INCREASED HEPATIC LIPASE ACTIVITY AND INCREASED DIRECT REMOVAL OF VERY-LOW-DENSITY LIPOPROTEIN REMNANTS IN WATANABE HERITABLE HYPERLIPEMIC (WHHL) RABBITS TREATED WITH ETHINYL ESTRADIOL

We studied the effects of ethinyl oestradiol on the serum concentrations and metabolism of very-low- and low-density lipoproteins (VLDL and LDL) in Watanabe heritable hyperlipidaemic (WHHL) homozygous rabbits, an animal model for familial hypercholesterolaemia. The results were compared with those in untreated homozygotes as well as in heterozygotes treated or not with ethinyl oestradiol. The gain in body weight was similar in all groups. Treatment with ethinyl oestradiol resulted in the homozygotes in an approx. 80% decrease in the concentrations of lipids and apoprotein B in the d < 1.019 lipoprotein fraction; those in the LDL fraction did not change. In the heterozygotes, basal serum lipids and apoprotein B levels in the d <1.019 fraction were low; ethinyl oestradiol treatment especially affected the LDL fraction (cholesterol -84%, apoprotein B -64%). Turnover experiments with I-125-labelled VLDL revealed that, on treatment with ethinyl oestradiol, the fractional catabolic rate in homozygous rabbits increased 2-fold. The secretion rates of lipids and protein in the d < 1.019 fraction as estimated after injection of Triton WR-1339 was not decreased. In homozygotes and heterozygotes increases in post-heparin hepatic lipase activity of 62 and 80% respectively were observed, with no changes in lipoprotein lipase activity. We conclude that ethinyl oestradiol induces in homozygous WHHL rabbits a direct removal of VLDL and VLDL remnants from the plasma, apparently due to an increase in hepatic lipase activity.