EFFECTS OF BA2-PIGMENT EPITHELIUM( AND CS+ ON APICAL MEMBRANE K+ CONDUCTANCE IN TOAD RETINAL)

Intracellular microelectrode techniques were employed to characterize the blocker sensitivity of the K+ conductance (g(K)) at the apical mem brane of the toad retinal pigment epithelium (RPE). Increasing the Kconcentration in the apical bath ([K+](o)) from 2 to 5 mM produced a r apid depolarization of the apical membrane potential (V-A). The additi on of 0.5 mM Ba2+ or 5 mM Cs+ to the apical bath rapidly depolarized V -A and increased the transepithelial resistance and ratio of apical-to -basolateral membrane resistance. In the presence of apical Ba2+ or Cs +, the response of V-A to Delta[K+](o) was markedly reduced, indicatin g that these ions are effective blockers of apical g(K). The Ba2+- and Cs+-induced decreases in the apparent apical-to-basolateral membrane conductance ratio were concentration dependent, with apparent dissocia tion constants of 17 mu M and 0.5 mM, respectively. The apparent block er sensitivity of apical g(K) is similar to that previously demonstrat ed for the inwardly rectifying K+ conductance in isolated toad RPE cel ls, suggesting that the inwardly rectifying K+ conductance comprises m uch of apical g(K).