The movement from blood to bile of passively injected autologous IgM antibody against horse erythrocytes was studied in rats. Both native and neuraminidase-treated antibody entered bile intact, with the peak titers for both measured between 60 and 90 min after injection. A small part (0.38%) of the injected dose of native antibody and 1.05% of the asialo-IgM antibody appeared in bile over 24 hr. These recoveries represented only a small fraction of the activity, which apparently disappeared from serum over the period. The pathways used by IgM antibody to enter bile were partially assessed. Prior injection of fetuin was found to abrogate the biliary secretion of native antibody and to significantly reduce the recovery of asialoantibody in bile.
In contrast, the presence of asialofetuin reduced (but not significantly) the secretion of native antibody and markedly changed the kinetics of appearance of asialo-IgM activity in bile. Some candidate routes to bile can be excluded as unlikely. The parahepatocellular pathway to bile canaliculi appears uninvolved because the secretion of antibody is slow and the inhibitory effects of fetuin argue against secretory component-mediated transport. Instead, two secretory pathways appear to be present. The first appears to be available to both native and asialo-IgM antibody, and may involve the peribiliary capillary plexus. The second route to bile is available to asialo-IgM antibody alone and is probably initiated by binding to the hepatic asialoglycoprotein receptor.