Induction of cytochrome P-450 in a selective subpopulation of hepatocytes.

The objective of this study was to determine whether the inductive effect of phenobarbital (PB) on liver cytochrome P-450 was the result of the action of this drug on all or some hepatocytes. For this purpose, a light (cell band I) and a heavy (cell band II) subpopulation of hepatocytes were separated from rat liver in a continuous density gradient. To determine the location of these hepatocytes in tissue, [14C]bromobenzene, which binds covalently to centrilobular hepatocytes, was administered. The specific activity (14C dpm/mg protein) was greater in cells of band I than in cells of band II, suggesting a predominant contribution of centrilobular hepatocytes to the lighter cell band. Microsomes were separated from each cell subpopulation after 3 days of PB administration and cytochrome P-450 was measured. Although a fivefold increment in cytochrome P-450 content of light hepatocytes was noted, the content of heavy hepatocytes was similar to that of the respective subpopulation in controls. Concomitantly, PB administered for 3 days induced the smooth endoplasmic reticulum of centrilobular hepatocytes only, as revealed by electron microscopy of whole tissue. These results indicated that PB induces cytochrome P-450 in a selective subpopulation of hepatocytes, most likely located near the terminal hepatic venule.

Bile secretion and liver cell heterogeneity in the rat.

The contribution of hepatocytes of different acinar zones to bile salt transport and to the secretion of the BSNDF was studied in the rat. Changes in the removal of 14C-taurocholate from blood, in the biliary secretion of taurocholate, and in canalicular flow were determined after damage of the periportal (acinar zone 1) or centrilobular (acinar zone 3) areas by allyl alcohol or bromobenzene, respectively. The extent of cell necrosis was quantitated by light microscopy, and the quality of the intracellular damage was assessed by electron microscopy. After either periportal or centrilobular damage, surviving cells responded to an intravenous infusion of taurocholate by secreting bile salts into bile at a rate similar to controls. However, following the administration of 14C-taurocholate at high concentrations and as a single bolus, the rate of removal of this isotope from blood was slower than in controls. Both experiments suggested that periportal and centrilobular hepatocytes had the capability for bile salt transport. Consequently, since the concentration of bile salts in sinusoidal blood at each zone determines the relative contribution of hepatocytes to bile salt transport, periportal cells probably transport the largest amount of bile salts reaching the acinus. Canalicular bile flow, on the other hand, decreased following centrilobular cell damage, and this was associated with a high concentration of bile salts in bile. This suggested that at bile salt loads near physiological concentrations, the predominant contribution of centrilobular hepatocytes is to the secretion of the BSNDF.