The Effect of Inorganic Orthophosphate on the Rates of Collagen Formation and Degradation in Bone and Cartilage in Tissue Culture

Using a tissue culture system, the effect of the concentration of inorganic orthophosphate [(Pi)] in the tissue culture medium on the synthesis and degradation of collagen in bone and cartilage was studied. A consistent observation was that the rate of synthesis of bone and cartilage collagen was markedly reduced when the [(Pi)] concentration of the tissue culture medium was lowered to 0.32 mm. In addition, less of the collagen that was synthesized was incorporated into the tissue. Increasing the [(Pi)] concentration beyond 1.30 mm (a concentration at which this effect was reversed) to concentrations as high as 3.23 mm of [(Pi)] did not significantly alter the rate of collagen synthesis in cartilage or bone. The [(Pi)] concentration in the tissue culture medium affected the rate at which the older structural collagen was degraded only when the rate of bone collagen resorption was elevated by increasing the O2 tension in the culture chamber and especially when, in addition to increasing the O2 tension, parathyroid extract was added to the medium. Under these circumstances, the rate of resorption of the older collagen was increased at [(Pi)] concentrations of 1.30 mm and 0.32 mm compared to the rates at [(Pi)] concentrations of 2.26 mm and 3.23 mm, but no significant differences were observed in the rates at these two higher concentrations of [(Pi)]. Under chamber conditions of 80% O2 with parathyroid extract added to the culture medium, the rate of bone collagen was reduced by approximately 50%, and 90% of the collagen that was synthesized was degraded and not incorporated into the tissue when the [(Pi)] in the tissue culture medium was reduced to 0.32 mm. In addition, approximately 25% of the collagen which was present in the tissue at the start of the experiment was degraded and released from the tissue during the incubation period. As a result, at the end of the 8-day period of tissue culture in this experiment, there was less bone collagen in the explants than at the start.