Basic cell metabolism of articular cartilage. Manometric studies

Basic metabolism generates energy by the anaerobic or aerobic metabolization of glucose. The articular cartilage obtains the substrate and oxygen from the synovial fluid. Their concentrations within the cartilage diminish with increasing proximity to the basal layer at the bone-cartilage boundary. The gradients depend upon the cell density and the consumption by the cells. The gradient of the partial oxygen pressure (PO2) could provide the conditions for the Pasteur effect, which, however, has not yet been clearly observed. Metabolic reactions of chondrocytes to positive or negative glucose gradient have not hitherto been examined, and are the object of the present study. The oxygen consumption (QO2) in the articular cartilage of pig femoral heads was measured manometrically (Warburg technique) in the absence of glucose and in its presence in concentrations of 1.25 to 10.0 mM. There was a close correlation between the QO2 and the concentration and consumption of glucose having the nature of a glucose-induced respiratory suppression (Crabtree effect). The consumption of glucose, which did not rise in proportion to its concentration, did not exceed 15.7 mumol/gdw/h. The maximum lactate production of 20 to 23 mumol/gdw/h was attained at 5 mM glucose and showed no further increase at higher substrate concentrations. This stoichiometric "lactate deficiency" means that the absorbed but not glycolyzed glucose is channeled into synthesizing and storage processes. In glucose-free incubation, despite aerobic conditions, lactate was utilized by the chondrocytes only at unphysiologically high concentrations greater than 10 mM. Potassium cyanide reduced the QO2 by only 80-90% as a sign of the activity of non-mitochondrial oxidases. With monoiodoacetate (MIA), respiration fell dramatically during the 4 h of substrate-free incubation. It can be concluded from the results that, under physiological conditions, the glucose level in the synovia reduces the consumption of oxygen in the well-glucose-supplied upper layer of the articular cartilage, thus allowing an oxidative compensation of the diminishing glycolysis in the basal zone. The Crabtree effect, which is demonstrated for the first time in chondrocytes, thus seems to have the role of an essential regulatory mechanism in the basic metabolism of the cartilage.