Alteration of Bile Canalicular Enzymes in Cholestasis. A POSSIBLE CAUSE OF BILE SECRETORY FAILURE

Abstract

A B S T R A C T Bile secretory failure (cholestasis) may result from several possible mechanisms involved in bile secretion. We have examined the possibility that abnormalities in enzyme content, composition, and turnover 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

Ethinylestradiol treatment induces multiple canalicular membrane transport alterations in rat liver.

“…Thus, our findings do not support the concept that selective alterations in the sinusoidal surface of hepatocytes play a unique role in the pathogenesis ofEE-induced intrahepatic cholestasis (7,10). Rather, our data support, but do not yet prove, the alternative possibility that assigns the primary cause(s) of EE-induced bile secretory failure to the canalicular membrane domain of hepatocytes (2,9,46,47). Most importantly, this study indicates that a decreased canalicular ATP-dependent bile salt transport capacity is primarily responsible for the EE-induced impairment of bile salt secretion in intact liver ( 1-3, 44, 46).…”

Alterations of hepatic Na + ,K + -ATPase and bile flow by estrogen: Effects on liver surface membrane lipid structure and function

“…This fraction has been called bile salt-independent bile flow, and recent studies have demonstrated a strong correlation between hepatic Na+, K+-ATPase activity and bile flow (10), supporting the hypothesis that this component of bile flow is regulated by the sodium pump. One drug consistently shown to reduce bile salt-independent bile flow is the synthetic estrogen derivative ethinyl estradiol (17a-ethinyl-1,3,5-estratriene-3,17f3-diol) (10)(11)(12)(13).The aim of the present study was to examine whether Na+, K+-ATPase activity is reduced after ethinyl estradiol treatment and, if so, what possible mechanisms might be involved. We recently found that ethinyl estradiol significantly increases hepatic cholesterol ester concentrations by activating hepatic microsomal cholesterol acyl-CoA transferase (14).…”

“…Liver surface membrane fractions were isolated according to Neville (17) through step 12 as described by Pohl et al (18). This surface membrane fraction is enriched in bile canalicular fronts, but also contains sinusoidal surface membranes (12,19). Microsomal fractions were prepared from the supernate of the liver surface membrane nuclear pellet by a modification of the procedure of Evans and Gurd (20) Abbreviations: ethinyl estradiol, 17a-ethinyl-1,3,5-estratriene-3,17,B-diol; ESR, electron spin resonance.…”

“…This fraction has been called bile salt-independent bile flow, and recent studies have demonstrated a strong correlation between hepatic Na+, K+-ATPase activity and bile flow (10), supporting the hypothesis that this component of bile flow is regulated by the sodium pump. One drug consistently shown to reduce bile salt-independent bile flow is the synthetic estrogen derivative ethinyl estradiol (17a-ethinyl-1,3,5-estratriene-3,17f3-diol) (10)(11)(12)(13).…”

“…This surface membrane fraction is enriched in bile canalicular fronts, but also contains sinusoidal surface membranes (12,19). Microsomal fractions were prepared from the supernate of the liver surface membrane nuclear pellet by a modification of the procedure of Evans and Gurd (20), previously described (21 Na+,K+-ATPase activity was determined in liver surface membrane fractions and homogenates according to the method of Ismail-Beigi and Edelman with minor modifications (12,23). A detailed account of the procedure used has been reported (9).…”