Left ventricular geometry, tissue composition, and residual stress in High Fat Diet Dahl-Salt sensitive rats.

BACKGROUND: Hypertension drives myocardial remodeling, leading to changes in structure, composition and mechanical behavior, including residual stress, which are linked to heart disease progression in a gender-specific manner. Emerging therapies are also targeting constituent-specific pathological features. All previous studies, however, have characterized remodeling in the intact tissue, rather than isolated tissue constituents, and did not include sex as a biological variable. OBJECTIVE: In this study we first identified the contribution of collagen fiber network and myocytes to the myocardial residual stress/strain in Dahl-Salt sensitive rats fed with high fat diet. Then, we quantified the effect of hypertension on the remodeling of the left ventricle (LV), as well as the existence of sex-specific remodeling features. METHODS: We performed mechanical tests (opening angle, ring-test) and histological analysis on isolated constituents and intact tissue of the LV. Based on the measurements from the tests, we performed a stress analysis to evaluate the residual stress distribution. Statistical analysis was performed to identify the effects of constituent isolation, elevated blood pressure, and sex of the animal on the output of both experimental measures and modeling results. RESULTS: Hypertension leads to reduced residual stress/strain intact tissue, isolated collagen fibers, and isolated myocytes in male and female rats. Collagen remains the largest contributor to myocardial residual stress in both normotensive and hypertensive animals. We identified sex-differences in both hypertensive and normotensive animals. CONCLUSIONS: We observed both constituent- and sex-specific remodeling features in the LV of an animal model of hypertension.

Perivascular adipose tissue contains functional catecholamines.

The sympathetic nervous system and its neurotransmitter effectors are undeniably important to blood pressure control. We made the novel discovery that perivascular adipose tissue (PVAT) contains significant concentrations of catecholamines. We hypothesized that PVAT contains sufficient releasable catecholamines to affect vascular function. HPLC, isometric contractility, immunohistochemistry, whole animal approaches and pharmacology were used to test this hypothesis. In normal rat thoracic aorta and superior mesenteric artery, the indirect sympathomimetic tyramine caused a concentration-dependent contraction that was dependent on the presence of PVAT. Tyramine stimulated release of NA, dopamine (DA) and the tryptamine serotonin (5-HT) from PVAT isolated from both arteries. In both arteries, tyramine-induced concentration-dependent contraction was rightward-shifted and reduced by the noradrenaline transporter inhibitor nisoxetine (1 µM), the vesicular monoamine transporter tetrabenazine (10 µM) and abolished by the α adrenoreceptor antagonist prazosin (100 nM). Inhibitors of the DA and 5-HT transporter did not alter tyramine-induced, PVAT-dependent contraction. Removal of the celiac ganglion as a neuronal source of catecholamines for superior mesenteric artery PVAT did not significantly reduce the maximum or shift the concentration dependent contraction to tyramine. Electrical field stimulation of the isolated aorta was not affected by the presence of PVAT. These data suggest that PVAT components that are independent of sympathetic nerves can release NA in a tyramine-sensitive manner to result in arterial contraction. Because PVAT is intimately apposed to the artery, this raises the possibility of local control of arterial function by PVAT catecholamines.

Chronic exposure to a high-fat diet affects stress axis function differentially in diet-induced obese and diet-resistant rats.

OBJECTIVE: Consumption of a high-fat (HF) diet is a contributing factor for the development of obesity. HF diet per se acts as a stressor, stimulating hypothalamo-pituitary-adrenal (HPA) axis activity resulting in elevated glucocorticoid levels; however, the mechanism behind this activation is unclear. We hypothesized that consumption of an HF diet activates HPA axis by increasing norepinephrine (NE) in the paraventricular nucleus (PVN) of the hypothalamus, leading to elevation in corticotrophin-releasing hormone (CRH) concentration in the median eminence (ME) resulting in elevated serum corticosterone (CORT). SUBJECTS: To test this hypothesis, diet-induced obese (DIO) and diet-resistant (DR) rats were exposed to either chow or HF diet for 6 weeks. MEASUREMENTS: At the end of 6 weeks, NE in the PVN was measured using HPLC, CRH in the ME, and CORT and leptin levels in the serum were measured using RIA and ELISA, respectively. The gene expression of tyrosine hydroxylase (TH), the rate-limiting enzyme in NE synthesis, and leptin receptor in brainstem noradrenergic nuclei were also measured. RESULTS: HF diet increased PVN NE in both DIO and DR rats (P<0.05). However, this was accompanied by increases in CRH and CORT secretion only in DR animals, but not in DIO rats. Leptin receptor mRNA levels in the brainstem noradrenergic areas were not affected in both DIO and DR rats. However, HF diet increased TH mRNA levels only in DIO rats. CONCLUSION: Significant differences occur in all the arms of HPA axis function between DIO and DR rats. Further studies are needed to determine whether this could be a causative factor or a consequence to obesity.

