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ACCELERATION OF CHOLINE UPTAKE AFTER DEPOLARIZATION‐INDUCED ACETYLCHOLINE RELEASE IN RAT CORTICAL SYNAPTOSOMES

Abstract— Activation of nerve elements in vivo and in vitro is associated with an increased rate of choline uptake by a Na+‐dependent high affinity transport system. Following the methodology of Barker (1976), rat cortical synaptosomes were depolarized (37°C, 10min) by 25mM‐KCl in the presence of CaCl2 (1 mM) or other divalent cations. After reisolation by centrifugation, the rate of 3H‐choline uptake (1.25μM) was measured by Millipore filtration. KCl treatment alone failed to accelerate the rate of uptake in the reisolated synaptosomes. CaCl2, BaC12 or SrCl2 (but not MgCl2 or MnCl2) were necessary (1 mM) to observe the KCl induced acceleration. Moreover, RbCl, but not LiCl or CsCl, also produced the calcium‐dependent rate enhancement in the reisolated synaptosomes. The conditions mediating the enhanced rate of choline uptake correlated strongly with those associated with neurotransmitter release. To test this possibility, synaptosomal acetylcholine content was measured in response to the various salt treatments. Treatment with KCI (25 mM) and CaCl2 (1 mM), but not KCl alone, reduced the synaptosomal acetylcholine content from 154 to 113pmol/mg protein. The respective rates of choline uptake increased about 60%. The increased rate was reversed by incubation with 50 μM‐choline followed by synaptosome reisolation. This procedure also normalized the acetylcholine content. In summary, the rate of choline uptake by the high affinity choline uptake system is inversely related to the synaptosomal acetylcholine content.