Decrease of intracellular chloride activity by furosemide in frog retinal pigment epithelium

We found that intracellular chloride activity in frog retinal pigment epithelium (RPE) was significantly higher than predicted from the equilibrium distribution across the cell membranes. When transepithelial chloride transport was inhibited by furosemide intracellular chloride activity decreased and approached electrochemical equilibrium. These data indicate that RPE possesses an active, furosemide-sensitive chloride transport across the apical membrane.