Implications of an anomalous intracellular electrical response in bullfrog corneal epithelium

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 differenceV_sc 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 theV_sc by 25mm K⁺ was reduced to one-quarter of the control change. If the Cl-permselective apical membrane resistanceR_o remained unchanged, the relative basolateral membrane resistanceR_i, which includes the lateral intercellular space, increased at the most by less than twofold after Ba²⁺. These effects in conjunction with the depolarization of theV_sc 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=R_o/R_i) 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 ofR_o/R_i 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 ofR_o thanR_iAlso an increase of lateral space resistance may offset the effect of decreasingR_i with 25mm K⁺. In contrast,R_o/R_i did transiently increase during voltage clamping of the apical membrane potential differenceV_o and exposure to 25mm K⁺ on the stromal side. This increase and subsequent decrease ofR_o/R_i supports the idea that increases in stromal K⁺ concentration may produce secondary membrane resistance changes. These effects onR_o/R_i 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 theV_sc by 6 mV whereas 10⁻⁴m ouabain depolarized theV_sc by 7 mV. Thus the basolateral membrane contains K⁺, Na⁺ and perhaps Cl⁻ pathways in parallel with the Na/K pump component.