Measurements of the Intracellular Potassium Activity of Retzius Cells in the Leech Central Nervous System Available to Purchase

The intracellular K activity of leech Retzius cells was measured using double-barrelled, liquid ion exchanger, microelectrodes. At the normal external K⁺ concentration of 4 mm (equivalent to 3 mm-K activity, assuming an activity coefficient of 0·75) the mean K activity was 101·3 ± 7·6 mm (S.D., n = 14) in the cell bodies, and 4·35 ± 0·4 mV (n = 27) in the extracellular spaces surrounding them, indicating a K⁺ equilibrium potential of – 80 mV. The mean membrane potential was –43·6±4·9 mV (n = 14). In a K-free external solution, or in the presence of 5×10⁻⁴ M-ouabain, the intracellular K activity decreased by up to 14 mm min⁻¹. This indicates an efflux of K⁺ ions across the cell membrane of approximately 2 × 10⁻¹⁰ mol cm⁻² s, and an apparent K⁺ permeability coefficient of 8 × 10⁻⁸ cms⁻¹. The cell membrane depolarized upon removal of K⁺ and upon addition of ouabain, and transiently hyperpolarized beyond its initial level on return to the normal external K⁺ concentration. The recovery from this hyperpolarization paralleled the increase of the intracellular K activity following the re-addition of K⁺. Our results suggest that, despite the high K⁺ permeability of the Retzius cell membrane, the intracellular K activity is maintained at a high level by an electrogenic pump.