ABSTRACT
A recent study demonstrated that in response to a feeding-induced metabolic acidosis, goldfish (Carassius auratus) adjust epithelial protein and/or mRNA expression in their kidney tubules for multiple transporters known to be relevant for acid–base regulation. These include Na+/H+ exchanger (NHE), V-type H+-ATPase (V-ATPase), cytoplasmic carbonic anhydrase, HCO3− transporters and Rhesus proteins. Consequently, renal acid output in the form of protons and NH4+ increases. However, little is known about the mechanistic details of renal acid–base regulation in C. auratus and teleost fishes in general. The present study applied the scanning ion-selective electrode technique (SIET) to measure proton flux in proximal, distal and connecting tubules of goldfish. We detected increased H+ efflux into the extracellular fluid from the tubule in fed animals, resulting from paracellular back-flux of H+ through the tight junction. By applying inhibitors for selected acid–base regulatory epithelial transporters, we found that cytosolic carbonic anhydrase and HCO3− transporters were important in mediating H+ flux in all three tubule segments of fed goldfish. Contrastingly, V-ATPase seemed to play a role in H+ flux only in proximal and distal tubules, and NHE in proximal and connecting tubules. We developed working models for transport of acid–base relevant equivalents (H+, HCO3−, NH3/NH4+) for each tubule segment in C. auratus kidney. While the proximal tubule appears to play a major role in both H+ secretion and HCO3− reabsorption, the distal and connecting tubules seem to mainly serve for HCO3− reabsorption and NH3/NH4+ secretion.
FOOTNOTES
Competing interests
The authors declare no competing or financial interests.
Author contributions
Conceptualization: S.F., D.K., C.M.W., M.J.O.; Formal analysis: S.F., D.K., M.J.O.; Investigation: S.F., D.K.; Writing - original draft: S.F.; Writing - review & editing: S.F., D.K., C.M.W., M.J.O.; Visualization: S.F.; Supervision: M.J.O.; Funding acquisition: C.M.W., M.J.O.
Funding
Financial support was provided by Discovery Grants from the Natural Sciences and Engineering Research Council of Canada (NSERC) to M.J.O. (NSERC PIN-2015-14139) and C.M.W. (NSERC PIN-2017-03843).
- Received January 31, 2019.
- Accepted May 21, 2019.
- © 2019. Published by The Company of Biologists Ltd
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