Transport and histochemical studies of bicarbonate handling by the alligator kidney

The alligator excretes a persistently alkaline urine despite consuming an acid-residue diet. The amount of bicarbonate excreted is greater than the amount filtered, evidencing tubular secretion of bicarbonate. The parallel urinary excretion of ammonium maintains external acid balance. To investigate putative renal mechanisms responsible for the concurrent excretion of large quantities of ammonium bicarbonate, we used acridine orange fluorescence methodology in microvesicles prepared from the proximal tubule brush border to assess the activity of the Na+-H+ antiporter, and histochemical methods (cobalt sulfide precipitation) to assess carbonic anhydrase localization. We found no evidence for the presence of a functioning Na+-H+ antiporter, the protein known to be responsible for the majority of bicarbonate reabsorption in mammals; Na+-H+ exchange in vesicles from the alligator kidney failed to exhibit saturation kinetics, showed no affinity for lithium, and was not inhibited by amiloride. Sensitive histochemical techniques failed to reveal carbonic anhydrase activity anywhere in the proximal tubule but detected an abundance of enzyme activity in the basolateral membranes and nuclei of distal tubular cells. In the connecting segment and collecting duct, cells without carbonic anhydrase alternated with cells containing carbonic anhydrase; in the latter, the enzyme was localized to the basolateral and luminal membranes, the nucleus and, to a lesser extent, throughout the cytoplasm. We conclude that the proximal tubule of the alligator kidney is devoid of the machinery necessary for the transport of large amounts of bicarbonate. The principal site at which bicarbonate is added to the final urine appears to be the distal tubule, at which site carbonic anhydrase is widespread.