Synaptosomes from rat brain accumulate choline by two kinetically distinct processes, a high-affinity uptake system [Michaelis constant (Km) = 1 X 10-6M], and a low-affinity system (Km = 9 X 10 -5M). The high-affinity uptake system requires sodium, and is associated with considerable formation of acetylcholine. The low-affinity uptake system is less dependent on sodium, and does not appear to be associated with a marked degree of acetylcholine formation. The high-affinity choline uptake appears to represent selective choline accumulation by cholinergic neurons.
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