Arterioles Of Spontaneously Hypertensive Rats Research Articles

Exercise Training Attenuates Sympathetic Activity and Improves Morphometry of Splenic Arterioles in Spontaneously Hipertensive Rats.

BackgroundAlterations in the structure of resistance vessels contribute to elevated systemic vascular resistance in hypertension and are linked to sympathetic hyperactivity and related lesions in target organs.ObjectiveTo assess the effects of exercise training on hemodynamic and autonomic parameters, as well as splenic arteriolar damages in male Wistar Kyoto (WKY) and Spontaneously Hypertensive Rats (SHR).MethodsNormotensive sedentary (WKYS) and trained (WKYT) rats, and hypertensive sedentary (SHRS) and trained (SHRT) rats were included in this study. After 9 weeks of experimental protocol (swimming training or sedentary control), arterial pressure (AP) and heart rate (HR) were recorded in freely moving rats. We assessed the autonomic control of the heart by sympathetic and vagal autonomic blockade. Morphometric analyses of arterioles were performed in spleen tissues. The statistical significance level was set at p < 0.05.ResultsResting bradycardia was observed in both trained groups (WKYT: 328.0 ± 7.3 bpm; SHRT: 337.0 ± 5.2 bpm) compared with their respective sedentary groups (WKYS: 353.2 ± 8.5 bpm; SHRS: 412.1 ± 10.4 bpm; p < 0.001). Exercise training attenuated mean AP only in SHRT (125.9 ± 6.2 mmHg) vs. SHRS (182.5 ± 4.2 mmHg, p < 0.001). The WKYT showed a higher vagal effect (∆HR: 79.0 ± 2.3 bpm) compared with WKYS (∆HR: 67.4 ± 1.7 bpm; p < 0.05). Chronic exercise decreased sympathetic effects on SHRT (∆HR: -62.8 ± 2.8 bpm) in comparison with SHRS (∆HR: -99.8 ± 9.2 bpm; p = 0.005). The wall thickness of splenic arterioles in SHR was reduced by training (332.1 ± 16.0 µm2 in SHRT vs. 502.7 ± 36.3 µm2 in SHRS; p < 0.05).ConclusionsExercise training attenuates sympathetic activity and AP in SHR, which may be contributing to the morphological improvement of the splenic arterioles.

Differential Effects of Short-Term Treatment with Two AT1 Receptor Blockers on Diameter of Pial Arterioles in SHR

Chronic treatment with angiotensin receptor blockers is largely accepted for protecting cerebral circulation during hypertension, but beneficial effects of short-term treatments are questionable, as highlighted by the recent SCAST trial. We compared the impact of 10 days treatment with candesartan (as SCAST) versus telmisartan (previously described to reverse arteriolar remodeling, chronic treatment) on pial arterioles of spontaneously hypertensive rats (SHR). We explored whether PPAR-gamma agonist activity or AT1 receptor blockade are involved in their differential effects. In the first study, 4-month-old male SHR were treated with telmisartan (TELMI, 2 mg/kg per day) or candesartan cilexetil (CANDE, 10 mg/kg per day) and compared to vehicle treated SHR and normotensive WKY. In a second study, SHR were treated with CANDE, pioglitazone (a PPAR-gamma agonist, PIO 2.5 mg/kg per day) or CANDE+PIO, compared to TELMI. Internal diameter of pial arterioles (ID, cranial window) was measured at baseline, during hemorrhage-induced hypotension, or following suffusion of Ang II (10−6 mol/L) or EDTA inactivation of smooth muscle cells (passive ID). PPAR-gamma and eNOS (target gene of PPAR-gamma) mRNA were evaluated in brain microvessels. For similar antihypertensive effects, TELMI (+44% versus SHR), but not CANDE, increased baseline ID. During hemorrhage, ID in TELMI group was similar to WKY, while ID in SHR and CANDE remained lower. In the second study, TELMI (+36%, versus SHR) and CANDE+PIO (+43%) increased baseline ID, but not CANDE or PIO alone. TELMI (−66%) and CANDE+PIO (−69%), but neither CANDE nor PIO alone, decreased Ang II-induced vasoconstriction. CANDE+PIO, but not CANDE, increased passive ID. In both studies, PPAR-gamma and eNOS expressions were higher in TELMI than CANDE. Short-term treatment with TELMI, but not with CANDE, reverses narrowing of pial arteriolar ID in SHR. This may involve PPAR-gamma related mechanisms, since CANDE+PIO treatment induced similar effects, and a better blockade of AT1 receptors.

