Phalloidin, administered to male rats for 7 days (500 ,sg per kg/day), increased the mean hepatic content of filamentous actin. Both bile flow and bile acid excretion diminished proportionally, whereas the bile-to-plasma ratios of Phalloidin, a cyclic peptide derived from the poisonous mushroom Amanita phalloides, specifically binds to purified muscle F-actin and prevents its depolymerization (1-3). Phalloidin increases the quantity of F-actin when added to isolated hepatocytes (4) or to isolated liver plasma membranes (2, 5, 6) and produces a marked increase in microfilaments in the pericanalicular region of hepatocytes when the alkaloid is administered chronically in low doses to rats (7). In the intact rat, chronic phalloidin administration, like other microfilament poisons, is associated with a diminution in bile production (7-9) and an extensive rearrangement of hepatocyte junctional elements observed by freeze-fracture electron microscopy (10).These changes in junctional structure suggest that microfilaments help regulate junctional development and support the notion that the junctional barrier may in turn influence the formation of bile. Because accumulating evidence suggests that the paracellular pathway is an important site for water, electrolyte, and solute entry into bile (11-14), the phalloidin-treated rat provides a useful experimental model to examine more closely the relationship among microfilament function, junctional structure, and bile secretion. In this study, we measure the effects of phalloidin on bile formation and biliary permeability to inulin and sucrose, two large molecular weight solutes that rapidly reach equilibrium between plasma and bile before liver water, suggesting that their entry into bile is via a paracellular route. Junctional structure and permeability were also examined by freezefracture studies and by the infusion of ionic lanthanum chloride at a time when increases in hepatic content of polymerized actin were also observed.
METHODSMale Sprague-Dawley rats (Charles River), initially weighing 175-225 g, were treated for 7 days with either phalloidin at 500 ug kg-' day-' in saline (16 animals) or saline alone as a control (16 animals) by intraperitoneal injection as described by Gabbiani et al. (7). Phalloidin was obtained from Boehringer Mannheim. On the eighth day, the rats were anesthetized by intraperitoneal injection of sodium phenobarbital at 50 mg (kg of body weight)-I, and the bile duct was cannulated with polyethylene tubing (PE10, Clay Adams). Body temperature was monitored by rectal probe and maintained at 370C by a heating lamp regulated by a constant temperature regulator (Yellow Springs Instrument). After 60-80 min, the abdomen was reopened and heparin (10,000 units kg-') was injected into the inferior vena cava before fixation of the liver by portal vein perfusion.Bile Flow and Bile Acid Excretion. Bile was collected for 80 min at 20-min intervals and expressed as !ul mind (g of body weight)-l and as gl min-1 (g of liver)-l. Bile acids were analyzed ...
