The role of BKCa channels on hyperpolarization mediated by hyperosmolarity in human articular chondrocytes

Chondrocytes, the only cell in cartilage, are subjected to hyperosmotic challenges continuously since extracellular osmolarity in articular cartilage increases in response to mechanical loads during joint movement. Hyperosmolarity can affect membrane transport, and it is possible that load modulates matrix synthesis through alterations in intracellular composition. In the present study, the effects of hyperosmotic challenges were evaluated using the whole-cell patch clamp technique, whole cell mode on freshly isolated human and bovine articular chondrocytes. In human chondrocytes, hypertonicity induced the activation of outward Ca2+-sensitive K+ currents, which were inhibited by iberiotoxin and TEA-Cl. The current induced by hypertonic switching (osmolarity from 300 to 400 mOsm/l) caused cell hyperpolarization (from -39 mV to -70 mV) with a reversal potential of -96 +/- 7 mV. These results suggest a role for Ca2+-activated K+ channels in human articular chondrocytes, leading to hyperpolarization as a consequence of K+ efflux through these channels. These channels could have a role in the articular chondrocyte's response to a hyperosmotic challenge and matrix metabolism regulation by load.