Volume-dependent taurine release from cultured astrocytes requires permissive [Ca(2+)](i) and calmodulin.
Cell swelling results in regulatory activation of multiple conductive anion pathways permeable toward a broad spectrum of intracellular organic osmolytes. Here, we explore the involvement of extracellular and intracellular Ca(2+) in volume-dependent [(3)H]taurine efflux from primary cultured astrocytes and compare the Ca(2+) sensitivity of this efflux in slow (high K(+) medium induced) and fast (hyposmotic medium induced) cell swelling. Neither Ca(2+)-free medium nor Ca(2+)-channel blockers prevented the volume-dependent [(3)H]taurine release. In contrast, loading cells with the membrane-permeable Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-AM suppressed [(3)H]taurine efflux by 65-70% and 25-30% under high-K(+) and hyposmotic conditions, respectively. Fura 2 measurements confirmed that BAPTA-AM, but not Ca(2+)-free media, significantly reduced resting intracellular Ca(2+) concentration ([Ca(2+)](i)). The calmodulin antagonists trifluoperazine and fluphenazine reversibly and irreversibly, respectively, inhibited the high-K(+)-induced [(3)H]taurine release, consistent with their known actions on calmodulin. In hyposmotic conditions, the effects were less pronounced. These data suggest that volume-dependent taurine release requires minimal basal [Ca(2+)](i) and involves calmodulin-dependent step(s). Quantitative differences in Ca(2+)/calmodulin sensitivity of high-K(+)-induced and hyposmotic medium-induced taurine efflux are due to both the effects of the inhibitors on high-K(+)-induced cell swelling and their effects on transport systems and/or signaling mechanisms determining taurine efflux.
Mongin
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Division of Neurosurgery, Albany Medical College, Albany, New York 12208, USA.
Cai
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Kimelberg
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D000818:Animals; D001253:Astrocytes / Q000166:cytology* / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002147:Calmodulin / Q000037:antagonists & inhibitors / Q000502:physiology*; D002478:Cells, Cultured; D002614:Chelating Agents / Q000494:pharmacology; D003470:Culture Media / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism*; D009994:Osmolar Concentration; D011188:Potassium / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013654:Taurine / Q000378:metabolism*; D014268:Trifluoperazine / Q000494:pharmacology
D002121:Calcium Channel Blockers; D002147:Calmodulin; D002614:Chelating Agents; D003470:Culture Media; D013654:Taurine; D014268:Trifluoperazine; D011188:Potassium; D002118:Calcium