Bicarbonate reabsorption and NHE-3 expression: Abundance and activity are increased in Henle's loop of remnant rats

Background. The bulk of bicarbonate reabsorption along the loop of Henle (LOH) is localized at the level of the thick ascending limb (TAL) and is mainly dependent on the presence of luminal Na+-H+ exchanger (NHE-3). We investigated whether the reduction of renal mass is associated with alterations in LOH bicarbonate transport coupled to changes in NHE-3 gene expression and in vivo activity. Methods. Sham-operated and remnant rats (4/6 nephrectomy) were studied 15 days after the surgery. To measure net bicarbonate reabsorption (JHCO(3)(-)) superficial loops were perfused by in vivo micropuncture. Perfusate was an end-like proximal solution containing H-3-methoxy-inulin. NHE-3 gene expression was quantified by competitive PCR using an internal standard of cDNA that differed from the wild-type NHE-3 by a deletion of 76 bp. Western blot experiments were performed on TAL suspension using anti-NHE-3 antibodies. Results. At various LOH bicarbonate loads, JHCO(3)(-) was constantly larger in remnant rats as compared to sham-operated animals. NHE-3 mRNA abundance was estimated to be 0.339 +/- 0.031 attomoles (amol)/ng(-1) total RNA in sham-operated (N = 5) and it increased to 0.465 +/- 0.023 in remnant rats (N = 5, P < 0.01). Western blot experiments showed a significant increase of NHE-3 protein abundance in TAL of remnant rats as compared to sham-operated animals. Finally, by means of a specific NHE-3 inhibitor, S-3226, in vivo microperfusion experiments demonstrated that NHE-3 in vivo activity along the LOH was substantially increased in remnant rats in addition to the non-NHE-3 bicarbonate transport. Conclusions. These data indicate that the reduction of renal mass increases mRNA, protein abundance and in vivo activity of NHE-3 along the TAL. This may explain, at least in part, the augmented transepithelial bicarbonate transport along the LOH. Such an effect will counterbalance the increased glomerular bicarbonate load, thus preventing urinary bicarbonate loss and mitigating the ensuing metabolic acidosis.