Section-specific expression of acid-base and ammonia transporters in the kidney tubules of the goldfish Carassius auratus and their responses to feeding.

In teleost fishes, renal contributions to acid-base and ammonia regulation are often neglected compared to the gills. In goldfish, increased renal acid-excretion in response to feeding was indicated by increased urine ammonia and inorganic phosphate concentrations and decreased urine pH. By micro-dissecting the kidney tubules and performing quantitative real-time PCR and/or immunohistochemistry, we profiled the section-specific expression of glutamate dehydrogenase (GDH), glutamine synthetase (GS), Na+ /H+ -exchanger 3 (NHE3), carbonic anhydrase II (CAIIa), V-H+ -ATPase subunit 1b, Cl- /HCO3 - -exchanger 1 (AE1), Na+ /HCO3 - -cotransporter 1 (NBC1), Na+ /K+ -ATPase subunit 1α, and Rhesus-proteins Rhbg, Rhcg1a and Rhcg1b. We show here for the first time that (i) the proximal tubule appears to be the major site for ammoniagenesis, (ii) epithelial transporters are differentially expressed along the renal tubule, and (iii) a potential feeding-related "acidic tide" results in the differential regulation of epithelial transporters, resembling the mammalian renal response to a metabolic acidosis. Specifically, GDH and NHE3 mRNAs were up- and GS down-regulated in the proximal tubule upon feeding, suggesting this section as a major site for ammoniagenesis and acid-secretion. The distal tubule may play a major role in renal ammonia secretion, with feeding-induced up-regulation of mRNA and protein for apical NHE3, cytoplasmic CAIIa, universal Rhcg1a and apical Rhcg1b, and down-regulation of basolateral Rhbg and AE1. Changes in mRNA expression of the Wolffian ducts and bladder suggest supporting roles in fine-polishing urine composition. The present study verifies an important renal contribution to acid-base balance and emphasises that studies looking at the whole kidney may overlook key section-specific responses.