Structural and functional membrane polarity in cultured monolayers of MDCK cells

MDCK cells form monolayers which have many of the properties usually found in transporting epithelia. The present article is devoted to the study of the structural and functional polarization of MDCK cells, which is one of the central features of transporting epithelia. The results show: (i) that MDCK monolayers transport 2.6 mumol hr-1 cm-2 of sodium in the apical to basolateral direction; (ii) the passive flux of this ion is relatively large (20.3 mole hr-1 cm-2), which is a characteristic of leaky epithelia; (iii) a large fraction of the penetration of sodium into the cells proceeds through an amiloride-sensitive channel, and the exit is operated mainly by a ouabain-sensitive pump; (iv) the net transport of sodium from the apical to the basolateral side agrees with the asymmetric labeling of the pumps with 3H-ouabain; (v) this asymmetric labeling agrees, in turn, with a higher concentration of intramembrane particles (IMPs) in freeze-fracture replicas of the basolateral side of the plasma membrane; (vi) the structural polarization of confluent MDCK cells is also revealed by the location of microvilli, occluding junctions, and pinocytotic vesicles; and (vii) the presence of a continuous ring formed by actin microfilaments visualized by immunofluorescence under the lateral aspect of the plasma membrane that may be related to the distribution of the occluding junctions, which act as barriers separating apical from basolateral membrane components.