The mechanism of Na+ transport by rabbit urinary bladder

The mechanism of Na+ transport in rabbit urinary bladder has been studied by microelectrode techniques. Of the three layers of epithelium, the apical layer contains virtually all the transepithelial resistance. There is radial cell-to-cell coupling within this layer, but there is no detectable transverse coupling between layers. Cell coupling is apparently interrupted by intracellular injection of depolarizing current. The cell interiors are electrically negative to the bathing solutions, but the apical membrane of the apical layer depolarizes with increasing Isc. Voltage scanning detects no current sinks at the cell junctions or elsewhere. The voltage-divider ratio, alpha, (ratio of resistance of apical cell membrane, Ralpha, to basolateral cell membrane, Rb) decreases from 30 to 0.5 with increasing Isc, because of the transport-related conductance pathway in the apical membrane. Changes in effective transepithelial capacitance with Isc are predicted and possibly observed. The transepithelial resistance, Rt, has been resolved into Ra, Rb, and the junctional resistance, Rj, by four different methods: cable analysis, resistance of uncoupled cells, measurements of pairs of (Rt, alpha) values in the same bladder at different transport rates, and the relation between Rt and Isc and between alpha and Isc. Rj proves to be effectively infinite (nominally 300 k omega muF) and independent of Isc, and Ra decreases from 154 to 4 omega muF with increasing Isc. In the resulting model of Na+ transport in "tight" epithelia, the apical membrane contains an amiloride-inhibited and Ca++-inhibited conductance pathway for Na+ entry; the basolateral membrane contains a Na+--K+-activated ATPase that extrudes Na+; intracellular (Na+) may exert negative feedback on apical membrane conductance; and aldosterone acts to stimulate Na+ entry at the apical membrane via the amiloride-sensitive pathway.