This study examined the production of nitric oxide (NO) in the renal c
ortex and medulla through the use of an in vivo microdialysis techniqu
e. Oxyhemoglobin (OxyHb) at a concentration of 3 mu mol/L was perfused
through the dialysis system to trap tissue NO. Methemoglobin (MetHb),
which was formed by NO oxidation of OxyHb in the dialysate, was spect
rophotometrically assayed at 401 nm. Because the oxidation of OxyHb to
produce MetHb is stoichiometric with NO, the production of NO can be
determined by the rate of MetHb formation. We found that NO concentrat
ion was significantly higher (P < .05) in the medulla (57.1 +/- 5.57 n
mol/L, n = 10) than in the cortex (31.2 +/- 5.7 nmol/L, n = 9). The mi
nimal detectable NO level of this assay is approximate to 10 nmol/L. I
ntravenous infusion of L-arginine (3 mg/kg per minute) for 30 minutes
produced a twofold to threefold increase in cortical and medullary NO;
N-G-nitro-L-arginine methyl ester (L-NAME) (10 mu g/kg per minute) de
creased NO by 33% in the renal cortex and by 46.5% in the renal medull
a. We have also compared under the same conditions the degradation pro
ducts of NO, nitrite, and nitrate in the renal cortex and medulla usin
g in vivo microdialysis combined with microtiter plate colorimetry. Ni
trite/nitrate concentration was significantly higher (P < .05) in the
medulla (2.7 +/- 0.6 mu mol/L, n = 4) than in the cortex (2.1 +/- 0.2
mu mol/L, n = 4). Infusion of L-arginine increased cortical and medull
ary nitrite/nitrate by 65% and 39%, respectively. L-NAME reduced corti
cal and medullary nitrite/nitrate by 18% and 23%, respectively. The re
sults indicate that the OxyHb-NO microdialysis trapping technique is a
highly sensitive in situ method for detecting regional tissue NO conc
entration and changes in the NO synthase activity in the kidney. These
studies have shown that NO concentration is higher in medullary tissu
e than in the cortex.