The 'intestinal-renal' arginine biosynthetic axis in the aging rat.

It has been suggested that L-arginine availability declines with advanced age, which could contribute to the endothelial dysfunction and decreased nitric oxide (NO) production that are features of aging. L-Arginine is made in the kidney and since the aging kidney develops progressive injury there may be decreased synthesis limiting availability. In this study we investigated the impact of aging on the regulation, at the gene level, of the various enzymes that synthesize L-arginine in the kidney (argininosuccinate synthetase and argininosuccinate lyase) and citrulline, the precursor of L-arginine made in the small intestine (phosphate-dependent glutaminase, carbamyl phosphate synthetase-1 and ornithine transcarbamylase). Studies were in young (3-5 months), middle-aged (11-13 months) and old (18-22 months) male and female Sprague-Dawley rats aged under barrier conditions. The plasma, renal cortical and brain cerebellar levels of L-arginine are unchanged in the old male rat, and expression of the genes involved in renal arginine synthesis and small intestinal citrulline synthesis is unchanged or upregulated with age in both males and females. This study shows that the synthesis of L-arginine is maintained with aging despite developing kidney damage. Therefore, the reduced NO generating capacity that occurs in aging must be due to downstream changes in the NO biosynthesis pathway, such as reduced abundance of NO biosynthetic enzymes.