The extraneuronal transport mechanism for noradrenaline (uptake2) avidly transports 1-methyl-4-phenylpyridinium (MPP+)

Abstract

The corticosterone-sensitive extraneuronal transport mechanism for noradrenaline (uptake2) removes the neurotransmitter from the extracellular space. Recently, an experimental model for uptake2 has been introduced which is based on tissue culture techniques (human Caki-1 cells). The present study describes some properties of uptake2 in Caki-1 cells and introduces a new substrate, i.e., 1-methyl-4-phenylpyridinium (MPP+). Experiments on Caki-1 cells disclosed disadvantages of tritiated noradrenaline as substrate for the investigation of uptake2. The initial rate of 3H-noradrenaline transport [kin = 0.58 microliter/(mg protein.min)] was low compared with other cellular transport systems and intracellular noradrenaline was subject to rapid metabolism (kO-methylation = 0.54 min-1). The neurotoxin MPP+ was found to be a good substrate of uptake2. Initial rates of specific 3H-MPP+ transport into Caki-1 cells were saturable, the Km being 24 micromol/l and the Vmax being 420 pmol/(mg protein.min). The rate constant of specific inward transport was 34 times higher [19.6 microliters/(mg protein.min)] than that of 3H-noradrenaline. The ratio specific over non-specific transport was considerably higher for 3H-MPP+ (12.6) than for 3H-noradrenaline (3.0). 3H-MPP+ transport into Caki-1 cells was inhibited by various inhibitors of uptake2. The highly significant positive correlation (p less than 0.001, r = 0.986, n = 7) between the IC50's for the inhibition of the transport of 3H-noradrenaline and 3H-MPP+, respectively, proves the hypothesis that MPP+ enters Caki-1 cells via uptake2. 3H-MPP+ is taken up via uptake2 not only by Caki-1 cells but also by the isolated perfused rat heart which is another established model of uptake2.(ABSTRACT TRUNCATED AT 250 WORDS)