INWARDLY RECTIFYING K-PIGMENT EPITHELIAL-CELLS( CURRENTS IN ISOLATED HUMAN RETINAL)

Purpose. K+ channels in the retinal pigment epithelium (RPE) play a nu mber of important roles, including the establishment of membrane poten tial, the transport of K+ between the subretinal space and choroid, an d the generation of the c-wave of the electroretinogram. Previous stud ies on amphibian RPE demonstrated that these functions are likely serv ed by an inwardly rectifying K+ channel. The aim of this study was to characterize inwardly rectifying K+ channels in cultured and freshly i solated adult human RPE (hRPE) cells. Methods. Single cells were dispe rsed enzymatically from primary cultures of adult hRPE or from fresh a dult hRPE-choroid. Ionic currents were recorded using either the perfo rated-patch or whole-cell configuration of the patch-clamp technique. Results. In 5 mM external K+, roughly 20% of cultured hRPE cells exhib ited a strong inwardly rectifying K+ conductance that passed inward bu t little outward current. This conductance increased when [K+](o) was increased and exhibited a voltage-dependent block by external Na+ at n egative potentials. In contrast, all freshly isolated hRPE cells exhib ited a mild inwardly rectifying K+ conductance that mediated substanti al outward current at physiological voltages. This conductance decreas ed when [K+](o) was increased and showed no voltage-dependent block by external Na+. Conclusions. The authors conclude that fresh hRPE cells express a mild inwardly rectifying K+ conductance. The operation of t his conductance at physiological voltages makes it a likely candidate for the resting K+ conductances of the apical and basolateral membrane s. Cultured hRPE cells express a functionally different channel type t hat may reflect a change in phenotype.