VESICLE TARGETING TO THE APICAL DOMAIN REGULATES BILE EXCRETORY FUNCTION IN ISOLATED RAT HEPATOCYTE COUPLETS

Background & Aims: Plasma membrane solute transport may be regulated i n many epithelial cells by vesicle traffic to and from the site of res idence of the transporter. The aim of this study was to determine if t his phenomenon may also play a role in the regulation of canalicular t ransport of bile acids. Methods: Confocal microscopy and image analysi s were performed to quantitatively assess changes in secretory capacit y and vesicle targeting in isolated rat hepatocyte couplets that had b een exposed to fluorescent bile acid after pretreatment with dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP) and/or nocodazole. Resu lts: DBcAMP stimulated bile acid secretion by 240% while significantly increasing canalicular circumference. Nocodazole decreased secretion by 410% and significantly decreased canalicular circumference. When DB cAMP was added to nocodazole-treated couplets, a slight but significan t increase was found in both fluorescent bile acid secretion and canal icular circumference as compared with nocodazole alone. Finally, DBcAM P stimulated translocation of vesicles to the canalicular membrane as determined by immunocytochemical localization of a putative bile acid transporter, Ca2+, Mg2+-ecto-adenosine triphosphatase. Conclusions: Th e findings support the view that apical membrane transport activity in the rat hepatocyte is highly regulated by the insertion of vesicles i nto this domain and that this process involves both microtubule-depend ent and -independent mechanisms.