A Possible Cause of Bile Secretory Failure

of liver plasma membrane constituents are altered in cholestasis. Severe and mild cholestasis were produced by 5 days of bile duct ligation and ethinyl estradiol administration, respectively. Bile duct ligation but not ethinyl estradiol treatments was associated with elevations of the serum bilirubin level and 5'-nucleotidase activity. However, basal bile flow and bilirubin transport maximum (T.) were significantly reduced after ethinyl estradiol treatment. Liver plasma membrane fractions rich in canalicular membranes were prepared from groups of rats in each of three categories: normal, after bile duct ligation, or ethinyl estradiol administration, and their respective controls. Electron microscopy and enzyme marker studies demonstrated plasma membrane fractions free of significant contamination. Plasma membrane fractions prepared from mild as well as severe cholestasis had increased alkaline phosphatase activity, and reduced 5'-nucleotidase and Mg'+-ATPase activities. Co'+-CMPase activity was unchanged. Kinetic analysis of 5'-nucleotidase and Mg'+-ATPase activities in plasma membrane fractions demonstrated reduced Vrna (but unaltered Ki). Reducted Vmax was unrelated to addition in vitro of di-or trihydroxy bile salts or ethinyl estradiol and, therefore, suggests that reduced activities in cholestasis are due to decreased enzyme content. Cholestasis was not associated with changes in the synthesis or degradation rate of pulse-labeled plasma This investigation was performed during tenure of special fellowship (AM 40777) to Dr. Simon. Dr. Simon's present address is the University of Colorado Medical Center, Division of Gastroenterology, Denver, Colo. 80220. Received for publication 25 Jutly 1972 and in revised form 27 November 1972. membrane proteins or alterations in the major protein bands separated on sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Plasma membrane cholesterol, phospholipid, and neutral sugar content was unaltered, but sialic acid content was significantly increased in both forms of cholestasis. Alterations in specific canalicular enzymes in two forms of cholestasis suggest that these changes may be involved in the pathogenesis of bile secretory failure, or may result from cholestasis.