Biomedical subjects
Publications and source records attributed to R Infante.
High acetone levels occur in uncontrolled diabetes and after isopropanol administration to rats. In both conditions, very low density lipoproteins (VLDL) secretion is depressed. Acetone, hoever, failed to affect the VLDL secretion rate by the isolated perfused rat liver, suggesting that this metabolite is not involved in impaired VLDL production in diabetes and after isopropanol administration.
Hepatic synthesis of apo-B and apo-C and their binding to nascent very low density lipoproteins (VLDL) have been studied in fat-fed rats. Apolipoproteins were located in hepatocyte organelles by light and electron microscopy after immunoenzymatic staining using peroxidase-conjugated antibodies. Our results indicate that apo-B and apo-C are synthesized by membrane-bound ribosomes. Both apoproteins seem to be adsorbed simultaneously to the lipid core of VLDL in the lumen of the endoplasmic reticulum channels, at the junction zone between rough and smooth endoplasmic reticulum. Some additional protein presumably binds nascent VLDL in the Golgi apparatus as judged by the strong positive reaction of lipoprotein particles with peroxidase-labeled antibodies. Finally our data show that significant amounts of apo-B and apo-C are bound to the sinusoidal plasma membrane in fed rat livers which probably represent remnants of lipoprotein of intestinal origin since membrane-bound apolipoproteins virtually disappeared 24 h after lymphatic duct cannulation. It is suggested that nascent VLDL (apo-C poor) could be enriched in apo-C from lipoprotein remnants at the space of Disse.
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Production of very low density lipoproteins by the liver depends on the cellular availability of fatty acids. It is stimulated by the uptake of free fatty acids from the plasma and by increased lipogenesis and is inhibited by actinomycin D, suggesting that RNA synthesis is involved in the regulation of apolipoprotein synthesis. This hypothesis has been investigated in rats in vivo and in isolated perfused livers with and without stimulation by fatty acid overload: [14C] orotate incorporation in liver polyribosomal RNA is 60 per cent greater in stimulated livers as compared to controls. This increase is primarily due to a higher incorporation in bound polysomes and in those containing at least six ribosomes and does not result from the inhibition of ribonuclease. RNase digestion of polysomal RNA (4.10(-10) M enzyme, 0 degrees C, 3 h) shows that there is twice as much radioactivity in the hydrolyzed RNA of stimulated livers as compared to controls. After partial purification of poly A-rich RNA by affinity chromatography, the mass yield and radioactivity are increased by 100 per cent in stimulated livers as compared to controls. In conclusion, de novo RNA synthesis seems to be necessary for fatty acid stimulation of VLDL production.
Stimulation of VLDL production by increasing fatty acid availability is now well established. However, a possible regulatory role of glycerol, another lipid precursor, in VLDL synthesis by the liver has not yet been substaniated. The present experiments investigate this problem using the isolated perfused rat liver. [14C] Glycerol uptake and metabolism were studied at two different glycerol concentrations: 1 mumol/perfusate (control) or 1.6 mmol/perfusate. VLDL production and lipid synthesis were investigated using [14C]leucine and several labelled fatty acids as precursors in control and glycerol-overloaded livers. Neoglycogenesis and lipogenesis from glycerol carbons are negligible in our conditions. The absolute amount of glycerol, but not the precentage, taken up by the liver, increased after raising its concentration in the perfusate. A major part of exogenous (plasmatic) glycerol was esterified with endogenous (non plasmatic) fatty acids. Incorporation of radioactive fatty acids into liver or plasma lipids was lower than in the the control group. Significant differences were observed between saturated and unsaturated fatty acids used as lipid precursors. Production of VLDL as assessed by radioactive leucine and fatty acid incorporation in the VLDL of the perfusate was depressed by glycerol. Glycerol partly inhibits the normal stimulation of VLDL production by plasmatic fatty acid overload.
The lipid composition and apopeptide patterns of plasma LDL and VLDL have been studied in a patient with LCAT deficiency, using two-dimensional immunoelectrophoresis. After absorption of a fat meal an elevation of VLDL apopeptides C was observed. Apo-LDL also showed an abnormal relative increase of apo-C. After heparin-induced lipolysis, a shift of apo-C from chylomicra and VLDL to LDL was noticed by the striking increase of apo-C in this last lipoprotein class. In view of the present and previously published results the possible mechanisms involved in lipoprotein catabolism in LCAT deficiency are discussed.
Ethacrynic acid (EA) was injected to rats with functional nephrectomy after a control period of steady-state bile flow sustained by taurocholate infusion. Biliary clearance of [14C]mannitol was measured in order to estimate canalicular bile flow and bile salt-independent fraction (BSIF). After EA infusion, bile flow increased by 56%; bile salt excretion rate decreased by 10%; electrolyte excretion rates all increased, principally Na+ and K+. Mannitol clearance increased in parallel with bile flow. The BSIF increased. EA was excreted into bile as a metabolite identified as the cysteine adduct of EA; its excretion rate was linearly correlated with the increment in bile flow. The results are consistent with the hypothesis that the biliary excretion of an EA derivative results in an osmotic water flow increasing the canalicular BSIF. Since EA ia a Na+-K+-ATPase inhibitor, it is necessary to reconsider the relationship between secretion of canalicular BSIF and active Na+ transport mediated by the Na+-K+-ATPase system.