The Effect of Fasting on the Secretion of Lipoproteins and Two Forms of Apo B by Perfused Rat Liver

Abstract

Abstract

Rat livers from animals fed ad lib or fasted 24 hr were perfused by the single pass technique with ³H-amino acids and label incorporation into lipoprotein apoproteins was measured. The lipoproteins were separated into very low density lipoproteins (d < 1.006), low-density lipoproteins (1.006 < d < 1.06), and high-density lipoproteins (1.06 < d < 1.21) by ultracentrifugation and the apoproteins of each density fraction were analyzed by gel filtration column chromatography in the presence of sodium dodecyl sulfate. The apoprotein fractions included apo B_h (higher-molecular-weight apo B), apo B₁ (lower-molecular-weight apo B), apo E, and apo C. Fasting decreased total hepatic apoprotein secretion from 75 to 53 μg·g⁻¹·hr⁻¹ and label incorporation into the apoproteins of very low density lipoproteins was reduced by 48% with the greatest reduction in both apo B fractions. There was a redistribution of apo B_h label into the low-density lipoproteins with fasting resulting in secretion of lipoproteins enriched in apo B_h. The high-density lipoprotein fraction contained 27 and 41% of the apo B₁ label isolated from liver perfusates of fasted and fed animals, respectively. The nascent high-density lipoprotein was enriched in labeled apo B₁ compared to low-density lipoproteins indicating that apo B in this density range represents a distinct metabolic fraction. With fasting the apo B_h and apo B₁ apoproteins behave independently in terms of the distribution of label incorporation into nascent hepatic lipoproteins.