Animal studies have suggested that
nitric oxide (NO) synthases (NOS) play a role in the regulation of
protein metabolism in
endotoxemia. We therefore investigated the role of inducible NOS (NOS2) on intestinal
protein and neuronal NOS (NOS1) and endothelial NOS (NOS3) on
amino acid metabolism. Three groups of mice were studied: 1) wild-type (WT), 2) NOS2 knockout (NOS2-KO), and 3) NOS2-KO +
N(omega)-nitro-l-arginine methyl ester (NOS2-KO +
l-NAME), both in nonstimulated and LPS-treated conditions. By infusion of the stable
isotopes l-[phenyl-(2)H(5)]Phe, l-[phenyl-(2)H(2)]Tyr, l-[guanidino-(15)N(2)]Arg, and l-[ureido-(13)C; (2)H(2)]
citrulline (
Cit), intestinal
protein,
amino acid, and Arg/NO metabolism were studied on the whole body level and across intestine. In nonstimulated situations, NOS2 deficiency increased whole body
protein turnover and intestinal Gln uptake and
Cit production. In NOS2-KO +
l-NAME, the above-mentioned changes were reversed. After LPS in WT, whole body NO and
Cit production increased. In contrast to this, LPS decreased net intestinal Gln uptake, whole body NO, and
Cit production in NOS2-KO mice. Treatment of NOS2-KO +
l-NAME with LPS was lethal in eight of eleven mice (73%). The surviving mice in this group showed a major drop in intestinal
protein breakdown and synthesis to almost zero. Thus both in baseline conditions and during
endotoxemia, the absence of NOS2 upregulated NOS1 and/or NOS3, which increased intestinal metabolism. The drop in intestinal
protein metabolism in the endotoxemic NOS2-KO +
l-NAME group might play a role in mortality in that group.