Connexin abundance in resistance vessels from the renal microcirculation in normo- and hypertensive rats

The expression of connexins in renal arterioles is believed to have a profound impact on conducted responses, regulation of arteriolar tonus and renal blood flow. We have previously shown that in renal preglomerular arterioles, conducted vasomotor responses are 40% greater in spontaneously hypertensive rats (SHR) than in normotensive Sprague–Dawley (SD) rats. Because conducted vasomotor responses depend on the cell–cell communication mediated through gap junctions, we hypothesized that the increased magnitude of conducted vasomotor response in SHR is associated with an increased amount of connexins in renal arterioles. To test this hypothesis, the amount of connexin 37 (Cx37), Cx40 and Cx43 was assessed in renal arterioles from normo- and hypertensive rats using quantitative immunofluorescence laser confocal miscroscopy. To account for differences in genetic background, we included both normotensive Wistar–Kyoto (WKY) and SD rats in the study. In all three strains of rats, and for all three isoforms, the expression of connexins was predominantly confined to the endothelial cells. We found a significantly increased abundance (240 ± 17.6%, p<0.05) of Cx37 in arterioles from WKY compared with SD and SHR. This high abundance of Cx37 was not related to blood pressure because normotensive SD demonstrated a level of Cx37 similar to that of SHR. Additionally, we found no evidence for an increased abundance of Cx40 and Cx43 in renal arterioles of SHR when compared with normotensive counterparts.

Connexin abundance in resistance vessels from the renal microcirculation in normo‐ and hypertensive rats

The expression of connexins in renal arterioles is believed to have a profound impact on conducted responses, regulation of arteriolar tonus and renal blood flow. We have previously shown that in renal preglomerular arterioles, conducted vasomotor responses are 40% greater in spontaneously hypertensive rats (SHR) than in normotensive Sprague-Dawley (SD) rats. Because conducted vasomotor responses depend on the cell-cell communication mediated through gap junctions, we hypothesized that the increased magnitude of conducted vasomotor response in SHR is associated with an increased amount of connexins in renal arterioles. To test this hypothesis, the amount of connexin 37 (Cx37), Cx40 and Cx43 was assessed in renal arterioles from normo- and hypertensive rats using quantitative immunofluorescence laser confocal microscopy. To account for differences in genetic background, we included both normotensive Wistar-Kyoto (WKY) and SD rats in the study. In all three strains of rats, and for all three isoforms, the expression of connexins was predominantly confined to the endothelial cells. We found a significantly increased abundance (240 +/- 17.6%, p<0.05) of Cx37 in arterioles from WKY compared with SD and SHR. This high abundance of Cx37 was not related to blood pressure because normotensive SD demonstrated a level of Cx37 similar to that of SHR. Additionally, we found no evidence for an increased abundance of Cx40 and Cx43 in renal arterioles of SHR when compared with normotensive counterparts.

Aldosterone causes vasoconstriction in coronary arterioles of rats via angiotensin II type-1 receptor: Influence of hypertension

Aldosterone is involved in many cardiovascular diseases with increased oxidative stress. Aldosterone-induced cardiac fibrosis is abolished by blockade of angiotensin II Type-1 (AT1) receptor. Recently, non-genomic vasoconstrictor effects of aldosterone were reported in various vascular beds. We tested the hypothesis that aldosterone stimulates angiotensin AT1 receptor, and causes vasoconstriction by increasing oxidative stress in coronary microcirculation. Coronary arterioles (60-120 microm) were isolated from spontaneously hypertensive rats (SHR) and control Wistar Kyoto (WKY) rats, aged 23-26 weeks. They were cannulated, and pressurized at 60 cm H2O. Effect of aldosterone (10(-15) to 10(-6) M) on coronary arteriolar diameter was examined. Aldosterone rapidly and dose-dependently decreased coronary arteriolar diameter in WKY rats and SHR (diameter changes, 8.4+/-0.7% vs 13.9+/-0.8%, P<0.05). Aldosterone-induced vasoconstriction was enhanced by 1.6-folds in SHR compared to WKY rats (P<0.05). Mineralocorticoid receptor antagonist spironolactone (10(-6) M) did not influence aldosterone-induced vasoconstriction. Selective angiotensin AT1 receptor blocker valsartan (10(-4) M) or candesartan (10(-7) M) abolished aldosterone-induced vasoconstriction. Similarly, superoxide dismutase (SOD, 300 U/ml), and NADPH oxidase inhibitor apocynin (10(-4) M) abolished it. Moreover, the vasoconstrictor effect of aldosterone disappeared in denuded vessels. Real-time quantitative RT-PCR revealed that angiotensin AT1 receptor mRNA level in coronary arterioles of SHR was upregulated by 1.5-folds compared to that in WKY rats (P<0.05). Aldosterone causes vasoconstriction in coronary arterioles, and this vasoconstrictor effect is enhanced by genetically defined hypertension. Aldosterone-induced vasoconstriction is mediated by angiotensin AT1 receptor presumably via oxidative stress.

