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The fish branchial epithelium : an immunological approach to ion transport protein localization Wilson, Jonathan Mark

Abstract

In my thesis the cellular and subcellular distributions of various ion transport proteins were determined in the fish branchial epithelium using an immunological approach employing non-homologous antibodies. The viabilities of current models of transepithelial ion transport in the gills of freshwater, and seawater fishes and an unusual amphibious air-breathing fish were tested. My thesis work primarily addresses the mechanisms involved in the translocation of Na+ and Cl- and their counterions, H+ and HCO3-, respectively and of NH4+. In freshwater fishes (tilapia: Oreochromis mossambicus; trout: Oncorhynchus mykiss) the vH+-ATPase colocalizes apically with an epithelial Na+ channel (ENaC)-like protein, however, the distribution of these proteins is restricted to pavement cells (PVCs) in tilapia while being found in both PVCs and mitochondria-rich (MR) cells in trout. An Na+/H+ exchanger-like (NHE) protein is also identified in the gill of the freshwater tilapia. In tilapia and coho salmon (Oncorhynchus kisutch), an apical band 3-like CI-/HCO3- anion exchanger (AE) is localized to MR cells demonstating the presence of a freshwater-type chloride cell (involved in Cl- uptake). In seawater fishes, the chloride cell has been well characterized as the site of active Cl- elimination. In my observations of seawater fishes (O.kisutch; turbot: Scophthamus maximus) and the brackishwater, air-breathing mudskipper fish (Periophthalmodon schlosseri), ion transport proteins involved in active Cl- elimination are localized to chloride cells (Na+,K+-ATPase, Na+:K+:2Cl- cotransporter (NKCC) and apical cystic fibrosis transmembrane receptor (CFTR)-like anion channel). The AE and NHE proteins that are potentially involved in acid-base regulation are localized to the chloride cell apical crypt and accessory cell, respectively, in the stenohaline turbot. However, in the euryhaline coho the apical AE of the freshwater MR cell is not observed in the seawater acclimated fish. In the mudskipper fish, I have shown that NHE, Na+,K+-ATPase, and carbonic anhydrase in gill MR cells contribute to active ammonia (NH4+) excretion in pharmacological studies. The branchial epithelium contains an unusual abundance of MR cells and immunolocalization studies show the presence of NHE-2 and 3-like isoforms, CFTR, and CA associated with the apical crypt and Na+,K+-ATPase and NKCC with the tubular system of these cells. And so it goes....

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