Paradoxical increase in nitric oxide synthase activity in hypercholesterolaemic rats with impaired renal function and decreased activity of nitric oxide

Background. We have shown that acute exposure of oxidized low-density lipop rotein (Ox-LDL) induces vasoconstriction in renal vessels and reduces glome rular filtration rate (GFR) in an isolated perfused rat kidney model by dec reasing the activity of nitric oxide (NO). L-arginine has a protective role against Ox-LDL-induced vasoconstriction. Micropuncture studies have demons trated that short-term diet-induced hypercholesterolaemia is associated wit h decreased GFR and renal blood flow and increased glomerular capillary pre ssure. This may be mediated by decreased activity of NO. Methods. Rats were made hypercholesterolaemic by supplementing the standard chow with 4% cholesterol and 1% sodium cholate. A group of rats on hyperch olesterolaemic diet also received L-arginine in the drinking water. After 4 and 6 weeks, blood samples and 24-h urine samples were collected for the m easurement of biochemical parameters. After 6 weeks, all rats were subjecte d to isolated perfusion of kidneys at a constant pressure of 100 mmHg. Duri ng isolated perfusion, the unused contralateral kidney was taken for morpho logical studies and for assessing the activity of nitric oxide synthase enz yme by P-NADPH diaphorase histochemistry. Results, Rats fed a high-cholesterol diet had LDL levels 3-6 times greater than the rats fed standard chow. Rats that received L-arginine in the drink ing water hail serum L-arginine levels 5-6 times greater than control rats. At 6 weeks, creatinine clearance was significantly lower in the rats on th e high-cholesterol diet compared to the rats on standard chow and rats on h igh-cholesterol diet plus L-arginine. Twenty-four-hour urinary total nitrat e and nitrite excretion in the hypercholesterolaemic rats was 1.5-2 times g reater than that of control rats. Twenty-four-hour urinary cGMP excretion w as significantly lower in the rats on a high-cholesterol diet, but in the r ats on high-cholesterol diet anti L-arginine, 24-h urinary cGMP excretion w as not significantly different from that of control rats. During isolated p erfusion of kidneys, renal perfusate flow was found to be significantly red uced in the kidneys taken from the rats on a high-fat diet compared to cont rols. L-arginine supplementation in the drinking water almost completely re versed the effect of a high-fat diet. Inulin clearance was also significant ly reduced in kidneys on a high-fat diet in contrast to controls but not in kidneys on high fat-diet and L-arginine. Basal cGMP excretion in urine was significantly lower in the kidneys taken from the rats on a high-fat diet compared to controls. L-arginine supplementation restored the basal cGMP ex cretion in these kidneys. NO synthase (NOS) enzyme activity as assessed by NADPH diaphorase activity showed that kidney sections taken from the rats o n a high-fat diet showed more intense staining, indicating increased activi ty compared to the kidney sections taken from the rats on a normal diet. Conclusion, Though activity of NO is diminished in hypercholesterolaemic ra ts with impaired renal function, there is a paradoxical increase in NO prod uction and NOS activity. L-arginine reverses the effects of a high-fat diet .