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Early changes in cholesteryl ester metabolism of the aorta during the regression of naturally occurring atherosclerotic lesions in pigeon aorta by ileal bypass surgery were examined. Three months after surgery, there was a decrease (50%) in the content of cholesteryl esters in the aorta. Increases in the activity of cholesteryl ester hypdrolase in the lysosomal (P less than 0.05) and the supernatant (P less than 0.01) fractions of the aorta also occurred at this time. There were no differences in the activity of cholesteryl ester synthetase and in the plasma levels of cholesterol and triglycerides between the ileal bypass group and the controls. These results suggest that ileal bypass surgery decreases the level of cholesteryl esters in the aorta, probably because of enhanced cholesteryl ester hydrolysis.

The in vitro effects of prostaglandins E1 and F1alpha on the activity of cholesteryl ester synthetase and cholesteryl ester hydrolase activities of the pigeon aorta were examined. It was found that prostaglandin E1 markedly inhibited the cholesteryl ester hydrolase activity in the supernatant fraction and slightly inhibited the cholesteryl ester synthetase activity. Prostaglandin F1alpha, however, modestly stimulated the cholesteryl ester hydrolase activity both in the microsomal and in the supernatant fraction of the aorta. These observations strongly warrant further studies on the role of prostaglandins in atherogenesis.

The effect of short-term (6 months) administration of conjugated equine estrogen (Premarin) on content and composition of the aortic sterols in male shite Carneau pigeons while they were on a cholesterol-free grain diet was investigated. Estrogen treatment resulted in a 38% increase (P less than 0.05) in free sterol concentration, with a 28.8% concomitant decrease (P less than 0.05) in the percent of cholesteryl esters. The total sterol concentration remained unchanged. This finding suggests that estrogens might influence the synthetic or hydrolytic (or both) processes that control the concentration of cholesteryl esters in the aorta. Fatty acid composition of steryl esters did not change significantly. The cholesterol content of plasma showed a mild reduction (14%) whereas the triglycerides increased significantly (30%).

Cholesteryl ester hydrolase activity was measured in the microsomal and supernatant fractions of the aorta of atherosclerosis-susceptible White Carneau and atherosclerosis-resistant Show Racer pigeons while on their normal cholesterol-free diets. Enzyme activities from both fractions showed fatty acid specificities for the hydrolysis of different cholesteryl esters in the following decreasing order: Linoleate greater than oleate greater than palmitate. At 9 months of age (the period of lipid accumulation) the microsomal enzyme activity in the Show Racer breed was significantly higher (P less than 0.001) than in the White Carneau breed, while the supernatant enzyme was slightly higher (P less than 0.05) in the White Carneaux at this age. In older birds (3 years of age) these differences in enzyme activities disappeared.