Synthesis and secretion of lipoproteins by primary cultures of rat hepatocytes.

Synthesis and secretion of VLDL and LDL by primary cultures of rat hepatocytes maintained in serum free medium have been studied. A time-dependent increase found in the [3H]leucine labelled lipoproteins which floated at a density of 1.006 g/ml indicate the secretion of VLDL into the medium. That the hepatocytes also secrete. LDL is shown by floatation of [3H]leucine labelled lipoproteins by sequential centrifugation at a density range of 1.006-1.06 g/ml. Electrophoretic and immunoprecipitation analysis show that about 60% and 65% respectively of 3H-radioactivity is associated with apoB in the two fraction of lipoproteins. At about 12hr 70-75% lipoproteins in the culture medium is in the VLDL density range and 25-30% is in the LDL density range. Conversion of secreted VLDL to LDL has also been shown by incubating hepatocytes with pre-labelled lipoproteins when there is a decrease in the fraction of VLDL range with a corresponding increase in the fraction of the LDL density range. Addition of glycosaminoglycans such as hyaluronic acid, chondroitin sulphate, and heparin into the medium cause significant increase in the synthesis and secretion of [3H]apoB into the medium indicating a possible secretory control of apoB by local reuptake.

Transfer of phospholipids and cholesterol from triglyceride-rich emulsions to HDL in rats treated with alloxan, propylthiouracil or ethanol

1. The transfer of lipids that constitute the surface of lipoprotein particles, phospholipids and unesterified cholesterol, from chylomicrons and VLDL to other lipoproteins, mainly HDL, was examined. 2. Emulsions known to mimic the metabolism of chylomicrons labeled with [3H]-phosphatidylcholine, [14C]-cholesterol or [3H]-triolein were injected through a cannula implanted into the carotid artery of male Wistar rats weighing 280-350 g. Plasma clearance of the radioactively labeled emulsion lipid constituents and transfer of surface lipids from the emulsions to native HDL and LDL were measured in plasma samples collected at 2-min intervals during 10 min. 3. The transfer was measured in rats with alloxan-induced diabetes (single intraperitoneal dose, 140 mg/kg body weight) or with propylthiouracil-induced hypothyroidism (0.1 per cent v/v in drinking water for 30 days), or given ethanol (20 per cent in drinking water) for a period of 30 days, and in control rats. 4. The entry of emulsion phospholipids into the HDL fraction was not affected by the different treatments (controls: 9.3 +/- 0.6 per cent of injected radioactivity, N = 8; diabetes: 12.5 +/- 2.4 per cent , N = 9; hypothyroidism: 10.9 +/- 0.9 per cent , N = 13; ethanol: 7.8 +/- 0.9 per cent , N = 5). However, phospholipid transfer to the LDL fraction was increased in diabetes (10.0 +/- 2.3 per cent ) and hypothyroidism (12.1 +/- 1.3 per cent ) compared to controls (6.7 +/- 0.9 per cent ). Transfer of unesterified cholesterol from the emulsions to LDL and HDL was increased in both diabetic and hypothyroid rats (controls, LDL: 1.6 +/- 0.6 per cent , HDL: 2.5 +/- 0.6, N = 5; diabetes, LDL: 5.3 +/- 1.2, HDL: 8.4 +/- 2.1 N = 5; hypothyroidism, LDL: 4.0 +/- 0.6, HDL: 3.6 +/- 0.4, N = 8). In ethanol-treated rats, transfer of surface lipids was similar to controls