Could hypertension possibly be adaptive?

1. We believe that the ultimate goal of cardiovascular regulatory mechanisms is not the regulation of arterial blood pressure (BP), but the maintenance of tissue blood flows commensurate with metabolic requirements. Thus, elevated BP can potentially contribute to optimizing tissue blood flows under select circumstances; for example, when there are primary defects in autoregulation of tissue blood flows. 2. The hypothesis that a primary defect in autoregulation of tissue blood flows may be responsible for the development of hypertension is presented. It is argued that, in this context, at least part of the rise in BP may be reflexly driven by a 'metaboreflex', a homeostatic mechanism acting to regulate tissue blood flows. 3. We argue that in the context of primary defects in autoregulation of tissue blood flows, the ability to generate and sustain a hypertensive phenotype increases the lifespan of species (i.e. if it were not for this adaptive hypertensive phenotype, death due to circulatory failure would occur much earlier). 4. Experimental and clinical evidence that indirectly supports the hypothesis is reviewed briefly and a means for testing this hypothesis is suggested.

Endothelin-1-induced elevation in blood pressure is independent of increases in sympathetic nerve activity in normotensive rats.

This study was performed to determine if endothelin-1 (ET-1)-induced pressor responses in urethane-anesthetized, normotensive rats are due to increased sympathetic nerve activity. Renal sympathetic nerve activity (RSNA) was used as an index of sympathetic nerve activity. ET-1 (30- 1000 pmol/kg) or sarafotoxin (S6c, ET B receptor agonist, 10-3,000 nmol/kg) given by bolus injection produced transient decreases in mean arterial pressure (MAP) and increases in RSNA and heart rate (HR). ET-1 caused a delayed but sustained increase in MAP that was not inhibited by acute sinoaortic denervation or alpha 1 -adrenergic receptor blockade. ET-1 never caused a sustained change in HR or RSNA. A-192621 (ET B receptor antagonist, 12 mg/kg) increased MAP (10-20 mm Hg) and decreased HR and RSNA. A-192621 blocked the transient decrease in MAP and increase in RSNA and HR caused by ET-1 and S6c. In A-192621-treated rats, ET-1, but not S6c, caused a sustained increase in MAP and decrease in HR and RSNA. After A-192621 treatment, ET-1 infusion caused a sustained elevation in MAP; HR and RSNA decreased only after the highest ET-1 dose. These results indicate that the initial increase in RSNA after ET-1 or S6c is secondary to ET B receptor-mediated vasodilation. Increased RSNA does not contribute to ET-1-induced pressor responses; these responses are likely due to vasoconstriction in normotensive, anesthetized rats. Finally, baroreceptor reflexes function after ET-1 or S6c treatment.

Role of endothelin ETB receptor activation in angiotensin II-induced hypertension: effects of salt intake.

We showed recently that endothelin (ET)A receptors are involved in the salt sensitivity of ANG II-induced hypertension. The objective of this current study was to characterize the role of endothelin ETB receptor activation in the same model. Male rats on fixed normal (2 meq/day) or high (6 meq/day) salt intake received a continuous intravenous infusion of ANG II or salt only for 15 days. During the middle 5 days of the infusion period, rats were given either the selective ETB receptor antagonist A-192621 or the nonselective endothelin receptor antagonist A-182086 (both at 24 mg x kg(-1) x day(-1) intra-arterially). Infusion of ANG II caused a greater rise in arterial pressure in rats on high-salt intake. The administration of A-192621 increased arterial pressure further in all rats. The chronic hypertensive effect of A-192621 was not significantly affected by salt intake or ANG II. The administration of A-182086 lowered arterial pressure chronically only in rats on normal salt intake receiving ANG II. Thus the salt sensitivity of ANG II-induced hypertension is not caused by changes in ETB receptor function.

