Protoporphyrin-induced Cholestasis in the Isolated In Situ Perfused Rat Liver

The pathogenesis of liver disease in protoporphyria has been presumed to result from the hepatic deposition of protoporphyrin. To examine the effects of protoporphyrin on hepatic bile flow and histopathology, studies were performed employing an isolated, in situ, rat liver perfusion system. Rat livers in the control group were perfused with 0-80 μmol sodium taurocholate/h. Rat livers in the experimental group were perfused with sodium taurocholate and (a) sufficient quantities of protoporphyrin to produce maximal canalicular secretion and (b) perfusate protoporphyrin concentrations of 0.01, 0.1, and 1 μM. The administration of protoporphyrin sufficient to achieve maximal canalicular secretion was found to significantly reduce bile flow in rats infused with 0, 40, and 80 μmol sodium taurocholate/h. Linear regression analysis defined the relationship between bile flow and biliary bile acid secretion and showed that the bile acid-independent fraction of bile flow was reduced (P < 0.01). Bile acid-dependent flow was unaffected and there was no significant difference in biliary bile acid secretion rates between control and protoporphyrin-perfused livers. Perfusion of rat livers with varying concentrations of protoporphyrin demonstrated the reduction of bile flow was dose-related. Analysis of perfusate enzyme activity did not reveal abnormalities that could account for the cholestasis. Studies to evaluate the effect of protoporphyrin on regional hepatic hemodynamics were inconclusive. Histopathological studies of control and protoporphyrin-perfused rat livers did not show abnormalities on light microscopy. However, canalicular dilatation, distortion, and loss of microvilli were present in the protoporphyrin-perfused livers examined by transmission electron microscopy. Although ultraviolet microscopy showed diffuse fluorescence of the hepatocytes and canaliculi of protoporphyrin-perfused livers, the deposition of protoporphyrin in amorphous or crystalline forms was notably absent in studies with polarizing and transmission electron microscopy. These studies provide evidence that protoporphyrin has hepatotoxic properties that affect the canalicular secretory apparatus. The mechanism(s) responsible for the injury require further clarification.