Fundamental influence of transmural stretch on endothelial function in rat aortic rings

Andrew MacKenzie2, Ian F. Gibson1 & Fiona J Dowell1. 1ICM, Glasgow University Vet School, Glasgow, UK; 2Bell College, Hamilton, UK.

Stretch of blood vessels is known to modify vascular function ( Franchi-Micheli et al., 2000 ) yet details of this remain unclear. In this study we investigated how transmural stretch influenced endothelial function in isolated rings of rat aorta. Specifically, how the functional activity of non-stimulated (basal) and agonist-stimulated production of nitric oxide (NO) was modified in vessels under two different levels of resting tension.

Rings of thoracic aorta, from female Wistar rats (200-250g), were mounted for isometric tension recording in organ baths under 1g or 5g resting tension. Concentration-response curves were generated to phenylephrine (PE, 10-10–10-5M) in endothelium-containing rings, in the absence and presence of the NO synthase inhibitor L-NAME (10-4M), and in endothelium-denuded preparations. Following induction of 70% PE-induced tone, vasodilation responses to acetylcholine (ACh, 10-9–10-5M) and the NO donor glycerol trinitrate (GTN, 10-10–10-6M) were recorded. Data are expressed as mean ± s.e. mean, n ³ 6; statistical comparisons were determined by ANOVA.

In endothelium-containing aortic rings the maximum response induced by PE was greater at 5g than 1 g stretch (1.85 ± 0.13g & 0.87 ± 0.12, respectively P<0.001). In rings under 1 g stretch the peak response to PE was enhanced profoundly in the presence of L-NAME (1.25 ± 0.11g, P<0.001) and also in preparations without endothelial cells (1.38 ± 0.24g, P<0.05), demonstrating that the basal release of NO has a significant depressor influence on PE-induced contraction. In contrast, the maximum contraction produced by PE in endothelial-containing rings under 5g stretch was unaffected by L-NAME (1.81 ± 0.24g, P>0.05) or by removal of the endothelium (1.93 ± 0.19g, P>0.05), indicating that the depressor influence of basally released NO is absent in tissue at the higher level of stretch. Following submaximal PE-induced contraction, the ability of ACh to induce relaxation was unaffected by the level of transmural stretch (maximum relaxation was 94.1±5.3 & 101±1.9% for 1 & 5g, respectively; P>0.05). The relaxation was absent in endothelium-containing vessels treated with L-NAME and in endothelial-denuded tissues, at both levels of stretch, demonstrating that ACh-induced relaxation is mediated entirely by NO in tissues under 1 and 5g resting tension. Furthermore these results demonstrate that, in contrast to basally released NO, the relaxant actions of stimulated NO are unaffected by stretch. In endothelial-denuded preparations the relaxation induced by low concentrations of GTN (10-9-10-8M) was greater in tissue of 1g resting tension than 5g (e.g. relaxation to GTN 3x10-8: 39.5±1.3 & 25.5±2.7g for 1 & 5g, respectively, P<0.01). However, the maximal relaxation to the NO donor was unaffected by the level of stretch. This data suggests that at the higher level of stretch smooth muscle cells may be less sensitive to NO.

These results demonstrate that increasing transmural stretch in rings of rat aorta from 1g to 5g abolishes the influence of basally released NO whilst, conversely, the relaxant action of agonist-stimulated activity of NO is not affected. In addition, the sensitivity of smooth muscle cells to exogenous NO may be modified by stretch.

Franchi-Micheli et al. (2000) Br. J. Pharmacol. 131, 1355-62.