Regulation of taurine transport in rat astrocytes by protein kinase C: role of calcium and calmodulin

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Abstract
Phorbol 12-myristate 13-acetate, a potential stimulator of protein kinase C (PKC), inhibited taurine uptake in rat astrocytes. This effect was mimicked by 1-oleoyl-2-acetyl-sn-glycerol, an endogenous stimulator of PKC, and by r-59949, an inhibitor of diacylglycerol kinase. Maximal inhibition was obtained at microM phorbol 12-myristate 13-acetate (PMA) after 1 h of treatment. This effect was prevented by pretreatment of the cells with chelerythrine, a potent and selective inhibitor of PKC. The transport of beta-alanine, an amino acid that shares the same transporter as taurine, was inhibited to a comparable extent. The effect of PMA was potentiated by cotreatment of the cells with thapsigargin or the Ca2+ ionophore A-23187. However, ethylene glycol-bis(beta-aminoethyl ether)-N,N,N1,N1-tetraacetic acid and verapamil did not prevent the PMA effect. Pretreatment of the cells with calmodulin antagonists W-13 or calmidazolium, prevented the PMA-induced inhibition of taurine uptake. This inhibition was not affected by cycloheximide, actinomycin D, colchicine, or cytochalasin D. The Na(+)-to-Cl(-)-to-taurine coupling ratio was unaffected. Dimethyl amiloride, a selective inhibitor of Na+/H+ antiport, was unable to prevent the effects of PMA. These effects were associated with a decrease in the maximal velocity and an increase in the Michaelis-Menten constant.