Method: Adult porcine TMJ disc and knee articular cartilage (AC) cells were enzymatically isolated and cultured on coverslips. Cell volume regulation measurements were performed by subjecting cells to a 20 minute 100 mOsm hypotonic shock (380 to 280 mOsm) using saline solutions. Differential interference contrast images were captured every 10 sec and cell volumes were estimated using a spherical model. Whole cell patch-clamp electrophysiology measurements were performed using an internal solution composed of (mM): 115 K-aspartate, 25 KCl, 5 NaCl, 1 CaCl2, 1 MgCl2, 10 EGTA, 10 HEPES, pH 7.2 KOH and 290 mOsm. Cells were voltage clamped at -60 mV and subjected to a series of 200 msec depolarizing steps.
Result: A passive response to hypotonic shock (~50% increase in cell volume) which reached a maximum within ~5 minutes was observed for both TMJ and AC cells, followed by an active regulatory volume decrease. Baseline electrophysiology measurements exhibited large outwardly rectifying voltage gated potassium currents and current voltage relationships revealed a resting membrane potential of ~38 mV for both TMJ and AC cells.
Conclusion: Cell volume measurements indicate that TMJ cells are capable of both passive and active cell volume regulation as has been previously shown for AC cells. Additionally, electrophysiology measurements suggest that voltage gated potassium currents regulate the resting membrane potential in TMJ cells under isosmotic conditions which has also been previously indicated for AC cells. Future measurements of membrane currents during osmotic shock on cells exposed to different nutrient conditions could illuminate possible mechanisms in the disease process.