Effects of candidate autocrine and paracrine mediators on growth responses in isolated rat arteries

Effect of mitogens on growth and contractile responses of rat small arteries: In vitro studies

Rat mesenteric and epigastric small arteries were cultured to investigate influences of mitogens on contractility, proliferation and protein synthesis. Wistar rat arteries were cultured in serum-free Dulbecco's Modified Eagle Medium, first, for 24 h to equilibrate and then for a further 24-48 h either in the absence or presence of test substances: angiotensin II (AII), 1 microM; AII, 1 microM + platelet derived growth factor BB-chain (PDGF-BB), 1 ng mL-1; PDGF-BB, 1 ng mL-1; PDGF-BB, 30 ng mL-1. No mechanical stress was applied. Viability was assessed by myography, protein synthesis by 6-h incorporation of 35S-methionine and proliferation by both 48-h 3H-thymidine-incorporation and immunohistochemical analysis using the thymidine analogue 5-bromo-2'-deoxyuridine. After 3 days in culture, the contractile responses of arteries to phenylephrine, serotonin, AII and PDGF-BB were preserved. Stimulation with PDGF-BB (30 ng mL-1) increased protein synthesis 1.5- (mesenteric) and 1. 9-fold (epigastric). Similarly, stimulation with PDGF-BB (30 ng mL-1) increased 3H-thymidine incorporation of unstimulated arteries 3.4- (mesenteric) and 2.8-fold (epigastric). The other treatments affected neither protein synthesis nor proliferation. Immunohistochemical analysis showed that the proliferation was occurring primarily in the adventitia and that the levels of apoptosis were unaltered by culture. The effects of AII and PDGF-BB on remodelling did not correlate with their contractile effects: epigastric arteries responded strongly to AII and PDGF-BB, while mesenteric arteries responded weakly. The results suggest that organ culture conditions which preserve contractile function may not be sufficient to preserve trophic mechanisms.

Angiogenesis and hypertension

BACKGROUND

The formation of new blood vessels is an important process in embryonic development and in physiological repair processes. Abnormalities in blood vessel growth have been associated with various pathologies.

HYPERTENSION AND IMPAIRED VASCULAR GROWTH

The basic observation underlying the hypothesis that essential hypertension is based on an impaired capacity for vascular growth is the nature of the structural alterations of microvascular beds in essential hypertension. Recent advances in understanding the molecular and cellular mechanisms of vascular growth suggest that the remodeling of individual vessels and vascular networks in hypertension may be a pathological variant of the formation of mature networks.

PATHOGENESIS OF IMPAIRED VASCULAR GROWTH

Genetic and fetal influences appear to have significant effects in determining impaired vascular development as an early cause of essential hypertension.

Vasomotor responses in chronically hyperperfused and hypoperfused rat mesenteric arteries

We evaluated the reactivity of small arteries after remodeling induced by elevated or reduced blood flow. In 6-wk-old rats, every other first-order side branch of the superior mesenteric artery was ligated near the bifurcation of second-order branches. Four weeks after surgery, vessels that had been exposed to high flow (HF) or low flow (LF) were isolated and mounted in a pressure myograph at 100 mmHg and were compared with vessels from sham-operated rats (Sham). In HF: 1) basal lumen diameter was increased; 2) sensitivity to norepinephrine, arginine vasopressin, and perivascular nerve stimulation was not modified; 3) maximal constrictor responses (delta diameter) to these stimuli and 125 mM K+ were increased; and 4) sensitivity and maximal dilator responses to sodium nitroprusside, acetylcholine, and flow were not modified. In LF: 1) basal diameter was reduced; 2) sensitivity to constrictor stimuli was not altered; 3) maximal responses to all vasoconstrictors except arginine vasopressin were reduced; and 4) sensitivity but not maximal dilator responses to sodium nitroprusside and acetylcholine was reduced. During acute flow-induced dilatations, lower shear stress was maintained in HF (48 +/- 7 dyn/cm2) than in Sham (63 +/- 10 dyn/cm2), but no shear stress regulation was observed in LF. These observations indicate that arterial structural responses to altered blood flow are accompanied by modified reactivity of the arterial smooth muscle, which entails changes in responsiveness to neurogenic and endothelium-dependent stimuli.

Apoptosis in aorta of deoxycorticosterone acetate-salt hypertensive rats: effect of endothelin receptor antagonism

BACKGROUND

Apoptosis or programmed cell death could be greater than normal in various cardiovascular disorders, particularly in the heart. Apoptosis might contribute to remodeling of blood vessels in hypertension and could participate in regulation of vascular hypertrophy/hyperplasia.

OBJECTIVE

To investigate apoptosis in deoxycorticosterone acetate (DOCA)-salt hypertension and to determine whether endothelin-1, whose expression is enhanced in these rats, plays a role in apoptosis.

