Blood-brain barrier disruption in the hypothalamus of young adult spontaneously hypertensive rats

Vascular permeability and endothelial glycocalyx were examined in young adult spontaneously hypertensive rats (SHR), stroke-prone SHR (SHRSP), and Wistar Kyoto rats (WKY) as a control, in order to determine earlier changes in the blood-brain barrier (BBB) in the hypothalamus in chronic hypertension. These rats were injected with horseradish peroxidase (HRP) as an indicator of vascular permeability. Brain slices were developed with a chromogen and further examined with cationized ferritin, a marker for evaluating glycocalyx. Staining for HRP was seen around vessels in the hypothalamus of SHR and SHRSP, but was scarce in WKY. The reaction product of HRP appeared in the abluminal pits of endothelial cells and within the basal lamina of arterioles, showing increased vascular permeability in the hypothalamus of SHR and SHRSP, whereas there were no leaky vessels in the frontal cortex of SHR and SHRSP, or in both areas of WKY. The number of cationized ferritin particles binding to the capillary endothelial cells was decreased in the hypothalamus of SHR and SHRSP, while the number decreased in the frontal cortex of SHRSP, compared with those in WKY. Cationized ferritin binding was preserved in some leaky arterioles, while it was scarce or disappeared in other leaky vessels. These findings suggest that BBB disruption occurs in the hypothalamus of 3-month-old SHR and SHRSP, and that endothelial glycocalyx is markedly damaged there without a close relationship to the early changes in the BBB.     

The influence of adrenal vein occlusion on whole-kidney hemodynamics in the spontaneously hypertensive rats.

It has been shown that occlusion of the adrenal vein causes an increase in renal vascular resistance in the ipsilateral kidney in Wistar Kyoto rats (WKY). The most probable mechanism of this phenomenon is the direct inflow of adrenal catecholamines to the kidney by the adrenal renal portal circulation (ARPC). As the number of vessels of the ARPC is bigger and the tonic sympathetic activity is higher in spontaneously hypertensive rats (SHR), the aim of the current study was to compare the effect of adrenal vein occlusion on renal vascular resistance between SHR and WKY. Mean arterial blood pressure and renal blood flow (RBF) were measured and renal vascular resistance (RVR) was calculated before and after closure of the adrenal vein. Occlusion of the adrenal vein significantly reduced RBF and increased RVR in both strains of rats. The rise of the RVR was significantly higher in SHR than in WKY. Therefore we assume that the hemodynamic responsiveness of the kidney due to increase in blood flow through ARPC is greater in SHR and may contribute to the development of arterial hypertension in this strain of rat.     

Vascular smooth muscle cells direct extracellular dysregulation in aortic stiffening of hypertensive rats

Aortic stiffening is an independent risk factor that underlies cardiovascular morbidity in the elderly. We have previously shown that intrinsic mechanical properties of vascular smooth muscle cells (VSMCs) play a key role in aortic stiffening in both aging and hypertension. Here, we test the hypothesis that VSMCs also contribute to aortic stiffening through their extracellular effects. Aortic stiffening was confirmed in spontaneously hypertensive rats (SHRs) vs. Wistar‐Kyoto (WKY) rats in vivo by echocardiography and ex vivo by isometric force measurements in isolated de‐endothelized aortic vessel segments. Vascular smooth muscle cells were isolated from thoracic aorta and embedded in a collagen I matrix in an in vitro 3D model to form reconstituted vessels. Reconstituted vessel segments made with SHR VSMCs were significantly stiffer than vessels made with WKY VSMCs. SHR VSMCs in the reconstituted vessels exhibited different morphologies and diminished adaptability to stretch compared to WKY VSMCs, implying dual effects on both static and dynamic stiffness. SHR VSMCs increased the synthesis of collagen and induced collagen fibril disorganization in reconstituted vessels. Mechanistically, compared to WKY VSMCs, SHR VSMCs exhibited an increase in the levels of active integrin β1‐ and bone morphogenetic protein 1 (BMP1)‐mediated proteolytic cleavage of lysyl oxidase (LOX). These VSMC‐induced alterations in the SHR were attenuated by an inhibitor of serum response factor (SRF)/myocardin. Therefore, SHR VSMCs exhibit extracellular dysregulation through modulating integrin β1 and BMP1/LOX via SRF/myocardin signaling in aortic stiffening.     

