Chloride transport in the proximal renal tubule.

Our knowledge of chloride transport along the nephron has greatly expanded. Whereas for a long time it was assumed that chloride ions were reabsorbed entirely passively with sodium--the "mendicant" role of chloride, more recent studies suggest that several distinct reabsorptive transport mechanisms operate in parallel. Accordingly, a new model of proximal chloride transport has evolved that includes both active, transcellular as well as passive, intercellular transport pathways. Transcellular chloride reabsorption involves anion exchange mechanisms in both the luminal and peritubular cell membranes, processes that also depend on sodium, hydrogen, and bicarbonate ions. Chloride transport is thus intimately related to sodium and fluid transport as well as to cell acid-base metabolism. Unresolved problems concern the relative magnitude of transcellular and paracellular chloride transport and the details of luminal and basolateral chloride translocation steps.