Intracellular potassium activity of isolated human and rabbit corneal epithelium

Single-barrelled open tip microelectrodes containing a potassium-selective liquid ion exchanger (Corning 477 317) were used to determine intracellular K-activity a_Kⁱ in squamous and basal cells of isolated rabbit and human corneal epithelium. The intracellular potassium activity a_Kⁱ during the five hours of incubation in Ringer solution was 114·3±23·6 mmol/l and 107·6±26·2 mmol/l for the basal cell layer of rabbit and human epithelia respectively. The corresponding values for the squamous cells of the superficial cell layer were 41·2±14·2 mmol/l and 50·2±11·1 mmol/l. Based on intracellular chemical concentrations of potassium c_Kⁱ an apparent activity coefficient could be calculated that is close to that of a dilute solution of the same ionic strength. This indicates that intracellular potassium is essentially free in both epithelia. Ouabain (5 × 10^?5 mol/l) added to the bathing solution of the rabbit cornea lowered the transmembranal voltage Ψ_M as well as the intracellular K-activities of squamous and basal cells. Three hours after application of ouabain a_Kⁱ of the basal cells had decreased to about one third of its initial value. Under all experimental conditions the potassium equilibrium potential E_K considerably exceeded Ψ_M.Our data indicate: (1) The high absolute values for a_Kⁱ imply that almost no potassium is bound or sequestered within the epithelial cells. (2) An active potassium uptake is necessary to explain the high K-activity in relation to the transmembranal voltage Ψ_M. (3) There is an intraepithelial gradient of a_Kⁱ within the corneal epithelia.