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Rates of energy metabolism of disc cells

Disc cells depend on diffusion from blood vessels at the disc margins for supply of nutrients. Loss of supply is thought to lead to disc degeneration, but how loss of supply affects nutrient concentrations in the disc is not known; nutrient concentrations within discs can normally only be calculated, since concentration measurements are invasive. However, realistic predictions cannot be made until there is data from measurements of metabolic rates at conditions found in the disc in vivo, i.e. at low levels of oxygen, glucose and pH.

Here in vitro measurements of metabolic rates of isolated bovine nucleus pulposus cells were made at varying levels of oxygen, glucose and pH to understand conditions centre of the disc. The objective was to obtain quantitative information on the interactions between oxygen and glucose concentrations and pH, and the rates of oxygen and glucose consumption and lactic acid production for disc nucleus cells. A metabolism chamber was designed to allow simultaneous recording of oxygen and glucose concentrations and of pH. These concentrations were measured electrochemically with custom-built glucose and oxygen sensors; lactic acid was measured biochemically. Bovine nucleus pulposus cells were isolated and inserted into the chamber, and simultaneous rates of oxygen and glucose consumption and of lactic acid production were measured over a range of glucose, oxygen and pH levels.

There were strong interactions between rates of oxygen consumption and lactic acid production and pH. At atmospheric oxygen levels, oxygen consumption rate at pH6.2 was 32% of that at pH7.4. The rate fell by 60% as oxygen concentration was decreased from 21% to 5% at pH7.4, but only by 20% at pH6.2. Similar interactions were seen for lactic acid production and glucose consumption rates; we found that glycolysis rates fell at low oxygen and glucose concentrations and low pH. Equations were derived which satisfactorily predict the effect of nutrient and metabolite concentrations on rates of lactic acid production rate and oxygen consumption. Disc cell metabolism in air and at pH7.4 differs markedly from that found in the disc nucleus in vivo, where low levels of oxygen, glucose and pH all co-exist. Modelling and experimental work needs to consider interactions on metabolism; measurements in vitro under standard laboratory conditions will not predict behaviour in vivo.

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Oxford University, Physiology Laboratory
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