Healthy adult dogs were studied for a defect in proteoglycan aggregation by immobilizing one limb for varying periods of time. Immobilization for 6 days resulted in a 41% reduction in proteoglycan synthesis by articular cartilage from the restrained knee compared with the contralateral control knee. After 3 weeks of immobilization, proteoglycan aggregation was no longer demonstrable in cartilage from the constrained limb. The aggregation defect was rapidly reversible and aggregates were again normal size 2 weeks after removal of a cast that had been worn for 6 weeks.In normal articular cartilage most of the proteoglycans (PG) exist in large aggregates that are noncovalently linked to hyaluronic acid (HA) (1). In osteoarthritis (OA) an aggregation defect exists within the tissue, since a greater than normal proportion of the PGs is not aggregated and aggregates which are present tend to be smaller than normal (2-5).Aggregation defects similar to those of OA in

The influence of static and intermittent stress on articular cartilage metabolism was examined in vitro. Full‐thickness plugs of cartilage from femoral condyles of normal adult dogs were cultured while static or cyclic stresses were applied for 2 hours. The plugs were then incubated under atmospheric pressure for 2 hours in medium containing radioactive label, to provide measurements of net synthesis of glycosaminoglycan (GAG) or protein. As a control, cartilage from the same knee was cultured in the apparatus at atmospheric pressure. When cartilage plugs were exposed to static stress, or to cyclic stresses at a duty cycle of 60 seconds on/60 seconds off, net GAG synthesis was suppressed to 30—60% of that in controls. In contrast, when a duty cycle of 4 seconds on/11 seconds off was used, GAG synthesis was increased by 34%. The duty cycle which increased GAG synthesis did not affect protein synthesis or tissue contents of DNA, uronic acid, or water. At the cycle which suppressed GAG synthesis, protein synthesis and uronic acid content were decreased, and water content was increased. As judged by uptake of 14C‐aminoisobutyric acid and 14C‐xylose, the above changes in GAG synthesis do not appear to have been due to changes in diffusion of nutrient molecules through the cartilage during loading.

Articular cartilage from the knees of 4 dogs whose ipsilateral paws had been transected 6 weeks earlier (knee,,,,,), and from their contralateral knees (knee,,), was examined. Knee,,,,, did not bear weight as a result of the surgical procedure but active motion of the joint, determined with an angular displacement monitor during walking, was comparable to that of knee,,.. In comparison to knee,, cartilage, knee,,,, samples showed decreases in thickness, Safranin-0 staining of the matrix, and uronic acid content (mean, 24.4%), and increase in water content (mean, 5.9%). Incorporation of into proteoglycans was 34-67% less in knee,,,,, than in knee,,, cartilage. Proteoglycan (PC) aggregation in knee,, cartilage was normal, whereas in knee,,... cartilage the bulk of the proteoglycans, and also those that had been newly synthesized ('5S-proteoglycans), did not exist in aggregates. This defect in aggregation was due, at least in part, to an abnormality in the hyaluronatebinding region of the core protein of the proteoglycans, since they did not interact in vitro with hyaluronic acid These changes are essentially identical to those shown

The effect of vigorous exercise on the reversibility of canine knee cartilage atrophy produced by immobilization OF the leg was studied. In comparison to cartilage from the contralateral control knees, cartilage from knees which had been immobilized in a cast For 6 weeks showed an increase in water content and decreases in thickness, Safranin 0 staining of the matrix, uronic acid content, and net proteoglycan synthesis. In addition, the ability OF both newly synthesized (3'S) and total tissue proteoglycans to interact with hyaluronic acid to Form aggregates was diminished; this was apparently due to an abnormality in the hyaluronate-binding region of the core proteins. If the casts were removed and the animals were then allowed to ambulate ad libitum for 3 weeks, a11 of these changes were reversed. However, knee cartilage from 3 dogs which had been run daily on a treadmill (6 miledday) for 3 weeks after removal of the casts exhibited continuing decreases in thickness, Safranin 0 staining, and uronic acid content (mean 31%), even though net proteoglycan synthesis was increased (mean 16%) in comparison to that in control cartilage from the contralateral (nonimmobilized) knee. Further- more, the abnormality in both 35!3-and total tissue proteoglycans which precluded their interaction with high molecular weight hyaluronic acid persisted. In this respect, the proteoglycans were indistinguishable from those obtained from knee cartilage immediately following 6 weeks in a cast.A number of reports have detailed the morphologic features of the degeneration that occurs in articular cartilage with immobilization of a limb (1-5). Few studies, on the other hand, have focused on the alterations in articular cartilage proteoglycans (PG) that result from immobility (6-7). In normal joint cartilage, most PG exist in large aggregates, in which a number of PG are noncovalently associated with a single filamentous molecule of hyaluronic acid (HA) in a linkage stabilized by tissue glycoproteins (8). PG account for most of the elasticity ofjoint cartilage and for its ability to resist compression (9). The biologic function of the aggregate has yet to be defined, but by virtue of its enormous size (over50 x ddtons) (lo), it presumably serves to contain the PG within the collagen meshwork of the cartilage.We have recently shown that the atrophy of knee cartilage which develops when the hind limb of a normal adult dog is immobilized in a cast for 6 weeks is accompanied by decreases in uronic acid content and net PG synthesis (7). Furthermore, newly synthesized PG from cartilage of the immobilized limb did not aggregate in vitro, due to an apparent abnormality in the HA-binding region of the PG core protein (7). All of these effects of immobilization were rapidly reversible if the casts were removed and the dogs were then allowed to ambulate ad libitum "on all fours" for 3 MATERIALS AND METHODSSource of tissue and procedure for immobilization and exercise. The right hind limbs of 7 adult mongrel dogs (25-30 kg) were immobilized against t...

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