Abstract-Acute, nonselective nitric oxide synthase inhibition in the pregnant rat decreases glomerular filtration rate and renal plasma flow, suggesting a role for nitric oxide in mediating renal vasodilation during pregnancy. As mid-gestation in the rat is associated with a significant increase in renal protein expression of inducible nitric oxide synthase, the aim of this study was to examine the role of inducible nitric oxide synthase in mediating renal hemodynamics changes at mid-gestation in the rat. At day 16 of pregnancy, glomerular filtration rate was significantly higher in pregnant rats compared with virgin rats (3.1Ϯ0.4 versus 2.7Ϯ0.3 mL/min, respectively; PϽ0.05), as was effective renal plasma flow (13.4Ϯ2.5 versus 10.9Ϯ2.2 mL/min, respectively; PϽ0.05). Acute administration of the inducible nitric oxide synthase selective inhibitor, AMT hydrochloride (750 nmol/h), markedly attenuated the increase in glomerular filtration rate observed in pregnant rats (2.3Ϯ0.2 mL/min, PϽ0.01 versus pregnant) without significantly altering glomerular filtration rate in virgin rats (2.1Ϯ0.2 mL/min). Acute AMT administration significantly decreased effective renal plasma flow in pregnant (8.9Ϯ1.8 mL/min, PϽ0.01 versus pregnant) and virgin rats (7.1Ϯ0.9 mL/min, PϽ0.05 versus virgin). Acute administration of EIT (380 nmol/h), another inducible nitric oxide synthase selective inhibitor, also attenuated pregnancy-induced increases in glomerular filtration rate (2.1Ϯ0.2, 2.8Ϯ0.3, and 2.3Ϯ0.3 mL/min; virgin, pregnant, and EIT, respectively) and effective renal plasma flow (8.5Ϯ1.1, 13.8Ϯ2.1, and 9.0Ϯ1.1 mL/min; virgin, pregnant, and EIT, respectively). Therefore, these findings suggest that inducible nitric oxide synthase may play an important role in mediating the renal hemodynamic changes that occur during normal pregnancy. Key Words: pregnancy Ⅲ nitric oxide synthase Ⅲ kidney Ⅲ hemodynamics Ⅲ rats N ormal pregnancy is associated with significant increases in renal hemodynamics as increases in glomerular filtration rate (GFR) and renal plasma flow (RPF) of Ͼ40% are observed in pregnant women. 1 In the rat, GFR and RPF are increased by Ͼ20% at mid-gestation with a return to prepregnant values by late pregnancy. 2,3 These increases in renal hemodynamics at mid-gestation in the rat are associated with significant increases in whole-body nitric oxide (NO) production 3,4 and renal protein expression of both inducible and neuronal nitric oxide synthase (iNOS and nNOS, respectively). 3 A role for NO in mediating renal vasodilation during pregnancy is suggested in a study by Danielson and Conrad in which acute nonselective NOS inhibition equalized GFR and effective RPF (ERPF) in virgin rats and pregnant rats at mid-gestation. 5 However, nonselective inhibition of all three NOS isoforms, endothelial (eNOS), neuronal (nNOS), and inducible (iNOS), does not elucidate which specific isoform is an important source of NO during pregnancy.Several lines of evidence suggest that NO generated by the iNOS isoform may play an important role in...