Captopril improves cerebrovascular structure and function in old hypertensive rats

We examined the effects of an angiotensin-converting enzyme inhibitor (ACEI), captopril, on cerebral arterioles in young and old spontaneously hypertensive rats (SHR). Animals were anesthetized with sodium pentobarbitone (60 mg kg(-1) day(-1)). We measured cerebral blood flow (CBF, arbitrary units) and cerebral arteriolar internal diameter (ID, mum) prior to and during stepwise hypotension (SH) in 6- (WKY-6) and 15-month-old (WKY-15) Wistar Kyoto rats and in age-matched SHR that were untreated (SHR-6 and SHR-15) or treated for 3 months with captopril (SHR-6C, 105+/-2 mg kg(-1) day(-1) and SHR-15C, 94+/-1 mg kg(-1) day(-1)). ID and cross-sectional area of the vessel wall (CSA) were measured in deactivated (EDTA) cerebral arterioles during a second SH. Captopril decreased the lower limit of CBF autoregulation (61+/-6 in SHR-6C and 51+/-2 in SHR-15C vs 52+/-6 in WKY-6 and 62+/-7 in WKY-15 and 83+/-14 mmHg in SHR-6 and 120+/-19 mmHg in SHR-15; P<0.05) and CSA (510+/-21 in SHR-6C and 585+/-25 in SHR-15C vs 529+/-12 in WKY-6 and 549+/-20 in WKY-15 and 644+/-38 mmHg in SHR-6 and 704+/-38 mmHg in SHR-15; P<0.05). Captopril increased cerebral arteriolar external diameter of SHR (105+/-5 in SHR-6C and 94+/-4 in SHR-15C vs 125+/-8 in WKY-6 and 108+/-3 in WKY-15 and 83+/-2 mmHg in SHR-6 and 80+/-2 mmHg in SHR-15 for a pial arteriolar pressure step of 35-39 mmHg; P<0.05). Captopril attenuated increases in cerebral arteriolar distensibility in young SHR. Thus, ACEIs attenuate eutrophic and hypertrophic inward remodeling of cerebral arterioles in young and old SHR, thus decreasing the lower limit of CBF autoregulation.

Role of protein kinase C in Ca channel blocker-induced renal arteriolar dilation in spontaneously hypertensive rats

The present study examined the role of L-/T-type Ca channels and the interaction between these channels and protein kinase C (PKC) in hypertension. The isolated perfused hydronephrotic rat kidney model was used to visualize directly the renal microvascular effects of L-/T-type Ca channel blockers (nifedipine and mibefradil, respectively). Nifedipine reversed the angiotensin II-induced constriction of afferent, but not efferent, arterioles in kidneys from Wistar-Kyoto rats (WKY), and similar magnitude in dilation was observed in spontaneously hypertensive rats (SHR). Although mibefradil elicited dilation of both arterioles, the afferent arteriolar dilation was less in SHR than in WKY (57+/-5% vs. 80+/-4% reversal at 1 micrommol/L). The pretreatment with staurosporine did not alter the angiotensin II-induced afferent arteriolar constriction in WKY, but attenuated this response in SHR. Furthermore, staurosporine enhanced the nifedipine-induced afferent arteriolar dilation (62+/-3% vs. 50+/-3% reversal at 10 nmol/L), and restored the attenuated afferent arteriolar response to mibefradil in SHR. The pretreatment with thapsigargin (a blocker of IP3-mediated intracellular calcium release) prevented the angiotensin II-induced afferent arteriolar constriction in WKY, but caused a significant constriction of afferent arterioles in SHR and efferent arterioles in WKY and SHR; in this setting, mibefradil did not alter efferent arteriolar tone. In conclusion, although both L-type (nifedipine) and T-type Ca channel blockers (mibefradil) exerted potent vasodilation of rat renal microvessels, these actions were modified by PKC, which determined the afferent arteriolar sensitivity to these blockers in SHR. Furthermore, the enhancement in nifedipine-induced afferent arteriolar dilation by staurosporine in SHR suggests that L-type Ca channel activity is augmented in hypertensive animals.

Heme oxygenase-mediated endothelial dysfunction in DOCA-salt, but not in spontaneously hypertensive, rat arterioles.

