Role of neuronal nitric oxide synthase in the macula densa

Background. There is evidence that macula densa nitric oxide (NO) inhibits tubuloglomerular feedback (TGF). However, TGF response is not altered in mi ce deficient in neuronal nitric oxide synthase (nNOS) (-/-). Furthermore, n NOS expression in the macula densa is inversely related to salt intake, yet micropuncture studies have shown that NOS inhibition potentiates TGF in ra ts on high sodium intake but not in rats on a low-salt diet. These inconsis tencies may be due to confounding systemic factors, such as changes in circ ulating renin. To further clarify the role of macula densa nNOS in TGF resp onse, independent of systemic factors, we tested the hypothesis that (1) TG F response is inversely related to sodium intake, and (2) during low sodium intake, NO produced by macula densa nNOS tonically controls the basal diam eter of the afferent arteriole (Af-Art). Methods. Af-Arts and attached macula densas were simultaneously microperfus ed in vitro. TGF response was determined by measuring Af-Art diameter befor e and after increasing NaCl in the macula densa perfusate. TGF response was studied in wild-type (+/+) and nNOS knockout mice (-/-), as well as in jux taglomerular apparatuses (JGAs) from rabbits fed a low-, normal-, or high-N aCl diet. Results. TGF responses were similar in nNOS +/+ and -/- mice. However, in n NOS +/+ mice, 7-nitroindazole (7-NI) perfused into the macula densa signifi cantly potentiated the TGF response (P=0.001), while in nNOS -/- mice, this potentiation was absent. In rabbits on three different sodium diets, TGF r esponses were similar and were potentiated equally by 7-NI. However, in JGA s from rabbits on a low-NaCl diet, adding 7-NI to the macula densa while pe rfusing it with low NaCl fluid caused Af-Art vasoconstriction, decreasing t he diameter by 14% (from 21.7 +/-1.3 to 18.6 +/-1.5 mum; P<0.001). This eff ect was not observed in JGAs from rabbits fed a normal(19.0<plus/minus>0.5 vs. 19.3 +/-0.8 mum after 7-NI) or high-NaCl diet (18.6 +/-0.7 vs. 18.4 +/- 0.7 mum). Conclusions. First, in this in vitro preparation, chronic changes in macula densa nNOS do not play a major role in the regulation of TGF. Compensatory mechanisms may develop during chronic alteration of nNOS that keep TGF rel atively constant. Second, nNOS regulates TGF response acutely. Third, the r esults obtained in the +/+ and -/- mice also confirm that the effect of 7-N I is due to inhibition of macula densa nNOS. Finally, during low sodium int ake (without induction of TGF), the regulation of basal Af-Art resistance b y macula densa nNOS suggests that NO in the macula densa helps maintain ren al blood flow during the high renin secretion caused by low sodium intake.