Spatial buffering of K+ by the retinal pigment epithelium in frog

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Abstract
Using K+-specific microelectrodes in the isolated retinal pigment epithelium preparation from frog eye, we have examined changes in extracellular K+ in the unstirred layer between the choroid and basal membrane. We found, using agents to depolarize apical and basal membranes, that membrane potential modulates K+ efflux from the basal membrane into the choroid. [K+] in the choroid was found to be higher than in the bathing medium. From the basal (scleral) surface of the choroid to the basal membrane of the retinal pigment epithelium, [K+] increased from 2 mM (bath concentration) to an average of 2.84 mM near the basal membrane. When [K+] was increased on the apical side, epithelial membranes depolarized and produced [K+] efflux from the basal membrane that led to an increase in choroidal [K+]. Ba2+, 2 mM, ouabain, 0.1 mM, also evoked increases in choroidal [K+] concomitant with membrane depolarization. Thus, apical Na+/K+ pump transport and K+ leakage due to tissue damage were eliminated as possible sources of observed increases in choroidal [K+] since apical Ba2+ and ouabain depolarize epithelial membranes without increasing apical [K+]. The effects of depolarizing agents were blocked by basal Ba2+, demonstrating the requirement for a large K+ conductance at the basal membrane. Theoretical analysis shows that shunt resistance and isolation of apical and basal compartments are limiting factors in the transfer of K+. In the retinal pigment epithelium, shunt resistance favors the transfer of electrical potentials between membranes while the paracellular pathway selectively impedes movement of K+ between compartments.(ABSTRACT TRUNCATED AT 250 WORDS)