Michael J. Mulvany

Remodeling and reparation of the cardiovascular system

Growth or altered metabolism of nonmyocyte cells (cardiac fibroblasts, vascular smooth muscle and endothelial cells) alters myocardial and vascular structure (remodeling) and function. However, the precise roles of circulating and locally generated factors such as angiotensin II, aldosterone and endothelin that regulate growth and metabolism of nonmyocyte cells have yet to be fully elucidated. Trials of pharmacologic therapy aimed at preventing structural remodeling and repairing altered myocardial structure to or toward normal in the setting of hypertension, heart failure and diabetes are reviewed. It is proposed that these are therapeutic goals that may reduce cardiovascular morbidity and mortality. Although this hypothesis remains unproved the primary goal of therapy should be to preserve or restore tissue structure and function.

Evidence for increased media thickness, increased neuronal amine uptake, and depressed excitation--contraction coupling in isolated resistance vessels from essential hypertensives.

The functional and morphologic characteristics of isolated subcutaneous resistance vessels (about 170 micron i.d.) from 15 untreated subjects with essential hypertension and 15 matched controls were examined. The vessels from the hypertensives had a 29% increase in the media-thickness-to-lumen-diameter ratio. The maximal force development to noradrenaline (NA) expressed as active pressure (an estimate of the pressure the vessels could have contracted against in vivo) was 30% higher in vessels from the hypertensives, while active media stress (force per square unit of smooth muscle) and sensitivity to NA was not significantly different. Increased active pressure, as well as unaltered active media stress and sensitivity, was seen for vasopressin, serotonin, angiotensin II, and K+. There was, however, an enhanced leftward shift of the NA sensitivity with cocaine (an inhibitor of the neuronal amine pump) in vessels from the hypertensives [pD2(+cocaine) and pD2(-cocaine) were 0.185 +/- 0.53) and 0.040 +/- 0.044, hypertensives and normotensives, respectively, p less than 0.05] suggesting an abnormality of presynaptic function in essential hypertension. Furthermore, the calcium sensitivity was depressed (pD2 was 4.197 +/- 0.050 and 4.381 +/- 0.068, hypertensives and normotensives, respectively, p less than 0.05), and the rate of relaxation was faster (p less than 0.05) in vessels from hypertensives, suggesting that excitation-contraction coupling might be depressed. The results suggest that the increased pressor response in essential hypertension can, to a large extent, be explained by altered vascular structure, while smooth muscle function is either unchanged or possibly depressed.

AN INCREASED CALCIUM SENSITIVITY OF MESENTERIC RESISTANCE VESSELS IN YOUNG AND ADULT SPONTANEOUSLY HYPERTENSIVE RATS

1 We have measured the calcium sensitivity in response to noradrenaline stimulation and potassium depolarization of isolated segments of 100 to 200 μm mesenteric resistance vessels from spontaneously hypertensive (SHR) and control Wistar‐Kyoto (WKY) rats. The rats were either young (4 wk) or adult (4 months), that is of ages before or after the SHRs had developed elevated blood pressure. Experiments were performed under conditions in which the effect of noradrenaline uptake by, and the potassium‐induced noradrenaline release from, the nerve terminals in the vessel walls was eliminated. 2 The response of the SHR and WKY vessels to noradrenaline under conditions where only the extracellular calcium appeared to have been removed was similar. When subsequently stimulated maximally by noradrenaline, the calcium‐sensitivity of the SHR vessels (Ca‐ED50 ≃ 0.1 mm) was greater than that of the WKY vessels (Ca‐ED50 ≃ 0.2 mm). When depolarized by potassium, all vessels were less sensitive to calcium and there was little difference in the calcium sensitivity of SHR and WKY vessels in either age group (Ca‐ED50 ≃ 0.8 mm). 3 The results suggest that whereas the potassium (potential)‐dependent calcium permeability of the SHR smooth muscle cell membrane is normal, the noradrenaline‐induced calcium permeability is abnormally high. The presence of this abnormality in the vessels from the young SHRs suggests that it may be a factor involved in the aetiology of hypertension in the SHR.