Vascular heme oxygenase (HO) metabolizes heme to form carbon monoxide. Carbon monoxide inhibits nitric oxide synthase and promotes endothelium-dependent vasoconstriction. We reported HO-1-mediated endothelial dysfunction in Dahl salt-sensitive hypertension. Previous studies suggested that salt-sensitive hypertensive rats, but not spontaneously hypertensive rats (SHR), display endothelial dysfunction. This study examines the hypothesis that HO-1-mediated arteriolar endothelial dysfunction develops in deoxycorticosterone acetate (DOCA)-salt hypertensive (DOCA) rats, but not in SHR. Uninephrectomized (isoflurane anesthesia) male Sprague-Dawley rats received DOCA injections and saline drinking solution for 4 wk. Rats subjected to sham surgery received vehicle injections and tap water. Blood pressure was elevated in DOCA rats and SHR compared with sham and Wistar-Kyoto (WKY) groups. Aortic HO-1 expression and blood carboxyhemoglobin levels were elevated in the DOCA group, but not in SHR. In isolated gracilis muscle arterioles, ACh caused concentration-related vasodilation in all groups, with attenuated maximum responses in DOCA, but not in SHR, arterioles. Acute pretreatment with an inhibitor of HO, chromium mesoporphyrin, restored ACh-induced responses in DOCA arterioles to sham levels. ACh responses remained the same in SHR and WKY arterioles after chromium mesoporphyrin treatment. These data show that HO-1 levels and activity are increased and arteriolar responses to ACh are decreased in DOCA rats, but not in SHR. Furthermore, in DOCA arterioles, an inhibitor of HO restores ACh-induced vasodilation to sham levels. These results suggest that elevated HO-1 levels and activity, not resulting from hypertension per se, contribute to endothelial dysfunction in DOCA rats.

Altered regulation of endothelin A receptor subtype in the cerebral arterioles in response to a Japanese-style diet, in stroke-prone hypertensive rats.

To investigate the expression of endothelin (ET)-1 and its receptors in the cerebral arterioles of stroke-prone (spSHR) and control spontaneously hypertensive rats (SHRs) and the changes in endothelin receptor subtypes A and B density elicited by a stroke-permissive diet, before the development of stroke. Six-week-old SHRs (n=11) and spSHRs (n=11) were assigned to either a regular or a "Japanese"-style diet, in addition to 1% NaCl in the drinking water, for 4 weeks. Cryosections (10 microm thick) of rat brain were assessed for endothelin receptor distribution and density by autoradiography with [125I]ET-1 (10(-10) mol/l) in the presence of cold ET-1 (10(-6) mol/l) or the peptide antagonists BQ-123 (10(-6) mol/l) or BQ-788 (10(-6) mol/l). Reverse transcriptase polymerase chain reaction (RT-PCR) and immunohistochemistry were used to detect specific mRNAs and localize immunoreactive ET-1 and ET(A) and ET(B). In both strains, immunoreactive ET-1 was detected in the endothelium of cerebral arterioles, and RT-PCR and autoradiography demonstrated the coexistence of both receptor subtypes in brain homogenates and the cerebral arteriole walls, respectively. With the regular diet, the ET(A) receptor density was lower in SHRs than in spSHRs (P = 0.007), whereas the ET(B) receptor density was similar (P = NS). The Japanese-style diet increased the density of ET(A) receptors (P = 0.006) in SHRs, but decreased it (P = 0.019) in spSHRs. No effect was seen on ET(B) receptor density. ET(A) and ET(B) receptor subtypes are expressed in the wall of cerebral arterioles of SHRs and spSHRs. The latter strain showed a marked increase in ET(A) receptor density under a regular diet, in addition to an altered regulation in response to a stroke-permissive diet.

Norepinephrine-induced calcium signaling pathways in afferent arterioles of genetically hypertensive rats.

This study provides new information about the relative importance of calcium mobilization and entry in the renal vascular response to adrenoceptor activation in afferent arterioles isolated from 7- to 8-wk-old Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Intracellular free calcium concentration ([Ca(2+)](i)) was measured in microdissected arterioles utilizing ratiometric photometry of fura 2 fluorescence. There was no significant strain difference in baseline [Ca(2+)](i). Norepinephrine (NE; 10(-6) and 10(-7) M) elicited immediate, sustained increases in [Ca(2+)](i). The general temporal pattern of response to 10(-6) M NE consisted of an initial peak and a maintained plateau phase. The response to NE was partially blocked by nifedipine (10(-6) M) or 8-(N,N-diethylamino) octyl-3,4,5-trimetoxybenzoate (TMB-8; 10(-5) M). A calcium-free external solution abolished the sustained [Ca(2+)](i) plateau response to NE, with less influence on the peak response. In the absence of calcium entry, TMB-8 (10(-5) M) completely blocked the calcium response to NE in WKY but not SHR, suggesting strain differences in mobilization. A higher concentration of TMB-8 (10(-4) M), however, blocked all discernible mobilization in both strains. We conclude that there are differences in Ca(2+) handling in renal resistance vessels between young WKY and SHR with respect to mobilization stimulated by alpha-adrenoceptors. Afferent arterioles of young SHR appear to have a larger inositol-1,4,5-trisphosphate-sensitive pool or release from a site less accessible to TMB-8.

