Dose response relations, biochemical mechanisms, and sex differences in the experimental fatty liver induced by tetracycline were studied in the intact rat and with the isolated perfused rat liver in vitro. In the intact male and female rat, no direct relationship was observed between dose of tetracycline and hepatic accumulation of triglyceride. With provision of adequate oleic acid as a substrate for the isolated perfused liver, a direct relation was observed between dose of tetracycline and both accumulation of triglyceride in the liver and depression of output of triglyceride by livers from male and female rats. Marked differences were observed between female and male rats with regard to base line (control) hepatic concentration of triglyceride and output of triglyceride. Accumulation of hepatic triglyceride, as a percent of control values, in response to graded doses of tetracycline, did not differ significantly between male, female, and pregnant rat livers. However, livers from female, and especially pregnant female rats, were strikingly resistant to the effects of tetracycline on depression of output of triglyceride under these experimental conditions. These differences between the sexes could not be related to altered disposition of tetracycline or altered uptake of oleic acid. Depressed hepatic secretion of triglyceride accounted only for 30 to 50% of accumulated hepatic triglyceride, indicating that additional mechanisms must be involved in the production of the triglyceride rich fatty liver in response to tetracycline.
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