Jin-Sheng Li

Resistance Artery Mechanics, Structure, and Extracellular Components in Spontaneously Hypertensive Rats Effects of Angiotensin Receptor Antagonism and Converting Enzyme Inhibition

BACKGROUND Altered vascular mechanics resulting from changes in collagen and integrins may influence resistance artery structure and function and, therefore, peripheral resistance and blood pressure in spontaneously hypertensive rats (SHR). METHODS AND RESULTS Effects of age, angiotensin-converting enzyme inhibition (fosinopril, 10 to 30 mg/kg per day), and AT(1)-receptor antagonism (irbesartan, 50 mg/kg per day) on vascular structure, mechanics, and composition were assessed in SHR. Systolic blood pressure was elevated in young SHR (130+/-2 mm Hg) compared with Wistar-Kyoto (WKY) rats (106+/-2 mm Hg). In adult SHR, the rise in systolic blood pressure (44+/-3 mm Hg) was blunted by fosinopril (18+/-1 mm Hg) and irbesartan (9+/-3 mm Hg). Lumen diameter of mesenteric resistance arteries was smaller and media/lumen ratio was greater in young and adult SHR versus WKY rats. Growth index was 24% in untreated adult SHR versus WKY rats; these values were -35% for fosinopril-treated and -29% for irbesartan-treated SHR versus untreated SHR. Isobaric wall stiffness was normal despite increased stiffness of wall components in adult SHR vessels. Irbesartan partially prevented stiffening of wall components in SHR. The collagen/elastin ratio was greater in adult SHR vessels (6.5+/-1.3) than in WKY (3.2+/-0.4) vessels. Expression of alpha(v)beta(3) and alpha(5)beta(1) integrins was increased in SHR aged 20 versus 6 weeks. Expression of alpha(5)beta(1) integrins was lower in young SHR, and alpha(v)beta(3) integrins were overexpressed in adult SHR versus WKY rats. Irbesartan and fosinopril attenuated differences in the collagen/elastin ratio and integrin expression. CONCLUSIONS Wall components of mesenteric resistance arteries stiffen with age in SHR. Interrupting the renin-angiotensin system has normalizing effects on integrin expression and composition, stiffness, and growth of the arterial wall.

In Vivo Study of AT1 and AT2 Angiotensin Receptors in Apoptosis in Rat Blood Vessels

In vitro experiments suggest that angiotensin II (Ang II) may cause growth via angiotensin type 1 (AT(1)) receptors and apoptosis via angiotensin type 2 (AT(2)) receptors. To answer the question of whether AT(1) or AT(2) receptor activation could induce apoptosis in the vasculature in vivo, Wistar rats were infused for 7 days with Ang II (120 ng. kg(-1). min(-1) subcutaneously) and treated with the AT(2) receptor antagonist PD 123319 (30 mg. kg(-1). d(-1) subcutaneously) or the AT(1) receptor antagonist losartan (10 mg. kg(-1). d(-1) orally). Apoptosis in thoracic aorta was quantified by radiolabeled DNA laddering and by terminal deoxynucleotide transferase-mediated dUTP nick end-labeling. The expression of p53, bax, bcl-2, and caspase-3, which play critical roles in apoptotic signaling, was examined by Western blot analysis. The mRNA expression of AT(1) and AT(2) receptors was determined by reverse transcription-polymerase chain reaction. The increase in systolic blood pressure and aortic growth induced by Ang II infusion was completely prevented by losartan alone or losartan given with PD 123319, whereas PD 123319 resulted in a greater increase in systolic blood pressure and aortic growth than Ang II alone. Radiolabeled DNA laddering showed that Ang II infusion+/-losartan or PD 123319 significantly increased apoptosis (147+/-8%, 178+/-20%, and 238+/-41%, respectively, P<0.05 compared with control). Expression of bax and active forms of caspase-3 was increased in the Ang II+PD 123319 group, whereas the expression of p53 and bcl-2 was not significantly different in all groups. The expression of AT(1) and AT(2) receptor mRNA was downregulated by losartan and PD 123319, respectively. Thus, when AT(1) or AT(2) receptors are stimulated in vivo, apoptosis is enhanced in the media of blood vessels. In the case of AT(1) receptor stimulation, this may occur secondary to vascular growth and modulate the latter. Both bax and caspase-3 participate in the pathways of apoptosis triggered by in vivo AT(1) receptor stimulation.

