Effect of agents which rearrange the cytoskeleton in vitro on the structure and function of hepatocytic canaliculi.

The integrity of the cytoskeletal structure of the bile canaliculus (BC) may be necessary for bile secretion. This includes actin filaments in microvilli, cytokeratins in the pericanalicular sheath and microtubules in the surrounding cytoplasm. We studied these cytoskeletal structures and also the secretory function of the hepatocytes in tissue culture using double-label fluorescent staining and the transhepatic transport and secretion of fluorescein diacetate and horseradish peroxidase. The hepatocytes were obtained from 14-day-old male rats. They were cultured in serum-free Williams's E medium, with insulin and dexamethasone added to induce differentiation. Four treatment groups of hepatocytes were studied: (a) colchicine (10(-4)M for 1 hour), (b) cytochalasin B (4 micrograms/ml for 1 hour), (c) ethanol (30 to 90 mM for 24 hours) and (d) controls. Colchicine caused the disappearance of the microtubules and completely inhibited the secretion of fluorescein diacetate and horseradish peroxidase into the BC. It did not affect the uptake or transport of fluorescein diacetate on horseradish peroxidase to the BC. Cytochalasin B disrupted the actin filaments and caused their aggregation around the BC. The canaliculi were dilated and the microvilli were decreased but the secretion was normal. Ethanol did not affect either the structure of the cytoskeleton or inhibit secretion. It is concluded that the secretory function of the BC requires the integrity of microtubules in cultured hepatocytes. The integrity of actin is not necessary for uptake, transport, or secretion as long as the pericanalicular sheath remains intact. Ethanol had no effect on the structure or the function of the cytoskeleton of the bile secretory apparatus.