ACETYLCHOLINE METABOLISM AND CHOLINE UPTAKE IN CORTICAL SLICES

Abstract— The uptake of [¹⁴C]choline was studied in cortical slices from rat brain after their incubation in a Krebs-Henseleit medium containing either 4.7 m m -KCl (low K), 25 m m -KCl (high K) or 25 m m -KCl without calcium (Ca free, high K). With 0.84 μ m -[¹⁴C]choline in the medium the uptake per gram of tissue was 0.62 nmol after incubation in low K medium, 1.13 nmol after incubation in high K medium and 0.78 nmol after incubation in a Ca free, high K medium. The differences caused by potassium were greater in fraction P₂ than in fractions P₁ and S₂. With 17 and 50 μ m -[¹⁴C]choline in the medium greater amounts of [¹⁴C]choline were taken up, but the effect of potassium on the uptake almost disappeared. The amount of radioactive material in fraction P₂ followed Michaelis-Menten kinetics with K _m values of 2.1 and 2.3 μ m after incubation in low and high K medium, respectively. Hemicholinium-3 only slightly inhibited choline uptake from a medium with 0.84 μ m -[¹⁴C]choline, but it abolished the extra-uptake induced by high K medium. The radioactivity in the slices consisted mainly of unchanged choline and little ACh was formed after incubation in low K medium, but after incubation in high K medium 50% of the choline taken up was converted into ACh. The hemicholinium-3 sensitive uptake of choline, the conversion of choline into ACh and the synthesis of total ACh, were stimulated about 7–8-fold by potassium. It is concluded that in cortical slices from rat brain all choline used for the synthesis of ACh is supplied by the high-affinity uptake system, of which the activity is geared to the rate of ACh synthesis.