Taurine-calcium interactions in frog rod outer segments: taurine effects on an ATP-dependent calcium translocation process

Frog rod outer segments (ROS) isolated in a Ca-free, EGTA-containing medium, showed a rapid ATP-dependent accumulation of 45Ca, GTP, CTP, ITP, UTP, GMP and beta-, gamma-methylene ATP did not substitute for ATP in energizing Ca uptake. This process required Mg, it was abolished in the presence of Ca ionophores A23187 and X537A and is not affected by external sodium. The Arrhenius activation energy was 7.9 kcal/mol and the pH optimum was approximately 7.2. The apparent Km for Ca uptake was 66 microM with a V max of 12.5 nmol/mg protein. 45Ca accumulation was reduced in illuminated ROS. The presence of 5-25 mM taurine, but not of GABA, glycine, histidine or proline, markedly enhanced Ca uptake by ROS.