Cellular mechanisms of renal potassium transport: proximal tubule and thick ascending limb. A, Proximal tubule potassium reabsorption is closely coupled to proximal sodium and water transport. Potassium is reabsorbed through both paracellular and cellular pathways. Proximal apical potassium channels are normally almost completely closed. The lumen of the proximal tubule is negative in the early proximal tubule and positive in late proximal tubule segments. Potassium transport is not specifically regulated in this portion of the nephron, but net potassium reabsorption is closely coupled to sodium and water reabsorption. B, In the thick ascending limb of Henle's loop, potassium reabsorption proceeds by electroneutral Na+-K+-2Cl- cotransport in the thick ascending limb, the low intracellular sodium and chloride concentrations providing the driving force for transport. In addition, the positive lumen potential allows some portion of luminal potassium to be reabsorbed via paracellular pathways [11]. The apical potassium channel allows potassium recycling and provides substrate to the apical Na+-K+-2Cl- cotransporter [12]. Loop diuretics act by competing for the Cl- site on this carrier.

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