Enhanced expression of the endothelin-1 gene in blood vessels of DOCA-salt hypertensive rats : Correlation with vascular structure

Endothelin (ET)-1 is a potent vasoconstrictor peptide which is also endowed with hypertrophic and mitogenic properties. Enhanced ET-1 gene expression in blood vessels of rats with hypertension induced by deoxycorticosterone acetate (DOCA)-salt has been previously demonstrated. The objective of this study was: (1) to investigate the individual effects of salt and DOCA, and of the development of hypertension in DOCA-salt-hypertensive rats, on ET-1 gene expression in blood vessels; and (2) to correlate the presence of enhanced ET expression and the severity of vascular hypertrophy. By providing salt and/or DOCA to rats which were or were not unilaterally nephrectomized, groups of rats with variable degrees of blood pressure elevation were generated. Increased abundance of ET-1 mRNA and a greater content of immunoreactive ET-1 were found with progression of hypertension in aorta and the mesenteric arterial bed only in unilaterally nephrectomized DOCA-salt-treated rats (DOCA-salt-hypertensive rats). Vascular expression of ET-1 was not enhanced in other DOCA-or salt-treated rats, even when blood pressure rose to a mean systolic pressure of 180 mm Hg. The wet weight of aorta and the mesenteric arterial bed, the media thickness, the media cross-sectional area, and the media-to-lumen ratio of mesenteric small arteries of all groups of rats exhibited a close correlation with systolic blood pressure. In DOCA-salt-hypertensive rats after 5 weeks, in which overexpression of ET-1 was maximal, the vascular morphometric parameters were excessive for the level of systolic blood pressure. However, in DOCA-salt-hypertensive rats treated with the combined ETA/ETBET receptor antagonist bosentan, vascular morphometry correlated closely with blood pressure, even though blood pressure was only slightly lower than than of untreated DOCA-salt-hypertensive rats. These data support the hypothesis that ET-1 gene overexpression in blood vessels may accentuate vascular hypertrophy in some forms of hypertension.

Endothelin receptor subtypes in resistance arteries from humans and rats

OBJECTIVE Endothelins bind to different receptor subtypes identified as ETA and ETB receptors, ETA receptors are present on smooth muscle cells of blood vessels. ETB receptors seem to predominate on endothelial cells of blood vessels, but may also be present on smooth muscle in some vascular beds. The aim of this study was to investigate the distribution of ETA and ETB receptors in human and in rat resistance arteries. METHODS Contractile responses to endothelin-1 and to the ETB agonist sarafotoxin S6c were investigated in human subcutaneous resistance arteries and in rat mesenteric resistance arteries mounted on a wire myograph. RESULTS Contractile responses to endothelin-1 were significantly reduced by the ETA selective antagonists BQ123 or BQ610 in human resistance arteries, and were almost abolished by the antagonists in rat arteries. The ETB agonist sarafotoxin S6c induced smaller responses than endothelin-1. When responses to sarafotoxin S6c were obtained in the presence of BQ610, the relatively small contractile response remained. It was slightly enhanced if the responses were obtained in the presence of the nitric oxide synthase inhibitor L-NAME or after removal of the endothelium. CONCLUSIONS Vasoconstrictor ETB receptors appear to be present in resistance vessels isolated from subcutaneous fat in humans and from the mesentery in rats, but their contribution to endothelin mediated constriction is small, particularly in rat resistance arteries. ETA receptors appear to be the more important endothelin receptor subtype in the human and rat resistance arteries examined.

Effect of antihypertensive treatment and N omega-nitro-L-arginine methyl ester on cardiovascular structure in deoxycorticosterone acetate-salt hypertensive rats.

