Cloning and expression of the epithelial sodium channel and its role in osmoregulation of aquatic and estivating African lungfish Protopterus annectens

摘要

The epithelial sodium channel (ENaC) is a sodium (Na+)-selective aldosterone-stimulated ion channel involved in Na+ transport homeostasis of tetrapods. We examined full-length cDNA sequences and tissue distributions of ENaC alpha, ENaC beta, and ENaC gamma subunits in the African lungfish Protopterus annectens. Protopterus ENaC (pENaC) comprises 3 subunits: pENaC alpha, pENaC beta, and pENaC gamma. pENaC alpha, pENaC beta, and pENaC gamma subunits are closely related to alpha, beta, and gamma subunits of the Australian lungfish Neoceratodus forsteri ENaC (nENaC), respectively. Three ENaC subunit mRNAs were highly expressed in the gills and moderately expressed in the kidney and rectum of P. annectens. During estivation for 2-4 weeks and 2-3 months, plasma Na+ concentration was relatively stable, but plasma urea concentration significantly increased in comparison with the control fish kept in a freshwater environment. Plasma aldosterone concentration and mRNA expression of the ENaCa. subunit gradually and significantly decreased in the gills and kidney after 2 months of estivation. Thus, aldosterone-dependent Na+ absorption via ENaC probably exists in the epithelial cells of osmoregulatory organs of lungfish kept in fresh water, whereas plasma Na+ concentration may be maintained by a mechanism independent of aldosterone-ENaC axis during estivation in lungfish. (C) 2014 Elsevier Inc All rights reserved.