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NUTRIENT SUPPLY TO CELLS OF THE INTERVERTEBRAL DISC ; EFFECT OF DIURNAL HYDRATION CHANGES

INTRODUCTION. The disc is the largest avascular structure in the body. Nutrients, essential for maintaining cell viability and activity are supplied by diffusion from the blood supply and diffuse through the matrix to the cells where they are metabolised; products of metabolism, particularly lactic acid are removed by the reverse route [1]. Metabolite concentrations in the central nucleus thus depend both on rates of diffusion and on rates of consumption or production by the cells. A fall in nutrient concentration to critical levels is thought to be a major cause of disc degeneration. Although it is apparent that this can arise as the result of a fall in blood supply or endplate calcification [2;3] changes in cellular demand can also cause nutrient concentrations to fall to dangerous levels[4]. One factor which could affect nutrient demand is the diurnal fall and regain of around 25 percent of the disc's fluid. Here we examine the influence of these changes in fluid content on diffusion of solutes through the matrix and on cellular metabolic rates. METHODS: Diffusion coefficients (D) of solutes ranging in molecular weights from 0.05-70kD were measured in the outer annulus and nucleus of bovine caudal intervertebral discs from 18-24 month steers. Low molecular weight solutes (sodium, lactate, glucose, sucrose) were labelled radioactively and solutes from 3-70 kD were fluorescently-labelled dextrans. D was determined from the concentration-gradient developed after spotting the tracer onto disc strips [5]. Since disc composition is symmetrical lengthwise through the mid-transverse plane and has no composition gradients along the cephalic-caudal plane, strips (1x2x7 mm) were excised from the outer annulus fibrosus (transverse plane) at the distance of 1mm from the edge of the disc and from central nucleus. Rates of glucose and oxygen consumption and of lactic acid production (R ) were measured from changes in medium concentrations measured electrochemically or enzymatically as described previously [1]. D and R were measured under initial tissue hydration and in tissue loaded to 3MN/m by equilibration in 15 % polyethylene glycol solution [6]. Results are presented as mean + s.e.m (n=4-6 independent experiments) and significance was tested using a two-sided Student's t-test. RESULTS The effect of molecular weight on solute diffusivity is shown in Figure 1. Diffusivity was found to fall 10 fold with increase in solute size from 0.09kD (lactate) to 3kD (dextran). However with further increase from 3kD to 70kD, diffusivity fell by only 50%. For small solutes, diffusivity in the nucleus was greater than in the annulus, but the differences between the two regions disappeared for higher MW solutes.