Involvement of glutamine, arginine, and polyamines in the action of ornithine alpha-ketoglutarate on macrophage functions in stressed rats

The ability of ornithine alpha-ketoglutarate (OKG) to enhance macrophage cy totoxicity in stress situations has been described, but the mechanisms invo lved remain unclear. It is known that OKG administration generates glutamin e (GLN), arginine (ARG), and polyamines. This study will (1) evaluate the e ffect of OKG on tumor necrosis factor alpha (TNF-alpha) secretion and nitri c oxide (NO.) production in macrophages from glucocorticoid (DEX)-treated r ats, and determine whether these effects can be reproduced by GLN or ARG su pplementations, and (2) use in vivo metabolic inhibitors methionine sulfoxi mine (inhibitor of GLN synthetase), S-methylthiourea (inhibitor of inducibl e nitric oxide synthase), and difluoromethylornithine (inhibitor of ornithi ne decarboxylase) to assess the roles of GLN, ARG, and polyamines in OKG ac tion. Controls received a mixture of nonessential amino acids (NEAA). GLN, ARG, and OKG all restored TNF-alpha secretion by Macrophages of glucocortic oid-treated rats. The same results were obtained with GLN and ARG supplemen tation However, the use of inhibitors clearly showed that OKG does not modu late TNF-alpha secretion by GLN, ARG, or polyamine pathways. We also observ ed that OKG enhanced NO. release by stimulated macrophages (DEX-OKG, 1.77 /- 0.64 vs. DEX-NEAA, 0.29 +/- 0.29 nmol/10(6) cells, P < 0.05). Using inhi bitors, it appears that this action of OKG is probably mediated via polyami ne synthesis and GLN. However, an oral administration of an equimolar amoun t of GLN failed to reproduce the OKG-mediated effect, possibly because OKG generates more GLN in the systemic circulation than GLN itself when these s ubstances are given orally. Our results underline the complexity of the mec hanism of action of OKG, which can differ according to the functions of eve n a single cell type.