Life and Death Cell Labeling in the Microcirculation of the Spontaneously Hypertensive Rat

Spontaneously hypertensive rats (SHRs) have elevated numbers of apoptotic cells. However, the extent and pattern of cell death at the microvascular level is unexplored. We developed a technique to determine early forms of cell death in vivo in the mesentery by use of the life/death indicator ethidium bromide (EB). The mesenteric microvasculature was superfused with 5 µM EB for a period of 3 min, rinsed and immediately viewed by digital fluorescence microscopy. EB-positive cell structures were observed both in the wall of microvessels as well as in the tissue parenchyma. The microvessels had about 2–4 EB-positive cell structures per 100 µm of vessel length. Larger arterioles (>25 µm) in the SHR had an increased EB-positive structure density. After normalization of the blood pressure in the SHR with adrenalectomy, no significant differences remained between Wistar-Kyoto (WKY) rats and SHRs. After dexamethasone treatment, the adrenalectomized SHRs had a higher EB-positive cell density in the smaller class of microvessels than the WKY rats. In addition, EB-positive cell fragments (0.5–2 µm) were observed in the mesentery microvessel wall, and with TUNEL labeling, they were demonstrated to represent DNA fragments. The percentage of microvessels with EB-positive fragments was higher in the SHR arterioles and capillaries. Capillaries and larger venules (>30 µm) in the SHR had higher levels of cell fragments per vessel length. After adrenalectomy, no significant differences remained between WKY rats and SHRs in any of the mcirovessel categories. When adrenalectomized rats were treated with dexamethasone, a higher number of EB-positive fragments was detected in the wall of SHR capillaries. These results indicate that the mesentery microcirculation in both strains is subject to an early and nonuniform pattern of cell death, as detected by EB, but is enhanced in selected individual microvascular segments of the SHR by a glucocorticoid-driven mechanism.

BLOOD FLOW SHEAR RATES IN ARTERIOLES OF SPONTANEOUSLY HYPERTENSIVE RATS AT EARLY AND ESTABLISHED STAGES OF HYPERTENSION

Alterations of blood rheological properties can affect blood flow shear rates and therefore alter changes in the interactions between blood and vascular wall components during the development of hypertension. This study was done to evaluate alterations of blood flow shear rates in resistance vessels during the development of genetic hypertension in rats. In the current study, measurements were carried out on spontaneously hypertensive rats (SHR) during an early (3 weeks of age) and an established stage (12 weeks of age) of hypertension development. Age matched normotensive Wistar Kyoto (WKY) rats were used as controls. Intravital television microscopy was used to quantitate blood flow shear rates in first-(1A), second-(2A) and third-order (3A) arterioles of the cremaster muscle. In the young SHRs mean arterial blood pressure was not different from age matched WKY rats, but there was a significant increase of shear rate values in all observed (1A, 2A, 3A) arterioles of SHRs. However, shear rate values were significantly less in arterioles (1A, 2A, 3A) of SHRs with an established hypertension compared to the 3-week-old SHR group. We conclude that shear rates are elevated in resistance vessels prior to an increase in mean arterial pressure during the development of genetic hypertension. These results suggest that a change in blood rheology may cause a change in peripheral vascular resistance and thus contribute to the pathogenesis of hypertension.

Endothelin and prostaglandin H(2)/thromboxane A(2) enhance myogenic constriction in hypertension by increasing Ca(2+) sensitivity of arteriolar smooth muscle.

