Effect of Na on bile acid uptake by isolated rat hepatocytes. Evidence for a heterogeneous system.

The effect of Na on cholate and taurocholate uptake was studied in isolated rat hepatocytes. Uptake of both bile acids can be divided into three components: a) a non-saturable component independent of Na, b) a Na-independent saturable and c) a Na-dependent saturable component. The two 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, rather than the Km, was affected by Na. These results suggest that more than one Na (probably two) are cosubstrates in the transport of one bile acid molecule and Na exerts 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 suggest that cholate and taurocholate share a common Na-independent transport system while there are at least two Na-dependent transport systems for taurocholate, one of which is shared by cholate.