Spatial and Diurnal Variations in Sodium Content Within Intervertebral Disc Tissue

Area of Science:

• Biophysics
• Biochemistry
• Biomaterials Science

Background:

• Daily cycles in intervertebral disc (IVD) osmolarity are biophysical signals regulating cellular metabolism.
• These osmotic cycles arise from the deformation of the charged, hydrated extracellular matrix (ECM).
• Spatial variations in ECM composition and fixed charge density (FCD) obscure the magnitude of these osmotic cycles.

Purpose of the Study:

• Validate laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) for assessing intra-tissue sodium content.
• Quantify temporal and spatial changes in intra-tissue sodium content during diurnal loading cycles within the IVD.

Main Methods:

• Modified nucleus pulposus (NP) tissue FCD via equilibrium dialysis.
• Assessed intra-tissue sodium using inductively coupled plasma-optical emission spectroscopy (ICP-OES) and LA-ICP-MS.
• Measured diurnal sodium changes in bovine IVD segments under simulated physiologic loading.

Main Results:

• LA-ICP-MS sodium measurements strongly correlated with ICP-OES, theoretical FCD, and iodine content.
• The NP region exhibited higher sodium content and greater diurnal fluctuations than the annulus fibrosus (AF).
• Pericellular matrix (PCM) sodium, normalized to ECM, was higher at end-of-day than end-of-night, suggesting elevated osmotic fluctuations for cells.

Conclusions:

• LA-ICP-MS is a validated method for assessing intra-tissue sodium content in the IVD.
• Significant diurnal sodium variations occur within the IVD, particularly in the NP and PCM.
• These findings highlight the dynamic osmotic environment experienced by IVD cells during daily loading.