Background Deoxycorticosterone acetate (DOCA)-salt hypertensive rats exhibit a very severe degree of cardiovascular hypertrophy, which may in part be mediated by overexpression of the endothelin-1 gene. Objective To examine the effects of the angiotensin I converting enzyme inhibitor cilazapril and of the calcium channel antagonist mibefradil, both of which may affect potential mechanisms responsible for hypertrophy of cardiovascular structures, and that of the nitric oxide synthase inhibitor Nw-nitro-L-arginine methyl ester (L-NAME), which may exert a paradoxical inhibitory effect on cardiovascular growth, on the severe cardiovascular hypertrophy of DOCA-salt hypertensive rats and on arterial expression of the endothelin-1 gene. Methods Small-artery structure was examined on a wire myograph and endothelin-1 messenger RNA (mRNA) was quantified by Northern blot analysis. Results Cilazapril did not affect blood pressure, cardiovascular structure or the increased abundance of endothelin mRNA of DOCA-salt hypertensive rats. Mibefradil treatment resulted in lower blood pressure, reduced cardiac hypertrophy, near-normal structure of conduit and small arteries and lower endothelin-1 mRNA abundance. L-NAME treatment resulted in higher blood pressure and increased severity of conduit artery hypertrophy, but reduced cardiac and small artery hypertrophy, and enhanced aortic endothelin- 1 mRNA. Conclusion These results suggest that the reninangiotensin hypertrophy in DOCA-salt hypertensive rats, which is not unexpected since plasma renin is suppressed in these rats. Calcium channel blockade may interfere with mechanisms underlying vascular hypertrophy in this model via blockade of calcium entry or by reducing vascular endothelin-1 gene expression when the blood pressure is lowered. L-NAME has been shown to exert a growth-inhibitory effect on small arteries and on the heart despite increasing blood pressure, probably independently from its ability to inhibit nitric oxide synthase, the latter of which is presumably involved in the blood pressure rise induced.

Differential effects of vasopressin and endothelin-1 on vascular contractile and calcium responses in pressurized small arteries from spontaneously hypertensive rats.

Objective To investigate the effects of vasopressin and endothelin-1 on the intracellular free calcium concentration ([Ca2+]i) and on contractile responses in endothelium-denuded resistance vessels of prehypertensive (5-week-old) and adult hypertensive (17-week-old) spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) rats. Methods Segments (2 mm long) of third-order branches of small mesenteric arteries were mounted in a perfusion myograph and maintained at 60mmHg pressure. Endothelium was removed by intraluminal passage of air. The vessel [Ca2+]i was measured by fura-2 fluorescence and contraction was determined using a video imaging system to record lumen diameter. Results Lumen diameter was significantly smaller in 5-and 17-week-old SHR than it was in age-matched WKY rats (5 week-old SHR versus WKY rats: 178 ± 4.0 versus 195 ± 4.3 μm; 17-week-old SHR versus WKY rats: 168 ± 7.0 versus 230 ± 3.1 μm). The basal [Ca2+]i was significantly higher in 5− and 17-week-old SHR than it was in age-matched WKY rats. Infusions of vasopressin and endothelin-1 increased [Ca2+]i and contractile responses in a dose-dependent manner in all groups. The vasopressin-induced change in [Ca2+]i was significantly greater in 5− and 17-week-old SHR than in age-matched controls. The sensitivity of [Ca2+]i to vasopressin was increased in adult SHR compared with WKY rats (pD2 9.0 ± 0.1 in SHR, 8.2 ± 0.3 in WKY rats). Vasopressin-stimulated contractile responses were increased in adult SHR. The endothelin-1-induced change in [Ca2+]i did not differ between WKY rats and SHR. The contractility of vessels in response to endothelin-1 infusion was similar in age-matched groups. Conclusions Endothelin-1-induced changes in [Ca2+]i and contractile responses in small arteries are similar in age-matched WKY rats and SHR, whereas responses to vasopressin are significantly enhanced in SHR compared with WKY rats. Thus [Ca2+]j signalling for vasopressin is more active than is that for endothelin-1. Vasopressin but not endothelin-1 might play a role in the development of hypertension in SHR.

