Spatial and Diurnal Variations in Sodium Content Within Intervertebral Disc Tissue

Intra-tissue sodium content assessed via LA-ICP-MS strongly correlated (<i>r</i> ≥ 0.95) with ICP-OES sodium content, theoretical FCD values, and LA-ICP-MS iodine content in equilibrated tissue. Diurnal sodium results demonstrate that at the macroscale the NP region possessed a greater sodium content than the annulus fibrosus (AF) and experienced greater diurnal changes ("end-of-day-NP" sodium [0.52 ± 0.18 mEq Na/g wet wt], "end of night-NP" sodium [0.34 ± 0.13 mEq Na/g wet wt]) than the AF which did not experience a diurnal change. At the microscale, average PCM sodium normalized to adjacent ECM sodium at the end-of-day (2.3 ± 0.96) was greater than at the end-of-night (1.5 ± 0.38), indicating cells embedded within their specialized PCM likely experience elevated osmotic fluctuations.