Prostacyclin synthase and phospholipases in the vascular wall of experimental hypertensive rats

To reveal the role of enzymes involved in PGI2 synthesis for vascular PGI2 generation in experimental hypertensive models, we defined PGI2 synthase and phospholipases activities in the aortic wall of two different experimental hypertensive rats, e.g. spontaneously hypertensive rats (SHR) and desoxycorticosterone acetate (DOCA)-salt hypertensive rats. In the stage of established hypertension both of the hypertensive models had a significantly large capacity of the vascular wall to produce PGI2, as compared to respective control rats. PGI2 synthase activities in the vascular wall were significantly increased by 27% for SHR and by 80% for DOCA-salt hypertensive rats. Moreover, the enzymatic activities were closely related to the blood pressure values for both of the models. On the other hand, phospholipase C or phospholipase A2 activities were increased or unchanged in SHR, respectively, whereas both of the phospholipases were significantly decreased in DOCA-salt hypertensive rats. Thus, it is indicated that PGI2 synthase is partly responsible for the increased PGI2 generation in the vascular wall of SHR and DOCA-salt hypertensive rats, and that vascular phospholipase C is playing a more important role in providing arachidonate for PGI2 synthesis in SHR.     

Vascular effects of 15-F2t-isoprostane in spontaneously hypertensive rats

F2-isoprostanes are a family of compounds derived from arachidonic acid by free radical-catalyzed peroxidation. Among F2-isoprostanes, 15-F2t-IsoP is a vasoconstrictor in animal and human vascular beds. Several recent studies found increased 15-F2t-IsoP levels in animal models of hypertension. However, no data is available on the vascular effect of 15-F2t-IsoP in such models. The contractile responses of 15-F2t-IsoP (10(-9) to 3 x 10(-5) mol/L) were tested on rat thoracic aortic rings in spontaneously hypertensive rats (SHR) compared with Wistar-Kyoto (WKY) rats. The contraction induced by 15-F2t-IsoP was not significantly different in aortic rings from WKY rats and SHR (Emax 139% +/- 5% vs. 134% +/- 6%, respectively) and was mediated through thromboxane A2-prostaglandin H2 receptor activation as shown by the rightward shift of the concentration-contraction curves in presence of GR 32191, a specific thromboxane A2-prostaglandin H2 receptor antagonist. Endothelial denudation increased the maximal contraction compared to intact rings induced by 15-F2t-IsoP in both WKY rats (170% +/- 20% vs. 139% +/- 5%, p < 0.05) and SHR (194% +/- 11% vs. 134% +/- 6%, p < 0.01), whereas pretreatment with Nomega-nitro-L-arginine (10(-4) mol/L) or with indomethacin (10(-5) mol/L) increased the maximal contraction to 15-F2t-IsoP in WKY rats but not in SHR. SHRs treated with an angiotensin-converting enzyme inhibitor, enalapril, for four weeks showed decreased maximal contraction to 15-F2t-IsoP in vessels with and without endothelium compared with untreated SHR. In conclusion, 15-F2t-IsoP-induced vasoconstriction is similar in SHR compared with WKY rats. Endothelium modulates 15-F2t-IsoP contraction in both strains. However, whereas this effect is mediated through nitric oxide- and cyclooxygenase-dependent pathways in WKY rats, other mediators are implicated in SHR.     

Heightened aberrant deposition of hard-wearing elastin in conduit arteries of prehypertensive SHR is associated with increased stiffness and inward remodeling

