The mechanism of Na+ transport by rabbit urinary bladder |
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Authors: | Simon A Lewis Douglas C Eaton Jared M Diamond |
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Institution: | (1) Physiology Department, University of California Medical Center, 90024 Los Angeles, California;(2) Present address: Department of Physiology and Biophysics, University of Texas Medical Branch, 77550 Galveston, Texas |
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Abstract: | 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, , (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, Rb, and the junctional resistance,R
j, by four different methods: cable analysis, resistance of uncoupled cells, measurements of pairs of (R
t, ) values in the same bladder at different transport rates, and the relation betweenR
t andI
sc and between andI
sc.R
j proves to be effectively infinite (nominally 300 k F) and independent ofI
sc, andR
a decreases from 154 to 4 k F 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. |
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Keywords: | |
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