Differential expression of canalicular membrane Ca2+/Mg2+-ecto-ATPase in estrogen-induced and obstructive cholestasis in the rat

Extracellular adenosine triphosphate (ATP) may regulate hepatocyte and chol angiocyte functions, and under some conditions it may have deleterious effe cts on bile secretion and cause cholestasis. The canalicular membrane enzym e Ca2+/Mg2+-ecto-ATPase (ecto-ATPase) hydrolyzes ATP/adenosine diphosphate (ATP/ADP) and regulates hepatic extracellular ATP concentration. Changes in liver ecto-ATPase in estrogen-induced cholestasis were examined in male ra ts receiving 17 alpha-ethinylestradiol (E groups) for 1, 3, or 5 days (5 mg /kg/day, sc) and compared with changes in rats subjected to obstructive cho lestasis (O groups) for 1, 3, or 8 days. Activity of ecto-ATPase, protein m ass in canalicular membranes and bile (estimated by Western blotting), stea dy state mRNA levels (by Northern blotting), and cellular and acinar distri butions of the enzyme (histochemistry and immunocytochemistry) were assesse d in these groups. Activity of ecto-ATPase, protein mass in isolated canali cular membranes, and enzyme mRNA levels were significantly increased in E g roup rats as compared with controls. In contrast, these parameters were mar kedly decreased in O group rats, and the enzyme protein was undetectable in bile. The ecto-ATPase histochemical reaction was markedly increased in the canalicular membrane of E group rats, extending from acinar zone 2 to zone 1,whereas it decreased in the O group. The ecto-ATPase immunocytochemical reaction was present in the canalicular membrane and pericanalicular vesicl es in control and E group hepatocytes, but it decreased in obstructive chol estasis and was localized only to the canalicular membrane. Thus, significa nt changes in liver ecto-ATPase were apparent in 17 alpha-ethinylestradiol- induced cholestasis that were opposite to those observed in obstructive cho lestasis. Assuming that the alterations observed in obstructive cholestasis are the result of the cholestatic phenomenon, we conclude that changes in ecto-ATPase in 17 alpha-ethinylestradiol-treated rats might be either prima ry events or part of an adaptive response in 17 alpha-ethinylestradiol-indu ced cholestasis.