Elastin is a major component of conduit arteries and a key determinant of vascular viscoelastic properties. Aberrant organization of elastic lamellae has been reported in resistance vessels from spontaneously hypertensive rats (SHR) before the development of hypertension. Hence, we have characterized the content and organization of elastic lamellae in conduit vessels of neonatal SHR in detail, comparing the carotid arteries from 1-wk-old SHR with those from Wistar-Kyoto (WKY) and Sprague Dawley (SD) rats. The general structure and mechanics were studied by pressure myography, and the internal elastic lamina organization was determined by confocal microscopy. Cyanide bromide-insoluble elastin scaffolds were also prepared from 1-mo-old SHR and WKY aortas to assess their weight, amino acid composition, three-dimensional lamellar organization, and mechanical characteristics. Carotid arteries from 1-wk-old SHR exhibited narrower lumen and greater intrinsic stiffness than those from their WKY and SD counterparts. These aberrations were associated with heightened elastin content and with a striking reduction in the size of the fenestrae present in the elastic lamellae. The elastin scaffolds isolated from SHR aortas also exhibited increased relative weight and stiffness, as well as the presence of peculiar trabeculae inside the fenestra that reduced their size. We suggest that the excessive and aberrant elastin deposited in SHR vessels during perinatal development alters their mechanical properties. Such abnormalities are likely to compromise vessel expansion during a critical period of growth and, at later stages, they could compromise hemodynamic function and participate in the development of systemic hypertension.     

Effects of postnatal growth and denervation on characteristics of the saphenous artery in SHR and WKY rats between 3 and 6 weeks of age: an in vitro study

Thigh vessels of spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats were unilaterally surgically denervated at 10 days of age by femoral nerve section. Denervated and contralateral control segments of saphenous arteries from 3- and 6-week-old rats were mounted in a small vessel myograph for study. Both strains showed growth changes in blood pressure, but there was no significant difference between WKY and SHR. Both strains also had significant growth changes in vessel dimensions and the in vitro measurements suggested that SHR vessels had a thicker wall. Denervation did not affect vessel size. Transmural nerve stimulation indicated loss of innervation due to the surgical procedure. In the denervated vessels, both norepinephrine (NE) and 5-hydroxytryptamine (5-HT) dose response curves were shifted to the left, indicating a postjunctional increase in sensitivity. Maximum tension developed was in the order K+ greater than 5-HT greater than NE. In comparing the two strains, vessels from 6-week-old SHR were less sensitive to 5-HT. Relaxation to acetylcholine was significantly decreased in denervated arteries from WKY, whereas in SHR the significant decrease occurred only at 3 weeks. Denervated vessels from both rat strains at 3 weeks showed greater relaxation to beta-receptor activation, but not at 6 weeks of age. Therefore, the absence of functional innervation resulted in altered function of the saphenous artery wall.     

Vascular receptors for angiotensin, vasopressin, and atrial natriuretic peptide in experimental hypertension

Angiotensin II (ANG II) and vasopressin (AVP) are two powerful vasoconstrictors, and atrial natriuretic peptide (ANP) is a potent vasorelaxant. The changes in the density or affinity of binding sites for these agents that may alter target organ responsiveness in hypertension are reviewed. ANG II binding in mesenteric arteries was unaltered in one-kidney, one-clip (1-K, 1-C) and in 2-K, 1-C hypertensive rats, while in deoxycorticosterone acetate (DOCA)-salt hypertensive rats ANG II binding to blood vessels was significantly increased. A role of mineralocorticoids to increase the number of vascular ANG II sites in some hypertensive models is suggested. In spontaneously hypertensive rats (SHR) ANG II receptors were increased in young rats in the prehypertensive stage with respect to Wistar-Kyoto (WKY) control rats, but normal in older rats. AVP binding in the vasculature of hypertensive rats was uniformly decreased in inverse correlation to plasma AVP levels, but vascular responsiveness to AVP was exaggerated. Inositol trisphosphate production by blood vessels of SHR in response to AVP showed that increased AVP receptor-coupled phospholipase C activity may mediate in part the exaggerated pressor response in spite of reduced or normal density of receptors for vasoconstrictor peptides. Vascular ANP sites in 2-K, 1-C, 1-K,1-C, and DOCA-salt hypertensive rats varied inversely with plasma concentrations of ANP. Normal densities of ANP receptors in saralasin-sensitive 2-K, 1-C hypertensive rats correlated with ANP sensitivity, while saralasin-insensitive 2-K, 1-C hypertensive rats, which did not respond to ANP, had significantly decreased density of ANP vascular receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prostacyclin synthase and phospholipases in the vascular wall of experimental hypertensive rats

