The effect of acute metabolic alkalosis on bicarbonate transport along the loop of Henle. The role of active transport processes and passive paracellular backflux

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 (JHCO3-) superficial loops were perfused by in vivo micropuncture. Perfusate was an end-like proximal solution containing 3H-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, JHCO3- 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.

A decrease in renal medullary tonicity stimulates anion transport in Henle's loop of rat kidneys

To investigate the effect of reduction in renal medulla osmolality on loop of Henle (LOH) net bicarbonate reabsorption, clearance and microperfusion experiments were performed on Sprague-Dawley rats. The decrease of renal medulla osmolality was induced by intravenous infusion of either a large dose of mannitol (mannitol protocol) or a hypotonic solution (hypotonic protocol) delivered at a rate to match the sodium and bicarbonate load of the control period. During the mannitol protocol, clearance data demonstrated a rise in glomerular filtration rate (GFR), renal plasma flow, urine pH, and fractional bicarbonate excretion. On the contrary, microperfusion experiments, performed in the absence of mannitol in the tubular perfusate, revealed a significant increase both in the absolute and fractional LOH bicarbonate transport. During the hypotonic protocol, there was a decrease in GFR, associated with an increase in fractional excretion of bicarbonate. In the microperfusion experiments, hypotonic saline, similar to mannitol, stimulated absolute and fractional LOH bicarbonate transport. Net reabsorption of chloride, measured under the same experimental conditions, was also found to be activated. Therefore, the intravenous infusion of hypotonic solution affected the LOH transepithelial net reabsorption of both bicarbonate and chloride. We hypothesize that the increase in the transport rate of these two anions, along the same segment and in similar experimental conditions, may be mediated, at least in part, by decreased medullary tonicity, which is one factor common both to hypertonic mannitol and hypotonic saline infusion.

The effects of respiratory alkalosis and acidosis on net bicarbonate flux along the rat loop of Henle in vivo

We have studied the effects of acute respiratory alkalosis (ARALK, hyperventilation) and acidosis (ARA, 8% CO2), chronic respiratory acidosis (CRA; 10% CO2 for 7-10 days), and subsequent recovery from CRA breathing air on loop of Henle (LOH) net bicarbonate flux (JHCO3) by in vivo tubule microperfusion in anesthetized rats. In ARALK blood, pH increased to 7.6, and blood bicarbonate concentration ([HCO3-]) decreased from 29 to 22 mM. Fractional urinary bicarbonate excretion (FEHCO3) increased threefold, but LOH JHCO3 was unchanged. In ARA, blood pH fell to 7.2, and blood [HCO3-] rose from 28 to 34 mM; FEHCO3 was reduced to < 0.1%, but LOH JHCO3 was unaltered. In CRA, blood pH fell to 7.2, and blood [HCO3-] increased to > 50 mM, whereas FEHCO3 decreased to < 0.1%. JHCO3 was reduced by approximately 30%. Bicarbonaturia occurred when CRA rats breathed air, yet LOH JHCO3 increased (by 30%) to normal. These results suggest that LOH JHCO3 is affected by the blood-to-tubule lumen [HCO3-] gradient and HCO3- backflux. When the usual perfusing solution at 20 nl/min was made HCO3- free, mean JHCO3 was -34.5 +/- 4.4 pmol/min compared with 210 +/- 28.1 pmol/min plus HCO3-. When a low-NaCl perfusate (to minimize net fluid absorption) containing mannitol and acetazolamide (2 x 10(-4) M, to abolish H(+)-dependent JHCO3) was used, JHCO3 was -112.8 +/- 5.6 pmol/min. Comparable values for JHCO3 at 10 nl/min were -35.9 +/- 5.8 and -72.5 +/- 8.8 pmol/min, respectively. These data indicate significant backflux of HCO3-along the LOH, which depends on the blood-to-lumen [HCO3-] gradient; in addition to any underlying changes in active acid-base transport mechanisms, HCO3- permeability and backflux are important determinants of LOH JHCO3 in vivo.