EFFECTS OF CYTOCHALASIN B ON THE UPTAKES OF MONOSACCHARIDES BY RAT BRAIN SYNAPTOSOMES

Abstract— Synaptosomal uptakes of a number of simple carbohydrates were strongly inhibited by cytochalasin B (K₁= 7-9 × 10^?8M). Phloretin (K₁= 2-4 × 10^?6M) and phloridzin (K₁= 3-4 × 10^?4M) were less inhibitory. Cytochalasin B competitively inhibited the uptakes of carbohydrates with saturable transport kinetics. Inhibition of sugar uptake was immediate on addition of cytochalasin B but was promptly reversed upon removal of the drug. Cytochalasin B had no effect on the efflux of D-glucos-amine or of the phosphorylated sugar, and did not affect intrasynaptosomal hexokinase(s). The synapto-somal uptakes of L-glucose, D-mannitol, L-fucose and the N -acetylated amino sugars were non-saturable and uninhibited by cytochalasin B. In the case of sugars which enter synaptosomes by both saturable and non-saturable processes, cytochalasin B could be used to selectively inhibit the saturable uptake components. The resultant non-saturable cytochalasin-insensitive uptake rates obtained were found to be widely different among the sugars studied, and must be corrected for in order to estimate accurate kinetic constants of the saturable processes.