To reveal the role of enzymes involved in PGI₂ synthesis for vascular PGI₂ generation in experimental hypertensive models, we defined PGI₂ synthase and phospholipases activities in the aortic wall of two different experimental hypertensive rats, e.g. spontaneously hypertensive rats (SHR) and desoxycorticosterone acetate (DOCA)-salt hypertensive rats. In the stage of established hypertension both of the hypertensive models had a significantly large capacity of the vascular wall to produce PGI₂, as compared to respective control rats. PGI₂ synthase activities in the vascular wall were significantly increased by 27% for SHR and by 80% for DOCA-salt hypertensive rats. Moreover, the enzymatic activities were closely related to the blood pressure values for both of the models. On the other hand, phospholipase C or phospholipase A₂ activities were increased or unchanged in SHR, respectively, whereas both of the phospholipases were significantly decreased in DOCA-salt hypertensive rats. Thus, it is indicated that PGI₂ synthase is partly responsible for the increased PGI₂ generation in the vasclar wall of SHR and DOCA-salt hypertensive rats, and that vascular phospholipase C is playing a more important role in providing arachidonate for PGI₂ synthesis in SHR.     

Antihypertensive treatment differentially affects vascular sphingolipid biology in spontaneously hypertensive rats

Background

We have previously shown that essential hypertension in humans and spontaneously hypertensive rats (SHR), is associated with increased levels of ceramide and marked alterations in sphingolipid biology. Pharmacological elevation of ceramide in isolated carotid arteries of SHR leads to vasoconstriction via a calcium-independent phospholipase A₂, cyclooxygenase-1 and thromboxane synthase-dependent release of thromboxane A₂. This phenomenon is almost absent in vessels from normotensive Wistar Kyoto (WKY) rats. Here we investigated whether lowering of blood pressure can reverse elevated ceramide levels and reduce ceramide-mediated contractions in SHR.

Methods and Findings

For this purpose SHR were treated for 4 weeks with the angiotensin II type 1 receptor antagonist losartan or the vasodilator hydralazine. Both drugs decreased blood pressure equally (SBP untreated SHR: 191±7 mmHg, losartan: 125±5 mmHg and hydralazine: 113±14 mmHg). The blood pressure lowering was associated with a 20–25% reduction in vascular ceramide levels and improved endothelial function of isolated carotid arteries in both groups. Interestingly, losartan, but not hydralazine treatment, markedly reduced sphingomyelinase-induced contractions. While both drugs lowered cyclooxygenase-1 expression, only losartan and not hydralazine, reduced the endothelial expression of calcium-independent phospholipase A₂. The latter finding may explain the effect of losartan treatment on sphingomyelinase-induced vascular contraction.

Conclusion

In summary, this study corroborates the importance of sphingolipid biology in blood pressure control and specifically shows that blood pressure lowering reduces vascular ceramide levels in SHR and that losartan treatment, but not blood pressure lowering per se, reduces ceramide-mediated arterial contractions.     

Increased vasodilator responsiveness to BRL 34915 in spontaneously hypertensive versus normotensive rats: contrast with nifedipine

The blood pressure-lowering potency and activity of BRL 34915, a new vasodilator and putative stimulator of potassium efflux from vascular smooth muscle, was investigated in conscious spontaneously hypertensive rats (SHR) and normotensive rats (NTR) after intravenous administration and compared with that of the calcium channel blocker, nifedipine. In SHR, BRL 34915 (3-100 micrograms/kg) or nifedipine (10-3000 micrograms/kg) produced similar reductions in mean arterial pressure of 58 +/- 3% and 55 +/- 3%, respectively. BRL 34915 (ED30% = 13.8 micrograms/kg) was 15.3 times more potent than nifedipine (ED30% = 207 micrograms/kg) in SHR. In contrast, only a 1.7-fold difference in potency was observed in NTR between BRL 34915 (ED30% = 123 micrograms/kg) and nifedipine (ED30% = 182 micrograms/kg). The potency ratio (ED30% NTR/ED30% SHR) for BRL 34915 was 8.83 whereas nifedipine had a ratio of 0.88, reflecting the greater responsiveness of the SHR to BRL 34915. Systemic hemodynamics were monitored in anesthetized SHR and NTR to determine the basis for the reductions in blood pressure. BRL 34915 (3-100 micrograms/kg iv) lowered mean arterial pressure in both groups solely by decreasing total peripheral vascular resistance, since no changes in cardiac output were observed. Relaxation responses were also obtained in phenylephrine-contracted isolated aortic strips from both strains of rat to ascertain whether differences in responsiveness existed at this level of the vasculature. No significant difference in the potency of BRL 34915 (3-10 microM) as a vasodilator was found in aortas from SHR or NTR. These results indicate that, unlike nifedipine, BRL 34915 is a more potent vasodepressor agent in SHR than in NTR and suggests that the potassium efflux stimulator may preferentially relax resistance vessels in the hypertensive rat.     