The myogenic response of skeletal muscle arterioles is enhanced in hypertension because of the release of endothelin (ET) and prostaglandin H(2) (PGH(2))/thromboxane A(2) (TxA(2)) from the endothelium. We hypothesized that ET and PGH(2)/TxA(2) modulate Ca(2+) signaling in arteriolar smooth muscle and thereby enhance myogenic constriction. Thus, simultaneous changes in intracellular Ca(2+) concentration in smooth muscle ([Ca(2+)](i)), measured by fura 2 microfluorometry (expressed as Ca(2+) fluorescence ratio [R(Ca)]), and diameter were obtained as a function of intraluminal pressure (P(i)) in isolated cannulated gracilis muscle arterioles (diameter approximately 120 micrometer) of normotensive Wistar rats (WR) and spontaneously hypertensive rats (SHR). In the absence of extracellular Ca(2+), increases in P(i) from 20 to 160 mm Hg increased the passive diameter of arterioles without changes in R(Ca). In the presence of extracellular Ca(2+) and endothelium, increases in P(i) elicited similar increases in R(Ca) (30+/-7% for control and 33+/-8% for SHR at 160 mm Hg) but a significantly (P<0.05) greater constriction of SHR arterioles compared with WR arterioles (at 160 mm Hg, 55+/-4% versus 38+/-2%, respectively, of passive diameter). In the absence of the endothelium, P(i)-induced changes in the R(Ca) and diameter of SHR and WR arterioles did not differ significantly. Also, a step increase in P(i) (from 80 to 140 mm Hg) elicited a similar increase in R(Ca) but greater constrictions in SHR versus WR arterioles. In the presence of the TxA(2) receptor inhibitor SQ29,548 and the ET(A) receptor inhibitor BQ123, there was no difference between responses of SHR and WR arterioles. In WR arterioles, increasing concentrations of KCl elicited a significant increase in R(Ca) (38+/-7% at 80 mmol/L) and completely constricted the arterioles. In contrast, constrictions to ET (52+/-7% at 3x10(-12) mol/L) and the TxA(2) agonist U46619 (40+/-8% at 3x10(-9) mol/L) were not accompanied by increases in R(Ca) at submaximal concentrations. Collectively, these findings suggest that in hypertension, endothelium-derived ET and PGH(2)/TxA(2) increase the Ca(2+) sensitivity of the contractile apparatus of arteriolar smooth muscle; thus, the similar increases in [Ca(2+)](i) in response to the elevation of intraluminal pressure elicit greater myogenic constriction.

Shear stress-induced release of prostaglandin H(2) in arterioles of hypertensive rats.

The nitric oxide-mediated portion of shear stress-induced dilation of rat gracilis muscle arterioles was shown to be impaired in spontaneously hypertensive rats (SHR). Because shear stress-induced dilation is primarily mediated by endothelium-derived prostaglandins in rat cremasteric arterioles, we hypothesized that in the cremasteric vascular bed the mediation of shear stress-induced dilation by prostaglandins is altered in hypertension. At a constant intraluminal pressure of 80 mm Hg, the active diameters of isolated rat cremasteric arterioles of normotensive 30-week-old Wistar-Kyoto rats (WKY) and SHR were 58.0+/-3.1 and 51.7+/-3.6 microm, respectively, whereas their passive diameters were 109.4+/-4.4 and 101.9+/-6.7 microm, respectively. Dilations to increases in shear stress elicited by increases in intraluminal flow (from 0 to 25 microL/min) were significantly less (P<0.05) in cremasteric arterioles isolated from SHR than from WKY. Arachidonic acid (10(-5) mol/L) elicited constrictions in SHR arterioles but dilations in WKY arterioles. The prostaglandin H(2)/thromboxane A(2) (PGH(2)/TxA(2)) receptor antagonist SQ 29,548 (10(-6) mol/L) significantly increased basal diameter by 11% and normalized the attenuated shear stress-induced dilation in SHR, whereas it did not affect basal diameter and arteriolar responses of WKY. Furegrelate, a specific inhibitor of TxA(2) synthase, did not affect the response in SHR. Also, SQ 29,548 reversed the arachidonic acid-induced constriction to dilation in SHR arterioles, whereas it did not affect the dilator response in WKY arterioles. Constrictions of arterioles of WKY and SHR to U46,619 (a PGH(2)/TxA(2) receptor agonist) were not different. These results demonstrate that in cremasteric arterioles of hypertensive rats, shear stress elicits an enhanced release of PGH(2), resulting in a reduced shear stress-dependent dilation. Thus, augmented hemodynamic forces can alter the shear stress-induced synthesis of prostaglandins, which may contribute to the elevated vascular resistance in hypertension.

Effect of calcium antagonists on glomerular arterioles in spontaneously hypertensive rats.

Through the use of microanatomic techniques, we investigated the effects of treatment with some dihydropyridine-type calcium antagonists (CAs) (ie, lercanidipine, manidipine, and nicardipine) and with the nondihydropyridine-type vasodilator hydralazine on hypertension-dependent glomerular injury and on the morphology of afferent and efferent arterioles in spontaneously hypertensive rats (SHR). Fourteen-week-old male SHR and age-matched normotensive Wistar-Kyoto rats were left untreated (control groups). Four additional groups of 14-week-old SHR were treated for 12 weeks with daily oral doses of 2.5 mg/kg lercanidipine, 5 mg/kg manidipine, 3 mg/kg nicardipine, or 10 mg/kg hydralazine. These treatments decreased systolic blood pressure values to a similar extent in SHR. Signs of glomerular injury, as characterized by glomerulosclerosis, hypertrophy, and an increased number of mesangial cells, were observed in control SHR. The treatment with CAs improved glomerular morphology and decreased the number of mesangial cells. Lercanidipine and manidipine were more effective than nicardipine in countering glomerular injury. In the SHR, both afferent and efferent arterioles revealed luminal narrowing, accompanied by increased wall thickness in efferent arterioles. The dihydropyridine-type derivatives that were tested decreased the luminal narrowing of afferent arterioles. Lercanidipine and manidipine countered the luminal narrowing of efferent arterioles. Hydralazine had no effect on hypertension-dependent glomerular injury or vascular changes. The present data indicate that lercanidipine and manidipine vasodilate afferent and efferent arterioles in SHR. A vasodilatory activity on efferent arteriole, which is not induced by the majority of CAs, may represent an useful property in the treatment of hypertension complicated by renal disease.

