EFFECT OF NA+ ON BILE-ACID UPTAKE BY ISOLATED RAT HEPATOCYTES - EVIDENCE FOR A HETEROGENEOUS SYSTEM

Abstract

The effect of Na+ on cholate and taurocholate uptake was studied in isolated rat hepatocytes. Uptake of both bile acids could be divided into 3 components: a non-saturable component independent of Na+; a Na+-independent saturable and a Na+-dependent saturable component. The 2 saturable components were inhibited by KCN, oligomycin and various other bile acids, which suggests carrier-mediated, energy-requiring processes. The relationship between Na+ concentration and the Na+-dependent uptake rate was sigmoidal. The maximal uptake rate, rathr than the Km, was affected by Na+. More than 1 Na+ (probably 2) were co-substrates in the transport of 1 bile acid molecule and Na+ may exert a stimulatory effect on the translocation of the bile acid-carrier complex across the membranes. The Na+-dependent uptake was inhibited by ouabain, suggesting that this uptake process is linked to the activity of membrane bound (Na+-K+)ATPase. Inhibition and competition studies suggested that cholate and taurocholate shared a common Na+-independent transport system while there were at least 2 Na+-dependent transport systems for taurocholate, 1 of which was shared by cholate.