Effects of high-sucrose feeding on insulin resistance and hemodynamic responses to insulin in spontaneously hypertensive rats

This study was designed to investigate the effects of a sucrose diet on vascular and metabolic actions of insulin in spontaneously hypertensive rats (SHR). Male SHR were randomized to receive a sucrose or regular chow diet for 4 wk. Age-matched, chow-fed Wistar-Kyoto (WKY) rats were used as normotensive control. In a first series of experiments, the three groups of rats had pulsed Doppler flow probes and intravascular catheters implanted to determine blood pressure, heart rate, and blood flows. Insulin sensitivity was assessed during a euglycemic hyperinsulinemic clamp performed in conscious rats. In a second series of experiments, new groups of rats were used to examine glucose transport activity in isolated muscles and to determine endothelial nitric oxide synthase (eNOS) protein expression in muscles and endothelin content in vascular tissues. Sucrose feeding was shown to markedly enhance the pressor response to insulin and its hindquarter vasoconstrictor effect when compared with chow-fed SHR. A reduction in eNOS protein content in muscle, but no change in vascular endothelin-1 protein, was noted in sucrose-fed SHR when compared with WKY rats, but these changes were not different from those noted in chow-fed SHR. Similar reductions in insulin-stimulated glucose transport were observed in soleus muscles from both groups of SHR when compared with WKY rats. In extensor digitorum longus muscles, a significant reduction in insulin-stimulated glucose transport was only seen in sucrose-fed rats when compared with the other two groups. Environmental factors, that is, high intake of simple sugars, could possibly potentiate the genetic predisposition in SHR to endothelial dysfunction and insulin resistance.     

Plasma concentrations of endothelin-1 in spontaneously hypertensive rats and DOCA-salt hypertensive rats

To investigate the possible involvement of endothelin-1 (ET-1), an endothelium-derived potent vasoconstrictor peptide, in the pathophysiology of hypertension, plasma ET-1 levels in 15-week-old spontaneously hypertensive rats (SHR) and DOCA-salt hypertensive rats were measured with a sandwich-type enzyme immunoassay. The vasocontractile effect of ET-1 in aortic helical preparations was significantly more sensitive in DOCA-salt hypertensive rats than in control sham-operated rats, but plasma levels of ET-1 did not differ between them. Plasma ET-1 levels in genetically hypertensive rats (SHR and stroke-prone SHR) were significantly lower than those in age-matched normotensive Wistar-Kyoto (WKY) rats. The plasma concentrations of big ET-1, a precursor of ET-1, in both SHR and SHR-SP were significantly lower than those of WKY, suggesting that the production of ET-1 is decreased in rats of genetic hypertension. Although the vascular reactivity to ET-1 increased in both DOCA-salt hypertensive and genetically hypertensive rats, present findings of the plasma ET-1 levels suggest that the role of ET-1 in the vascular control system may be different in DOCA-salt hypertensive rats and genetically hypertensive rats.     

Hypertension is associated with marked alterations in sphingolipid biology: a potential role for ceramide

Background

Hypertension is, amongst others, characterized by endothelial dysfunction and vascular remodeling. As sphingolipids have been implicated in both the regulation of vascular contractility and growth, we investigated whether sphingolipid biology is altered in hypertension and whether this is reflected in altered vascular function.

