Differential expression and enzymatic properties of the Na⁺,K⁺-ATPase α3 isoenzyme in rat pineal glands

We have used immunoblotting and biochemical techniques to analyze expression of Na⁺,K⁺-ATPase α and β subunits in rat pineal glands. Western blot analysis of pineal microsomal membrane fractions with antisera specific for each of the three rat α and two rat β subunits revealed similar levels of expression of α1 and α3 subunits in pineal glands of 5-day-old rats. High levels of α3 and β2 subunits and low levels of α1 subunits were detected in adult glands. No α2 or β1 subunits were detectable at either developmental stage. Examination of the enzymatic properties of the pineal gland α3 isoform suggests that this enzyme is a ouabain-sensitive ATPase whose activity is dependent upon Na⁺ and K⁺. This ATPase exhibited a lower apparent K_m for Na⁺ than the kidney α1 isoenzyme and did not show positive cooperative Na⁺ activation. Our results suggest that the activity of the Na⁺,K⁺-ATPase α3 isoenzyme may be adapted to function under conditions of hyperpolarizing transmembrane potentials.