METHODS

We administered two orally active endothelin-A (ET[A])-selective receptor antagonists, A-127,722.5 (30 mg/kg per day) and LU 135,252 (50 mg/kg per day), to establish whether antigrowth effects of these ET(A) antagonists are in part mediated through apoptosis. Apoptosis was evaluated by radiolabeling of 3' OH ends of fragmented DNA, extracted from aortas, using terminal deoxynucleotidyl transferase, to show the presence of internucleosomal DNA splicing as 'DNA laddering'. Its presence was confirmed by in-situ end-labeling.

RESULTS

Systolic blood pressure was slightly but significantly lower in treated than it was in untreated DOCA-salt hypertensive rats by a mean of 26 mmHg (P < 0.01) after 4 weeks of treatment with A-127,722.5 and by 19 mmHg (P < 0.01) in rats treated with LU 135,252. Aortic cross-sectional area (CSA) was significantly greater (P < 0.001) in DOCA-salt rats than it was in uninephrectomized controls. This increased CSA was normalized by both ET(A) antagonists. DOCA-salt rats exhibited a greater degree of apoptosis (evaluated by DNA 'laddering') in aorta (353 +/- 14 pixels/microg DNA) than did control rats (232 +/- 10 pixels/microg DNA, P < 0.01). The magnitude of apoptosis was significantly greater (P < 0.01) in aorta of endothelin-antagonist-treated than it was in aorta of untreated DOCA-salt hypertensive rats. In-situ end-labeling confirmed that more apoptosis had occurred in the media of aorta from DOCA-salt hypertensive rats and the further increase found after treatment with the ET(A) antagonists.

CONCLUSION

An increase in apoptosis occurs in aorta of DOCA-salt hypertensive rats, probably as a physiologic counterpart of growth in this hypertensive model. ET(A) antagonists may act in part by accentuating the apoptosis, thereby inducing a blunting of vascular growth, which could also contribute to their antihypertensive effects.

Structural properties of rat mesenteric small arteries after 4-wk exposure to elevated or reduced blood flow

We determined the structure of mesenteric small arteries after chronic elevation and chronic reduction of blood flow. In 6-wk-old rats, we ligated second-order side branches of every other first-order side branch of the superior mesenteric artery. This persistently reduced blood flow (-90%) in the vessels feeding into the ligated trees and elevated blood flow (+80%) in the nonligated mesenteric artery side branches. Four weeks after surgery, vessels that had been exposed to high blood flow (HF) or low blood flow (LF) and vessels from sham-operated rats (Sham) were isolated and mounted in a pressure myograph system. At an intraluminal pressure of 100 mmHg, the internal diameter at rest was larger in HF (533 +/- 23 microm) and smaller in LF (262 +/- 14 microm) than in Sham vessels (427 +/- 15 microm). Also, wall and media cross-sectional areas were larger in HF and smaller in LF than in Sham vessels (media: 22 +/- 1, 11 +/- 2, and 16 +/- 1 x 10(3) microm2, respectively), but circumferential wall stress did not differ among groups. DNA content was significantly increased in HF vessels (+100%) and was not modified in LF vessels. Maximal vasoconstrictions elicited by high potassium or norepinephrine were slightly increased in HF vessels but were reduced by 50% in LF vessels. Thus chronic changes in blood flow give rise to structural changes that normalize circumferential wall stress. Elevated blood flow resulted in outward hypertrophic remodeling involving hyperplasia. Reduced blood flow resulted in inward hypotrophic remodeling accompanied by hyporeactivity of the arterial smooth muscle.

Effect of chronic ETA-selective endothelin receptor antagonism on blood pressure in experimental and genetic hypertension in rats

1. Chronic treatment with a combined ETA/ETB endothelin receptor antagonist has been shown to reduce blood pressure in experimental rat models of hypertension in which endothelin-1 gene overexpression occurs in the walls of blood vessels, particularly small, resistance-sized arteries, but not in those genetic or experimental models of hypertension in which there is no overexpression of vascular endothelin-1. Failure of some experimental models of hypertension to respond to treatment with the combined ETA/ETB endothelin antagonist may be due in part to blockade of vasorelaxant endothelial ETB receptors which could in theory reduce the efficacy of endothelin antagonism. 2. In this study the orally active ETA-selective endothelin antagonists A-127722.5 and LU 135252 were used in chronic experiments on deoxycorticosterone acetate (DOCA)-salt hypertensive rats (which overexpress vascular endothelin-1 and respond with blood pressure lowering to combined ETA/ETB endothelin receptor antagonism), on spontaneously hypertensive rats (SHR) (which do not overexpress vascular endothelin-1 and do not respond with blood pressure lowering to the combined ETA/ETB receptor antagonist), and in 1-kidney 1 clip Goldblatt (1-K IC) hypertensive rats (which present mild overexpression of vascular endothelin-1 but do not respond with blood pressure lowering to the combined ETA/ETB receptor antagonist). Additionally, it has been suggested that interruption of the renin-angiotensin system may sensitize responses to endothelin antagonism. Accordingly, SHR were treated with an angiotensin converting enzyme inhibitor, cilazapril, in addition to the ETA receptor antagonist. 3. Blood pressure of DOCA-salt hypertensive rats was lowered by a mean of 24 and of 27 mmHg (P < 0.01) by A-127722.5 after 4 weeks of treatment, when given orally at two different doses (10 and 30 mg kg-1 day-1), and by 18 mmHg by LU 135252 50 mg kg-1 day-1. 4. SHR treated with A-127722.5 for 8 weeks starting at 12 weeks of age exhibited the same progressive rise in blood pressure as untreated SHR. Addition of cilazapril resulted in similar reduction of blood pressure in A-127722.5-treated and untreated SHR. 5. Treatment of 1-K IC hypertensive rats with the dose of LU 135252 which lowered blood pressure in DOCA-salt hypertensive rats did not cause any reduction in blood pressure relative to untreated rats. 6. These results demonstrate that treatment with either dose of the selective ETA receptor antagonists A-127722.5 or LU 135252 caused reductions in blood pressure similar to those obtained for a combined ETA/ETB endothelin antagonist. Blood pressure was lowered only in hypertensive rats known to overexpress vascular endothelin-1 (DOCA-salt hypertensive rats) but not in those which do not (SHR) or only have mild vascular overexpression of endothelin-1 gene (1-K 1C hypertensive rats). Reduction in activity of the renin-angiotensin system in SHR did not sensitize blood pressure to potential hypotensive effects of an ETA-selective receptor antagonist.

