Functional up-regulation of basolateral Na+-dependent HCO3- transporter NBCn1 in medullary thick ascending limb of K+-depleted rats

The electroneutral, Na+-coupled HCO3- cotransporter (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 HCO3- uptake across the basolateral cell membrane is an important buffering mechanism during the transcellular transport of NH4+ 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 NH4+ 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 HCO3- influx in normal and K+-depleted rats. Hypokalaemia induced a threefold up-regulation of Na+-coupled HCO3- 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 NH4+, and strengthen our previous suggestion that NBCn1 is an important player in the excretion of NH4+. They also indicate that it is the delivery of luminal NH4+ rather than systemic changes of pH that determine the expression of NBCn1.