Effects of calcium, calmodulin, protein kinase C and protein tyrosine kinases on volume-activated taurine efflux in human erythroleukemia cells

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Abstract
The effects of calcium, calmodulin, protein kinase C (PKC) and protein tyrosine kinase (PTK) modulators were examined on the volume-activated taurine efflux in the erythroleukemia cell line K562. Exposure to hypoosmotic solution significantly increased taurine efflux and intracellular calcium concentration ([Ca2+]i). The Ca2+ channel blockers La3+ (1 mM), verapamil (200 μM) and nifedipine (100 μM) inhibited the hypoosmotically-induced [Ca2+]i increase by more than 90%, while the volume-activated taurine efflux was inhibited by 61.3 ± 9.5, 74.1 ± 9.3 and 38.0 ± 1.5%, respectively. Furthermore, the calmodulin inhibitors W7 (50 μM) and trifluoperazine (10 μM) and the Ca2+/calmodulin-dependent protein kinase II inhibitor KN-62 (2 μM) significantly blocked the volume-activated taurine efflux by 93.4 ± 2.7, 77.9 ± 3.5 and 61.3 ± 15.8%, respectively. In contrast, the PKC inhibitor staurosporine (200 nM) or the PKC activator phorbol 12-myristate 13-acetate (100 nM) did not have significant effects on the volume-activated taurine efflux. However, pretreatment with PTK inhibitors genistein, tyrphostin A25, and tyrphostin A47 blocked the volume-activated taurine efflux. These results suggest that the volume-activated taurine efflux in K562 cells may not directly involve Ca2+, but may require the presence of calmodulin and/or PTK.