Methods and Findings

In isolated carotid arteries from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats, shifting the ceramide/S1P ratio towards ceramide dominance by administration of a sphingosine kinase inhibitor (dimethylsphingosine) or exogenous application of sphingomyelinase, induced marked endothelium-dependent contractions in SHR vessels (DMS: 1.4±0.4 and SMase: 2.1±0.1 mN/mm; n = 10), that were virtually absent in WKY vessels (DMS: 0.0±0.0 and SMase: 0.6±0.1 mN/mm; n = 9, p<0.05). Imaging mass spectrometry and immunohistochemistry indicated that these contractions were most likely mediated by ceramide and dependent on iPLA₂, cyclooxygenase-1 and thromboxane synthase. Expression levels of these enzymes were higher in SHR vessels. In concurrence, infusion of dimethylsphingosine caused a marked rise in blood pressure in anesthetized SHR (42±4%; n = 7), but not in WKY (−12±10%; n = 6). Lipidomics analysis by mass spectrometry, revealed elevated levels of ceramide in arterial tissue of SHR compared to WKY (691±42 vs. 419±27 pmol, n = 3–5 respectively, p<0.05). These pronounced alterations in SHR sphingolipid biology are also reflected in increased plasma ceramide levels (513±19 pmol WKY vs. 645±25 pmol SHR, n = 6–12, p<0.05). Interestingly, we observed similar increases in ceramide levels (correlating with hypertension grade) in plasma from humans with essential hypertension (185±8 pmol vs. 252±23 pmol; n = 18 normotensive vs. n = 19 hypertensive patients, p<0.05).

Conclusions

Hypertension is associated with marked alterations in vascular sphingolipid biology such as elevated ceramide levels and signaling, that contribute to increased vascular tone.     

Contribution of nitric oxide in the mechanisms of flow-dependent vasodilation in normo- and hypertensive rats

The aim of this study was to evaluate the role of nitric oxide (NO) in the mechanisms of arterial distensibility and intravascular pressure stability in normotensive and spontaneously hypertensive rats. The experiments were performed on the anesthetized male Wistar, Wistar-Kyoto (WKY), and spontaneously hypertensive rats (SHR). The abdominal aorta was cannulated and perfused with variable blood flow rates with subsequent determination of major characteristics of regional vascular function. The blockade of nitric oxide (NO) synthase resulted in the increase in hydraulic resistance of the hindlimb vascular bed in all series of the experiments. It was associated with the decrease in the intravascular pressure stability. The obtained results provide further evidence for an important role of NO in the formation of conductivity and stability of the arterial pressure both in normo- and hypertensive rats. However, the involvement of NO in the phenomenon of flow-dependent vasodilation in SHR is unlikely. The major difference between SHR and normotensive rats involved the ability of the resistive arteries of SHR to enhance vascular conductivity in response to blood flow enhancement. Presumably, there are some unidentified additional factors that are involved in the flow-dependent vasodilation in SHR.     

Insulin resistance in spontaneously hypertensive rats is associated with endothelial dysfunction characterized by imbalance between NO and ET-1 production

Insulin stimulates production of NO in vascular endothelium via activation of phosphatidylinositol (PI) 3-kinase, Akt, and endothelial NO synthase. We hypothesized that insulin resistance may cause imbalance between endothelial vasodilators and vasoconstrictors (e.g., NO and ET-1), leading to hypertension. Twelve-week-old male spontaneously hypertensive rats (SHR) were hypertensive and insulin resistant compared with control Wistar-Kyoto (WKY) rats (systolic blood pressure 202 +/- 11 vs. 132 +/- 10 mmHg; fasting plasma insulin 5 +/- 1 vs. 0.9 +/- 0.1 ng/ml; P < 0.001). In WKY rats, insulin stimulated dose-dependent relaxation of mesenteric arteries precontracted with norepinephrine (NE) ex vivo. This depended on intact endothelium and was blocked by genistein, wortmannin, or N(omega)-nitro-l-arginine methyl ester (inhibitors of tyrosine kinase, PI3-kinase, and NO synthases, respectively). Vasodilation in response to insulin (but not ACh) was impaired by 20% in SHR (vs. WKY, P < 0.005). Preincubation of arteries with insulin significantly reduced the contractile effect of NE by 20% in WKY but not SHR rats. In SHR, the effect of insulin to reduce NE-mediated vasoconstriction became evident when insulin pretreatment was accompanied by ET-1 receptor blockade (BQ-123, BQ-788). Similar results were observed during treatment with the MEK inhibitor PD-98059. In addition, insulin-stimulated secretion of ET-1 from primary endothelial cells was significantly reduced by pretreatment of cells with PD-98059 (but not wortmannin). We conclude that insulin resistance in SHR is accompanied by endothelial dysfunction in mesenteric vessels with impaired PI3-kinase-dependent NO production and enhanced MAPK-dependent ET-1 secretion. These results may reflect pathophysiology in other vascular beds that directly contribute to elevated peripheral vascular resistance and hypertension.     

