Mechanisms contributing to fluid-flow-induced Ca²⁺ mobilization in articular chondrocytes

We previously showed that fluid flow, which chondrocytes experience in vivo and which results in a variety of morphological and metabolic changes in cultured articular chondrocytes, can also stimulate a rise in intracellular calcium concentration ([Ca²⁺](i)). However, the mechanism by which Ca²⁺ is mobilized in response to flow is unclear. In this study, we investigated the roles of intracellular Ca²⁺ stores, G-proteins, and extracellular ATP in the flow-induced Ca²⁺ response in bovine articular chondrocytes (BAC). Cells loaded with the Ca²⁺ sensitive dye Fura-2 were exposed to steady flow at 34 ml/min (37 dynes/cm²) in a parallel plate flow chamber. Whereas ryanodine and caffeine had no effect, both neomycin and thapsigargin significantly decreased the Ca²⁺(l) response to flow, suggesting a role for Ca²⁺ store release, possibly through an inositol 1,4,5-trisphosphate (IP₃)-dependent mechanism. Twenty-four-hour treatment with pertussis toxin also significantly decreased the response, suggesting that the mechanism may be G-protein regulated. In addition, ATP release by chondrocytes does not appear to mediate the flow-induced Ca²⁺ response because suramin, a P2 purinergic blocker, had no effect. These results suggest that BAC respond rapidly to changes in their mechanical environment, such as increased fluid flow, by a mechanism that involves IP₃ stimulated Ca²⁺(i) release and G- protein activation.