17beta-estradiol restores endothelial nitric oxide release to shear stress in arterioles of male hypertensive rats.

Endothelial nitric oxide (NO)-mediated responses are impaired in arterioles of male spontaneously hypertensive rats (SHR), but they are still present in female SHR. We hypothesized that in vitro incubation of arterioles of male SHR with estrogen will restore NO-mediated responses by upregulation of endothelial NO synthase. Responses to increases in perfusate flow (from 0 to 25 microL/min) and to the calcium ionophore A23187 (5 x 10(-8) to 10(-6) mol/L), norepinephrine (NE; 10(-7) to 3 x 10(-7) mol/L), sodium nitroprusside (SNP; 10(-8) to 10(-6) mol/L), and adenosine (ADO; 10(-6) to 5 x 10(-5) mol/L) were studied in cannulated and pressurized gracilis muscle arterioles ( approximately 75 microm in diameter) isolated from 12-week-old male SHR before and after incubation with 10(-9) mol/L 17beta-estradiol (17beta-E(2)) for 16 to 18 hours. After incubation with 17beta-E(2), basal diameter of arterioles was significantly increased (by approximately 10%), and flow-induced dilation was significantly enhanced (79.8+/-2.9 versus 103.7+/-3.7 microm at 25 microL/min), resulting in a lowered shear stress (62.0+/-9.1 versus 32.5+/-4.2 dyne/cm(2)). Also, vasoconstrictions to A23187 were reversed to dilations (-18.7+/-2.2 versus 18.8+/-1.7 microm), and constrictions to NE were significantly attenuated (-30.7+/-3.0 versus -21.2+/-2.8 microm). These alterations were eliminated by ICI 182,780 (10(-7) mol/L), an estrogen receptor antagonist; 5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole (10(-5) mol/L), a transcription inhibitor; or N(omega)-nitro-L-arginine methyl ester (10(-4) mol/L), an inhibitor of NO synthase, whereas they were not affected by aminoguanidine (5 x 10(-5) mol/L), a specific inhibitor of inducible NO synthase. Arteriolar responses were not altered by incubation with 17alpha-estradiol. Estrogen, via a receptor-mediated pathway, upregulates endothelial NO synthase gene expression, leading to increased NO production, and restores the regulation of wall shear stress in arterioles of male SHR.

Internephron coupling by conducted vasomotor responses in normotensive and spontaneously hypertensive rats.

We compared conducted vasomotor responses in juxtamedullary microcirculation in normotensive Sprague-Dawley (SD) and spontaneously hypertensive rats (SHR). The goals of the study were as follows: 1) decide whether internephron coupling is facilitated by conducted vasomotor responses; 2) determine whether the magnitude of induced vasoconstriction decreases with increasing distance from the stimulation site; and 3) determine whether the response is stronger in SHR than in SD rats. Microapplication of KCl to the distal afferent arteriole caused local vasoconstriction that was rapidly conducted (speed > 3.0 mm/s) into the cortical radial artery and neighboring afferent arterioles in SD and SHR. The strength of the response was significantly greater (approximately 40%, P < 0.025) in SHR than SD, and the magnitude decreased monotonically with increasing distance from the stimulation site in both strains. Mechanical length constants were similar in SD and SHR (approximately 325 microm), indicating that the signal responsible for the effect decays at the same rate in both strains. We conclude that internephron coupling strength is significantly greater in SHR and that internephron coupling is due to vascular events conducted along the preglomerular vasculature.

Effects of an angiotensin-converting enzyme inhibitor, a calcium antagonist, and an endothelin receptor antagonist on renal afferent arteriolar structure.

