In Inactin-anesthetized Wistar rats with an intact renal innervation, intratubular nitro-L-arginine methyl ester (L-NAME, 10(-4) M) increased proximal fluid uptake (J(va), at 2.47 +/- 0.61 x 10(-4) mm(3). mm(-2). s(-1)) by 17% (P < 0.05), whereas coadministration with sodium nitroprusside (SNP, 10(-4) M) decreased J(va) by 18% (P < 0.01). Similar manipulation of NO generation was without effect in groups of Wistar rats subjected to acute renal denervation. Intratubular aminoguanidine (10(-4) M), a selective inducible nitric oxide synthase (NOS) blocker, had no effect on J(va) in intact kidneys of Wistar rats, but the neuronal NOS (nNOS) blocker, 7-nitroindazole (10(-4) M and 10(-6) M) increased J(va) by 19-23% (both P < 0.001). In stroke-prone spontaneously hypertensive rats (SHRSP), J(va) values in the innervated kidneys were lower (P < 0.05) than in the corresponding Wistar groups and were unchanged by intratubular L-NAME or L-NAME plus SNP. The tonic attenuation of proximal epithelial transport by NO was dependent on the renal sympathetic nerves and appeared to be generated by the nNOS isoform of the enzyme. This role of NO was not evident in the SHRSP.

Abstract-This study investigated the role of NO in mediating the renal sympathetic nerve-mediated increases in proximal tubular fluid reabsorption (Jva). In inactin-anesthetized Wistar rats, renal sympathetic nerve stimulation (15 V, 2 ms) at 0.75 and 1.0 Hz did not change blood pressure or glomerular filtration rate but did decrease urine flow and sodium excretion in a frequency-related fashion by 40% to 50% at 1.0 Hz (both, PϽ0.01). Renal nerve stimulation in control animals increased Jva by 11% at 0.75 Hz (PϽ0.05) and 31% at 1.0 Hz (PϽ0.01). Intraluminal N-nitro-L-arginine methyl ester (L-NAME) resulted in a higher basal Jva (19%, PϽ0.05), and renal nerve stimulation had no effect on Jva. When L-NAME plus sodium nitroprusside was present intraluminally, however, there were frequency-dependent increases in Jva that were similar in pattern and magnitude to the control rats. Introduction of the relatively selective nNOS blocker 7-nitroindazole intraluminally, at 10 Ϫ6 and 10 Ϫ4 M, raised basal Jva by 18% and 24%, respectively (PϽ0.01), and renal nerve stimulation did not change Jva. Intraluminal aminoguanidine (10 Ϫ4 M), a relatively selective iNOS blocker, did not affect basal Jva, which remained unchanged during renal nerve stimulation. These data are consistent with NO exerting a tonic inhibitory action on the basal levels of Jva, which, in part, is caused by NO generated by the nNOS isoform. Moreover, the findings have revealed that the presence of NO is necessary to ensure that renal nerves can stimulate fluid reabsorption by the proximal tubules, requiring NO generated from both nNOS and iNOS. Key Words: nitric oxide Ⅲ renal nerves Ⅲ antinatriuresis Ⅲ sodium Ⅲ kidney T he renal sympathetic nerves play an important role in regulating renin release, tubular sodium and water reabsorption, and renal vascular resistance. Increasing levels of renal nerve activity progressively recruit these functions; that is, at low frequencies there is renin release. Thereafter, increased tubular reabsorption of fluid becomes apparent, and it is only at the higher frequencies that there are reductions in renal blood flow and glomerular filtration rate. 1,2 Noradrenaline is released from the renal sympathetic nerve endings and, as at other sympathetic neuroeffector junctions, has an autoinhibitory feedback action, mediated via presynaptic ␣ 2 -adrenoceptors, to attenuate the level of neurotransmitter release. 3 At the postsynaptic level, noradrenaline acts on the ␣ 1 -adrenoceptors of the epithelial cells to stimulate sodium, and hence water, transport by activation of the Na ϩ /K ϩ -ATPase at the basolateral membrane and the Na ϩ /H ϩ -exchanger at the apical membrane. 4,5 Recently, there has been increasing interest in the role of NO in the control of renal function. NO synthase (NOS) catalyzes the generation of NO, which stimulates cyclic GMP production to modify specific aspects of renal function. 6 At least 3 isoforms of NOS have been identified so far: inducible NOS (iNOS), endothelial NOS (eNOS), and neuronal NOS (nNOS),...

