Basolateral membrane K permselectivity and regulation in bullfrog cornea epithelium

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Abstract
In the isolated bullfrog cornea epithelium, under short-circuit conditions the regulation of the K permeability of the basolateral membrane was studied with conventional and K-selective microelectrodes in Cl-free Ringers In Cl-free Ringers, the transcellular current is less than 1 μA/cm2, allowing estimation of the basolateral membrane electromotive force from measurements of the membrane voltage (V sc). The apparent basolateral membrane K conductance was determined from measurements of the effects of single ion substitutions of K for Na on theV sc. An increase of K from 2.5 to 25mm on the stromal side depolarized the membrane voltage by 29 mV, whereas additional increases to 56 and 100mm resulted in depolarizations consistent with a Nernstian prediction. In the range between 25 and 56mm K, these decreases in membrane voltage were smaller after either decreasing the stromal-side pH from 8.1 to 7.2 or substitution of sulfate with gluconate. In contrast, preincubation with 0.1mm oubain did not change the membrane voltage depolarizations over any of the K ranges between 2.5 and 100mm. Equivalent circuit analysis, based on the effects of nystatin on the electrical parameters, was, used to validate the changes in the apparent basolateral, membrane K conductance following increases in [K], substitution of SO4 with gluconate and Na:K pump inhibition. An increase in the [K] to 120mm decreased the basolateral membrane resistance nearly three-fold, whereas gluconate substitution resulted in a 2.5-fold increase of the basolateral membrane resistance. This resistance increased an additional 2-fold after exposure to 5mm Ba. However, exposure to 0.1mm oubain had no significant effect on this resistance. Therefore, there is an agreement between the results of circuit analysis and the magnitude of membrane voltage depolarization resulting from increases in [K], gluconate substitution and pump inhibition with ouabain. Na:K pump inhibition with ouabain caused the K activity to decline slightly after 30 min from 98±7 to 83±8mm, which is consistent with a small basolateral membrane Na conductance. The estimated K permeability ranged from 3.7×10−7 to 1.1×10−6 cm/sec. The less than Nernstian predicted decline of the membrane voltage between 2.5 and 25mm K and the small basolateral membrane Na conductance suggest that the basolateral membrane is also permeable to another unknown ion.