Cl−−HCO3− exchange in rat renal basolateral membrane vesicles

The present studies examined the mechanism of bicarbonate transport across basolateral membrane vesicles prepared from rabbit renal cortex. Isotopic sodium uptake was stimulated by bicarbonate when compared with gluconate (2.5 nmol/mg protein per 5 s versus 1.4 nmol/mg protein per 5 s), and this process was inhibited by disulfonic stilbenes. Imposition of an interiorpositive potassium diffusion potential further stimulated isotopic sodium uptake to 3.4 nmol/mg protein per 5 s, an effect that occurred only in the presence of bicarbonate and was blocked by disulfonic stilbenes. Kinetic analysis of the rate of bicarbonatedependent sodium uptake as a function of sodium concentration revealed saturable stimulation with a V... of 2.7 nmol/mg protein per 2 s and a Km of 10.4 mM. The effect of bicarbonate concentration on bicarbonate-dependent sodium uptake was more complex. The present results demonstrate an electrogenic (negatively charged) sodium/bicarbonate cotransporter in basolateral membrane vesicles from the rabbit renal cortex. The electrogenicity implies a stoichiometry of at least two bicarbonate ions for each sodium ion.

Section: Methodsmentioning

confidence: 56%

Electrogenic sodium/bicarbonate cotransport in rabbit renal cortical basolateral membrane vesicles.

Akiba¹,

Alpern²,

Eveloff³

et al. 1986

“…Bicarbonate absorption by proximal renal tubules is considered to be mediated by Na+/H' exchange and perhaps H+-dependent ATPase in the apical membranes (20,36), and Na+/ HCO3 cotransport (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) and 14CO3/Cl-exchange (35,42) in the basolateral membrane. Of these transporters, the Na+/H' antiporter in the brush border membrane and Nae/HCO3 cotransporter in the basolateral membrane are considered to play major roles in regulating the intracellular pH ofproximal tubular cells (17).…”

Section: Resultsmentioning

confidence: 99%

Parallel adaptation of the rabbit renal cortical sodium/proton antiporter and sodium/bicarbonate cotransporter in metabolic acidosis and alkalosis.

Akiba¹,

Rocco²,

Warnock³

1987

119

Recent studies have shown that the bicarbonate reabsorptive capacity ofthe proximal tubule is increased in metabolic acidosis. For net bicarbonate reabsorption &o be regulated, there may be changes in the rate of apical H+ secretion as well as in the basolateral base exit step. The present studies examined the rate of Na+/H+ exchange (acridine orange method) and Na+/HCO3 cotransport (22Na uptake) in apical and basolateral membranes prepared from the rabbit renal cortex by sucrose density gradient centrifugation. NH4C loading was used to produce acidosis (arterial pH, 7.27±0.03), and Cl-deficient diet with furosemide was used to produce alialosis (arterial pH, 7.51±0.02). Maximal transport rate (V..) of Na+/H' antiporter and Na+/HCO3 cotransporter were inversely related with plasma bicarbonate concentration from 6 to 39 mM. Furthermore, the maximal transport rates of both systems varied in parallel; when V. for the Na+/ HCO3 cotransporter was plotted against V,,x for the Na+/H+ antiporter for each of the 24 groups of rabbits, the regression coefficient (r) was 0.648 (P < 0.001). There was no effect of acidosis or alkalosis on affinity for Na+ of either transporter. We conclude that both apical and basolateral H+/HCO3 transporters adapt during acid-base disturbances, and that the maximal transport rates of both systems vary in parallel during such acid-base perturbations.

“…While these studies were initially interpreted to imply that 80-100% of apical membrane proton secretion is mediated by Na+/H+ exchange, this interpretation has recently been complicated by the demonstration of a Na+-coupled basolateral membrane bicarbonate exit step (24)(25)(26)(27)(28)(29)(30). Based on the present results, the observed 80-100% inhibition of J.co, in previous studies can be explained by a 65% inhibition of apical membrane proton secretion (Na+/H+ antiporter) and > 90% inhibition ofbasolateral membrane bicarbonate exit (Na+/(HCOj)>1 symporter).…”

Section: Discussionmentioning

confidence: 99%

“…The interpretation of these studies is complicated by the fact that the major basolateral membrane H+/HCO-transport mechanism in the proximal tubule is Na+-coupled and rheogenic (carrying negative charge) (24)(25)(26)(27)(28)(29)(30). Removal of Na+ from the luminal and capillary fluids inhibits basolateral membrane bicarbonate exit, which alkalinizes the cell (25, 26, secretory mechanism.…”

Section: Introductionmentioning

confidence: 99%

Role of the Na+/H+ antiporter in rat proximal tubule bicarbonate absorption.

Preisig¹,

Ives²,

Cragoe³

et al. 1987

160

103

Amiloride and the more potent amiloride analog, 54N-t-butyl) amiloride (t-butylamiloride), were used to examine the role of the Na+/H+ antiporter in bicarbonate absorption in the in vivo microperfused rat proximal convoluted tubule. Bicarbonate absorption was inhibited 29, 46, and 47% by 0.9 mM or 4.3 mM amiloride, or 1 mM t-butylamiloride, respectively. Sensitivity of the Na+/H+ antiporter to these compounds in vivo was examined using fluorescent measurements of intracellular pH with (2',7')-bis(carboxyethyl)-(5,6)-carboxyfluorescein (BCECF). Amiloride and t-butylamiloride were shown to be as potent against the antiporter in vivo as in brush border membrane vesicles. A model of proximal tubule bicarbonate absorption was used to correct for changes in the luminal profiles for pH and inhibitor concentration, and for changes in luminal flow rate in the various series. We conclude that the majority of apical membrane proton secretion involved in transepithelial bicarbonate absorption is mediated by the Na+-dependent, amiloride-sensitive Na+/H+ antiporter. However, a second mechanism of proton secretion contributes significantly to bicarbonate absorption. This mechanism is Na+-independent and amiloride-insensitive.