Narrowed afferent arteriolar diameter in young, spontaneously hypertensive rats (SHR) may be a contributor to later development of high blood pressure. Thus, treatment that causes dilation of the afferent arterioles in SHR may inhibit the redevelopment of high blood pressure when treatment is withdrawn. We treated SHR with an ACE inhibitor (cilazapril, 5 to 10 mg/kg per day, high; 1 mg/kg per day, low), a calcium antagonist (mibefradil, 20 to 30 mg/kg per day), and an endothelin receptor antagonist (bosentan, 100 mg/kg per day) from age 4 to 20 weeks. Untreated SHR and Wistar-Kyoto rats were also investigated. At 20 weeks, the rats were killed, and morphology of the afferent arterioles was studied. Other SHR (untreated, high cilazapril, low cilazapril, mibefradil) were treated in exactly the same way and then followed to 32 weeks without treatment. The morphometric studies showed that cilazapril increased the lumen diameter in the afferent arterioles and decreased the media-lumen ratio in a dose-dependent manner. On withdrawal of cilazapril treatment, the reduction in blood pressure persisted. Mibefradil tended to increase afferent arteriolar diameter, whereas it did not alter media-lumen ratio. The persistent effect on blood pressure was only moderate after withdrawal of mibefradil. Bosentan had no effect on renal afferent arteriolar structure or blood pressure. In conclusion, cilazapril was more effective than mibefradil in altering afferent arteriolar structure and caused the most persistent effect on blood pressure after treatment withdrawal. The association of increased afferent arteriolar diameter and lower blood pressure level after withdrawal of treatment may suggest a pathogenic role for afferent arteriolar diameter in the development of high blood pressure in SHR.

Both nitric oxide and prostaglandin-mediated responses are impaired in skeletal muscle arterioles of hypertensive rats.

To investigate the role played by endothelium-derived dilator factors in the regulation of peripheral vascular resistance by determining whether the dysfunction of the endothelium contributes to the reduced dilator responsiveness of skeletal muscle arterioles in hypertension. The endothelial function of isolated, cannulated, pressurized (at 80 mmHg) gracilis muscle arterioles (45-50 microns diameter) of normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) was compared by utilizing vasoactive agents of known action. Acetylcholine (ACh, 10(-9), 10(-8) and 5 x 10(-8) mol/l) and sodium nitroprusside (SNP, 10(-8), 10(-7) and 10(-6) mol/l) elicited similar dilations in arterioles of WKY rats and SHR. Substance P (10(-9), 10(-8) and 5 x 10(-8) mol/l) caused significantly less dilation (by approximately 70%) of SHR arterioles compared with WKY rat arterioles. The calcium ionophore A23187 (5 x 10(-8), 5 x 10(-7) and 10(-6) mol/l) elicited dilations in WKY rat arterioles (9.1 +/- 1.1, 24.0 +/- 1.5, and 39.0 +/- 3.4%, respectively), whereas it evoked constrictions (6.5 +/- 1.1, 14.9 +/- 1.5, and 25.5 +/- 1.6%, respectively) in SHR arterioles. Removal of endothelium, inhibition of prostaglandin synthesis (indomethacin) or blockade of prostaglandin H2 (PGH2) receptors (by SQ 29548) eliminated A23187-induced constrictions of SHR arterioles. The nitric oxide synthase blocker, NG-nitro-L-arginine elicited a significantly greater inhibition of substance P-induced dilations and a greater reduction in basal diameter of WKY rat arterioles than it did in those from SHR. These data suggest that, in SHR arterioles, the synthesis and/or action of nitric oxide is, or are, impaired and the metabolism of arachidonic acid is altered, resulting in an enhanced production of PGH2. The simultaneous dysfunction of these two dilator pathways of arteriolar endothelium could contribute significantly to the enhanced peripheral resistance observed in hypertension.

Effects of lisinopril on the structure of renal arterioles.

We investigated the effect of long-term administration of the angiotensin-converting enzyme inhibitor lisinopril on renal arterioles in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) using a morphometric method and vascular cast technique. Rats were treated with lisinopril beginning at 4 weeks of age. At 15 weeks of age, the kidney vessels were fixed when maximally relaxed. Resin was perfused into the right kidney to make a cast of the renal vasculature. The opposite kidney was used for the morphometric study to evaluate structural changes of the vascular wall. The vascular cast study demonstrated a significant reduction in the lumen diameter of the afferent but not the efferent arterioles in SHR compared with those in WKY. In lisinopril-treated rats, the afferent arteriolar lumen diameters were significantly larger than those of the respective control groups in both strains. However, treatment did not affect the lumen diameter of efferent arterioles in either strain. The morphometric study revealed that the cross-sectional area of afferent arteriolar media was significantly smaller in SHR than WKY, suggesting that the impaired growth of the afferent arteriolar media was involved in the narrowed afferent arteriolar lumen in SHR. The presence of significantly smaller media-lumen ratio, greater media cross-sectional area, and larger internal as well as external diameters of the afferent arterioles in treated SHR than in untreated rats suggested that lisinopril treatment normalizes the structure of the afferent arterioles in SHR by vascular reverse remodeling and by inducing media growth.