Effects of taurine on calcium binding and accumulation in rabbit hippocampal and cortical synaptosomes

The effect of taurine on Ca(2+) binding and uptake was studied with rabbit brain cortical and hippocampal synaptosomes. Taurine (25 mM) increased by 25% the high affinity (45)Ca(2+) binding in the cortical fraction and by 55% in hippocampal synaptosomes but had no effect on low affinity Ca(2+) binding. Taurine decreased significantly the fluorescence of the chlorotetracycline-hydrophobic Ca(2+) chelate probe in both synaptosomal fractions which suggests a shift of bound Ca(2+) from the hydrophobic to the hydrophilic part of the membranes. The uptake of (45)Ca(2+) by rabbit brain synaptosomes, when measured in control and 65 mK K(+)-containing media, was not influenced by taurine. However, taurine inhibited significantly the (45)Ca(2+) uptake in synaptosomes incubated in media containing moderately increased K(+) concentrations (14 and 20 mM K(+)). The effects of taurine are discussed in conjunction with its stabilizing effect on excitable membranes.