Daily phenobarbital (PB) injections, on 3-7 consecutive days, induce an intense proliferation of smooth endoplasmic reticulum (ER) associated with a decrease of the glucose-6-phosphatase activity. This situation first affects the centrolobular hepatocytes, enhancing the degree of liver lobule heterogeneity. This experimental model was used for isolation and further subfractionation of hepatocytes on Ficoll density gradients, as described in the preceding paper. Profiles of protein, DNA, RNA, glycogen, phosphorylase, and glucose-6-phosphatase were determined all along the gradient. Two liver cell populations were distinguished: (a) light hepatocytes (mean density 1.10) present the same morphological characteristics as centrolobular cells, i.e., an abundant smooth ER composed of tubular elements, numerous small mitochondria, and few glycogen particles; (b) heavy hepatocytes (mean density 1.14) are characterized by large and compact glycogen areas and prominent rough endoplasmic cisternae, as are the perilobular cells. After incubation in the Wachstein-Meisel medium, Centrolobular hepatocytes exhibit dispersed reaction sites of glucose-6-phosphatase activity, whereas perilobular cells present a continuous and intense reaction. Morphometric determinations were carried out for both cell populations. Centrolobular PB hepatocytes are considerably enlarged (mean diameter: 23.7 mum); perilobular hepatocytes have a significantly smaller mean diameter of 19.2 mum, which is close to the values of control liver cells.

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