A multisubstrate mechanism of striatal dopamine uptake and its inhibition by cocaine

A study of Na+ and Cl- as co-substrates in dopamine uptake into striatal suspensions and inhibition of dopamine uptake by cocaine was made by monitoring the initial velocity of the uptake of exogenously added non-radioactively labeled dopamine using a rotating disk electroanalytical technique with 50 msec resolution. Dopamine, in the concentration range of 0.025 to 4.00 microM, was found to be taken up rapidly into the tissue phase of striatal suspensions following the apparent zero order rate law for the first 25 sec. The observed, dopamine concentration-dependent, initial velocity data were first analyzed graphically using the Eadie-Hofstee transformation of the Michaelis-Menten kinetic equation and, subsequently, using all of the velocity data and the results of the graphical analyses, by non-linear curve fitting. Dopamine uptake was found to be first order in dopamine with a Vmax of 582 pmol/sec/g wet weight and a Km of 1.2 microM. The results of experiments in which choline and isethionate were substituted for Na+ and Cl-, respectively, suggested that the uptake process is second order in Na+ and first order in Cl-. Multisubstrate analyses of the initial velocities of uptake over the concentration range of 0.025 to 1.5 microM dopamine suggested that the mechanism of binding of dopamine to the uptake carrier is a partially random, sequential mechanism where dopamine or Na+ binds first with the uptake carrier and Cl- binds last. Cocaine was found to uncompetitively inhibit dopamine uptake and competitively inhibit both Na+ and Cl- binding (apparent Km values: 131 and 51 mM, respectively), suggesting that the mechanism of cocaine inhibition may be to bind to the dopamine occupied uptake carrier complex at the Na+ binding site.