Acid-base transport along the distal nephron: mechanism(s) and regulation.

The cortical distal tubule, a very heterogeneous segmant sarting at the macula densa and ending with the collecting duct, is responsible for the reabsorpton of 5 to 8% of the filtered bicarbonate. The rate of ttransport is not saturated and is regulated by several factors: hormones, potassium homeostasis, systemic acid-base balance, protein content of the diet and state of hydration. In very particular circumstances the bicarbonate reabsorption can be converted in secretion, but this is limited to the late portion of the distal tubule (initial collecting duct). The molecular mechanism involved in H+ secretion and hence in HCO3- reabsorptin have been identified in the Na+-H+ exchanger, the proton ATPase and the H+-K+ ATPase . Their distribution along the distal tubule and therefore their contribution to the overall H+ secretion is highly dependent on the segment involved (early vs late).