Characterization of the antihypertensive effect of a thiazide diuretic in angiotensin II-induced hypertension.

OBJECTIVES: The antihypertensive effect of thiazide diuretics in angiotensin II induced hypertension has never been characterized. In the current study, we sought to determine the effect of a thiazide diuretic on arterial pressure and renal fluid excretion in rats receiving a chronic intravenous infusion of angiotensin II while on fixed normal or high sodium intakes. DESIGN AND METHODS: Male rats were chronically instrumented with arterial and venous catheters for drug injection and direct daily measurements of blood pressure and heart rate. Rats were maintained on high salt intake (HS), 6 mEq/day, or on normal salt intake (NS), 2 mEq/day. Rats were randomly assigned to four groups: HS and NS with 15 day angiotensin II infusion (5 ng/min) and HS and NS without angiotensin II infusion. Trichlormethiazide (TCM), a thiazide diuretic, was orally administered, approximately 10 mg/kg per day, for the middle 5 days of angiotensin II infusion. RESULTS: Only HS rats receiving angiotensin II infusion became hypertensive. Angiotensin II infusion did not produce changes in heart rate, sodium balance or water balance. Chronic administration of TCM significantly reduced mean arterial pressure (MAP) within 24 h in HS rats receiving angiotensin II, but did not affect MAP in any other group. TCM produced a similar loss of Na+ and water in all rats. Blood volumes and plasma electrolytes did not change during the study. CONCLUSIONS: The antihypertensive effects of thiazide diuretics are not due exclusively to volume depletion. We propose that salt and water loss caused by TCM may lower MAP by impairment of salt-sensitive pressor mechanisms activated by angiotensin II.

Nitric oxide-independent effects of tempol on sympathetic nerve activity and blood pressure in normotensive rats.

The role of the sympathetic nervous system in 4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl (tempol)-induced cardiovascular responses in urethane-anesthetized, normotensive rats was evaluated. Tempol caused dose-dependent (30-300 micromol/kg iv) decreases in renal sympathetic nerve activity (RSNA), mean arterial blood pressure (MAP), and heart rate (HR). Similar responses were obtained after sinoaortic denervation and cervical vagotomy. These responses were not blocked following treatment with the nitric oxide synthase inhibitor NG-nitro-L-arginine (2.6 mg x kg(-1) x min(-1) iv for 5 min) or the alpha2-adrenergic receptor antagonist idazoxan (0.3 mg/kg iv bolus). Idazoxan blocked the effects of clonidine (10 miccrog/kg iv) on HR, MAP, and RSNA. Hexamethonium (30 mg/kg iv) inhibited RSNA, and tempol did not decrease RSNA after hexamethonium. The effects of tempol on HR and MAP were reduced by hexamethonium. In conclusion, depressor responses caused by tempol are mediated, partly, by sympathoinhibition in urethane-anesthetized, normotensive rats. Nitric oxide does not contribute to this response, and the sympathoinhibitory effect of tempol is not mediated via alpha2-adrenergic receptors. Finally, tempol directly decreases HR, which may contribute to the MAP decrease.

Role of ET(A) receptors in experimental ANG II-induced hypertension in rats.

The objectives were to determine if ANG II-induced hypertension is maintained by activation of endothelin type A (ET(A)) receptors by endogenous ET-1 and if this effect is influenced by salt intake. Male rats were maintained on high sodium intake (HS; 6 meq/day) or on normal sodium intake (NS; 2 meq/day). Hypertension was produced by intravenous infusion of ANG II (5 ng/min) for 15 days. Five-day oral dosing with the selective ET(A)-receptor antagonist ABT-627 (~2 mg. kg(-1). day(-1)) reduced mean arterial pressure (MAP) to baseline levels in rats on HS receiving ANG II infusion, but it did not affect MAP in normotensive HS controls. In rats on NS, ABT-627 only transiently decreased MAP in rats receiving ANG II and slightly reduced MAP in normotensive controls. ABT-627 produced mild retention of sodium and water in NS rats receiving ANG II, but not in any other group. These results indicate that ET-1 plays a role in ANG II-induced hypertension via activation of ET(A) receptors and that this role is more prominent in rats on HS.

