Aneurysmal enlargement of the aorta during regression of experimental atherosclerosis.

We explored the relationship between regression of diet-induced atherosclerosis and aneurysmal enlargement of the aorta in cynomolgus monkeys. Atherosclerotic plaques were induced in 17 monkeys by feeding them a diet containing 2% cholesterol and 25% peanut oil for 6 months (group I, n = 6; group III, n = 6) or 12 months (group II, n = 5). Regression was induced in group III by feeding a regression diet consisting of 0.25% cholesterol and 15% corn oil in a standard chow diet, for 6 months after the 6-month induction period. Serum cholesterol was 788 +/- 80 mg/dl after 6 months of induction, 508 +/- 53 mg/dl after the 12-month induction period, and 198 +/- 15 mg/dl in the regression group at 12 months. Aortas were fixed in situ under conditions of controlled pressure perfusion, and transverse sections of the unopened vessels were taken at standard levels in the midthoracic and abdominal aortic segments. The area encompassed by the internal elastic lamina was taken as a measure of artery size. Plaques were abundant in abdominal and thoracic sections after the 6- and 12-month induction periods, and no significant difference was observed in lumen area or artery size between the groups. The ratio of abdominal to thoracic aortic plaque area was markedly reduced in the regression group (0.3 +/- 0.2 for regression compared with 0.6 +/- 0.3 for 6-month induction and 1.3 +/- 0.2 for 12-month induction animals; p less than 0.05 for both). A twofold increase was observed in abdominal aortic lumen area in the regression group (10.0 +/- 1.5 mm2 for regression compared with 5.6 +/- 0.7 mm2 for the 6-month and 4.2 +/- 0.7 mm2 for the 12-month induction groups; p less than 0.05 for both) as well as a twofold increase in internal elastic lamina area (10.5 +/- 1.5 mm2 compared with 6.0 +/- 0.7 mm2 for the 6-month and 5.9 +/- 0.8 mm2 for the 12-month induction group; p less than 0.05 for both). Aortic enlargement in the regression group was accompanied by a reduction in media thickness in the abdominal aorta. No significant vessel enlargement or alteration in media thickness occurred in the thoracic aorta. One of six regression animals (17%) had a threefold enlargement of the abdominal aorta and was thought to have a manifest aneurysm.(ABSTRACT TRUNCATED AT 400 WORDS)