Experiments were undertaken to evaluate the role of intravital epifluorescent TV microscopy in the study of the hepatic transport of sodium fluorescein and fluorescently labeled sodium glycocholate in rats and hamsters.
It was found that the apparent sinusoid to canaliculus transport time for fluorescently labeled sodium glycocholate was only half of that for sodium fluorescein, 0.50 and 0.92 sec, respectively. The sinusoid to bile transport time was 35 sec: for fluorescently labeled sodium glycocholate and 90 sec for sodium fluorescein. There were also marked differences in zonal clearance of these two substances. Sodium fluorescein was removed from Zone 1 much faster than from Zone 3, while fluorescently labeled sodium glycocholate was cleared at the same rate by hepatocytes in all three acinar zones.
This study provides direct evidence that there are zonal differences in hepatic transport rates for certain compounds. It also demonstrates that epifluorescent TV microscopy has the potential to provide quantitative data on the transport rates of bile acids and other molecules into, through and out of the liver.
The liver acinus, the functional microcirculatory unit of the liver, is conventionally divided into three zones (1, 2). Zone 1 is localized primarily around the terminal portal tracts and Zone 3 around the terminal hepatic venules. Zone 2 is an intermediate zone between Zones 1 and 3. It has been suggested that this zonal organization around the hepatic microvasculature has functional consequences. Cells situated close to the terminal portal venules are exposed to incoming blood rich in oxygen and nutrients, while cells around the terminal hepatic venules receive blood with relatively lower concentrations of those substances normally taken up by the hepatocytes and sinusoidal lining cells. Consequently, the functional roles of the different microcirculatory zones may be different, i.e., the cells in different zones may function differently in the metabolism and transport