Enhanced expression of endothelin-1 gene in resistance arteries in severe human essential hypertension

BACKGROUND

Endothelins are potent vasoconstrictors, and may also act as mitogens and hypertrophic agents. Expression of a member of this family of peptides, endothelin-1, is enhanced in the endothelium of blood vessels of rats with severe forms of hypertension, even in the absence of elevated plasma endothelin levels. In some of these hypertensive models enhanced endothelin-1 gene expression may contribute to vascular hypertrophy of small arteries and to elevation of blood pressure.

OBJECTIVE

To establish whether endothelin-1 may play a role in essential hypertension in humans, in whom plasma levels are known to be usually within normal limits, by examining the expression of the endothelin-1 gene in resistance-size arteries of normotensive subjects, and in humans with mild and severe hypertension.

METHODS

Using in-situ hybridization, the abundance of endothelin-1 messenger RNA transcripts was evaluated in small arteries of subcutaneous gluteal fat obtained by biopsy in normotensive and hypertensive patients.

RESULTS

Vessels from five normotensive subjects and four untreated mild essential hypertensive patients did not exhibit topographically localized specific labeling with the antisense human endothelin-1 probe. Biopsies from four untreated hypertensive patients with moderate-to-severe blood pressure elevation, in contrast, showed a heavy density of grains on endothelial cells of small arteries of gluteal subcutaneous fat, corresponding to hybridization of the antisense human endothelin-1 complementary RNA probe with endothelin-1 messenger RNA.

CONCLUSION

Some patients with moderate-to-severe essential hypertension, similar to some experimental rat models with severe blood pressure elevation, exhibit enhanced endothelial expression of the endothelin-1 gene. This is the first demonstration that overexpression of the endothelin-1 gene may occur in the vascular wall in a small sample of this subset of hypertensive patients. This pathophysiologic phenomenon could play a role in blood pressure elevation and perhaps in the pathogenesis of vascular hypertrophy. Treatment with endothelin receptor antagonists may offer a novel therapy for these moderate-to-severe hypertensive patients.

The endothelium of resistance arteries: physiology and role in hypertension

The endothelium plays a very important role in the regulation of vascular function by way of its barrier role, by interaction with circulating cells such as platelets, which may release vasoactive or growth regulating agents, and through production of substances which modulate vascular tone and smooth muscle cell growth, and which may also exert antithrombotic effects. The endothelium of resistance arteries, vessels critically involved in generating resistance to flow and which play an important role in hypertension, has been studied mainly from the point of view of generation of agents which regulate vascular tone and growth. Endothelium-derived relaxing factors such as nitric oxide, prostacyclin, hyperpolarizing factors (EDHF) and possibly C-type natriuretic peptide (CNP), are counteracted by endothelium-derived contracting factors, which include endothelins and contracting factors (EDCF) which are less well characterized and appear to be cyclooxygenase products. In experimental hypertension in animals, and in human essential hypertension, these mechanisms may be altered. There may be a reduced generation of endothelium-derived nitric oxide and enhanced production of EDCF. Some of the mechanisms involved in the role these agents play in the physiology of resistance arteries and pathologically in hypertension will be reviewed.

Modulation of endothelin receptor expression in human vascular smooth muscle cells by interleukin-1 beta

Endothelin may play a role in atherosclerosis as it causes smooth muscle cell proliferation and its levels are elevated in patients with atherosclerosis. We report that interleukin-1 beta is a potent inducer of endothelin receptor expression in cultured human vascular smooth muscle cells. The effect is dose- and time-dependent and is due to an increase in receptor number. Endothelin receptor mRNA levels are also elevated. Interleukin-1 beta is a major regulator of endothelin release so, together with its effects on endothelin receptors, it may be responsible for a generalized activation of the endothelin system in diseased vessels.