Sodium Entry into Tubule Cells

The proximal tubule cells have a negative intracellular potential of -70 mV relative to both luminal fluid and peritubular fluid, and the cells have a low intracellular sodium concentration (< 30 mmol). So sodium movement from the luminal fluid into the cell is in the direction of a large electrical gradient as well as a chemical concentration gradient. Thus, sodium entry into the cell occurs passively. However, this entry of sodium is mediated by carrier proteins that also transport other solutes simultaneously. As noted above, these solutes may be transported with the sodium ions (symport) or in exchange for the sodium (antiport). Many solutes are transported by Na+-linked symport and anti-port. Most of the sodium (80%) entering the tubule cells does so in exchange for H+ secretion. In turn, H+ secretion into the tubular lumen, leads to the re-absorption of both CI-and HCOi (Figure RE.6a).

Apiccl mçfnbrano mCJtitjforia of cell °f«!l

Apiccl mçfnbrano mCJtitjforia of cell °f«!l

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Figure RE.6a Sodium handling in the early proximal tubule

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