Synthesis of hyaluronate in cultured bovine articular cartilage

The synthesis and distribution of hyaluronate and proteoglycan were studied in bovine articular cartilage in short-term explant culture with [3H]acetate and H2(35)SO4 as precursors. The incorporation of [3H]acetate into hyaluronate and sulphated glycosaminoglycans was linear with time, except that hyaluronate synthesis showed a marked lag at the beginning of the incubation. [3H]Hyaluronate represented 4-7% of the total [3H]glycosaminoglycans synthesized over a 6 h period. However, the distributions of [3H]hyaluronate and 3H-labelled sulphated glycosaminoglycans were different: about 50% of the newly synthesized [3H]hyaluronate appeared in the medium, compared with less than 5% of the 3H-labelled sulphated proteoglycans. A pulse-chase experiment revealed that the release of newly synthesized [3H]hyaluronate from cartilage was rapid. No difference was observed in the distribution of [3H]hyaluronate between medium and tissue by cartilage from either the superficial layer or the deep layer of articular cartilage. When articular cartilage was incubated with 0.4 mM-cycloheximide, proteoglycan synthesis was markedly inhibited, whereas the synthesis of hyaluronate was only partially inhibited and resulted in more of the newly synthesized hyaluronate being released into the medium. Analysis of the hydrodynamic size of [3H]hyaluronate isolated from cartilage on Sephacryl-1000 revealed one population that was eluted as a broad peak (Kav. less than 0.7), compared with two populations (Kav. greater than 0.5 and less than 0.5) appearing in the medium of cultures. These data suggest that hyaluronate is synthesized in excess of proteoglycan synthesis and that the hyaluronate that is not complexed with proteoglycans is rapidly lost from the tissue.