Nitric oxide regulates mitochondrial respiration and functions of articular chondrocytes

Objective. Biologic effects of nitric oxide (NO) have been shown to increas e under hypoxic conditions. Because the oxygen tension in joint cavities of patients with arthritis is fairly low, biologic effects of NO would be exp ected to be significantly large in these compartments. This study was under taken to investigate the effects of NO on the energy metabolism and functio ns of articular chondrocytes under different oxygen tension conditions. Methods. Articular chondrocytes from rabbits were cultured under various ox ygen concentrations in the presence or absence of NO and NOC18, an NO donor . Cellular respiration was measured using a Clark-type oxygen electrode. Le vels of ATP in the cells were determined according to the luciferin-lucifer ase method. Cellular synthesis of proteoglycans was determined by measuring the incorporation of radioactivity (derived from S-35-labeled SO4) into gl ycosaminoglycans. Expression of stress-related proteins was evaluated by We stern blotting analysis using specific antibodies. Results, Respiration and ATP synthesis of cultured chondrocytes were inhibi ted by NO, particularly under low oxygen concentrations, The presence of ei ther NO or specific inhibitors of mitochondrial electron transport suppress ed the synthesis of proteoglycans without affecting cell viability. When ex posed to NO, cellular levels of heat-shock protein 70 (hsp70) and heme oxyg enase 1 (HO-1) increased markedly. The presence of inhibitors of mitochondr ial electron transport also increased cellular levels of hsp70 and HO-1. Conclusion, These results suggest that NO generated in the joint might inhi bit energy metabolism and the synthesis of proteoglycans of chondrocytes, t hereby modulating pathophysiologic processes occurring in patients with art hritis.