Matrix-bound growth factors are released upon cartilage compression by an aggrecan-dependent sodium flux that is lost in osteoarthritis by Keppie, SJ, Mansfield, JC, Tang, X, Philp, CJ, Graham, HK, Önnerfjord, P, Wall, A, McLean, C, Winlove, CP, Sherratt, MJ, Pavlovskaya, GE, Vincent, TL