Changes in intracellular calcium concentration in response to hypertonicity in bovine articular chondrocytes

Intracellular calcium concentration ([Ca2+]i) in articular chondrocytes changes during mechanical challenges associated with joint movements, because of the fluctuation of the extracellular osmotic environment during joint loading. Matrix synthesis by chondrocytes is modulated by loading patterns, possibly mediated by variations in intracellular composition, including [Ca2+]i. The present study has employed the Ca(2+)-sensitive fluoroprobe Fura-2 to determine the effects of hypertonic shock on intracellular Ca2+ concentration ([Ca2+]i) and to characterise the mechanisms involved in the response for isolated bovine articular chondrocytes. In cells subjected to a hypertonic shock, [Ca2+]i rapidly increased by approximately 300%, reaching a maximal value within 50 s following the hypertonic shock with a recovery of more than 90% towards the initial [Ca2+]i within 5 min. The effect was inhibited by removal of extracellular Ca2+ ions, but not by thapsigargin, indicating that the rise in [Ca2+]i is only a result of influx from the extracellular medium. The rise was insensitive to inhibitors of L-type voltage-activated Ca2+ channels, TRPV channels or stretch-activated cation channels. Non-specific inhibitors of Ca2+ channels like CdCl2, NiCl2, LaCl3 and ZnCl2 significantly attenuated the response, although the extent in which CdCl2 and NiCl2 (both of them inhibitors of annexin-mediated Ca2+ fluxes) inhibited the response was significantly greater. The rise was also sensitive to KBR7943, inhibitor of NCE reverse mode and trifluoperazine, inhibitor of the activity of annexins. Hypertonic shock also produced also hyperpolarisation of chondrocytes (Em measured by means of Di-BA-C4(3), a membrane potential sensitive dye), which was inhibited by TEA-Cl and BaCl, but was not affected by changing the extracellular solution to Ca(2+)-free HBS. Inhibition of hyperpolarisation completely abolished the [Ca2+]i rise following hypertonic shock. Treatment with retinoic acid, which can increase the activity of annexins as Ca2+ transport pathways caused a significant increase in [Ca2+]i. The recovery of [Ca2+] was inhibited by benzamil and was dependent on extracellular Na+, but was unaffected by Na-orthovanadate, an inhibitor of plasma Ca(2+)-ATPase. We conclude that in response to hypertonic shock, NCE reverse mode and annexins are the pathways responsible for the [Ca2+]i increase, while forward mode operation of NCE is responsible for the subsequent extrusion of Ca2+ and recovery of [Ca2+]i towards initial values.