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First published online May 1, 2006
Journal of Experimental Biology 209, 1928-1943 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02190
Characterization of a branchial epithelial calcium channel (ECaC) in freshwater rainbow trout (Oncorhynchus mykiss)
1 Department of Biology, University of Ottawa, 30 Marie Curie, Ottawa, ON
K1N 6N5, Canada
2 Department of Biology, McMaster University, 1280 Main Street W, Hamilton,
ON L8S 4K1, Canada
3 Department of Biological Sciences, University of Alberta, Edmonton,
Alberta T5G 2E9, Canada
4 Institute of Cellular and Organismic Biology, Academia Sinica, Taipei,
Taiwan
* Author for correspondence (e-mail: sfperry{at}science.uottawa.ca)
Accepted 28 February 2006
The entry of calcium (Ca2+) through an apical membrane epithelial calcium channel (ECaC) is thought to a key step in piscine branchial Ca2+ uptake. In mammals, ECaC is a member of the transient receptor potential (TRP) gene family of which two sub-families have been identified, TRPV5 and TPRV6. In the present study we have identified a single rainbow trout (Oncorhynchus mykiss) ECaC (rtECaC) that is similar to the mammalian TRPV5 and TRPV6. Phylogenetic analysis of the protein sequence suggests that an ancestral form of the mammalian genes diverged from those in the lower vertebrates prior to the gene duplication event that gave rise to TRPV5 and TRPV6.
The putative model for Ca2+ uptake in fish proposes that the mitochondria-rich cell (also termed ionocyte or chloride cell) is the predominant or exclusive site of transcellular Ca2+ movements owing to preferential localisation of ECaC to the apical membrane of these cells. However, the results of real-time PCR performed on enriched gill cell populations as well as immunocytochemistry and in situ hybridisation analysis of enriched cells, cell cultures and whole gill sections strongly suggest that ECaC is not exclusive to mitochondria-rich cells but that it is also found in pavement cells. Not only was ECaC protein localized to areas of the gill normally having few mitochondria-rich cells, but there was also no consistent co-localization of ECaC- and Na+/K+-ATPase-positive (a marker of mitochondria rich cells) cells. Taken together, the results of the present study suggest that although ECaC (mRNA and protein) does exist in trout gill, its cellular distribution is more extensive than previously thought, thus suggesting that Ca2+ uptake may not be restricted to mitochondria-rich cells as was proposed in previous models.
Key words: TRPV, mitochondria-rich cell, pavement cell, calcium uptake, chloride cell, rainbow trout, Oncorhynchus mykiss
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