Functional up-regulation of basolateral Na+-dependent HCO3- transporter NBCn1 in medullary thick ascending limb of K+-depleted rats.
The electroneutral, Na(+)-coupled HCO(3)(-) co-transporter (NBCn1) is expressed in the basolateral membrane of rat medullary thick ascending limb (mTAL) segments and is up-regulated strongly during metabolic acidosis. We have suggested previously that NBCn1-mediated HCO(3)(-) uptake across the basolateral cell membrane is an important buffering mechanism during the transcellular transport of NH(4)(+) in the mTAL. To investigate this further we treated rats with a low-K(+) diet for 4 days, a manoeuvre known to increase proximal tubular ammoniagenesis and delivery of luminal NH(4)(+) to the mTAL. Hypokalaemia strongly increased immunolabelling of basolateral NBCn1 in the mTAL of the inner stripe of the outer medulla. Immunoblotting revealed an eightfold up-regulation of NBCn1 protein in K(+)-depleted rats. Subsequently we used in vitro perfusion of isolated mTAL and BCECF imaging to investigate Na(+)-coupled HCO(3)(-) influx in normal and K(+)-depleted rats. Hypokalaemia induced a threefold up-regulation of Na(+)-coupled HCO(3)(-) influx (1.64+/-0.28 vs. 0.47+/-0.05 pH units/minute, n=8). The Na(+)-dependent alkalinization was also significantly larger in K(+)-depleted rats (0.38+/-0.04 vs. 0.23+/-0.03 pH units). These data indicate that K(+)-depleted rats respond with a strong up-regulation of NBCn1 protein and function, probably to cope with the higher tubular load of NH(4)(+), and strengthen our previous suggestion that NBCn1 is an important player in the excretion of NH(4)(+). They also indicate that it is the delivery of luminal NH(4)(+) rather than systemic changes of pH that determine the expression of NBCn1.
Institute of Physiology, University of Aarhus, Olé Worms Allé 160, 8000 Aarhus C, Denmark.
Jakob K
JK
Jakobsen
Elvin
E
Odgaard
Weidong
W
Wang
Marie Louise
ML
Elkjaer
Søren
S
Nielsen
Christian
C
Aalkjaer
Jens
J
Leipziger
D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D015153:Blotting, Western; D004032:Diet; D005260:Female; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D007679:Kidney Medulla / Q000378:metabolism*; D008138:Loop of Henle / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D027982:Sodium-Bicarbonate Symporters / Q000096:biosynthesis*; D015854:Up-Regulation
D001639:Bicarbonates; D000644:Quaternary Ammonium Compounds; C422004:Slc4a4 protein, rat; D027982:Sodium-Bicarbonate Symporters; D011188:Potassium