Bicarbonate transport by amphibian nephron.

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.