Na+/K+-ATPase [alpha]-subunit (nka[alpha]) Isoforms and Their mRNA Expression Levels, Overall Nka[alpha] Protein Abundance, and Kinetic Properties of Nka in the Skeletal Muscle and Three Electric Organs of the Electric Eel, Electrophorus electricus

This study aimed to obtain the coding cDNA sequences of Na.sup.+ /K.sup.+ -ATPase [alpha] (nka[alpha]) isoforms from, and to quantify their mRNA expression in, the skeletal muscle (SM), the main electric organ (EO), the Hunter's EO and the Sach's EO of the electric eel, Electrophorus electricus. Four nka[alpha] isoforms (nka[alpha]1c1, nka[alpha]1c2, nka[alpha]2 and nka[alpha]3) were obtained from the SM and the EOs of E. electricus. Based on mRNA expression levels, the major nka[alpha] expressed in the SM and the three EOs of juvenile and adult E. electricus were nka[alpha]1c1 and nka[alpha]2, respectively. Molecular characterization of the deduced Nka[alpha]1c1 and Nka[alpha]2 sequences indicates that they probably have different affinities to Na.sup.+ and K.sup.+ . Western blotting demonstrated that the protein abundance of Nka[alpha] was barely detectable in the SM, but strongly detected in the main and Hunter's EOs and weakly in the Sach's EO of juvenile and adult E. electricus. These results corroborate the fact that the main EO and Hunter's EO have high densities of Na.sup.+ channels and produce high voltage discharges while the Sach's EO produces low voltage discharges. More importantly, there were significant differences in kinetic properties of Nka among the three EOs of juvenile E. electricus. The highest and lowest V.sub.max of Nka were detected in the main EO and the Sach's EO, respectively, with the Hunter's EO having a V.sub.max value intermediate between the two, indicating that the metabolic costs of EO discharge could be the highest in the main EO. Furthermore, the Nka from the main EO had the lowest K.sub.m (or highest affinity) for Na.sup.+ and K.sup.+ among the three EOs, suggesting that the Nka of the main EO was more effective than those of the other two EOs in maintaining intracellular Na.sup.+ and K.sup.+ homeostasis and in clearing extracellular K.sup.+ after EO discharge.