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)-inducib le cytochrome P-450 (P-450) 2B1/2 and the PB-induced proliferation of endop lasmic 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 perive nular, midzonal and periportal hepatocytes from adult male rats injected wi th 80 mg/kg of PB once a day for 3 days by quantitative immunohistochemistr y, microphotometry and morphometry, respectively. Then, the density of P-45 0 2B1/2 or total P-450 molecules in the ER membrane (number of molecules pe r mu m(2) of ER) was calculated by dividing P-450 2B1/2 or total P-450 cont ent by the ER area followed by multiplying Avogadro's number. P-450 2B1/2 c ontent in hepatocyte cytoplasm increased markedly in perivenular (15 times) , midzonal (20 times) and periportal hepatocytes (10 times) after PB inject ion. The area of ER increased all hepatocytes (1.7-1.9 times) after PB inje ction. Thus the density of P-450 2B1/2 in the ER membrane increased markedl y 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 t he 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 re duction in molecules of other P-450 isoforms after PB injection (substituti onal increase), whereas the value increased in midzonal and periportal hepa tocytes without reduction in other P-450 isoforms (additive increase).