Transcellular Chloride Pathways in Ambystoma Proximal Tubule |
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| Authors: | S Abdulnour-Nakhoul EL Boulpaep |
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| Institution: | (1) Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, CT 06520-8026, USA, US |
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| Abstract: | The transport mechanisms of Ambystoma proximal tubule that mediate transcellular Cl− absorption linked to Na+ were investigated in isolated perfused tubules using Cl−-selective and voltage-recording microelectrodes. In control solutions intracellular activity of Cl− (a
i
Cl
) is 11.3 ± 0.5 mm, the basolateral (V
1
), apical (V
2
), and transepithelial (V
3
) potential differences are −68 ± 1.2 mV, +62 ± 1.2 mV and −6.4 ± 0.3 mV, respectively. When Na+ absorption is decreased by removal of organic substrates from the lumen, a
i
Cl
falls by 1.3 ± 0.3 mm and V
2
hyperpolarizes by +11.4 ± 1.7 mV. Subsequent removal of Na+ from the lumen causes a
i
Cl
to fall further by 2.3 ± 0.4 mm and V
2
to hyperpolarize further by +15.3 ± 2.4 mV. The contribution of transporters and channels to the observed changes of a
i
Cl
was examined using ion substitutions and inhibitors. Apical Na/Cl or Na/K/2Cl symport is excluded because bumetanide, furosemide
or hydrochlorothiazide have no effect on a
i
Cl
. The effects of luminal HCO−
3 removal and/or of disulfonic stilbenes argue against the presence of apical Cl-base exchange such as Cl-HCO3 or Cl-OH. The effects of basolateral HCO−
3 removal, of basolateral Na+ removal and/or of disulfonic stilbenes are compatible with presence of basolateral Na-independent Cl-base exchange and Na-driven
Cl-HCO3 exchange. Several lines of evidence favor conductive Cl− transport across both the apical and basolateral membrane. Addition of the chloride-channel blocker diphenylamine-2-carboxylate
to the lumen or bath, increases the a
i
Cl
by 2.4 ± 0.6 mm or 2.9 ± 1.0 mm respectively. Moreover, following inhibition by DIDS of all anion exchangers in HCO−
3-free Ringer, the equilibrium potential for Cl− does not differ from the membrane potential V
2
. Finally, the logarithmic changes in a
i
Cl
in various experimental conditions correlate well with the simultaneous changes in either basolateral or apical membrane
potential. These findings strongly support the presence of Cl− channels at the apical and basolateral cell membranes of the proximal tubule.
Received: 14 November 1997/Revised: 6 July 1998 |
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| Keywords: | : Proximal tubule — Chloride channels — Chloride-base exchange — Na-driven Cl-HCO3 exchange — Transcellular chloride transport — Disulfonic stilbenes |
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