Effect of an ET(B)-selective and a mixed ET(A/B) endothelin receptor antagonist on venomotor tone in deoxycorticosterone-salt hypertension.

OBJECTIVES: Because the ET(B) receptor is important in venoconstriction, we examined the effects of a selective ET(B) receptor antagonist (A-1 92621) and a mixed ET(A/B) receptor antagonist (A-182086) on endogenous endothelin-1 (ET-1) contributions to elevated venomotor tone in deoxycorticosterone acetate-salt (DOCA-salt) hypertension. METHODS: Changes in venomotor tone were assessed using repeated measurements of mean circulatory filling pressure (MCFP) in awake, uninephrectomized, DOCA-salt-treated rats and uninephrectomized sham rats following intravenous (i.v.) injections of the ET(B) antagonist (12 mg/kg i.v.) or the ET(A/B) antagonist (12 mg/kg i.v.) alone, or 1 h before ganglion blockade with hexamethonium (30 mg/kg i.v.). RESULTS: DOCA-salt rats were hypertensive and exhibited higher MCFP than sham normotensive rats. The ET(A/B) receptor antagonist lowered mean arterial blood pressure (MABP) in DOCA-salt and sham rats, but MCFP fell in DOCA-salt rats only. The ET(B) antagonist produced no changes in MCFP while MABP increased in both groups. Pre-treatment of DOCA-salt rats, but not sham rats, with either antagonist produced greater declines in MCFP following hexamethonium than after hexamethonium alone. CONCLUSIONS: The present study confirms previous findings of elevated MCFP in DOCA-salt hypertensive rats compared to normotensive rats, but is the first to show that venomotor tone is affected by the actions of endogenous ET-1 acting at ET(B) receptors to modulate sympathetic input to the veins, as well as direct actions of ET-1 on vascular smooth muscle (VSM) ET(A) receptors. We also showed that mixed ET(A/B) receptor antagonism was effective in lowering MCFP and MABP in DOCA-salt hypertensive rats.

Factors affecting endothelin-induced venous tone in conscious rats.

There may be a relation between altered venous function, endothelin (ET)-1, and an impairment in the activity of endothelial-derived nitric oxide (NO) and prostanoids in salt-dependent hypertension. The present study examined the effects of salt intake on ET-induced changes in venomotor tone and the effects of blockade of NO synthase with N(G)-nitro-L-arginine methyl ester (L-NAME) and of cyclooxygenase with indomethacin on venomotor tone caused by the ET(B) selective agonist sarafotoxin 6c (S6c) in awake rats. Rats were anesthetized for permanent placement of catheters for measurements of arterial and venous pressures. A silicone balloon catheter was also fixed in the right atrium to produce brief circulatory arrest. Venomotor tone was estimated from measurements of mean circulatory filling pressure (MCFP) in conscious rats. There were no differences in mean arterial pressure, heart rate, or MCFP responses to short-term administration of ET-1 or S6c at different levels of salt intake. L-NAME or indomethacin did not change MCFP or the response of MCFP to short-term injection of S6c. In conclusion, neither basal MCFP nor integrated venomotor responses to short-term injection of ET-1 or S6c were altered by short-term changes in salt intake, blockade of NO synthase or cyclooxygenase. These data do not support the hypothesis that increased salt intake alters reactivity of veins to ET-1, NO, or prostanoids.

Differential localization of P2 receptor subtypes in mesenteric arteries and veins of normotensive and hypertensive rats.

