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First published online November 17, 2006
Journal of Experimental Biology 209, 4591-4596 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02558

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Fooling a freshwater fish: how dietary salt transforms the rainbow trout gill into a seawater gill phenotype

Steve F. Perry^1,*, Luis Rivero-Lopez¹, Brian McNeill¹ and Jonathan Wilson²

¹ Department of Biology, Centre for Advanced Research in Environmental Genomics, University of Ottawa, 30 Marie Curie, Ottawa, Ontario K1N 6N5, Canada
² CIIMAR, Rua dos Bragas 177, 4050-123 Porto, Portugal

View larger version (63K): [in a new window]   Fig. 1. Elevated dietary salt intake increases the number of mitochondria-rich cells and Na+/K+-ATPase activity in trout gill. (A,B) Representative images depicting Na+/K+-ATPase (NKA) immunofluorescence (red, arrows) in gill sections from (A) control and (B) salt-fed fish; nuclei are stained blue. (C) Morphometric analysis (N=6 for each group) demonstrated a significant increase (P=0.006) in the fractional surface area of mitochondria-rich (NKA immunopositive) cells in the salt-fed (filled bars) fish. (D) Branchial NKA activity (N=10 for each group) was increased threefold (P=0.011) by salt feeding (filled bars). Scale bars, 10 µm. Asterisks in C and D indicate statistically significant difference from control values.

View larger version (31K): [in a new window]   Fig. 2. Elevated dietary salt intake increases the expression of genes involved in ion secretion in seawater. (A,B) The results of western blot analysis revealed a marked increase in Na+/K+-ATPase (NKA; band at 115 kDa) and Na+/K+/2Cl- co-transporter (NKCC1; band at 160 kDa) protein in fish fed a high salt diet (filled bars; N=4 for each group). Blots were cropped for presentation purposes. (C) The levels of NKA, NKCC1 and cystic fibrosis membrane transmembrane conductance regulator (CFTR) mRNAs in salt-fed fish (N=14) were increased 5- to 15-fold relative to control fish (N=9). Asterisks in B and C indicate statistically significant difference from control values (P<0.05).

View larger version (118K): [in a new window]   Fig. 3. Elevated dietary salt intake increases the expression of NKCC1 in branchial mitochondria-rich cells. Localisation of Na+/K+-ATPase (NKA; red), NKCC1 (green) and nuclei (blue) in gill sections from (A) control or (B,C) salt-fed fish. Areas of co-localisation of NKA with NKCC1 appear as yellow/orange. D is a representative image depicting a gill section from which both primary antibodies were omitted. Arrows in A indicate cells exhibiting NKA immunofluorescence, and in B co-localisation of NKA and NKCC1. Scale bars, 10 µm.

View larger version (73K): [in a new window]   Fig. 4. Elevated dietary salt intake results in the appearance of seawater-like chloride cells. Representative transmission electron micrographs of gill mitochondria-rich cells (MRC) from (A,B) high-saltfed or (C) control trout. The MRCs can be identified by their abundance of mitochondria (m) and tubular elements. In the high-salt-fed trout, a large mitochondria-rich chloride cell (CC) is flanked by a smaller accessory cell (AC). The apical surfaces of these two adjacent cells form a shallow crypt; this area has been enlarged 2x in B. (C) In the control trout, two large MRCs are shown with microplicae elaborating their apical membranes. Pavement cells (PVC) with less pronounced microplicae are seen between (asterisk) and to the left. This area has been enlarged 2x in the inset. Arrows and arrowheads indicate the MRC-PVC and CC-AC tight junctions, respectively. The lateral membranes of MRCs are indicated by crossed arrows. Scale bars, 1 µm.