Expansion of aortic aneurysms is reduced by propranolol in a hypertensive rat model

Purpose: It has been suggested that propranolol has unique effects that slow aneurysm expansion by remodeling the structural proteins of the aorta. These effects are believed to be independent of blood pressure reduction, a hypothesis we tested in this investigation with a rat model of abdominal aortic aneurysm (AAA). Methods: With an established model, AAA were induced in normotensive Wistar-Kyoto (WKY) rats and genetically hypertensive Wistar-Kyoto (WKHT) rats by perfusing an isolated segment of the infrarenal aorta with elastase. A propranolol dose-response was studied for each strain: (1) saline solution controls (n = 18); (2) propranolol, 10 mg/kg subcutaneously (n = 18); (3) propranolol, 30 mg/kg (n = 14). Systolic blood pressure was determined by tail plethysmography before operation and on day 14, as well as by direct recording at surgery and on day 14. Rats were killed at 14 days, and aneurysm diameter was measured. Results: The initial tail BP was 129 ± 22 mm Hg in WKY animals and 158 ± 21 mm Hg in WKHT animals (p < 0.0001). Tail BP and intraaortic systolic, diastolic, and mean blood pressure (BP) were not significantly decreased by propranolol treatment in either strain of rats. However, BP tended to rise in WKY rats, whereas it fell slightly in WKHT rats. Initial aortic size in all animals was 1.06 ± 0.12. The final aortic size in untreated, hypertensive rats was more than twice that of untreated normotensive controls: 1:WKHT, 3.0 ± 0.73 mm, 1:WKY, 6.9 ± 3.5 mm (p < 0.01). After treatment with both doses of propranolol, hypertensive aneurysms were significantly smaller than the untreated WKHT group (p < 0.05) and not significantly different from aneurysms in all groups of normotensive animals: 2:WKY, 3.1 ± 1.13 mm, 2:WKHT, 4.0 ± 1.81 mm; 3:WKY, 4.1 ± 0.41 mm, 3:WKHT, 2.9 ± 1.24 mm. There was no significant difference in aortic size between the three normotensive WKY groups. Conclusions: Hypertension increases the size of aortic aneurysms in this experimental model. Propranolol significantly reduces the size of experimental AAA in hypertensive animals independently of the dose and by a mechanism that may be unrelated to simple BP reduction.