Basolateral membrane Na+/H+ exchange enhances HCO3- absorption in rat medullary thick ascending limb: evidence for functional coupling between basolateral and apical membrane Na+/H+ exchangers.
The role of basolateral membrane Na+/H+ exchange in transepithelial HCO3- absorption (JHCO3) was examined in the isolated, perfused medullary thick ascending limb (MTAL) of the rat. In Na(+)-free solutions, addition of Na+ to the bath resulted in a rapid, amiloride-sensitive increase in intracellular pH. In MTALs perfused and bathed with solutions containing 146 mM Na+ and 25 mM HCO3-, bath addition of amiloride (1 mM) or 5-(N-ethyl-N-isopropyl) amiloride (EIPA, 50 microM) reversibly inhibited JHCO3 by 50%. Evidence that the inhibition of JHCO3 by bath amiloride was the result of inhibition of Na+/H+ exchange included the following: (i) the IC50 for amiloride was 5-10 microM, (ii) EIPA was a 50-fold more potent inhibitor than amiloride, (iii) the inhibition by bath amiloride was Na+ dependent, and (iv) significant inhibition was observed with EIPA as low as 0.1 microM. Fifty micromolar amiloride or 1 microM EIPA inhibited JHCO3 by 35% when added to the bath but had no effect when added to the tubule lumen, indicating that addition of amiloride to the bath did not directly inhibit apical membrane Na+/H+ exchange. In experiments in which apical Na+/H+ exchange was assessed from the initial rate of cell acidification following luminal EIPA addition, bath EIPA secondarily inhibited apical Na+/H+ exchange activity by 46%. These results demonstrate basolateral membrane Na+/H+ exchange enhances transepithelial HCO3- absorption in the MTAL. This effect appears to be the result of cross-talk in which an increase in basolateral membrane Na+/H+ exchange activity secondarily increases apical membrane Na+/H+ exchange activity.
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Department of Medicine, University of Texas Medical Branch, Galveston 77555, USA.
George
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Watts
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D000042:Absorption; D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology*; D004232:Diuretics / Q000494:pharmacology; D004848:Epithelium / Q000187:drug effects / Q000502:physiology; D066298:In Vitro Techniques; D007679:Kidney Medulla / Q000502:physiology*; D007684:Kidney Tubules / Q000502:physiology*; D007700:Kinetics; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000494:pharmacology; D017923:Sodium-Hydrogen Exchangers / Q000187:drug effects / Q000378:metabolism*
D001639:Bicarbonates; D004232:Diuretics; D017923:Sodium-Hydrogen Exchangers; D000584:Amiloride; D012964:Sodium; C039614:ethylisopropylamiloride