NaCl entry mechanisms in the luminal membrane of the renal tubule

Academic Article

Abstract

  • Isolated cells and membrane vesicle preparations have provided useful tools for characterizing the details of ion-translocating mechanisms at the cellular and subcellular level. This approach has been especially helpful for studies of the electrically 'leaky' segments of the nephron because the transcellular routes of transport can be examined without the complicating features of paracellular shunt pathways. Several modes of coupled NaCl transport across the luminal membrane of the proximal tubule and thick ascending limb have been described with this approach. All forms of coupled NaCl transport are examples of secondarily active transport. The primary active transport are driven by the Na + gradient across the luminal membrane, and are, therefore, examples of secondarily active transport. The primary active transport step is the maintenance of the Na + gradient by the basolateral Na + -K + -ATPase. Directly coupled NaCl transport systems are best described in the thick ascending limb, and include NaCl cotransport (the 'ternary' complex model) and 1Na + -1K + -2Cl - cotransport. Another form of NaCl transport is indirectly coupled by the pH gradient across the luminal membrane, and accomplishes net NaCl cotransport by the parallel operation of Na + /H + and Cl - /HCO 3 - exchangers. The parallel exchanger model is discussed with reference to preferential HCO 3 - absorption, HCO 3 - secretion, and the apparent coupling ratio between net H + secretory rates and Na + absorptive rates. Parallel exchange may contribute to NaCl absorption in the cortical segments of the nephron in conjunction with the elevated renal cortical CO 2 tensions at those sites. Electrical coupling is another mechanisms by which NaCl entry may be coupled across the luminal membrane. In this instance, a transcellular anion current flow would accompany rheogenic Na + transport if the electrical potential across the luminal membrane were depolarized by Na + entry and if an effective anion exit site existed in the basolateral membrane. The overall understanding of transepithelial transport requires an integration of the ion-translocating mechanisms described at the membrane and cellular level with measurements of electrochemical driving forces at each membrane site and the regulatory mechanisms that function at the level of the intact tubule.
  • Authors

    Author List

  • Warnock DG; Eveloff J
  • Volume

  • 11
  • Issue

  • 6