Lipids, lipoproteins and hepatic 3-hydroxy-3-methylglutaryl CoA reductase activities of rabbits during adaptation to atherogenic semi-purified diets.

Young adult, male, New Zealand white rabbits were pair-fed low fat, semi-purified diets containing either casein or soy-protein isolate and sucrose or dextrose for 174 days. While casein feeding resulted in hypercholesterolemia, adaptation occurred beginning between 90-120 days of feeding as evidenced by a reduction in mean serum cholesterol concentrations. At the end of the dietary period no atherosclerotic lesions were seen in aortas of rabbits in any of the dietary groups. Serum free and esterified cholesterol concentrations averaged two-fold greater with casein-feeding than those of rabbits fed the soy-protein paired diets. Differences due to carbohydrate type were minimal. Elevations of both free and esterified cholesterol in the low density lipoprotein fraction (1.011 less than d less than 1.031 g/ml, LDL1) and of free cholesterol in the high density fraction (1.079 less than d less than 1.19 g/ml, HDL) were observed. The major cholesterol carrying lipoprotein after adaptation was LDL1 although HDL free cholesterol was increased in the casein groups. Hepatic free cholesterol content was only moderately increased while cholesteryl ester concentrations were elevated three-fold. Hepatic 3-hydroxy-3-methylglutaryl CoA reductase activities were similar among all rabbits at this time. Gall bladder wet weights, including contents, were significantly different in the casein fed animals. Reasons for these differences may include alterations in hepatic acyl-CoA cholesterol acyl transferase (ACAT) activities, lipoprotein production and turnover and/or the rate or extent of gallbladder emptying.