Structural and mechanical adaptations in rat aorta in response to sustained changes in arterial pressure

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Abstract
Structural and mechanical adaptations in response to sustained changes in arterial pressure were studied on abdominal aorta of the male rat. Two models were used: 1. Aortic ligature (L), immediately below the renal arteries producing hypotension distal to the knot (duration before sacrifice 6 weeks or 3 months). 2. One‐clip renal hypertensive rats (H) (duration 6 weeks). Normotensive sham‐operated rats (C) served as controls. At sacrifice mean tail artery pressure was L: 58±1, C: 110±3, and H: 163±5 mmHg (SE, n=6). Segments of abdominal aorta were mounted in vitro for determination of their length–tension relations (activation: High‐K+ solution with 2.5 mM Ca2+). At end of experiments the vessels were supramaximally stimulated at optimal circumference (Io) for active force (activation: High‐K+ solution with 10 mM Ca2+, and 10‐5 M noradrenaline), and then fixated for light and electron microscopy. Passive and active length–tension relations were shifted towards lower and higher circumference values for hypo‐ and hypertensive vessels, respectively. The Io values were L: 3.60±0.13, C: 4.44±0.19, and H: 4.91±0.29 mm. The media thickness at I0 was reduced in L: 56.0±3.3, and increased in H: 81.3±2.4 compared to C: 73.4±1.8 μm. Maximal active wall stress was L: 46.6±9.8, C: 74.2±7.0, and H: 83.8±7.7 mN/mm2. Intracellular volume (ICV) in the media was L: 30±2, C: 45±3, and H: 44±1% (n=4 for each). The results indicate that the vascular smooth musle in the H group has an unaltered force generating ability. The low active stress in L could to a large extent be explained by the decreased ICV. In conclusion, the arterial vessel wall of these animals, with no genetic predisposition for hypertension, adjusts smooth muscle content and length–force relations in a characteristic way in response to sustained alterations in transmural pressure.

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