AJP - Renal Physiology, Vol 251, Issue 5 865-F872, Copyright © 1986 by American Physiological Society
Bicarbonate transport by amphibian nephron
C. B. Yucha and L. C. Stoner
To determine the site of bicarbonate reabsorption, tubular fragments from
five different segments of the salamander nephron (Ambystoma) were perfused
in vitro. Bicarbonate contents (total CO2) of tubular fluid were determined
by microcalorimetry. Bicarbonate was not transported by the diluting
segment or the mid-distal tubule. Although proximal tubule fragments did
appear to reabsorb bicarbonate (10.8 +/- 3.7 pmol X mm-1 X min-1, P less
than 0.01, n = 14), the chemical gradients observed were small with respect
to the sensitivity of our methods. In the late distal tubule (LDT),
bicarbonate reabsorption averaged 28.9 +/- 8.2 pmol X mm-1 X min-1 (P less
than 0.01, n = 16). Transport in this segment was inhibited by
ethoxzolamide or by perfusing with a sodium-free solution. Ethoxzolamide
had no effect on the trans-epithelial voltage. Thus bicarbonate is
reabsorbed by the LDT of amphibia via a mechanism that is dependent on
carbonic anhydrase and the presence of luminal sodium. The 13 initial
collecting tubules (ICT) studied did not appear to reabsorb bicarbonate. On
the other hand, some ICT developed substantial bicarbonate gradients.
Therefore this nephron segment has the capacity to reabsorb bicarbonate.
