Transport and metabolism of choline in synaptosomes: ionic requirement.

100pM-choline and incubated with lOOp~-[Me-~'C]choline only one energy of activation was observed (40.0kJ/mol) over the whole range of temperatures studied (6-40°C; Table 1). Similarly in nerve-cell cultures it was observed that, after incubation with concentrations of choline analogous to the ones present in the growth medium (30m~) , only one energy of activation was found (Massarelli et al., 1979). The results should be correlated with the changes in the endocellular choline pool when the synaptosomes are incubated in the absence of choline (Massarelli & Wong, 1980) and indicate that after preincubation in the absence of choline and incubation with very low concentrations of choline (lphi) the energy of activation showed a value not exceeding 29kJ/mol, which is rather low compared with other active-transport processes. Below 23-25OC, when the synaptosomal membranes are known to change their state, the uptake and the transport of choline appear to require more energy. This does not happen when the endocellular concentrations of choline are increased after preincubation and incubation with 1 0 0 ~ ~ choline.