1 We have examined the effects of nitric oxide inhibition, indomethacin and the dual lipoxygenase/ cyclo-oxygenase inhibitor, 3-amino-l-[m-(trifluoromethyl)-phenyl]-2-pyrazoline (BW755C), on the responses of small mesenteric arteries of Wistar rats, with and without endothelium, to noradrenaline, potassium chloride, endothelin-1, acetylcholine and sodium nitroprusside. 2 Noradrenaline, potassium chloride and endothelin-l caused concentration-dependent contraction of small mesenteric arteries. Indomethacin (14 iM) attenuated the contractile response to both noradrenaline and potassium chloride. The inhibitory action of indomethacin persisted in vessels treated with CHAPS. 3 Acetylcholine produced concentration-dependent relaxation in these vessels. Indomethacin (14JM) had no significant effect on the acetylcholine concentration-response relationship. 4 NG-nitro-L-arginine methyl ester (L-NAME, 100 LM) potentiated the contractile response to both noradrenaline and potassium chloride and inhibited acetylcholine-induced relaxation. Indomethacin attenuated the effects of L-NAME. 5 BW755C inhibited the contractile response to noradrenaline and potassium chloride but not to endothelin-1. The inhibitory effects of BW755C persisted in the presence of indomethacin and in vessels treated with CHAPS. 6 BW755C enhanced endothelium-dependent relaxation, as assessed by the response to acetylcholine. In the presence of indomethacin, BW755C produced a shift to the right of the concentration-response curve to acetylcholine. 7 Inhibition of nitric oxide synthase with L-NAME, reversed the inhibitory effect of BW755C on noradrenaline-and potassium-induced contraction. L-NAME and BW755C in combination resulted in a shift to the right of the concentration-response curve to acetylcholine. 8 Sodium nitroprusside produced concentration-dependent relaxation of the vessels. Endothelium removal reduced the maximum relaxation to nitroprusside. BW755C did not alter the response to sodium nitroprusside in vessels with or without endothelium. 9 These data support the existence of two vasoconstrictor products of arachidonic acid released during contraction of small mesenteric arteries with noradrenaline and potassium chloride: a cyclo-oxygenase product and a lipoxygenase product both of which appear to be largely endothelium-independent.

1 We have examined the effects of inhibition of nitric oxide synthase, cyclo-oxygenase and lipoxygenase on the responses of renal arcuate arteries of Wistar rats, with and without endothelium, to noradrenaline, potassium chloride, endothelin-l, acetylcholine and sodium nitroprusside. 2 Noradrenaline, potassium chloride and endothelin-1 caused concentration-dependent contraction of the vessels. Indomethacin (14 fLM) attenuated the contractile response to noradrenaline and to potassium chloride. The inhibitory effect of indomethacin persisted following endothelial removal. 3 Acetylcholine produced concentration-dependent relaxation of the vessels which was potentiated by indomethacin (14 gM). 4 NG-nitro-L-arginine methyl ester (L-NAME, 100 gM) did not affect the contractile response to either noradrenaline or potassium chloride but abolished relaxation to acetylcholine. In addition, L-NAME abolished the affects of indomethacin on acetylcholine-induced relaxation and noradrenaline-and potassium chloride-induced contraction. 5 BWC755C attenuated noradrenaline and potassium chloride-induced contraction. This effect persisted in the presence of indomethacin. 6 In vessels pretreated with CHAPS, BW755C inhibited both noradrenaline and potassium chlorideinduced contraction. In these vessels BW755C had no additional inhibitory effect to indomethacin on noradrenaline-and potassium-induced contraction.7 Inhibition of nitric oxide synthase with L-NAME (1001iM) attenuated the effect of BW755C on noradrenaline-and potassium-induced contraction. 8 BW755C alone did not affect endothelium-dependent relaxation as assessed by the response to acetylcholine. However, in the presence of indomethacin, BW755C inhibited acetylcholine-induced relaxation. 9 BW755C did not affect endothelium-independent relaxation as assessed by the response to sodium nitroprusside in vessels with or without endothelium. 10 These data support the existence of two vasoconstrictor products of arachidonic acid released during contraction of renal arcuate arteries with noradrenaline and potassium chloride. A cyclooxygenase product which appears to be endothelium-independent and the other an endotheliumdependent lipoxygenase product.