Ca²⁺-sensitive phosphoinositide hydrolysis is activated in synovial cells but not in articular chondrocytes

Cell-to-cell diffusion of second messengers across intercellular channels allows tissues to co-ordinate responses to extracellular stimuli. Intercellular diffusion of inositol 1,4,5-trisphosphate, locally produced by focal stimulations, sustains the propagation of intercellular Ca²⁺ waves, by stimulating the release of intracellular Ca²⁺ in neighbouring cells. We previously demonstrated that in cultured articular chondrocytes and HIG-82 synovial cells, studied with digitial fluorescence video imaging, mechanical stimulation of a single cell induced intercellular Ca²⁺ waves dependent on the presence of gap junctions. In the absence of extracellular Ca²⁺ the propagating distance of the wave decreased significantly in HIG-82 cells, but appeared unaffected in chondrocytes. We now show that both cells types express connexin 43 and a similar functional coupling, thus suggesting that the different Ca²⁺ sensitivity of intercellular waves is not due to major differences in gap junction constituent proteins. In HIG-82 synoviocytes, but not in chondrocytes, the Ca²⁺ ionophore ionomycin stimulated phosphoinositide hydrolysis in a concentration-dependent manner, an effect strictly dependent on the presence of extracellular Ca²⁺, suggesting the expression, in these cells, of a Ca²⁺-sensitive phospholipase C activity. Such an activity could be stimulated also by Ca²⁺ influx induced by P_2Y receptor activation and considerably amplifies ATP-induced inositol phosphate (InsP) production. In contrast, Ca²⁺ influx did not affect considerably the response of chondrocytes to ATP stimulation. In HIG-82 cells, the combined application of ionomycin and ATP maximally stimulated InsP synthesis, suggesting the involvement of two independent mechanisms in inositol phosphate generation. These results suggest that in HIG-82 synovial cells the recruitment of a Ca²⁺-sensitive phospholipase C activity could amplify the cell response to a focally applied extracellular stimulus, thus providing a positive feedback mechanism for intercellular wave propagation.