Background: Sodium-potassium-adenosinetriphosphatase (Na,K-ATPase) is the primary membrane enzyme responsible for the reabsorption of sodium ions in the kidney. It is known that in the nephron the major subunit isoforms of Na,K-ATPase are alpha 1 and beta 1. Previous reports on the presence of alpha 2 and alpha 3 isoforms in the kidney were mixed and controversial.
Methods: Techniques of ultrathin cryosectioning and immunoelectron microscopy were used to study the distribution of alpha subunit isoforms (alpha 1, alpha 2, alpha 3) and beta subunit (beta 1 isoform) of Na,K-ATPase in renal tubular cells. Western blot analysis was used to show the presence of the alpha 3 isoform in the extract of kidney mitochondria.
Results: We were able to confirm the previous finding that the alpha 1 isoform and the beta 1 isoform were the preponderant isoforms of the alpha and beta subunits of Na,K-ATPase in the basolateral membrane. In addition, we unexpectedly found the presence of the alpha 3 isoform in the mitochondria of rat renal tubular cells. The alpha 2 and alpha 3 isoforms were not observed in either the apical or basolateral membrane.
Conclusions: Both immunoelectron microscopy and Western blot analysis of the rat kidney mitochondria confirm the presence of the alpha 3 isoform of Na,K-ATPase in the rat kidney mitochondria. The function of this enzyme in the mitochondria is not clear at this time.