Changes in the density of cytochrome P-4502B1/2 molecules in the membrane of endoplasmic reticulum in perivenular, midzonal and periportal hepatocytes of rats after administration of phenobarbital

To examine the relationship between induction of phenobarbital (PB)-inducible cytochrome P-450 (P-450) 2B1/2 and the PB-induced proliferation of endoplasmic reticulum (ER) in hepatocytes, we measured the amount of P-450 2B1/2, amount of total P-450 isoforms, and area of ER in the cytoplasm of perivenular, midzonal and periportal hepatocytes from adult male rats injected with 80 mg/kg of PB once a day for 3 days by quantitative immunohistochemistry, microphotometry and morphometry, respectively. Then, the density of P-450 2B1/2 or total P-450 molecules in the ER membrane (number of molecules per mu m(2) of ER) was calculated by dividing P-450 2B1/2 or total P-450 content by the ER area followed by multiplying Avogadro's number. P-450 2B1/2 content in hepatocyte cytoplasm increased markedly in perivenular (15 times), midzonal (20 times) and periportal hepatocytes (10 times) after PB injection. The area of ER increased all hepatocytes (1.7-1.9 times) after PB injection. Thus the density of P-450 2B1/2 in the ER membrane increased markedly in hepatocytes in the three zones, whereas that of total P-450 increased slightly. The proportion of P-450 2B1/2 density to total P-450 density in the ER membrane increased markedly in perivenular hepatocytes and moderately in midzonal and periportal hepatocytes after PB injection. Thus P-450 2B1/2 molecules in the ER membrane increased in perivenular hepatocytes with reduction in molecules of other P-450 isoforms after PB injection (substitutional increase), whereas the value increased in midzonal and periportal hepatocytes without reduction in other P-450 isoforms (additive increase).