title: H+ and HCO3- transporters in the medullary thick ascending limb of the kidney: molecular mechanisms, function and regulation.
abstract: The H+ and HCO3- transporters present in the medullary thick ascending limb (MTAL) of the kidney are involved in several functions, such as transepithelial transport, defense of cell pH and cell volume. Apical H+ secretion occurs via the NHE-3 and NHE-2 isoforms of the Na+/H+ exchanger, and H(+)-ATPase. The apical Na+/H+ exchanger is responsible for most of the apical step of transepithelial HCO3- absorption and is unresponsive to cell acidification under isosmotic conditions. Basolateral HCO3- efflux mechanisms may occur via the Cl-/HCO3- exchanger and via the cotransporters K+/HCO3- (in the rat) and Na-3HCO3- (in the mouse). However, the role of each transporter in transepithelial HCO3- absorption is currently unknown. Inhibition of the basolateral Na+/H+ exchanger (NHE-1) paradoxically inhibits the apical Na+/H+ exchanger. This cross talk is independent of cell pH and may involve variations in cell volume. Arginine vasopressin (AVP) and hyperosmolality induce a differential regulation of basolateral NHE-1 and the apical Na+/H+ exchanger. They stimulate the basolateral NHE-1, and the resulting cell alkalinization probably stimulates the pHi-sensitive AE2, which restores cell volume by cellular uptake of NaCl. They also inhibit the apical Na+/H+ exchanger, which reduces net HCO3- absorption and thus may prevent interstitial fluid alkalinization. Chronic metabolic acidosis markedly increases HCO3- absorptive capacity of MTAL, by stimulating at least the synthesis of apical NHE-3 protein, as in the proximal tubule. Conversely, chronic metabolic alkalosis reduces the apical NHE-3 transport activity by decreasing the synthesis of NHE-3 protein. The paradoxical increase in HCO3- absorptive capacity of MTAL observed in the model of chronic NaHCO3-load alkalosis should be due to other factors overcoming the inhibitory effect of alkalosis on NHE-3.
authors.affiliation: Université Pierre et Marie Curie, Institut National de la Santé et de la Recherche Médicale U 356, Paris, France.
authors.forename: M
authors.initials: M
authors.lastname: Paillard
mesh_terms: D000137:Acid-Base Imbalance / Q000378:metabolism; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport / Q000187:drug effects; D007679:Kidney Medulla / Q000378:metabolism*; D008138:Loop of Henle / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers / Q000502:physiology
chemical_list: D001639:Bicarbonates; D017923:Sodium-Hydrogen Exchangers