High-affinity choline uptake under various pH conditions: investigation of synaptosomal fluorescence

The choline transport into synaptosomes obtained from the rat brain striatum was studied in the presence and absence of Na+ ions within the pH range 3.9-9.1. The intrinsic tryptophan fluorescence was investigated in synaptosomes at varying pH. Three pH ranges with different interpositions of the Na(+)-dependent and Na(+)-independent uptake curves were revealed: acid, alkaline, and weak-acid regions. It is assumed that pH 6-7 may be important in regulation of the cholinergic function because of the complicated interpositions of the two curves. The Km and Vmax values of both the Na(+)-dependent and Na(+)-independent processes for the striatal fraction were calculated using Lineweaver-Burk plots at three pH values. Some differences in the intrinsic tryptophan fluorescence of the synaptosomal membrane were observed in the presence and absence of Na+ ions in the pH range 6-7, but they were not statistically significant. At the variations of pH the alterations in the choline transport system did not correspond to the total structural changes in the synaptosomal membrane, when monitored by intrinsic tryptophan fluorescence. The pH-evoked choline transport changes may be due to specific alterations in the choline transport-related proteins.