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 Ca(2+) waves, by stimulating the release of intracellular Ca(2+) 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 Ca(2+) waves dependent on the presence of gap junctions. In the absence of extracellular Ca(2+) 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 Ca(2+) sensitivity of intercellular waves is not due to major differences in gap junction constituent proteins. In HIG-82 synoviocytes, but not in chondrocytes, the Ca(2+) ionophore ionomycin stimulated phosphoinositide hydrolysis in a concentration-dependent manner, an effect strictly dependent on the presence of extracellular Ca(2+), suggesting the expression, in these cells, of a Ca(2+)-sensitive phospholipase C activity. Such an activity could be stimulated also by Ca(2+) influx induced by P(2Y) receptor activation and considerably amplifies ATP-induced inositol phosphate (InsP) production. In contrast, Ca(2+) 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 Ca(2+)-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.
Capozzi
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Dipartimento di Biochimica, Biofisica e Chimica delle Macromolecole, Università di Trieste, via Licio Giorgieri 1, I-34127 Trieste, Italy.
Tonon
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D'andrea
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D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D020013:Calcium Signaling; D002358:Cartilage, Articular / Q000166:cytology / Q000378:metabolism*; D019902:Chondrocytes / Q000166:cytology / Q000378:metabolism*; D018031:Connexin 43 / Q000096:biosynthesis; D017630:Connexins / Q000096:biosynthesis; D003600:Cytosol / Q000378:metabolism; D006868:Hydrolysis; D015759:Ionomycin / Q000494:pharmacology; D010716:Phosphatidylinositols / Q000378:metabolism*; D010812:Physical Stimulation; D011817:Rabbits; D013583:Synovial Membrane / Q000166:cytology / Q000378:metabolism*; D010738:Type C Phospholipases / Q000378:metabolism
D018031:Connexin 43; D017630:Connexins; D010716:Phosphatidylinositols; D015759:Ionomycin; D000255:Adenosine Triphosphate; D010738:Type C Phospholipases