A high-salt diet enhances nitric oxide (NO)-induced inhibition of transport in the thick ascending limb (THAL). Long exposures to NO inhibit Na-K-ATPase in cultured cells. We hypothesized that NO inhibits THAL Na-K-ATPase after long exposures and a high-salt diet would augment this effect. Rats drank either tap water or 1% NaCl for 7-10 days. Na-K-ATPase activity was assessed by measuring ouabain-sensitive ATP hydrolysis by THAL suspensions. After 2 h, spermine NONOate (SPM; 5 M) reduced Na-K-ATPase activity from 0.44 Ϯ 0.03 to 0.30 Ϯ 0.04 nmol P i ⅐ g protein Ϫ1 ⅐ min Ϫ1 in THALs from rats on a normal diet (P Ͻ 0.03). Nitroglycerin also reduced Na-K-ATPase activity (P Ͻ 0.04). After 20 min, SPM had no effect (change Ϫ0.07 Ϯ 0.05 nmol Pi ⅐ g protein Ϫ1 ⅐ min Ϫ1 ). When rats were fed high salt, SPM did not inhibit Na-K-ATPase after 120 min. To investigate whether ONOO Ϫ formed by NO reacting with O 2 Ϫ was involved, we measured O 2 Ϫ production. THALs from rats on normal and high salt produced 35.8 Ϯ 0.3 andϪ production differed, we studied the effects of the O 2 Ϫ scavenger tempol. In the presence of 50 M tempol, SPM did not inhibit Na-K-ATPase after 120 min (0.50 Ϯ 0.05 vs. 0.52 Ϯ 0.07 nmol Pi ⅐ g protein Ϫ1 ⅐ min Ϫ1 ). Propyl gallate, another O 2 Ϫ scavenger, also prevented SPM-induced inhibition of Na-K-ATPase activity. SPM inhibited pump activity in tubules from rats on high salt when O 2 Ϫ levels were increased with xanthine oxidase and hypoxanthine. We concluded that NO inhibits Na-K-ATPase after long exposures when rats are on a normal diet and this inhibition depends on O 2 Ϫ . NO donors do not inhibit Na-K-ATPase in THALs from rats on high salt due to decreased O 2 Ϫ production.superoxide; nitric oxide; Na transport; hypertension NITRIC OXIDE (NO) is an important regulator of renal function, promoting natriuresis and diuresis (9). Intrarenal administration of NO synthesis inhibitors lowers urinary Na excretion (11-13, 17, 19, 20, 22, 38), whereas NO donors induce natriuresis (18,19). NO-induced natriuresis and diuresis can occur in the absence of hemodynamic changes, indicating that NO directly inhibits Na and water absorption along the nephron. In vitro studies have shown that NO blunts transport in isolated proximal tubules (1), cortical collecting ducts (3, 39, 40), inner medullary collecting ducts (2), and thick ascending limbs (2, 34, 35). Additionally, NO has been reported to inhibit Na transport in several cultured renal cells (5,15,16). We reported that the inhibition of net Na absorption caused by NO in the thick ascending limb and cortical collecting duct is due to reduced activity of the transporters responsible for Na entry into the cell, namely, Na/H exchange (4) and Na-K-2Cl cotransport (29) in the thick ascending limb and amiloridesensitive Na channels in the cortical collecting duct (40). We did not find inhibition of basolateral Na-K-ATPase after 20 min of exposure. However, others showed that NO inhibits Na-K-ATPase activity in cultured cells after longer exposures and that inhibition ...