Volume-dependent taurine release from cultured astrocytes requires permissive [Ca2+]iand calmodulin

Abstract

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 Ca2+ in volume-dependent [3H]taurine efflux from primary cultured astrocytes and compare the Ca2+ sensitivity of this efflux in slow (high K+ medium induced) and fast (hyposmotic medium induced) cell swelling. Neither Ca2+-free medium nor Ca2+-channel blockers prevented the volume-dependent [3H]taurine release. In contrast, loading cells with the membrane-permeable Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane- N, N, N′, N′-tetraacetic acid (BAPTA)-AM suppressed [3H]taurine efflux by 65–70% and 25–30% under high-K+ and hyposmotic conditions, respectively. Fura 2 measurements confirmed that BAPTA-AM, but not Ca2+-free media, significantly reduced resting intracellular Ca2+concentration ([Ca2+]i). The calmodulin antagonists trifluoperazine and fluphenazine reversibly and irreversibly, respectively, inhibited the high-K+-induced [3H]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 [Ca2+]iand involves calmodulin-dependent step(s). Quantitative differences in Ca2+/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.