Uptake of radiocalcium by nerve endings isolated from rat brain: kinetic studies

1 The uptake of radiocalcium by nerve-ending particles isolated from rat brain was studied in vitro by means of a rapid lanthanum quenching technique. 2 The observed uptake fits a theoretical three-compartment model with two separate uptake phases, a fast, initial phase followed by a late, slow phase. This holds true during control conditions as well as during high-potassium stimulation. 3 The uptake as a function of the external calcium concentration can be described in terms of Michaelis-Menten kinetics during high-potassium stimulation. Under control conditions the fit is clearly applicable but statistically not as good as during potassium stimulation. 4 The affinity for the uptake of calcium remains unchanged under control conditions while during high-potassium stimulation the affinity drastically decreases during the late, slow phase of uptake. 5 During high-potassium stimulation the maximal velocity of calcium uptake is twice that during control conditions. This holds true for both the fast and the slow phases of the uptake. 6 Mg2+ has an inhibitory effect on the uptake, the inhibition being more effective during high-potassium stimulation. Tetrodotoxin has a slight inhibitory effect additional to that extended by Mg2+ during the initial phase of uptake into high potassium stimulated synaptosomes.