Evidence for mediated transport of chloride in cat cerebral cortex in vitro

Summary

The kinetics of the uptake of chloride by slices of cat cerebral cortex incubated in vitro have been studied for a wide range of concentrations of chloride and potassium in the incubation media.

• 1. In studies with ³⁶Cl at a constant concentration of external K⁺, the rate of entry of chloride into slices of cat cerebral cortex (after equilibration with the fluid of the extracellular space) followed Michaelis-Menten kinetics with V_max=7.7 μmoles/g/min and K_m=245 mM. The kinetic data so described differ significantly from a curve describing simple diffusion (P < 0.02).

• 2. A clear dependence on concentration of K⁺ in the incubation medium was demonstrated. When external chloride was held constant at 6.8 mM and concentrations of potassium were varied over a range of 27–100 mM, the apparent rate of entry of chloride followed Michaelis-Menten kinetics with V_max=0.191 μmoles/ g/min and K_m=30.3 mM.

• 3. Under similar equilibrium conditions, the rate of influx of chloride was not statistically different from the rate of efflux, and the value for extracellular space (355 μl/g) derived by extrapolation from efflux data was of an identical order of magnitude to the value (352 μl/g) obtained from influx data. These values are essentially identical to those previously determined for chloride space of cat cerebral cortex in vivo (Bourke, Greenberg and Tower 1965).

• 4. The total, membrane-delimited content of chloride in cerebral cortex, as defined in these studies in vitro, was a function of the concentration of K⁺ in the incubation medium.