Influence of serum on secretion of hyaluronic acid by synovial cells. Its possible relevance in arthritis.

Fluid from normal synovial joints contains about 3 mg./ml. hyaluronic acid (Hamerman and Schuster, 1958). Otherwise, it resembles an ultrafiltrate of plasma, but with a total protein concentration of 13 mg./ml. (Balazs, Watson, Duff, and Roseman, 1967), which is about one-fifth of that in plasma. With the exception of a tiny fraction bound to hyaluronic acid (Sandson and Hamerman, 1962), the proteins are believed to be identical with those in plasma, although the proportions of the individual proteins differ (Binette and Schmid, 1965). In the inflamed joints of rheumatoid arthritis, the protein concentration rises toward that in plasma. The hyaluronic acid often becomes more dilute, but is much more abundant since the amount of synovial fluid is usually much increased (Balazs and others, 1967). Increased synovial fluid is associated with synovial proliferation in this and other forms of chronic arthritis, suggesting that inflammatory exudation might provide a common factor which stimulates synovial growth and secretion of hyaluronic acid. However, the increased content of hyaluronic acid in inflamed joints might arise also from decreased breakdown or disappearance from the joint, about which little is known, and it would be very difficult to study either aspect of hyaluronic metabolism in intact joints. A recent study of the influence of human serum upon growth of homologous synovial cells (Clarris and Fraser, 1968a) provided an opportunity to examine the effects of serum upon the secretion of hyaluronic acid by these cells in vitro. Samples from these experiments were stored at i5oC. pending the final development of an accurate microassay for hyaluronic acid in cell-culture medium. This paper describes the effects of serum upon the secretory activity of synovial cells in vitro. Material and Methods Growth media were obtained from experiments reported earlier in detail (Clarris and Fraser, 1968a). In each experiment, replicate cultures were established in 20 per cent. foetal calf serum for 24 hours after subculture, and were then randomly allotted for further growth in Medium 199 containing serum, either fresh or previously heat-inactivated for 30 minutes at 56°C., at concentrations of 10 and 50 per cent. The experiments were terminated after 46 to 74 hours. Two additional experiments were performed with concentrations of heat-inactivated serum ranging from 5 to 80 per cent. (Table III, below). Cell numbers were determined at the beginning and end of the experiments from which the following indices were calculated.