K+ transport in ‘Tight’ epithelial monolayers of MDCK cells

Abstract

Bidirectional transepithelial K⁺ flux measurements across ‘high-resistance’ epithelial monolayers of MDCK cells grown upon millipore filters show no significant net K⁺ flux. Measurements of influx and efflux across the basal-lateral and apical cell membranes demonstrate that the apical membranes are effectively impermeable to K⁺. K⁺ influx across the basal-lateral cell membranes consists of an ouabain-sensitive component, an ouabain-insensitive component, an ouabain-insensitive but furosemide-sensitive component, and an ouabain-and furosemide-insensitive component. The action of furosemide upon K⁺ influx is independent of (Na⁺−K⁺)-pump inhibition. The furosemide-sensitive component is markedly dependent upon the medium K⁺, Na⁺ and Cl⁻ content. Acetate and nitrate are ineffective substitutes for Cl⁻, whereas Br⁻ is partially effective. Partial Cl⁻ replacement by NO₃ gives a roughly linear increase in the furosemide-sensitive component. Na⁺ replacement by choline abolishes the furosemide-sensitive component, whereas Li⁺ is a partially effective replacement. Partial Na⁺ replacement with choline gives an apparent affinity of ∼7mm Na, whereas variation of the external K⁺ content gives an affinity of the furosemide-sensitive component of 1.0mm. Furosemide inhibition is of high affinity (K _1/2=3 μm). Piretanide, ethacrynic acid, and phloretin inhibit the same component of passive K⁺ influx as furosemide; amiloride, 4,-aminopyridine, and 2,4,6-triaminopyrimidine partially so. SITS was ineffective. Externally applied furosemide and Cl⁻ replacement by NO ₃ ⁻ inhibit K⁺ efflux across the basal-lateral membranes indicating that the furosemide-sensitive component consists primarily of K∶K exchange.