NET TAURINE TRANSPORT AND ITS INHIBITION BY A TAURINE ANTAGONIST

P-2-fractions were isolated from rat brain, and used to study net taur ine transport. The fractions were incubated in increasing concentratio ns of [H-3]taurine and the intraterminal concentration measured by liq uid scintillation and amino acid analysis. The membrane potential of t he isolated fractions was estimated using Rb-86(+) as a marker for int racellular K+. Taurine was synthesized in the P-2-fraction when incuba ted in taurine free medium. At external taurine concentrations below 3 70 mu M a significant amount of the endogenous taurine was released to the incubation medium. Net taurine uptake into the P-2-fraction was a chieved at external taurine concentrations exceeding 370 mu M The taur ine antagonist hyl-3-methyl-4H,1,2,4-benzothiadiazine-1,1-dioxide (TAG ) competitively inhibited taurine and [H-3]taurine transport into the P-2-fraction. As the external concentration of taurine was increased, the accumulation of Rb-86(+) into the P-2-fraction was facilitated. Th is indicated an increasing hyperpolarization of the neuronal membrane as taurine transport shifted from release towards uptake. TAG reduced the hyperpolarization that paralleled taurine accumulation, in a dose dependent manner. Our results indicate that relatively low transmembra nal gradients of taurine may be maintained by an electrogenic taurine transporter having a large transport capacity. Such a transporter may well serve the needs of osmotic regulation, i.e. to transport large am ounts of taurine in any direction across the neuronal membrane.