Effect of thyroid hormone on the distribution and activity of Na, K-ATPase in ventricular myocardium

Employing detergent-free sucrose-density gradient fractionation method we isolated cholesterol-rich lighter membrane fractions containing ∼10% of protein, ∼30% of cholesterol in membranes of ventricular myocardium. Cholesterol-rich lighter membrane fractions contain >70% of Na, K-ATPase and caveolins 1 and 3 and <10% of β-actin. Treatment of hypothyroid rats with T₃ increased the relative abundance of both α1 and β1 Na, K-ATPase subunits in total membranes by 4- to 5-fold (with no change in caveolin-3), and resulted in 1.9-fold increase in enzyme activity. T₃-induced Na, K-ATPase subunits were preferentially distributed to the lighter fractions (#s 4, 5 and 6); and increased abundance of α1 and β1 were 34-70% and 43-68%, respectively. We conclude that the activity of Na, K-ATPase is not uniform in cardiac membranes, and while a significant amount of Na, K-ATPase is present in cardiac cholesterol-rich membrane fractions, the intrinsic activity is significantly less than the enzyme present in relatively cholesterol-poor membranes.