The mechanism of Na⁺ transport by rabbit urinary bladder

Summary

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 increasingI _sc. Voltage scanning detects no current sinks at the cell junctions or elsewhere. The voltage-divider ratio, α, (ratio of resistance of apical cell membrane,R _a, to basolateral cell membrane,R _b) decreases from 30 to 0.5 with increasingI _sc, because of the transportrelated conductance pathway in the apical membrane. Changes in effective transepithelial capacitance withI _sc are predicted and possibly observed. The transepithelial resistance,R _t, has been resolved intoR _a, R_b, and the junctional resistance,R _j, by four different methods: cable analysis, resistance of uncoupled cells, measurements of pairs of (R _t, α) values in the same bladder at different transport rates, and the relation betweenR _t andI _sc and between α andI _sc.R _j proves to be effectively infinite (nominally 300 kΩ μF) and independent ofI _sc, andR _a decreases from 154 to 4 kΩ μF with increasingI _sc. 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.