Plasma Triglyceride Concentrations

Reduced plasma levels of high density lipoprotein (HDL) cholesterol are associated with increased risk for coronary heart disease. Although plasma HDL levels are, in general, inversely related to plasma triglyceride (TG) concentrations, a small proportion of individuals with low HDL cholesterol concentrations have normal plasma TG levels. We wished to determine whether subjects with low plasma levels of HDL cholesterol could be characterized by common abnormalities of lipoprotein metabolism independent of plasma TGs. Therefore, we studied the metabolism of low density lipoprotein (LDL) apolipoprotein B (apo B) and HDL apolipoprotein A-I (apo A-I) in subjects with low plasma HDL cholesterol concentrations with or without hypertrigh/ceridemia. Nine subjects with low plasma HDL cholesterol levels and normal levels of plasma TGs and LDL cholesterol were studied. Autologous I-LDL and I-HDL were injected intravenously, and blood samples were collected for 2 weeks. LDL apo B and HDL apo A-I levels were measured by specific radioimmunoassays. Fractional catabolic rates (FCRs, pools per day) and production rates (PRs, milligrams/kilogram day) for each apolipoprotein were determined. The results were compared with those obtained previously in nine subjects with low plasma HDL cholesterol levels and hypertrigh/ceridemia and in seven normal subjects. The normal subjects had an HDL apo A-I FCR (mean±SD) of 0.21 ±0.04. Despite large differences in plasma TG levels, the HDL apo A-I FCRs were similar in the low-HDL, normal-TG group (0 JO±0.09) and the low-HDL, high-TG group (0J3±0.10), although only the latter value was significantly increased versus control subjects (p< 0.03). Increased apo A-I FCRs were associated with reduced HDL apo A-I levels in both groups of patients. Apo A-I PRs were similar in all groups. In contrast, LDL apo B PR was increased approximately 50% in the low-HDL, normal-TG group (193±6.6; p<0.01) compared with normal subjects (12.5±2.6). There was a strong trend toward a greater LDL apo B PR in the low-HDL, high-TG group (17.6±4.5;p=0.06 versus normal subjects) as well. LDL apo B FCRs were similar in all three groups. LDL apo B concentrations were also increased in the group with low HDL cholesterol and normal TG levels. Both groups with low HDL cholesterol levels had cholesterol-depleted LDL and HDL particles. In summary, reduced levels of plasma HDL cholesterol were generally associated with accelerated fractional removal of HDL apo A-I from plasma, increased production of plasma LDL apo B, and evidence of increased cholesteryl ester transfer out of LDL and HDL. The presence of these similar metabolic abnormalities whether or not plasma TG levels were increased suggests that increased apo B production may be a central defect in these patients and that low plasma HDL levels may be closely linked to increased plasma levels of apo B-containing lipoproteins independent of circulating levels of plasma TG. (Arteriosclerosis and Thrombosis 1993;13:842-851)