Regulation of biliary cholesterol secretion is independent of hepatocyte canalicular membrane lipid composition: a study in the diosgenin-fed rat model

Background/Aims: Phosphatidylcholine (PC) and sphingomyelin (SM) are the major phospholipids on the outer leaflet of the hepatocyte canalicular membrane. Since cholesterol preferentially associates with SM in detergent-resistant microdomains, we hypothesized that canalicular membrane lipid composition could modulate secretion of the sterol into bile. Methods: Male Wistar rats were fed for 10 days with a control diet with or without the plant sterol diosgenin (1% w/w) to induce biliary cholesterol hypersecretion. Thereafter, lipid compositions and phospholipid molecular species were determined in fistula bile and highly enriched canalicular membrane fractions. Results: Despite four-fold higher biliary cholesterol output in diosgenin-fed rats, no differences were observed between canalicular membranes of diosgenin and control groups with respect to cholesterol/phospholipid ratios (0.58 vs 0.62), phospholipid classes and acyl chain compositions of SMs (16:0>24:1>24:0>22:0>18:0>23:0>20:0>24:2), or PCs (mainly diacyl 16:0–18:2, 16:0–20:4, 18:0–20:4, and 18:0–18:2). In contrast to canalicular PCs, bile contained more hydrophilic species (mainly diacyl 16:0–18:2 and 16:0–20:4), without differences between both groups. In vitro resistance of purified canalicular membrane fractions against detergents such as Triton X-100 and taurocholate was also similar in both groups. Conclusions: Diosgenin-induced biliary cholesterol hypersecretion occurs in the absence of changes of canalicular membrane lipids. Our data therefore do not support a major role of canalicular membrane lipid composition in regulation of biliary cholesterol secretion.