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Low-friction surfaces are critical for efficient joint articulation. The tribological properties of articular cartilage have been studied extensively in native tissue and joints. Despite their importance, very few studies have examined the frictional properties of tissue-engineered cartilage. We have therefore reviewed the relationship between composition, structure and friction in tissue-engineered cartilage.

Adenylate cyclase [ATP pyrophosphate lyase (cyclizing), EC 4.6.1.1] was shown to be present in cultured human articular chondrocytes. Optimal conditions of incubation time, protein and substrate concentrations and pH were determined in whole cell lysates. Maximal activity occurred at pH 8.5 with no decrease in activity up to pH 10.0. Adenylate cyclase activity of particulate membrane preparations was enhanced by the addition of crude cytosol preparations. The prostaglandins E1, E2, F1 alpha, F2 alpha, D2, B1, B2, A1 and A2, as well as adrenaline and isoprenaline, stimulated adenylate cyclase derived from either adult or foetal chondrocytes. No significant stimulation was observed in the presence of human calcitonin or glucagon. Bovine parathyroid hormone always significantly stimulated the adenylate cyclase derived from foetal chondrocytes, but not from adult chondrocytes. Preincubation of the chondrocytes in culture with indomethacin and with or without supernatant medium from cultured mononuclear cells increased the responsiveness of the adenylate cyclase to prostaglandin E1.

1. The addition of 50 000g cytosol preparations of isolated human platelets, cultured rat osteogenic sarcoma or cultured bone cells to particulate preparations of adenylate cyclase, from the same or unrelated tissues, caused marked enhancement of the hormone-stimulated enzyme activities. 2. The degree of enhancement obtained by addition of the cytosol preparations was similar to that observed on addition of GTP. 3. The enhancing activity of the three cytosol types was found to be sensitive to digestion by trypsin and alkaline phosphatase, partially heat-labile and partially inactivated by exposure to charcoal. 4. Gel filtration studies indicated an apparent molecular weight of 20 000–30 000. Further, the 20000-30000-mol.wt. fractions obtained by gel filtration could enhance the adenylate cyclase activity of particulate preparations derived from unrelated cell types. 5. The results suggest a common or similar adenylate-cyclase-enhancing factor or factors, protein in nature, present in the three cytosol types.