ATP acts at P2 receptors to contract blood vessels and reactivity to vasoconstrictor agents is often altered in hypertension. This study was designed to identify P2 receptors in mesenteric arteries and veins and to determine whether ATP reactivity is altered in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Computer-assisted video microscopy was used to measure vessel diameter in vitro. ATP was a more potent constrictor of veins (EC(50) = 2.7 microM) than arteries (EC(50) = 196 microM) from normotensive rats; there was no change in ATP reactivity in vessels from DOCA-salt rats. The P2X1 receptor agonist alpha,beta-methylene ATP (alpha,beta-MeATP, 0.03-3 microM) contracted arteries but not veins. ATP-induced contractions in arteries were blocked by alpha,beta-MeATP (3 microM) desensitization. 2-Methylthio-ATP (0.1-10 microM), an agonist that can act at P2Y1 receptors, did not contract arteries or veins, whereas UTP, an agonist at rat P2Y2/P2Y4 receptors, contracted veins (EC(50) = 15 microM) and arteries (EC(50) = 24 microM). UTP-induced contractions of veins cross-desensitized with ATP, whereas UTP-induced contractions in arteries were unaffected by alpha,beta-MeATP-desensitization. The P2X/P2Y1 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4-disulfonic acid blocked ATP-induced contractions of arteries (IC(50) = 4.8 microM) but not veins. Suramin, an antagonist that blocks P2Y2 receptors, partly inhibited ATP- and UTP-induced contractions of veins. Immunohistochemical studies revealed P2X1 receptor immunoreactivity in arteries but not veins. These data indicate that mesenteric vascular reactivity to ATP is not altered in DOCA-salt hypertension. ATP acts at P2X1 and P2Y2 receptors to contract mesenteric arteries and veins, respectively, whereas in arteries UTP acts at an unidentified P2 receptor.

Effects of salt intake and angiotensin II on vascular reactivity to endothelin-1.

Hypertension produced by chronic infusion of angiotensin II (Ang II) is significantly blunted by blockade of endothelin-1 (ET-1) ET(A) subtype receptors. Furthermore, this model is salt-sensitive, and the antihypertensive response to ET(A) receptor blockade is more pronounced in animals on high salt intake. The goal of these experiments was to evaluate the effect of salt intake and Ang II on vascular reactivity to ET-1. In superior mesenteric arteries from normal male rats, studied in a standard muscle bath, incubation for 1 h with a subcontractile concentration of Ang II (10(-10) M) did not affect concentration-response curves to ET-1. Pressor responses in vivo to 2-h infusions of Ang II (5 ng/min) in rats maintained on normal or high salt intake were abolished by pretreatment with the ET(A) receptor antagonist ABT-627. The antagonist had no effect on pressor responses to phenylephrine (PE). In other experiments, rats maintained on either high or normal salt intake received continuous infusion of Ang II (5 ng/min i.v.) for 7 days, and then their superior mesenteric arteries were tested in the muscle bath. The maximum contractile response to ET-1 in arteries from Ang II-infused rats on normal salt intake was larger than in arteries from rats not receiving Ang II. Conversely, maximum responses to ET-1 in arteries from Ang II-infused rats on high salt intake were depressed compared with controls. No differences in vascular reactivity to PE were found. Thus, chronic in vivo exposure to Ang II results in specific salt-dependent changes in vascular reactivity to ET-1.

Mechanisms of increased venous smooth muscle tone in desoxycorticosterone acetate-salt hypertension.

The purpose of the present study was to identify mechanisms that contribute to increased venous smooth muscle tone in desoxycorticosterone acetate (DOCA)-salt hypertension in rats. Male Sprague-Dawley rats were uninephrectomized, received subcutaneous implants of DOCA, and drank 1% sodium chloride/0.2% potassium chloride solutions. Sham-operated rats received only uninephrectomy and drank tap water. Three to 4 weeks later, arterial and venous catheters were implanted for measurements of arterial and central venous pressures, respectively, and a silicone balloon catheter was permanently fixed in the right atrium to produce brief circulatory arrest. Venous smooth muscle activity was estimated on the basis of repeated measurements of mean circulatory filling pressure in conscious rats resting in their home cages. DOCA-salt-treated rats were hypertensive and had elevated mean circulatory filling pressure compared with normotensive sham-operated rats. Blockade of the endothelin subtype A receptor with 1 mg/kg ABT-627 IV decreased arterial blood pressure and mean circulatory filling pressure significantly more in hypertensive rats than in normotensive rats. Ganglionic blockade with 30 mg/kg hexamethonium IV also decreased arterial blood pressure and mean circulatory filling pressure more in hypertensive than in normotensive rats. Pretreatment with ABT-627 did not affect subsequent hemodynamic responses to ganglionic blockade. We conclude that venous smooth muscle tone is increased in DOCA-salt hypertension through the independent actions of both endogenous endothelin-1 acting on subtype A receptors and sympathetically mediated venoconstrictor activity.

