Experiments were performed in neuronal (nNOS)-and endothelial nitric oxide synthase (eNOS)-deficient mice to study the role of nitric oxide (NO) in macula densa control of renin secretion in vivo and in the isolated, perfused mouse kidney. Acute and chronic administration of loop diuretics was used as a method to stimulate macula densa-mediated renin secretion. Increases in plasma renin concentration (PRC) in response to a 3-day infusion of bumetanide (50 mg⅐kg Ϫ1 ⅐day Ϫ1 ) or an acute injection of furosemide (50 mg/kg ip) were not markedly altered in nNOSϪ/Ϫ mice. Responses to furosemide were also maintained in eNOSϪ/Ϫ mice, but the administration of N -nitro-L-arginine methyl ester (L-NAME) markedly attenuated the PRC response to furosemide in these mice. In the isolated kidney preparation, bumetanide caused similar relative increases in renin secretion in kidneys of wild-type, nNOSϪ/Ϫ, and eNOSϪ/Ϫ mice. Bumetanide only marginally increased renin secretion in L-NAME-treated kidneys, but the bumetanide effect was normalized by S-nitroso-N-acetyl-penicillamine. Basal PRC was significantly reduced in male nNOSϪ/Ϫ mice compared with nNOSϩ/ϩ (189 Ϯ 28 vs. 355 Ϯ 57 ng ANG I ⅐ml Ϫ1 ⅐h Ϫ1 ; P ϭ 0.017). There was no significant difference in PRC between eNOSϩ/ϩ and eNOSϪ/Ϫ mice. Basal renin secretion rates in perfused kidneys isolated from nNOSϪ/Ϫ or eNOSϪ/Ϫ mice were markedly reduced compared with wild-type controls. Our data suggest that NO generated by macula densa nNOS does not play a specific mediator role in macula densa-dependent renin secretion. However, NO independent of its exact source permits the macula densa pathway of renin secretion to function normally. plasma renin; isolated, perfused kidney; neuronal nitric oxide synthase knockout; endothelial nitric oxide synthase knockout; furosemide; bumetanide THE MACULA DENSA (MD) CELLS LOCATED at the terminal end of the loop of Henle are believed to play an important role in both the regulation of preglomerular resistance and the control of renin secretion. Commensurate with their distinct function as sensor cells in these local regulatory pathways, MD cells have a number of unique characteristics that distinguish them from neighboring cells of the cortical thick ascending limb. One of the more striking of these discriminating features is the expression of substantial amounts of neuronal nitric oxide synthase (nNOS) and the ability to generate nitric oxide (NO) (31, 53).A considerable body of experimental evidence indicates that the expression of nNOS in the MD and the expression and secretion of renin by juxtaglomerular (JG) granular cells are altered in parallel by a variety of interventions. Reductions in oral salt intake (4,42,46, 50), renal hypoperfusion, and pharmacological blockade of Na-K-2Cl cotransport of the thick ascending limb (4, 43) all stimulate the expression of both proteins. Furthermore, changes in NOS activity have been demonstrated to affect renin secretion, and the majority of studies suggest a stimulatory role for NO (4,20,35,40,42). It ha...