Passive ionic properties of frog retinal pigment epithelium

Summary

The isolated pigment epithelium and choroid of frog was mounted in a chamber so that the apical surfaces of the epithelial cells and the choroid were exposed to separate solutions. The apical membrane of these cells was penetrated with microelectrodes and the mean apical membrane potential was −88 mV. The basal membrane potential was depolarized by the amount of the transepithelial potential (8–20mV). Changes in apical and basal cell membrane voltage were produced by changing ion concentrations on one or both sides of the tissue. Although these voltage changes were altered by shunting and changes in membrane resistance, it was possible to estimate apical and basal cell membrane and shunt resistance, and the relative ionic conductanceT _i of each membrane. For the apical membrane:T _K⋟0.52,T _{HCO 3}∼=0.39 andT _Na∼=0.05, and its specific resistance was estimated to be 6000–7000Ω cm². From the basalT _K∼=0.90 and its specific resistance was estimated to be 400–1200Ω cm². From the basal potassium voltage responses the intracellular potassium concentration was estimated at 110mm. The shunt resistance consisted of two pathways: a paracellular one, due to the junctional complexes and another, around the edge of the tissue, due to the imperfect nature of the mechanical seal. In well-sealed tissues, the specific resistance of the shunt was about ten times the apical plus basal membrane specific resistances. This epithelium, therefore, should be considered “tight”. The shunt pathway did not distinguish between anions (HCO₃ ⁻, Cl⁻, methylsulfate, isethionate) but did distinguish between Na⁺ and K⁺.