Enhanced expression of endothelin-1 gene may cause blood pressure-independent vascular hypertrophy

Summary: Endothelin-1 (ET-1) has hypertrophic and mitogenic properties. Enhanced ET-1 gene expression in blood vessels of deoxycorticosterone acetate (DOCA)/ salt hypertensive rats and DOCA/salt spontaneously hypertensive rats (SHRs) was previously demonstrated. In this study, the effect on ET-1 gene expression in blood vessels and on vascular hypertrophy of the development of hypertension of DOCA/salt hypertensive rats, and that of salt and DOCA, were investigated in Sprague-Dawley rats and in SHRs. Increased abundance of ET-1 mRNA and a greater content of immunoreactive ET-1 were found with progression of hypertension in the aorta and the mesenteric arterial bed only in DOCA/salt hypertensive rats and in DOCA/salt SHRs. Vascular expression of ET-1 was not enhanced in DOCA- or salt-treated rats, even when blood pressure rose to a mean systolic pressure of 210 mm Hg. The media thickness and the media cross-sectional area of mesenteric resistance arteries of all groups of rats, including SHRs and Wistar-Kyoto (WKY) rats, exhibited a close correlation with systolic blood pressure. In DOCA/salt hypertensive rats after 5 weeks and in DOCA/salt SHRs in which significant over-expression of ET-1 was present in blood vessels, vascular morphometric parameters were excessive for the level of systolic blood pressure. In DOCA/salt hypertensive rats and DOCA/salt SHRs treated with the combined ETA/ ETB endothelin receptor antagonist bosentan, vascular morphometry correlated more closely with blood pressure, even though the blood pressure was only slightly lower than that of untreated rats. Lumen diameter correlated with blood pressure in all groups, including those overexpressing ET-1. These data support the hypothesis that ET-1 gene overexpression in blood vessels may accentuate vascular hypertrophy, but not remodeling, in DOCA/salt hypertensive rats and DOCA/salt SHRs in excess of that caused by blood pressure elevation per se.

ENDOTHELIN-1 GENE EXPRESSION AND VASCULAR HYPERTROPHY IN DOCA-SALT HYPERTENSION COMPARED TO SPONTANEOUSLY HYPERTENSIVE RATS

1. Enhanced endothelin‐1 gene expression has been found in blood vessels of deoxycorticosterone acetate (DOCA)‐salt hypertensive rats. In this study, the effects of salt, DOCA and the development of hypertension in DOCA‐salt hypertensive rats on the expression of the endothelin‐1 gene in blood vessels and on vascular hypertrophy were compared in Sprague‐Dawley (SD) rats and spontaneously hypertensive rats (SHR).

Hypertension induced by brain grafts from fetal spontaneously hypertensive rats.

Hypothalami from fetal rats were grafted into the third ventricle of four strains of adult rats. Grafts from spontaneously hypertensive rats (SHR), in contrast to grafts from Wistar-Kyoto (WKY) rats, induced an elevation of systolic blood pressure and a thickening of the media of resistance arteries, along with corresponding alterations in the contractile properties of these vessels. However, no cardiac hypertrophy was observed. The resistance arteries of rats grafted with hypothalamic from SHR also displayed functional alterations that were similar to what is typically found in the resistance arteries of young prehypertensive SHR, ie, an increase in the sensitivity to cocaine and an impairment in the ability to relax in the presence of acetylcholine. This suggests that the brain may play a causal role in these alterations. Histological examination of sections of brains grafted with previously labeled tissue revealed that (1) there was no brain area that was systematically infiltrated by grafts from SHR and not by grafts from WKY rats; (2) the volume of the transplants appeared larger 2 weeks after the graft than the volume of the tissue originally implanted; and (3) grafts from SHR were slightly larger, displayed more individual foci, and extended farther along the anteroposterior axis than grafts from WKY rats. In addition, glial cultures derived from the hypothalami of SHR had a higher in vitro growth rate than equivalent cultures from WKY rats. It is therefore possible that the ability of brain grafts from SHR to induce hypertension is related to a higher proliferative and/or migratory potential of nonneuronal cells within the hypothalamus.