Abstract
The amiloride-sensitive epithelial sodium channel (ENaC) has previously been shown to be involved in the maintenance of body fluid volume and in Na+ absorption across the skin and urinary bladder in amphibians. However, the function and distribution of ENaC have not been clearly described in amphibian kidney. We therefore cloned the ENaC α-subunit cDNA from kidney of the marine toad, Bufo marinus. The ENaC mRNA and protein were abundantly expressed in the kidney and in the urinary bladder and ventral pelvic skin. In an immunohistochemical study, the ENaC α-subunit protein was specifically localized to the apical membrane of the principal cells but not the intercalated cells from the late distal tubule to the collecting duct in the kidney or in the apical area of cells of urinary bladder epithelia. When toads were acclimated to dry and hyper-saline environments, the levels of ENaC mRNA expression in the kidney and urinary bladder decreased under hyper-saline acclimation, but not under dry conditions. Immunohistochemical observations indicated that the levels of ENaC protein expression were much lower in the apical area of renal distal tubules and urinary bladder epithelia of hyper-saline acclimated toad compared with controls. The present study suggests that Bufo ENaC is significantly expressed and functions during Na+ reabsorption in the apical membrane domain in the distal nephron of normal and desiccated toads. Natriuresis may be caused by decreases in ENaC expression and its trafficking to the cell surface in the distal nephron, a response to prevent excessive Na+ reabsorption in hyper-saline-acclimated toads.
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Acknowledgements
The authors thank Prof. H. Ota (Tropical Biosphere Research Center, University of the Ryukyu) and Dr. Y. Hokama (Agricultural Experiment Station of Okinawa Prefecture) for their kind help and suggestions with collecting animals. They are also grateful to Prof. T. Wakasugi (Department of Life and Environmental Science, University of Toyama) for his valuable advice regarding the molecular biological study.
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This work was supported in part by a research grant (a priority project in Science Faculty) from Toyama University and by a Ground-based Research Announcement for Space Utilization promoted by the Japan Space Forum to M.U.; N.K. is grateful to the Japan Society of the Promotion of Science (JSPS) for a young scientist’s fellowship.
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Konno, N., Hyodo, S., Yamada, T. et al. Immunolocalization and mRNA expression of the epithelial Na+ channel α-subunit in the kidney and urinary bladder of the marine toad, Bufo marinus, under hyperosmotic conditions. Cell Tissue Res 328, 583–594 (2007). https://doi.org/10.1007/s00441-007-0383-9
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DOI: https://doi.org/10.1007/s00441-007-0383-9