Effects of fluoride on in vitro calcification of tendon matrix

Ca2+ and Pi uptake induced in vitro by a collagenous matrix derived from bovine tendon is inhibited by 1 X 10(-6) to 2 X 10(-5) M NaF and stimulated by 2 X 10(-5) to 2 X 10(-3) M NaF. Fluoride uptake occurs only over the latter concentration range. The uptake of Ca2+, Pi, and F-1 progresses toward a limiting extent at which the molar Ca/P and Ca/F values are 1.6 to 1.7 and 4.5 to 5.7, respectively. Although the matrix-bound mineral, previously formed in the absence of NaF, readily undergoes dissolution when exposed to a Ca2+- and P-free medium of pH less than 7.4, the bound mineral phase formed in the presence of NaF does not. We conclude that fluoroapatite is the primary matrix-bound mineral. The uptake of fluoride, Ca2+. amd Pi by both uncalcified and previously calcified matrices is inhibited by methylenediphosphonate and by phosphonoacetate as is calcification in the absence of NaF. Kinetic studies indicate that formation of a CaP complex precedes the uptake of F-1 and suggest that F-1 and OH-1 compete for interaction with that CaP complex during the calcification process. We concluded that fluoroapatite formation induced by the collagenous matrix occurs by a multistep pathway comparable to that proposed previously for hydroxyapatite formation.