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First published online July 6, 2005
Journal of Experimental Biology 208, 2719-2729 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01688
Gene expression after freshwater transfer in gills and opercular epithelia of killifish: insight into divergent mechanisms of ion transport
1 Department of Zoology, University of British Columbia, Vancouver BC,
Canada V6T 1Z4
2 Department of Biology, Georgia Southern University, Statesboro, GA
30460-8042, USA
3 Department of Physiology and Pharmacology, James Cook University, Cairns,
QLD 4879, Australia
4 Department of Biology, McMaster University, Hamilton ON, Canada L8S
4K1
* Author for correspondence (e-mail: scott{at}zoology.ubc.ca)
Accepted 17 May 2005
We have explored the molecular basis for differences in physiological
function between the gills and opercular epithelium of the euryhaline
killifish Fundulus heteroclitus. These tissues are functionally
similar in seawater, but in freshwater the gills actively absorb
Na+ but not Cl–, whereas the opercular epithelium
actively absorbs Cl– but not Na+. These
differences in freshwater physiology are likely due to differences in absolute
levels of gene expression (measured using real-time PCR), as several proteins
important for Na+ transport, namely
Na+,H+-exchanger 2 (NHE2), carbonic anhydrase 2 (CA2),
Na+,HCO3–cotransporter 1, and V-type
H+-ATPase, were expressed at 3- to over 30-fold higher absolute
levels in the gills. In gills, transfer from 10% seawater to freshwater
increased the activity of Na+,K+-ATPase by twofold (from
12 h to 7 days), increased the expression of NHE2 (at 12 h) and CA2 (from 12 h
to 7 days), and decreased the expression of NHE3 (from 12 h to 3 days). In
opercular epithelium, NHE2 was not expressed; furthermore,
Na+,K+-ATPase activity was unchanged after transfer to
freshwater, CA2 mRNA levels decreased, and NHE3 levels increased. Consistent
with their functional similarities in seawater, killifish gills and opercular
epithelium expressed Na+,K+-ATPase 
1a,
Na+,K+,2Cl–cotransporter 1 (NKCC1),
cystic fibrosis transmembrane conductance regulator (CFTR)
Cl– channel and the signalling protein 14-3-3a at similar
absolute levels. Furthermore, NKCC1 and CFTR were suppressed equally in each
tissue after freshwater transfer, and 14-3-3a mRNA increased in both. These
results provide insight into the mechanisms of ion transport by killifish
gills and opercular epithelia, and demonstrate a potential molecular basis for
the differences in physiological function between these two organs.
Key words: Fundulus heteroclitus, carbonic anhydrase, Na+, H+-exchanger, Na+, HCO3–cotransporter, fish
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