A MODEL OF THE SODIUM DEPENDENCE OF DOPAMINE UPTAKE IN RAT STRIATAL SYNAPTOSOMES

Initial velocity of uptake of dopamine (DA) has been measured in rat s triatal synaptosomes as a function of both [DA] and [Na]. Carrier medi ated uptake is totally dependent on external sodium. The data were fit ted to a rapid equilibrium model which has been found in previous stud ies to fit, with appropriate simplification, uptake data for glutamate , GABA, and choline in several brain regions under varying conditions. This model also gives a good fit to the dopamine data. The minimal be st fit simplification of this model allows for DA uptake along with tw o sodium ions and predicts that apparent maximal velocity of uptake sh ould increase with [Na], while the Michaelis-Menten constant should de crease. The minimal best fit model for DA, and a number of kinetic par ameters which quantitate the model, are compared to those for the GABA , glutamate, and choline transporters. The results are consistent with a symmetrical, rapid equilibrium model, which has been presented prev iously for other neurotransmitters and precursors (18). This model off ers a unifying basis for understanding the sodium and membrane potenti al dependence of neurotransmitter transport and the possible participa tion of transporters in depolarization induced release throughout the CNS.