Effect of vasodilatators on the structure of the aorta in normotensive ageing rats

The effects of hypertension and its treatment on the function and structure of arteries have been extensively studied, but no data are yet available on the effect of hypotensive drugs on the large arteries in normotensive rats. Two groups of 11-month-old normotensive breeder male rats were treated for 5 months, one with an angiotensin converting enzyme (ACE) inhibitor (MK-421, 2 mg/kg per day; n = 10) and the other with dihydralazine (15 mg/kg per day; n = 8). A group of 14 rats served as controls. Blood pressure was recorded every 14 days by the tail-cuff method. At the end of the treatment period (5 months), the rats were killed under anaesthesia and the descending thoracic aorta was removed and fixed. The different components of the aorta were assessed by automated morphometric image analysis after specific coloration of elastin (orceine), collagen (sirius red) and nuclei (haematoxylin after periodic acid oxidation. Both the ACE inhibitor and dihydralazine caused similar decreases in systolic blood pressure (SBP) compared with controls. This hypotensive effect was associated with a reduction in medial thickness, from 120 +/- 15 microns in controls to 104 +/- 9 microns in ACE inhibitor-treated and 103 +/- 10 microns in dihydralazine-treated normotensive rats. Elastin density significantly increased in the two treatment groups but the greatest increase was in the dihydralazine-treated group (P less than 0.001). Elastin fibre thickness increased significantly in the dihydralazine-treated group only. Collagen density was not significantly modified by either treatment.(ABSTRACT TRUNCATED AT 250 WORDS)