Changes in the concentrations and distributions of apolipoproteins of the aging rat.

The hyperlipidemia associated with aging was characterized in the rat by comparing the plasma lipid, lipoprotein, and apolipoprotein profiles of adult (12 weeks old) and old (96 weeks old) male rats. Compared with those of the adult rats, the VLDL concentrations of the old rats were reduced, but IDL, LDL, and HDL concentrations were elevated. Despite a reduced VLDL concentration, concentrations of triglycerides in the plasma of the old rats were elevated. This phenomenon was attributed to an enrichment of triglyceride in the other lipoprotein fractions. In the old rats, hypercholesterolemia was the result of elevated IDL- and HDL-cholesterol whereas elevated plasma concentrations of apolipoproteins B and E were attributed to elevated LDL and HDL concentrations, respectively. Although concentrations of apolipoproteins A-I and A-IV did not change significantly in the plasma of the old rats, the distribution pattern of the apoA-IV was altered dramatically. Compared with the adult rats, a shift of apoA-IV in the HDL to the "lipoprotein-free" fraction was observed in the old rats, as measured by agarose gel chromatography. The data demonstrate that the hyperlipidemia in the old rats is associated with selective changes in the apolipoprotein profile.