Interaction of the beta adrenergic receptor antagonist bucindolol with serotonergic receptors.

Bucindolol is a nonselective beta-adrenergic receptor antagonist that has additional vasodilating properties. Because some beta-adrenergic receptor antagonists such as cyanopindolol are used as 5-HT1A/5-HT1B receptor antagonists, we tested the hypothesis that bucindolol can interact with 5-HT receptors. Both in vitro and in vivo methods were used to examine the interaction of bucindolol with 5-HT receptors relevant to the cardiovascular system-the 5-HT1A, 5-HT1D, 5-HT2A, and 5-HT2B receptors-and with alpha1-adrenergic receptors. In binding studies, bucindolol displayed high affinity for the 5-HT1A receptor (Ki, 11 nM), modest affinity for the 5-HT2A receptor (Ki, 382 nM), and no measurable affinity for the 5-HT1D receptor; binding affinity for the 5-HT2B receptor was not studied. Bucindolol also displayed significant binding affinity (Ki, 69 nM) for the alpha1-adrenergic receptors. Alpha1-Adrenergic receptor antagonist activity was confirmed by the ability of bucindolol (1 mg/kg) to act as a competitive antagonist against 0.01-30 microg/kg phenylephrine-induced pressor responses in conscious rats. In conscious permanently instrumented rats, bucindolol (0.1-3.0 mg/kg, i.v.) did not cause bradycardia similar to that elicited by the 5-HT1A-receptor agonist 8-OH-DPAT (3-300 microg/kg, i.v.), nor did bucindolol (1 mg/kg) block the 8-OH-DPAT-induced bradycardia. Bucindolol (10(-9)-10(-5) M) did not cause relaxation in the PGF2alpha-contracted, endothelium-intact porcine coronary artery, nor did bucindolol (10(-5) M) block 5-HT-induced coronary artery relaxation, indicating that bucindolol does not have significant interactions at the 5-HT1D receptor. Bucindolol also displayed no agonist activity at the 5-HT2A and 5-HT2B receptor (endothelium-denuded rat thoracic aorta and rat stomach fundus, respectively), but did act as a weak 5-HT2A-receptor antagonist (-log K(B) [M] = 5.4+/-0.1) and 5-HT2B-receptor antagonist (-log K(B) [M] = 7.8+/-0.1). Thus, these data suggest that bucindolol lacks the ability to activate the 5-HT1A, 5-HT1D, 5-HT2A, and 5-HT2B receptor, but can block alpha1-adrenergic receptors and act as a weak 5-HT2A- and 5-HT2B-receptor antagonist. The relevance of these serotoninergic effects as it pertains to the mechanism of bucindolol-induced vasodilation is unknown.

Mechanisms of endothelin-induced venoconstriction in isolated guinea pig mesentery.

In the present study, endothelin (ET) agonists and receptor selective antagonists were used to characterize ET receptors mediating constriction in guinea pig mesenteric veins (250-300 micrometers diameter) in vitro. The contribution of ET-evoked vasodilator release to venous tone was also explored. Computer-assisted video microscopy was used to monitor vein diameter. Endothelin-1 (ET-1), endothelin-3 (ET-3), and sarafotoxin 6c (S6c) produced sustained concentration-dependent contractions with a rank order agonist potency of ET-1 = S6c > ET-3. Indomethacin (1 microM) and Nomega-nitro-L-arginine (100 microM) enhanced ET-1 and S6c responses. The ETA selective antagonists BQ-610 (100 nM) and PD156707 (10 nM) shifted ET-1 concentration-response curves rightward and decreased maximal ET-1 responses, without changing S6c responses. The ETB selective antagonist BQ-788 (100 nM) shifted S6c responses rightward but produced no change in ET-1 responses. Combined application of BQ-788 and BQ-610 or BQ-788 and PD 156707 produced a rightward shift in ET-1 responses that was greater than shifts produced by BQ-610 or PD 156707 alone. In conclusion, smooth muscle in guinea pig mesenteric veins expresses ETA and ETB receptors coupled to contractile mechanisms. Activation of endothelial ETB receptors results in release of vasodilators, primarily nitric oxide.

