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

Abstract

The ability of ornithine alpha-ketoglutarate (OKG) to enhance macrophage cytotoxicity in stress situations has been described, but the mechanisms involved remain unclear. It is known that OKG administration generates glutamine (GLN), arginine (ARG), and polyamines. This study will (1) evaluate the effect of OKG on tumor necrosis factor alpha (TNF-alpha) secretion and nitric oxide (NO*) production in macrophages from glucocorticoid (DEX)-treated rats, and determine whether these effects can be reproduced by GLN or ARG supplementations, and (2) use in vivo metabolic inhibitors methionine sulfoximine (inhibitor of GLN synthetase), S-methylthiourea (inhibitor of inducible nitric oxide synthase), and difluoromethylornithine (inhibitor of ornithine decarboxylase) to assess the roles of GLN, ARG, and polyamines in OKG action. Controls received a mixture of nonessential amino acids (NEAA). GLN, ARG, and OKG all restored TNF-alpha secretion by macrophages of glucocorticoid-treated rats. The same results were obtained with GLN and ARG supplementation. However, the use of inhibitors clearly showed that OKG does not modulate TNF-alpha secretion by GLN, ARG, or polyamine pathways. We also observed 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 inhibitors, it appears that this action of OKG is probably mediated via polyamine synthesis and GLN. However, an oral administration of an equimolar amount of GLN failed to reproduce the OKG-mediated effect, possibly because OKG generates more GLN in the systemic circulation than GLN itself when these substances are given orally. Our results underline the complexity of the mechanism of action of OKG, which can differ according to the functions of even a single cell type.