (JGA)-derived nitric oxide (NO) despite enhanced expression of NO synthase (NOS) isoforms in the JGA. We tested the hypothesis that the enhanced TGF response is due to inactivation of NO by oxygen radicals(O-2(-)). SHR had significantly (P < 0.05) greater expression of the peroxynitrate reaction product, nitrotyrosine, in renal cortex. A membrane-permeant, metal-independent superoxide dismutase mimetic, tempol, was used to test the functional role of O-2(-). Maximum TGF responses, assessed from changes in proximal stop-flow pressure (P-SF) during orthograde loop of Henle (LH) perfusion of artificial tubular fluid (ATF), were enhanced in SHR [Wistar-Kyoto rat (WKY) 8.8 +/- 0.4(n = 30 nephrons) vs. SHR 10.8 +/- 0.4 mmHg(n = 39 nephrons), P < 0.001]. TGF responses of SHR were unresponsive to microperfusion of 7-nitroindazole (7-NI, 10(-4) M), which is an inhibitor of neuronal NOS (nNOS) [WKY 8.3 +/- 0.3 to 10.8 +/- 0.4 (n = 8, P < 0.001) vs. SHR 10.0 +/- 0.7 to 10.5 +/- 0.8 mmHg (n = 8; not significant)]. Microperfusion of tempol (10-4 M) into the efferent arteriole (EA) supplying the peritubular capillaries (PTC) blunted TGF. The response to tempol was significantly (P < 0.05) greater in SHR [Delta TGF in WKY 19 +/- 6% (n = 10) vs. SHR 32 +/- 3% (n = 10)]. Microperfusion of the NO donor compound S-nitroso-N-acetyl-penicillamine (SNAP, 10(-7)-10(-4) M) via the LH blunted TGF, but the sensitivity of the response was impaired significantly (P < 0.05) in SHR nephrons. PTC perfusion of tempol (10(-4) M) normalized the response to loop perfusion of both SNAP and 7-NI in SHR nephron to levels in WKY (during tempol, Delta P-SF with 7-NI in WKY 8.9 +/- 0.6 to 11.4 +/- 0.8; n = 12 vs. SHR 9.5 +/- 0.5 to 12.5 +/- 0.4 mmHg; n = 16). In conclusion, TGF responses are enhanced in SHR, in part due to a diminished role for NO from nNOS in blunting TGF due to enhanced O-2(-) formation. O-2(-) in the JGA enhances TGF responses by inactivation of locally generated NO.