Implications of an anomalous intracellular electrical response in bullfrog corneal epithelium

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Abstract
The ionic dependencies of the transepithelial and intracellular electrical parameters were measured in the isolated frog cornea. In NaCl Ringer's the intracellular potential differenceVsc measured under short-circuit conditions depolarized by nearly the same amount after either increasing the stromal-side KCl concentration from 2.5 to 25mm or exposure to 2mm BaCl2 (K+ channel blocker). With Ba2+ the depolarization of theVsc by 25mm K+ was reduced to one-quarter of the control change. If the Cl-permselective apical membrane resistanceRo remained unchanged, the relative basolateral membrane resistanceRi, which includes the lateral intercellular space, increased at the most by less than twofold after Ba2+. These effects in conjunction with the depolarization of theVsc by 62 mV after increasing the stromal-side K+ from 2.5 to 100mm in Cl-free Ringer's as well as the increase of the apparent ratio of membrane resistances (a=Ro/Ri) from 13 to 32 are all indicative of an appreciable basolateral membrane K+ conductance. This ratio decreased significantly after exposure to either 25mm K+ or Ba2+. The decline ofRo/Ri with 25mm K+ appears to be anomalous since this decrease is not consistent with just an increase of basolateral membrane conductance by 25mm K+, but rather perhaps a larger decrease ofRo thanRiAlso an increase of lateral space resistance may offset the effect of decreasingRi with 25mm K+. In contrast,Ro/Ri did transiently increase during voltage clamping of the apical membrane potential differenceVo and exposure to 25mm K+ on the stromal side. This increase and subsequent decrease ofRo/Ri supports the idea that increases in stromal K+ concentration may produce secondary membrane resistance changes. These effects onRo/Ri show that the presence of asymmetric ionic conductance properties in the apical and basolateral membranes can limit the interpretative value of this parameter. The complete substitution of Na+ withn-methyl-glucamine in Cl-free Ringer's on the stromal side hyperpolarized theVsc by 6 mV whereas 10−4m ouabain depolarized theVsc by 7 mV. Thus the basolateral membrane contains K+, Na+ and perhaps Cl− pathways in parallel with the Na/K pump component.

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