Down-regulation of nitric oxide synthase in the diabetic rabbit kidney: potential relevance to the early pathogenesis of diabetic nephropathy.

OBJECTIVES: Nephropathy is a well-recognised complication of diabetes mellitus (DM). The aim of this study was to investigate the effect of DM on the density and distribution of nitric oxide (NO) synthase (NOS) in the rabbit kidney. Quantification of the NOS radioligand on slide-mounted sections was compared with the nitroblue tetrazolium reaction, where the intensity of the reaction varies with the nicotinamide adenine dinucleotide diaphorase (NADPH-d) activity of NOS.

MATERIALS AND METHODS: DM was induced with alloxan in six New Zealand White (NZW) rabbits. Plasma creatinine, urea and electrolytes were monitored at monthly intervals. The kidneys were removed following 6 months of DM. Transverse serial sections were cut and low-resolution autoradiography was performed using a radioligand for NOS ([(3)H]-NOARG). Histochemical localisation of NADPH-d activity was also performed. Densitometric analysis was performed on the autoradiographs and the results compared with those obtained from six age-matched control rabbits.

RESULTS: There was a significant (p < 0.01) rise in plasma creatinine levels in the diabetic rabbits, although the mean values remained within the reference range. There was a significant (p < 0.0001) down-regulation of NOS binding sites in both the cortex and medulla of the DM kidney when compared with the controls. A similar decrease in NADPH-d activity was seen in the diabetic renal cortex and medulla. In addition, NADPH-d activity also appeared to be reduced in the diabetic glomeruli when compared with controls.

CONCLUSIONS: NOS binding sites and NADPH-d activity are significantly decreased in the DM renal cortex and medulla. These changes are associated with a mild deterioration in renal function and may be an early event that could subsequently play a role in the progression of DM nephropathy. Manipulating the NO pathway during the early stages of DM nephropathy may be beneficial.