Selective modulation of hepatic and ileal Na+-K+-ATPase by bile salts in the rat

Sodium-potassium adenosinetriphosphatase (Na+-K+-ATPase) is modulated by functional demands. We determine whether Na+-K+-ATPase specific activity was changed by oral administration of different bile salts and whether upregulation in the liver is due to increased numbers of catalytic units. In rats after bile duct drainage for 18 h, Na+-K+-ATPase activity was reduced to 50% of control in liver and ileum but unchanged in jejunum and kidney. Increased Na+-K+-ATPase activity after short-term feeding of bile salts was noted only following trihydroxy bile salts, i.e., taurocholate (100 mg/100 g body wt) increased hepatic Na+-K+-ATPase 143% and ileum 138% above control, whereas jejunum and kidney were unchanged. Chronic feeding of trihydroxy bile salts for 4 days increased hepatic Na+-K+-ATPase (214-260%) and alkaline phosphatase (189-274%), whereas 5'-nucleotidase and Mg2+-ATPase activities were unchanged from control. Plasma membrane Na+-K+-ATPase activity significantly increased as early as 4 h after taurocholate administration, whereas homogenate activity did not rise until 16 h; both reached a new steady state between 24 and 48 h. Sixteen hours after bile salt feeding, increased Na+-K+-ATPase activity was blocked by cycloheximide, and in the liver increased enzyme activity (179%) was associated with a comparable change in sodium-dependent [gamma-32P]ATP binding (162%) to liver plasma membrane fractions. These studies show Na+-K+-ATPase activity adapts selectively in liver and ileum following administration of trihydroxy bile salts, and the process involves increased density of Na+-K+ pump sites on the liver plasma membrane.