MEASUREMENT OF PARACELLULAR EPITHELIAL CONDUCTIVITY BY CONDUCTANCE SCANNING

A new method, conductance scanning, allows determination of local para -and transcellular conductivities in flat epithelia. Experiments were performed on kidney distal tubule cells, MDCK clone C11, which form mo nolayers on permeable supports. Above the apical surface, local voltag e drops generated by a sinusoidal current clamp were recorded by means of a scanning microelectrode. Data were collected above cell centres and tight junctions The scanning Signal was always significantly highe r above the tight junctions, but was uniformly distributed along the j unctions. For determination of conductivities two procedures were appl ied. Method 1: the supraepithelial potential distribution was computed for given trans-and paracellular currents at all positions of the ele ctrode. In a fit algorithm, the currents were varied until the calcula ted potential difference equalled the voltage measured. Method 2: afte r collecting scanning data in control Ringer's, intercellular space wi dth was reduced by mucosal addition of 40 mM sucrose and a second set of data was obtained at decreased paracellular, but presumably unchang ed transcellular, conductivity. From these data, trans-and paracellula r conductivities were calculated. Results of both methods were in exce llent agreement. Confluent MDCK-C11 monolayers exhibited a transepithe lial conductivity of 13 mS/cm(2). The transcellular pathway contribute d 2.6 mS/cm(2) (20%) and the paracellular pathway 10.5 mS/cm(2) (80%) to the total conductivity. Collapse of the lateral intercellular space s decreased the paracellular conductivity to 4 mS/cm(2) (60%). Conflue nt MDCK-C11 monolayers constitute true ''leaky'' epithelia with homoge neously distributed trans-and paracellular conductivities. In conclusi on, conductance scanning fills a methodical gap, which hitherto impede d the functional characterzation of tight junctions.