DISRUPTION OF ACTIN ORGANIZATION BY CYTOCHALASIN-D DOES NOT IMPAIR BILIARY-SECRETION OF ORGANIC-ANIONS IN THE RAT

The bile canaliculi of hepatocytes contract spontaneously, and it is h ypothesized that this canalicular motility provides a propulsive force for normal intrahepatic bile flow. Cytochalasin disrupts actin polyme rization, inhibits contraction, and decreases bile now. We investigate d whether this cholestasis was associated with impaired canalicular se cretion. Isolated rat hepatocyte doublets, with and without incubation with 2 mu mol/L cytochalasin D (cytD), were superfused, under first-o rder conditions, to steady state with fluorescein isothiocyanate-label ed glycocholic acid (FITC-GC) and carboxy-4',5'-dimethylfluorescein di acetate (CMFD), which are fluorescent substrates for the bile acid and the non-bile acid organic anion transport pathways, respectively. Flu orescent microscopic images were quantified and the data analyzed by n oncompartmental and compartmental kinetic methods. cytD dilated the ca nalicular spaces fivefold but did not change the proportion of doublet s that secreted either probe. Cytochalasin did not affect the mean cel lular transit times of FITC-GC (2.8 and 2.5 minutes for control and cy tochalasin-treated groups, respectively) and of carboxy-4',5' -dimethy lfluorescein (3.8 and 3.7 minutes, respectively). Analysis with a thre e-compartment model gave estimates of the rate constants for canalicul ar secretion: 0.21 +/- 0.04 and 0.22 +/- 0.03 min(-1) in control and t reated cells, respectively, for FITC-GC, and 0.14 +/- 0.01 and 0.16 +/ - 0.02 min(-1), respectively, for carboxy dimethylfluorescein. When ki netics are first-order, the canalicular secretion of organic anions is not altered by actin disruptive agents, suggesting that actin filamen ts do not modulate the function or distribution of these transporters. This suggests that impaired contractility rather than impaired canali cular secretion is the mechanism of cytD-induced cholestasis.