The changes occurring in hepatocytes of F-344 male rats during a 3-wk treatment with a hypolipidemic agent, 1-methyl-4-piperidyl-bis [p-chlorophenoxy]acetate (SaH 42-348), have been evaluated by morphometric and biochemical methods. The twofold increase in liver weight resulted from a significant increase in hepatocyte cytoplasm as well as a moderate increase in the number of liver cells. The peroxisome population and SER played an overwhelming part in the hypertrophy of hepatocytic cytoplasm. The relative volume and the surface density of peroxisomes volume resulted from an increased ninefold and sevenfold, respectively. The increase in the collective peroxisome volume resulted from an increase in both the number and the average volume of peroxisomes. The SER also demonstrated a substantial increase in these values. The relative volume and surface density of mitochondria were not significantly altered in comparison to controls, while these values for RER decreased onefold. Studies on the lobular distribution of cytoplasmic organelles before and during treatment revealed that the relative volume and surface density of peroxisomes and SER increased from periportal to centrilobular cells of the hepatic lobule, whereas mitochondrial values decreased from periportal to centrilobular cells. The RER values were fairly constant in different parts of the hepatic lobule. The increase in peroxisome and SER volume and surface area was first evident within the first 3 days of SaH 42-348 treatment and these values continued to increase, reaching a steady state within 2 wk. The time course of increase in catalase and carnitine acetyltransferase activities correlated with the morphometric data on the peroxisomes. After cessation of SaH 42-348 treatment, the peroxisome values decreased rapidly within the first 3 days and reached control levels within 1 wk. Moderate reduction in SER values occurred after withdrawal of the drug, but these values remained higher than controls even after 2 wk, suggesting that the reduction in the amount of circulating peroxisome proteins may result in empty SER channels. On the 4th day of drug withdrawal a significant increase in the relative volume and surface density of lysosomes was observed, suggesting that these organelles may play some part in the removal of cellular membranes. However, the rapid reduction in peroxisome values after SaH 42-348 withdrawal appears to be due to cessation of enhanced peroxisome protein synthesis.

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