Taurine efflux through the volume-sensitive organic osmolyte/anion channel (VSOAC) is activated by hypotonic shock. The signalling pathways that link membrane stretch or the reduction in intracellular osmolarity to activation of taurine efflux remain unclear. Tyrosine phosphorylation is implicated in this process since the tyrosine kinase inhibitor tyrphostin a23 reduces osmosensitive taurine efflux in astrocytes (Mongin et al. 1999). Significantly, members of the mitogen-activated protein kinase (MAPK) family are coactivated with taurine efflux by hyposmolarity. To determine whether MAPKs are involved in regulating taurine efflux in the glioma cell line C6, inhibitors of the three key MAPKs were tested: the extracellular-signal regulated kinase (ERK) inhibitor PD 98059, the p38 inhibitor SB 202190 and the c-Jun NH₂-terminal kinase (JNK) inhibitors dicumarol (Krause et al. 2001) and SP 600125. JNK inhibitors were also tested on the epithelial cell line HeLa to determine whether JNK was implicated in other cell lines. ¹⁴C-taurine efflux was measured using methods previously described (Belsey et al. 2002). Since taurine efflux can be inhibited by blocking the pore of VSOAC or by inhibiting its activation pathway, the test compounds were either pre-incubated or directly applied to HeLa and C6 cells.

At 100 mM, tyrphostin a23 inhibited taurine efflux in HeLa after pre-incubation and when applied directly by 25.0 ± 1.6 and 32.2 ± 10.0 % (± S.E.M.; n = 3 for all data), respectively. It also inhibited taurine efflux in C6 cells after pre-incubation and when applied directly by 25.4 ± 4.9 and 37.2 ± 1.7 %, respectively. Taurine efflux is primarily activated by a decrease in osmolarity rather than membrane distension (Deleuze et al. 2000), unlike ERK and p38, which are primarily activated by membrane distension. Consistent with this notion, we found only a minimal effect on taurine efflux when both ERK and p38 were inhibited in C6 glioma. JNK is hypotonically activated, although little is known about its involvement in modulating taurine efflux. At 100 mM, dicumarol inhibited taurine efflux in C6 after pre-incubation and when applied directly by 72.6 ± 4.3 and 80.3 ± 2.3 %, respectively. It also blocked taurine efflux in HeLa by 90.3 ± 3.5 %, although it had only a minimal effect following pre-incubation. The more specific JNK inhibitor SP 600125 had minimal effect in HeLa when pre-incubated or when directly applied, supporting the notion that JNK plays no role in taurine efflux in this cell line and indicating that any blockade effect by dicuramol is not related to its effect on JNK. However, pre-incubating SP 600125 in C6 produced 50.7 ± 4.0 % block at 100 µM. Taken together, these results suggest that JNK plays a role in the osmosensitive taurine efflux pathway but this involvement is cell-type specific, since JNK only appears to play a role in C6, not HeLa.