Lipoprotein secretion in lean and obese Zucker female rats in vivo and in a single-pass-perfused liver preparation

Abstract

The plasma lipoprotein composition as well as lipoprotein synthesis and secretion were studied in vivo and in a single-pass-perfused liver preparation in lean and obese Zucker rats. Compared with their lean littermates the levels in the plasma of very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL) + low density lipoprotein (LDL) and high density lipoprotein (HDL) were increased 4-, 2-and 2.5 fold, respectively, in obese rats. In these rats both VLDL and IDL + LDL were enriched in triglycerides, while the HDL were enriched in cholesterol. Although the VLDL and IDL + LDL protein concentrations were the same in lean and obese rats, the HDL protein concentration was 3-fold greater in the obese rats. Both the lean and obese rats incorporated similar amounts of [^l4C]leucine into total liver protein. However, obese rats incorporated 2.5-fold and 6-fold more [^l4C]leucine into VLDL and HDL in vivo, 2.7-fold and 1.7 fold more [³⁵S]methionine in VLDL and HDL present in the perfusate, than did lean rats. The perfusate [³⁵S]-labelled apoproteins (apo-B₁₀₀, B₄₈; apo-E, apo-AI, apo-AIV and apo-C) were separated by gel electrophoresis and identified by autoradiography. Incorporation of [³H]glycerol into liver, VLDL, IDL + LDL and HDL triglycerides was 2-, 48-, 13- and 1.5-fold higher in obese than in lean rats, respectively. The [³H]-labelled triglycerides in VLDL and IDL + LDL present in the perfusate was 5.4-fold and 4.4-fold more in obese rat. There was no difference in the incorporation of [³H]glycerol into triglycerides of perfusate- HDL between the two genotypes of rats. Thus, the hypertriglyceridaemia observed in obese Zucker rats results from very high synthetic rates of both the lipid and protein moieties of plasma lipoproteins. Before this study, no report of the simultaneous triglycerides and protein synthesis in vivo and in a single-pass-perfused liver preparations had been reported.