Location of Resistance Arteries

Thickening and narrowing of resistance arteries must, by definition, be key elements in the control of the cardiovascular system. However, the precise location of resistance arteries is difficult to establish. This is due to technical problems related to the small size of the vessels, to the measurement conditions disturbing the hemodynamics, and to the status of the animals while the measurements are being made. Furthermore, due to large data heterogeneity, previous studies do not give unequivocal information concerning the pressure profile in the vascular system, or the level of arterial diameter responsible for blood flow. Finally, and importantly, there is little evidence regarding the conscious state, which is thus a major limitation to understanding the mechanisms of blood distribution and the pathogenesis for disease processes such as genetic hypertension. This review first summarizes briefly the techniques which are available for identifying resistance arteries and the inherent technical limitations which are involved. The review then provides a critical assessment of the available data, both as regards measurement of local blood pressures and as regards control of peripheral resistance. The evidence suggests that, at least as regards rats and other small animals, feed arteries as well as more distal microvessels contribute to the maintenance and regulation of blood flow and resistance. Evidence from larger animals is however lacking, and it is thus unclear if resistance function should be based on arterial diameter or anatomic location. Furthermore, evidence concerning man is not available. We therefore conclude the review with suggestions for future research in this area.

Development of blood pressure in spontaneously hypertensive rats after withdrawal of long-term treatment related to vascular structure.

We have studied the effects of long-term treatment with different antihypertensive drugs on blood pressure and mesenteric resistance vessel structure of spontaneously hypertensive rats (SHR), both during treatment and after withdrawal of treatment. Young SHR were treated from 4 to 24 weeks with five different drugs: perindopril (1.5mg/kg per day), captopril (60mg/kg per day), hydralazine (25mg/kg per day), isradipine (42mg/kg per day) and metoprolol (130mg/kg per day). At 24 weeks, 24-h mean blood pressures (MBP), measured invasively, were 121 mmHg (perindopril), 137mmHg (captopril), 140mmHg (hydralazine), 149mmHg (isradipine) and 146 mmHg (metoprolol), compared to control values of 177 mmHg (SHR) and 132 mmHg (Wistar-Kyoto rats, WKY). Mesenteric resistance vessel structure, measured as media: lumen ratio (m:1), was also reduced to different extents: to WKY-level by perindopril (m:1 = 4.4%), to midway between SHR- and WKY-levels by captopril, hydralazine and isradipine (m: 1 = 5.9%), and not at all by metoprolol (m: 1 = 6.8%). When treatment was discontinued, low MBP (ca 151 mmHg) persisted for 12 weeks in rats treated with the angiotensin converting enzyme inhibitors (perindopril and captopril), but rose rapidly in rats which had received the other treatments. At 3–12 weeks after withdrawal of treatment vascular structure was closely correlated with the blood pressure expected from the SHR- and WKY-control values, as were the left ventricle: body weight ratios. The results suggest that the ability of a drug to control vascular structure during treatment is not in itself a predictor of a persistent effect on blood pressure after withdrawal of treatment. They also pointed to the usual close relationship between vascular structure and blood pressure in the absence of treatment.

Small artery structure is an independent predictor of cardiovascular events in essential hypertension.

Objective Structural abnormality of resistance arteries is a characteristic pathophysiological phenomenon in essential hypertension and can be assessed in vitro as an increase in the media: lumen ratio (M: L) of isolated small arteries. We have investigated whether M: L is a risk predictor in uncomplicated essential hypertensive patients. Recently, high M: L was demonstrated as a prognostic marker in patients at high cardiovascular risk, including normotensive type 2 diabetic patients. Since diabetes is associated with pressure-independent changes in M: L, the relevance of this finding to essential hypertension has been uncertain. Methods We conducted a follow-up survey of 159 essential hypertensive patients, who had previously been submitted to a M: L evaluation while participating in a clinical trial. They composed a homogeneous moderate-risk group, with no concomitant diseases, and represented 1661 years of follow-up. Results Thirty patients suffered a documented predefined cardiovascular event during follow-up. Increased relative risk (RR) was associated with M: L ≥ 0.083 (mean level of the hypertensive cohort), RR = 2.34 [95% confidence interval (CI) 1.11–4.95], and with M: L ≥ 0.098 (mean level of a normotensive control group + 2SD), RR = 2.49 (95% CI 1.21–5.11). Both results remained significant (RR = 2.19, 95% CI 1.04–4.64, and RR = 2.20, 95% CI 1.06–4.56, respectively) when adjusted for Heart Score level (10-year mortality risk-estimate, integrating age, gender, systolic blood pressure, cholesterol and smoking). Conclusion Abnormal resistance artery structure independently predicts cardiovascular events in essential hypertensive patients at moderate risk.

