REGULATION OF TAURINE TRANSPORT IN RAT ASTROCYTES BY PROTEIN-KINASE-C- ROLE OF CALCIUM AND CALMODULIN

Phorbol 12-myristate 13-acetate, a potent 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 i nhibition was obtained at 1 mu M 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 tra nsporter as taurine, was inhibited to a comparable extent. The effect of PMA was potentiated by cotreatment of the cells with thapsigargin o r the Ca2+ ionophore A-23187. However, ethylene glycol-bis(beta-aminoe thyl ether)-N,N,N',N'-tetraacetic acid and verapamil did not prevent t he 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 increas e in the Michaelis-Menten constant.