Simultaneous measurements of ionic currents and leucine uptake at the amino acid cotransporter KAAT1 expressed in Xenopus laevis oocytes

t: The transport properties of the intestinal amino acid cotransporter KAAT1, heterologously expressed in Xenopus oocytes, were studied using simultaneous voltage-clamp and tritiated leucine uptake measurements. While addition of 1 mM leucine to oocytes kept at -80 mV in presence of Na+ or K+ caused an increase in holding current, in presence of Li+ the current was reduced. Uptake measurements in voltage-clamp conditions showed that a comparable accumulation of amino acid occurred in all three ionic conditions and irrespective of the direction and amount of the current change. The ratio of moles of transferred charge to moles of transported amino acid ranges from 1.45 for K+ to 3.52 for Li+. A hypothetical interpretation involving the coexistence of two populations of transporters, one operating in the uncoupled mode and the other in the substrate transport mode is discussed.