Vascular receptors for atrial natriuretic peptide in hypertension

Atrial natriuretic peptide (ANP) is secreted by the heart in response mainly to atrial distension and circulates in plasma in picomolar concentrations. It binds to receptors in blood vessels which it relaxes, renal glomeruli where it induces increased glomerular filtration rate, renal papilla to produce natriuresis, adrenal glomerulosa celts to inhibit aldosterone secretion, and median eminence and pituitary where it may inhibit vasopressin secretion. In experimental models of hypertension plasma levels of ANP are uniformly elevated, except in spontaneously hypertensive rats, in which plasma ANP may only rise transiently. The action of ANP on smooth muscle cells of the blood vessel wall results in production of cyclic GMP, which appears to be the second messenger producing relaxation of pre-contracted blood vessels. Mechanisms other than cGMP generation have been proposed but remain unproven as mediators of ANP action. Receptors for ANP in blood vessels are of two subtypes: B-receptors (or R₁-receptors), which contain guanylate cyclase in their structure, and C-receptors (or R₂-receptors), which have not been shown to the present to be biologically active. Our studies on vascular ANP receptors are reviewed. In several experimental models of hypertension such as saralasin-insensitive 2-kidney, 1-clip and 1-kidney, 1-clip Goldblatt hypertensive rats and in DOCA-salt hypertensive rats, we have found elevated plasma ANP, as well as decreased binding and ANP-induced vascular relaxation and blood pressure-lowering effects of ANP. Both the B and C ANP receptors appear decreased in density, even after acid washing of membranes to remove any retained circulating ANP. In SHR we have found that plasma ANP was higher than in control WKY rats only transiently at 8 weeks. Binding was significantly lower in 4 and 8 week-old SHR, but cGMP generation and relaxation produced by ANP were increased in the 4 week-old SHR but normal at 8, 12 or 16 weeks. Expression of B-receptors was exaggerated in 4 week-old SHR relative to C receptors in comparison to age-matched WKY and Wistar rats. These results may underly the normalization of blood pressure found in SHR when a small dose of ANP is infused intravenously, in contrast to other models of experimental hypertension which appear to be more resistant to ANP-induced blood pressure lowering effects. In humans with essential hypertension, plasma ANP was increased in patients with moderate to severe uncontrolled high blood pressure, associated with echocardiographic evidence of left ventricular hypertrophy. In these patients, platelet ANP binding was significantly reduced. If these sites resemble vascular ANP sites in their behavior, severely hypertensive patients may be less sensitive to ANP, which may contribute to blood pressure elevation.     

Role of Musclin in the Pathogenesis of Hypertension in Rat

Musclin is a novel skeletal muscle-derived secretory factor found in the signal sequence trap of mouse skeletal muscle cDNAs. Musclin possesses a region homologous to the natriuretic peptide family. Thus, musclin is found to bind with the natriuretic peptide clearance receptors. However, the role of musclin in vascular regulation remains unclear. In this study, we aim to investigate the direct effect of musclin on vascular tone and to analyze its role in hypertension using the spontaneously hypertensive rats (SHR). In aortic strips isolated from SHR, musclin induced contractions in a dose-dependent manner. We found that the musclin-induced vasoconstriction was more marked in SHR than in normal rats (WKY). Moreover, this contraction was reduced by blockade of natriuretic peptide receptor C using the ab14355 antibody. Therefore, mediation of the natriuretic peptide receptor in musclin-induced vasoconstriction can be considered. In addition, similar to the natriuretic peptide receptor, expression of the musclin gene in blood vessels was higher in SHR than in WKY. Injection of musclin markedly increased the blood pressure in rats that can be inhibited by anti-musclin antibodies. Musclin-induced vasoconstriction was more pronounced in SHR than in WKY as in its expression. Taken together, these results suggest that musclin is involved in blood pressure regulation. The higher expression of musclin in hypertension indicates that musclin could be used as a new target for the treatment of hypertension in the future.