TIME-DEPENDENT CHANGES IN THE RESPONSE OF CARTILAGE TO STATIC COMPRESSION SUGGEST INTERSTITIAL PH IS NOT THE ONLY SIGNALING MECHANISM

The goal of the present study was to reexamine the role of interstitia l pH in regulating the biosynthetic rate in cartilage tissue by addres sing two research questions: (a) Do small, short-term changes in inter stitial pH, induced independently by two different mechanisms (namely, by controlling the pH of the medium or by mechanical compression), re sult in biosynthetic rates commensurate with those expected from the ' 'natural'' relationship between interstitial pH and biosynthesis? and (b) Are the effects of changes in the pH of the medium or in compressi on the same for short-term (14-hour) and long-term (60-hour) exposures ? Biosynthetic rates were estimated from incorporation of sulfate and proline into explants of bovine epiphyseal cartilage during the final 14 hours of culture. These rates decreased with decreasing pH of the m edium, with increasing compression, and with decreasing native glycosa minoglycan content; or? expressed in terms of interstitial pH, acidifi cation induced by compression or by lowering the pH of the medium resu lted in a decreased biosynthetic rate, whereas interstitial acidificat ion effected by increasing glycosaminoglycan content enhanced it. When the time for which tissue was exposed to changes in the pH of the med ium was increased from 14 to 60 hours, the relationship between the bi osynthetic rate and the pH remained constant whereas the relationship between the biosynthetic rate and compression was reversed. These data suggest that the transduction mechanisms underlying the response to p H of the medium and compression differ and that some adaptation or sti mulation by modest levels of compression can occur with longer exposur es. Interstitial pH is not the sole determinant of biosynthesis, and i t cannot really account for the long-term response of cartilage tissue to static compression.