Soundararajan Venkatesan

Lipoprotein separation in a novel iodixanol density gradient, for composition, density, and phenotype analysis

Separation of lipoproteins by traditional sequential salt density floatation is a prolonged process (∼72 h) with variable recovery, whereas iodixanol-based, self-generating density gradients provide a rapid (∼4 h) alternative. A novel, three-layered iodixanol gradient was evaluated for its ability to separate lipoprotein fractions in 63 subjects with varying degrees of dyslipidemia. Lipoprotein cholesterol, triglycerides, and apolipoproteins were measured in 21 successive iodixanol density fractions. Iodixanol fractionation was compared with sequential floatation ultracentrifugation. Iodixanol gradient formation showed a coefficient of variation of 0.29% and total lipid recovery from the gradient of 95.4% for cholesterol and 84.7% for triglyceride. Recoveries for VLDL-, LDL-, and HDL-cholesterol, triglycerides, and apolipoproteins were approximately 10% higher with iodixanol compared with sequential floatation. The iodixanol gradient effectively discriminated classic lipoproteins and their subfractions, and there was evidence for improved resolution of lipoproteins with the iodixanol gradient. LDL particles subfractionated by the gradient showed good correlation between density and particle size with small, dense LDL (<25.5 nm) separated in fractions with density >1.028 g/dl. The new iodixanol density gradient enabled rapid separation with improved resolution and recovery of all lipoproteins and their subfractions, providing important information with regard to LDL phenotype from a single centrifugation step with minimal in-vitro modification of lipoproteins.

Effects of growth hormone treatment on very-low-density lipoprotein apolipoprotein B100 turnover in adult hypopituitarism

Adult hypopituitarism is associated with hyperlipidemia, mainly due to an increase of very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) levels. Recent studies have shown that such patients exhibit increased hepatic secretion of VLDL apolipoprotein B100 (VLDL apo B100). To examine the effects of growth hormone (GH) replacement on VLDL apo B100 turnover, 13 GH-deficient hypopituitary patients (8 women and 5 men; aged 47 +/- 3 years, mean +/- SEM; body mass index [BMI], 30 +/- 2 kg/m2) entered a double-blind placebo-controlled study for 6 months (GH 0.125 IU/kg/wk for 4 weeks, and then 0.25 IU/kg/wk). GH was subsequently used in all patients for a further 6 months. A 6-hour [1-13C] leucine infusion was administered at baseline and at 6 months. The secretion rate of VLDL apo B100 was derived by kinetic analysis following quantitation of isotopic enrichment by gas chromatography/mass spectrometry. The GH-treated group (6 patients) demonstrated a similar fractional secretion rate (FSR) for VLDL apo B100 at 0 and 6 months. The pool size and absolute secretion rate (ASR) also were unaffected significantly by GH therapy. No significant changes were observed in the placebo group (7 patients). Treatment with GH for 6 months caused an increase in the high-density lipoprotein (HDL) cholesterol concentration (13 patients, 1.27 +/- 0.13 v 1.16 +/- 0.10 mmol/L, respectively, P = .05), whereas total cholesterol and triglyceride concentrations did not change. Nonesterified fatty acids (NEFAs) increased during GH therapy (471 +/- 43 micromol/L at 6 months v 349 +/- 49 micromol/L at baseline, P < .0005). The data suggest that GH does not affect VLDL apo B100 turnover in a significant way.

Very—low-density lipoprotein apolipoprotein B100 kinetics in adult hypopituitarism☆☆☆

Hypopituitarism is associated with hyperlipidemia, the mechanisms of which are not fully known. One possible mechanism is an increased hepatic secretion of very-low-density lipoprotein (VLDL) apolipoprotein B100 (apo B100). To investigate this, 13 hypopituitary patients (seven women and six men; age, 46 +/- 3 years [mean +/- SEM]; body mass index [BMI], 29 +/- 2 kg/m2) and 13 matched controls (seven women and six men; age, 43 +/- 3 years; BMI, 28 +/- 2 kg/m2) were investigated in a stable-isotope study. [1-(13)C]leucine (1 mg/kg body weight) was administered, followed by a continuous 6-hour infusion of [1-(13)C]leucine (at a rate of 1 mg/kg/h). Patients had a similar fractional secretion rate (FSR) of VLDL apo B100 versus controls (0.37 +/- 0.05 v 0.38 +/- 0.06 pools/h, respectively), but they had a significantly larger pool size (3.4 +/- 0.3 v 1.9 +/- 0.3 mg/kg) and higher absolute secretion rate ([ASR] 27.8 +/- 2.9 v 16.0 +/- 2.5 mg/kg/d). The increase in hepatic VLDL production may explain the lipid abnormalities found in hypopituitarism. Fasting circulating nonesterified fatty acids (NEFAs) were decreased in the patients (284 +/- 26 v 664 +/- 92 micromol/L, P < .001) despite the increase in VLDL secretion. An inverse relationship was observed between the NEFA level and VLDL apo B100 FSR in the patients (r(s) = -.85, P < .005).