H(+)-K(+)-ATPase activity in rat collecting duct segments

111

Net HCO transport in the rabbit kidney cortical collecting duct (CCD) is mediated by simultaneous H ϩ secretion and HCO secretion, most likely occurring in ␣ -and ␤ -intercalated cells (ICs), respectively. The polarity of net HCO transport is shifted from secretion to absorption after metabolic acidosis or acid incubation of the CCD. We investigated this adaptation by measuring net HCO flux before and after incubating CCDs 1 h at pH 6.8 followed by 2 h at pH 7.4. Acid incubation always reversed HCO flux from net secretion to absorption, whereas incubation for 3 h at pH 7.4 did not. Inhibition of ␣ -IC function (bath Cl Ϫ removal or DIDS, luminal bafilomycin) stimulated net HCO secretion by ‫ف‬ 2 pmol ր min per mm before acid incubation, whereas after incubation these agents inhibited net HCO absorption by ‫ف‬ 5 pmol ր min per mm. Inhibition of ␤ -IC function (luminal Cl Ϫ removal) inhibited HCO secretion by ‫ف‬ 9 pmol ր min per mm before incubation, whereas after incubation HCO absorption was stimulated by only ‫ف‬ 3 pmol ր min per mm. After acid incubation, luminal SCH28080 inhibited HCO absorption by only 5-15% vs the ‫ف‬ 90% inhibitory effect of bafilomycin. In outer CCDs, which contain fewer ␣ -ICs than midcortical segments, the reversal in polarity of HCO flux was blunted after acid incubation. We conclude that the CCD adapts to low pH in vitro by downregulating HCO secretion in ␤ -ICs via decreased apical Cl Ϫ ր base exchange activity and upregulating HCO absorption in ␣ -ICs via increased apical H ϩ -ATPase and basolateral Cl Ϫ ր base exchange activities. Whether or not there is a reversal of IC polarity or recruitment of ␥ -ICs in this adaptation remains to be established. (

Section: Discussionsupporting

confidence: 80%

Adaptation of rabbit cortical collecting duct HCO3- transport to metabolic acidosis in vitro.

Tsuruoka¹,

Schwartz²

1996

111

“…This stimulation is inhibited in late distal tubule by SCH 28080, a drug shown to inhibit with great specificity the gastric type K,H-ATPase (45). It has been reported that SCH 28080 also blocks H+ secretion and K+ reabsorption in K-depleted rats in the outer and inner medullary collecting duct (42,46) and also in cortical collecting duct in HCO--secreting conditions (46), and the present results are compatible with the existence of KH-ATPase also in late distal tubules. On the other hand, no evidence for functionally measurable K /H + activity was found in our rats on a control diet.…”

Section: Methodsmentioning

confidence: 99%

Renal bicarbonate reabsorption in the rat. IV. Bicarbonate transport mechanisms in the early and late distal tubule.

Wang¹,

Malnic²,

Giebisch³

et al. 1993

Bicarbonate transport was studied in vivo by separate microperfusion experiments of early and late distal tubules. Total CO2 was measured by microcalorimetry and fluid absorption by 3H-inulin. Significant bicarbonate absorption was observed in all experimental conditions. Bicarbonate transport was loaddependent upon increasing the luminal bicarbonate concentration from 15 to 50 mM in both early and late distal tubule segments and remained constant at higher concentrations at a maximum rate of 100-110 pmol/min per mm. At low lumen bicarbonate concentrations (15 mM), higher rates of bicarbonate absorption were observed in early (32.9±4.57 pmol/min per mm) as compared to late distal tubules (10.7±3.1 pmol/ min per mm). Amiloride and ethyl-isopropylamiloride both inhibited early but not late distal tubule bicarbonate absorption whereas acetazolamide blocked bicarbonate transport in both tubule segments. Fluid absorption was significantly reduced in both tubule segments by amiloride but only in early distal tubules by ethyl-isopropylamiloride. Substitution of lumen chloride by gluconate increased bicarbonate absorption in late but not in early distal tubules. Bafilomycin Al, an inhibitor of HATPase, inhibited late and also early distal tubule bicarbonate absorption, the latter at higher concentration. After 8 d on a low K diet, bicarbonate absorption increased significantly in both early and late distal tubules. Schering compound 28080, a potent H-K ATPase inhibitor, completely blocked this increment of bicarbonate absorption in late but not in early distal tubule. The data suggest bicarbonate absorption via Na+-H+ exchange and H-ATPase in early, but only by amiloride-insensitive H' secretion (H-ATPase) in late distal tubules. The study also provides evidence for activation of K+-H+ exchange in late distal tubules of K depleted rats. Indirect evidence implies a component of chloride-dependent bicarbonate secretion in late distal tubules and suggests that net bicarbonate transport at this site results from bidirectional bicarbonate movement. (J. Clin.

“…In potassium-depleted animals H/K-ATPase activity markedly increases and appears to play an important role in distal potassium reabsorption. Recent studies suggest that the H/K-ATPase may influence distal acidification in both potassium-depleted and normokalemic animals as well as during metabolic acidosis and respiratory acidosis (26)(27)(28).…”

Section: Methodsmentioning

confidence: 99%

Regulation of collecting tubule adenosine triphosphatases by aldosterone and potassium.

Eiam‐Ong¹,

Kurtzman²,

Sabatini³

1993

To examine the precise role of potassium and aldosterone on acid-base composition and on collecting tubule ATPases, glucocorticoid-replete adrenalectomized rats were replaced with zero, physiological, or pharmacological doses of aldosterone and were fed varying potassium diets to produce hypokalemia, normokalemia, or hyperkalemia. Radiochemical measurement of ATPase activities showed that collecting tubule H/KATPase changed inversely with potassium and not with aldosterone whereas H-ATPase changed directly with aldosterone but not with potassium. When both enzymes changed in the same direction, alterations in acid-base composition were profound; however, when these two acidifying enzymes changed in opposite directions or when only one enzyme changed, the effect on acid-base balance was modest. Serum bicarbonate was -45 meq/liter when aldosterone was high and potassium was low, it was only 29 meq/liter when aldosterone was high but potassium was normal or when aldosterone was normal and potassium was low. Our observations may help explain the metabolic alkalosis of primary aldosteronism in which aldosterone excess and hypokalemia are combined and the metabolic acidosis of aldosterone deficiency in which hypoaldosteronism and hyperkalemia are paired. The present study also demonstrated that aldosterone plays the major role in controlling Na/K-ATPase activity in cortical collecting tubule. Hypokalemia stimulates Na/K-ATPase activity in the medullary collecting tubule; this stimulatory effect of hypokalemia supports the hypothesis that the enzyme is present on the apical membrane at this site. (J.