Small Artery Remodeling Depends on Tissue-Type Transglutaminase

Remodeling of small arteries is essential in the long-term regulation of blood pressure and blood flow to specific organs or tissues. A large part of the change in vessel diameter may occur through non–growth-related reorganization of vessel wall components. The hypothesis was tested that tissue-type transglutaminase (tTG), a cross-linking enzyme, contributes to the inward remodeling of small arteries. The in vivo inward remodeling of rat mesenteric arteries, induced by low blood flow, was attenuated by inhibition of tTG. Rat skeletal muscle arteries expressed tTG, as identified by Western blot and immunostaining. In vitro, activation of these arteries with endothelin-1 resulted in inward remodeling, which was blocked by tTG inhibitors. Small arteries obtained from rats and pigs both showed inward remodeling after exposure to exogenous transglutaminase, which was inhibited by addition of a nitric oxide donor. Enhanced expression of tTG, induced by retinoic acid, increased inward remodeling of porcine coronary arteries kept in organ culture for 3 days. The activity of tTG was dependent on pressure. Inhibition of tTG reversed remodeling, causing a substantial increase in vessel diameter. In a collagen gel contraction assay, tTG determined the compaction of collagen by smooth muscle cells. Collectively, these data show that small artery remodeling associated with chronic vasoconstriction depends on tissue-type transglutaminase. This mechanism may reveal a novel therapeutic target for pathologies associated with inward remodeling of the resistance arteries.

Vascular remodelling of resistance vessels: can we define this?

Time for primary review 31 days. As pointed out originally by Folkow [1], many of the haemodynamic features associated with essential hypertension can be accounted for by alterations in the structure of the resistance vessels. In vivo haemodynamic experiments (where forearm blood flow was measured with the vasculature fully relaxed under conditions of reactive hyperaemia) showed that the relaxed peripheral resistance in essential hypertensive patients was increased, that the pressor response to maximal concentrations of agonists was increased, but that the threshold concentration of agonists which caused vascular contraction was not altered. From simple physical reasoning, it was suggested that these findings could be accounted for by a slight change in the structure of the resistance vessels, such that there is a decrease in the lumen diameter and an increase in the wall (or media) thickness to lumen diameter ratio [2–5]. It is, however, only recently that it has become possible to quantify resistance vessel structure, and thus to describe the results of the remodelling process with any precision. This development has inevitably brought with it controversy about the terminology which is appropriate for characterizing the remodelling process. In this review, I will briefly summarize a recent scheme which has been proposed to define the forms of remodelling, and the current information concerning the remodelling of resistance vessel structure. I will then discuss the current debate concerning the terminology to be used. The review is restricted to the morphology of individual vessels, and not to possible changes in branching vascularity [6]. The resistance vessels exert their function through the resistance which they present to the blood flow. The resistance is determined by the lumen diameter (to the fourth power according to the Poiseuille relation), and the lumen diameter is a function of the passive and active mechanical … * Tel.: +45-8942-1726; fax: +45-8612-8804; e-mail: mm@farm.aau.dk

Changes in noradrenaline sensitivity and morphology of arterial resistance vessels during development of high blood pressure in spontaneously hypertensive rats

SUMMARY We have investigated whether differences seen in the pharmacological and morphological properties of mesenteric resistance vessels from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) controls are also present in vessels from young SHR and WKY rats in which there is little difference in blood pressure (BP). Segments of small arteries (lumen diameter 150

Role of membrane potential in the response of rat small mesenteric arteries to exogenous noradrenaline stimulation.

m) were taken from a specific location in the mesenteric bed of 6-, 12-, and 24-week-old SHR and WKY rats, and mounted on a myograph capable of directly measuring their tension. Vessels were set to internal circumference L, = 0.8 Lin where Ll00 was an estimate of the internal circumference the vessels would have had when relaxed in situ and under a transmural pressure of 100 mm Hg. At all ages, compared with WKY vessels, the effective lumen diameter, I, = L,/x, was smaller in the SHR vessels. However, media hypertrophy was seen only in vessels from 12- and 24-week-old SHRs. In physiological salt solution the noradrenaline sensitivity of all vessels was similar (EDM * 2.4 nM). However, inhibition of neuronal uptake with cocaine revealed that at all ages the noradrenaline sensitivity of the vascular smooth muscle cells in the SHR vessels was greater than that of the cells in the WKY vessels. The results also suggested that the neuronal noradrenaline uptake was greater in the SHR vessels at all ages. The main increase in BP in the SHRs occurred between the ages of 6 week and 12 weeks. The results are therefore consistent with the hypothesis that differences in the structure of the resistance vessels are among the factors responsible for the development and maintenance of genetic hypertension. However, they point also to the possible involvement of differences in the noradrenaline sensitivity of the smooth muscle cells in the resistance vessel wails.