Ammonium carriers in medullary thick ascending limb.
Absorption of NH(4)(+) by the medullary thick ascending limb (MTAL) is a key event in the renal handling of NH(4)(+), leading to accumulation of NH(4)(+)/NH(3) in the renal medulla, which favors NH(4)(+) secretion in medullary collecting ducts and excretion in urine. The Na(+)-K(+)(NH(4)(+))-2Cl(-) cotransporter (BSC1/NKCC2) ensures approximately 50-65% of MTAL active luminal NH(4)(+) uptake under basal conditions. Apical barium- and verapamil-sensitive K(+)/NH(4)(+) antiport and amiloride-sensitive NH(4)(+) conductance account for the rest of active luminal NH(4)(+) transport. The presence of a K(+)/NH(4)(+) antiport besides BSC1 allows NH(4)(+) and NaCl absorption by MTAL to be independently regulated by vasopressin. At the basolateral step, the roles of NH(3) diffusion coupled to Na(+)/H(+) exchange or Na(+)/NH(4)(+) exchange, which favors NH(4)(+) absorption, and of Na(+)/K(+)(NH(4)(+))-ATPase, NH(4)(+)-Cl(-) cotransport, and NH(4)(+) conductance, which oppose NH(4)(+) absorption, have not been quantitatively defined. The increased ability of the MTAL to absorb NH(4)(+) during chronic metabolic acidosis involves an increase in BSC1 expression, but fine regulation of MTAL NH(4)(+) transport probably requires coordinated effects on various apical and basolateral MTAL carriers.
Attmane-Elakeb
A
A
Institut National de la Santé et de la Recherche Médicale Médicale Unité 426, Institut Fédératif Régional Xavier Bichat, Faculté de Médecine Xavier Bichat, 75870 Paris Cédex 18, France.
Amlal
H
H
Bichara
M
M
D000818:Animals; D017920:Antiporters / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D027682:Cation Transport Proteins; D002462:Cell Membrane / Q000502:physiology; D007679:Kidney Medulla / Q000502:physiology*; D008954:Models, Biological; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D028021:Sodium-Potassium-Chloride Symporters; D014667:Vasopressins / Q000502:physiology
D017920:Antiporters; D002352:Carrier Proteins; D027682:Cation Transport Proteins; D000644:Quaternary Ammonium Compounds; D028021:Sodium-Potassium-Chloride Symporters; C043130:ammonium-potassium antiporter; D014667:Vasopressins