Abstract
To clarify ion-absorbing functions and molecular mechanisms of mitochondria-rich (MR) cells, Mozambique tilapia (Oreochromis mossambicus) were acclimated to artificial freshwaters with normal or lowered Na+ and/or Cl- concentration: (1) normal Na+/normal Cl- (control); (2) normal Na+/low Cl-; (3) low Na+/normal Cl-; and (4) low Na+/low Cl-. Scanning electron microscopy (SEM) revealed that concave and convex apical surfaces of MR cells predominantly developed in low Na+ and low Cl- waters, respectively, whereas small apical pits predominated in control conditions. Expression of Na+/H+ exchanger-3 (NHE3) mRNA in the gills was increased in low Na+ waters (low Na+/normal Cl- and low Na+/low Cl-, whereas that of Na+/Cl- cotransporter (NCC) expression was upregulated in low Cl-, but not in low Na+/low Cl-. Immunofluorescence staining showed that enlarged NHE3-immunoreactive apical regions were concave or flat in low Na+ waters, whereas NCC-immunoreactive regions were enlarged convexly in low Cl- waters. Using SEM immunocytochemistry the distribution of NHE3/NCC was compared with SEM images obtained simultaneously, it was further demonstrated that NHE3 and NCC were confined to concave and convex apical surfaces, respectively. These results indicated that small apical pits developed into concave apical surfaces to facilitate Na+ uptake through NHE3, and into convex apical surfaces to enhance Na+/Cl- uptake through NCC. Our findings integrated morphological and functional classifications of ion-absorbing MR cells in Mozambique tilapia.
Highlights
Euryhaline teleosts inhabiting both freshwater and seawater maintain their plasma osmolalities within narrow physiological ranges, equivalent to about one-third seawater osmolality
Our previous study showed that both apical Na+/H+ exchanger-3 (NHE3) and Na+/Cl– cotransporter (NCC), rather than V-ATPase, are importantly involved in ion uptake in gill MR cells of tilapia acclimated to hypotonic environments (Inokuchi et al, 2008)
In fish acclimated to artificial freshwaters with lowered Na+ and/or Cl– concentration, we examined the apical morphology of MR cells, expression of NHE3 mRNA and NCC mRNA in the gills, and their immunolocalization in MR cells
Summary
Euryhaline teleosts inhabiting both freshwater and seawater maintain their plasma osmolalities within narrow physiological ranges, equivalent to about one-third seawater osmolality. MR cells in freshwater-adapted teleosts are responsible for ion uptake, but the mechanisms involved are still poorly understood (Marshall, 2002; Hirose et al, 2003; Perry et al, 2003; Evans et al, 2005; Hwang and Lee, 2007). In freshwater-adapted fish, three ion-transport proteins have been proposed as the apical pathways for sodium uptake in MR cells. It has been demonstrated that Na+/Cl– cotransporter (NCC) is located in the apical membrane of MR cells in the embryonic skin of freshwater-adapted tilapia (Hiroi et al, 2008). Our previous study showed that both apical NHE3 and NCC, rather than V-ATPase, are importantly involved in ion uptake in gill MR cells of tilapia acclimated to hypotonic environments (Inokuchi et al, 2008)
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