Wall tension and contraction of the aorta in 6‐month‐old spontaneously hypertensive rats

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Abstract
1 The present study aimed at comparing the influence of passive tension on the effect exerted by noradrenaline on the thoracic aorta of 6‐month‐old Wistar and spontaneously hypertensive rats (SHR). 2 Concentration–response curves to noradrenaline were obtained in aorta rings, at two levels of passive tension: 3 and 0.5 g. 3 The maximal responses (in percentage of the maximal response to noradrenaline obtained in the beginning of the experiment at a tension of 2 g) were significantly larger (P < 0.05) at 3 g than at 0.5 g in both kinds of rats: 171 versus 69%, respectively, for SHR; 139 versus 76%, respectively, for Wistar rats. 4 When expressed as mg of active tension per mg of tissue, the maximal contraction at both 3 and 0.5 g was smaller in SHR than in Wistar rats (at 3 g: 64.6 ± 6.7, n=6 versus 122.3 ± 19.1, n=8, respectively, P < 0.05; at 0.5 g: 24.0 ± 1.0, n=6 versus 49.0 ± 5.9, n=8, respectively, P < 0.05). 5 Maximal responses to noradrenaline were markedly decreased by cytochalasin B (50 μM) (to 15.2 ± 6.0%, n=6, at 3 g and 2.8 ± 1.9%, n=6, at 0.5 g in SHR; to 11.8 ± 2.3%, n=4, at 3 g and 4.3 ± 1.3%, n=4, at 0.5 g in Wistar rats). Cytochalasin B at a lower concentration (4 μM) produced a less marked decrease in responses to noradrenaline in both strains of rats. The presence of cardiovascular structural changes in SHR was confirmed by the fact that the heart weight (mg):body weight (g) was higher in SHR (3.37 ± 0.06, n=10) than in Wistar rats (2.40 ± 0.12, n=11) (P < 0.05). 6 It is concluded that in 6‐month‐old SHR the contractile capacity of the aortic tissue is reduced. However, the differential sensitivity of aortic smooth muscle at the two different levels of tension remains present. This difference may depend on filament interaction. Contractions to noradrenaline in the rat aorta are highly dependent on actin polymerization.