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

Effect of pressure overload and its recovery on the rat carotid artery: change of vascular reactivity and remodeling process

Compared to chronic hypertension, little is known about the pathophysiology of acute hypertension and its recovery. To characterize this, we investigated the functional and structural properties of the common carotid arteries (CCA) from 35 rats. We established a unilateral and reversible carotid arterial hypertension model using the partial transverse aortic constriction (TAC) technique. By TAC, the right CCAs were made to endure a pressure-overload environment, while the left CCAs remained under normotension. The TACs were removed 2 weeks later, which unloaded the hypertensive effects. We compared the contractile, histological, and molecular responses of the CCA before TAC, during TAC (the hypertension period), and after removal of TAC (the recovery period). Vessel contractility was nearly abolished during 2 weeks of TAC. The recovery process from hypertension showed an initial hypercontractile period within a week. The relaxation response due to acetylcholine, as measured during the recovery period, showed a longer time course than the contractility for recovering its magnitude. During the hypertension period, the media thickness increased and this persisted throughout the recovery period. Apoptosis of the endothelial layer was significantly increased during the hypertension period and this disappeared 2 weeks after recovery. Expression of endothelial NO synthase was not detectable at the end of the hypertension period, but this gradually returned to the basal level after 2 weeks of recovery. Although increased contractility is usually expected in chronic hypertensive vessels, an abrupt pressure overload decreases contractility and the endothelium-dependent relaxation. It also increases endothelial apoptosis and the media thickness. These findings have clinical relevance, and they could be applied to human acute and severe hypertension and its recovery.

Effects of lipopolysaccharide on vascular reactivity and mortality in rats

1 The effects of intraperitoneal (i.p.) lipopolysaccharide on vascular reactivity to noradrenaline in rat aorta under different conditions of passive tension, as well as on mortality in normotensive and hypertensive rats, were studied. 2 Concentration-response curves to noradrenaline were obtained in aorta rings, at two levels of passive tension: 3 and 0.5 g, from control and lipopolysaccharide-treated Wistar rats. Contractile responses were expressed as percentage of the maximal response to noradrenaline obtained in the beginning of the experiment at a resting tension of 2 g. The maxima were significantly larger (P<0.05) at 3 g than at 0.5 g in both groups of rats: 117.8 vs. 62.3%, respectively, for control animals; 85.8 vs. 32.5%, respectively, for lipopolysaccharide-treated rats. 3 The 24-h mortality after the i.p. administration of lipopolysaccharide was lower in spontaneously hypertensive rats (1/12; 8%), when compared with control Wistar-Kyoto rats (5/11; 45%). However, mortality was higher in Wistar-Kyoto made hypertensive by 8-day administration of corticosterone (6/6; 100%). 4 We conclude that a differential sensitivity to noradrenaline of aortic smooth muscle at two different levels of passive tension is still present in lipopolysaccharide-treated animals. Chronic hypertension in SHR rats is associated with resistance to the lethal effects of lipopolysaccharide, whereas abrupt-onset hypertension induced by corticosterone leads to an increased mortality. 5 These results are compatible with the myofibrillary hypothesis, which explains vascular hyper-reactivity in chronic arterial hypertension, by postulating that a more favourable relative position (and/or proportion) for actin and myosin occurs, whereas in states of vascular hyporeactivity, such as vasodilatory shock, the opposite phenomenon may exist.

Functional importance of the actin cytoskeleton in contraction of bovine iris sphincter muscle

1. The contractile capacity of smooth muscle cells depends on the cytoskeletal framework of the cell. The aim of this study was to determine the functional importance of both the actin and the tubulin components of the cytoskeleton in contractile responses of the bovine isolated iris sphincter muscle. 2. In each preparation, two contractions to the muscarinic agonist carbachol were obtained. The maximum responses of the first contractions were taken as 100%. The second contractions to carbachol were elicited in the presence of either cytochalasin B (50 and 5 microm), an inhibitor of the actin cytoskeleton, or colchicine (100 microm), an inhibitor of the tubulin cytoskeleton (30 min incubation). 3. Cytochalasin B, at a concentration of 50 microm, significantly decreased the contractions induced by carbachol, with the maximum response reduced to 21.8 +/- 6.6% (n = 12) of the initial maximum. The maximal contractions to carbachol in the presence of colchicine reached 96.2 +/- 7.9% (n = 9) of the initial contraction, which was not significantly different from control second responses to carbachol with neither drug present, which reached 113.3 +/- 7.6% (n = 7). 4. The effect of cytochalasin B was dose-dependent, since at a lower concentration of 5 microm, the drug decreased the maximum contraction to carbachol to 60.3 +/- 8.8% (n = 6). The effect of cytochalasin B was at least partially reversible, since after the use of the higher concentration of 50 microm, contractions to carbachol increased to 62.3 +/- 15.5% (n = 4) of the maximal response, after 1 h repeated washing of the preparations. 5. Cytochalasin D, at a concentration of 50 microm, completely abolished the contractions induced by carbachol (n = 4). 6. These findings suggest that in bovine iris sphincter muscle, contractions to carbachol are highly dependent, from a functional point of view, on actin polymerization, and not, to any important degree, on the polymerization of tubulin.