This study tested the hypothesis that nitric oxide (NO)-mediated renal vasodilation due to the activity of the inducible nitric oxide synthase (iNOS) contributes to glomerular hyperfiltration in diabetic rats. Two weeks after induction of diabetes mellitus by streptozotocin, mean arterial BP (MAP), GFR (inulin clearance), and renal plasma flow (RPF) (para-aminohippurate clearance) were measured in conscious instrumented rats. Diabetic rats had elevated GFR (3129 +/- 309 microl/min versus 2297 +/- 264 microl/min in untreated control rats, P < 0.05) and RPF (10526 +/- 679 microl/min versus 8005 +/- 534 microl/min), which was prevented by chronic insulin treatment. Intravenous administration of 0.1 and 1 mg of L-imino-ethyl-lysine (L-NIL), an inhibitor of iNOS, did not affect MAP, GFR, or RPF, either in diabetic or control rats. A higher L-NIL dose (10 mg) increased MAP and decreased RPF in diabetic rats significantly (n = 6, P < 0.05), but not in controls (n = 6). In addition, 0.1 mg of NG-nitro-L-arginine methyl ester (L-NAME), a nonselective blocker of NOS isoforms, decreased GFR (2389 +/- 478 microl/min) and RPF (7691 +/- 402 microl/min) in diabetic animals to control levels, while renal hemodynamics in normoglycemic rats were not altered. Higher L-NAME doses (1 and 10 mg) reduced GFR and RPF in diabetic and control rats to identical levels. In glomeruli isolated from diabetic and control rats, neither iNOS mRNA nor iNOS protein expression was detected. In contrast, increased protein levels of endothelial constitutive NOS (ecNOS) were found in glomeruli of diabetic rats compared with controls. By immunohistochemistry, ecNOS but not iNOS staining was observed in the endothelium of preglomerular vessels and in diabetic glomeruli. These results support the notion that increased NO availability due to greater abundance of ecNOS contributes to the pathogenesis of glomerular hyperfiltration in early experimental diabetic nephropathy. In contrast, we found no functional or molecular evidence for increased glomerular expression and activity of iNOS in diabetic rats.