To examine the interaction of NO and Cyclooxygenase (COX-2) and the signaling pathway involved, primary cultured Rabbit cTAL were used. In these cells, immunoreactive COX-2 and vasodilatory prostaglandins were increased by a NO donor, SNAP (2.5±0.3 fold control, n=6, p<0.01). SNAP increased expression of phosphorylated p38 (pp38), (2.4±0.3 fold control; n=5; p<0.01), which was inhibited by the p38 inhibitor, SB203580 (1.3±0.1 fold control, n=5, p<0.01). SB203580 inhibited SNAP-induced COX-2 expression (1.4±0.2 fold control, n=6, NS vs. control) as well as levels of PGE2 (5.9±1.2 to 1.5±0.9 fold) and 6-keto PGF1 (5.7±2.5 to 2.2±0.6 fold). In cTAL cells transfected with a luciferase reporter driven by the wild type mouse COX-2 promoter, SNAP stimulated luciferase activity, which was reversed by SB203580 (control vs. SNAP vs. SNAP+SB203580:1.4±0.2, 8.3 ±1.4 and 0.4 ±0.1 fold control respectively, n=4, p<0.01). EMSA indicated that SNAP stimulated NFB binding activity in cTAL that was also inhibited by the p38 inhibitor. SNAP was not able to stimulate a mutant COX-2 promoter construct that is not activated by NFB (0.9 ±0.1, 1.2±0.1 and 1.0 ±0.2 respectively, n=4, NS). Low chloride increased COX-2 expression (2.7±0.4 fold control, n=6, p<0.01) and pp38 expression (2.8±0.3 fold; n=5, p<0.01), which were reversed by the specific NOS inhibitor, 7Ni. Administration of a low salt diet increased immunoreactive COX-2 and nNOS in the macula densae and surrounding cTAL of kidneys of wild type mice but did not significantly elevate COX-2 expression in nNOS -/- mice. In summary, these studies indicate that in cTAL, NO can increase COX-2 expression in cTAL and macula densa through a p38-dependent signaling pathways via activation of NFB.