5-HT2B-receptor antagonist LY-272015 is antihypertensive in DOCA-salt-hypertensive rats.

We previously demonstrated a change in the receptors mediating 5-hydroxytryptamine (5-HT)-induced contraction in arteries of deoxycorticosterone acetate (DOCA)-salt-hypertensive rats. Specifically, contraction to 5-HT is mediated primarily by 5-HT2A receptors in arteries from normotensive sham rats and by both 5-HT2A and 5-HT2B receptors in arteries from hypertensive rats. We hypothesized that the 5-HT2B receptor may play a role in maintaining the high blood pressure of DOCA-salt-hypertensive rats, and herein we provide data connecting in vitro and in vivo findings. The endothelium-denuded isolated superior mesenteric artery of DOCA-salt rats displayed a marked increase in maximum contraction to the newly available 5-HT2B-receptor agonist BW-723C86 compared with that of arteries from sham rats, confirming that the 5-HT2B receptor plays a greater role in 5-HT-induced contraction in arteries from DOCA-salt rats. In chronically instrumented rats, the 5-HT2B-receptor antagonist LY-272015 (0.3, 1.0, and 3.0 mg/kg iv at 30-min intervals) was given cumulatively 1 time/wk during 4 wk of continued DOCA-salt treatment. LY-272015 did not reduce blood pressure of the sham-treated rats at any time or dose. However, LY-272015 (1.0 and 3. 0 mg/kg) significantly reduced mean blood pressure in a subgroup of week 3 (-20 mmHg) and week 4 DOCA-salt (-40 mmHg) rats that had extremely high blood pressure (mean arterial blood pressure approximately 200 mmHg). Blockade of 5-HT2B receptors by in vivo administration of LY-272015 (3.0 mg/kg) was verified by observing reduced 5-HT-induced contraction in rat stomach fundus, the tissue from which the 5-HT2B receptor was originally cloned. These data support the novel hypothesis that 5-HT2B-receptor expression is induced during the development of DOCA-salt hypertension and contributes to the maintenance of severe blood pressure elevations.

Role of endothelin in hypertension of experimental chronic renal failure.

Surgical ablation of renal mass leads to a reduction in kidney function and commonly to the development of hypertension and chronic renal failure (CRF) in rats. The objective of this study was to determine whether endothelin (ET)-1 is involved in the maintenance of the hypertension that accompanies loss of renal mass. First, we demonstrated the antihypertensive efficacy of PD 155080, a selective, orally active ET(A) receptor antagonist, in a group of rats made hypertensive by continuous intravenous infusion of ET-1 (2.5 pmol x kg(-1) x min[-1]) for 7 days. ET-1 produced a sustained hypertension and PD 155080 (56.4 micromol/kg [25mg/kg] BID PO) normalized blood pressure (BP) during the 5 days of drug administration. In a second experiment, Sprague-Dawley rats underwent a 5/6 reduction in renal mass (RRM); 4 weeks later, PD 155080 administered for 7 days resulted in a sustained reduction in BP. Sham-operated rats also showed a slight hypotensive response to PD 155080 administration. Plasma urea nitrogen, plasma creatinine, urinary protein excretion, and creatinine clearance were not altered by PD 155080 administration in RRM or sham rats. In a third experiment, we investigated the contribution of the renin-angiotensin system to BP control in RRM rats given PD 155080. In these rats, PD 155080 reduced BP during 5 treatment days, and this antihypertensive effect was not altered by coadministration of the angiotensin-converting enzyme inhibitor enalapril in the drinking water (508 micromol/L [250 mg/L]). These results demonstrate that (1) ET-1 plays a role in established RRM hypertension through activation of the ET(A) receptor subtype, (2) lowering blood pressure with PD 155080 in RRM rats does not adversely affect renal function, and 3) the antihypertensive effect of ET(A) receptor antagonism is not opposed by the renin-angiotensin system.

Neither platelet activating factor nor leukotrienes are critical mediators of liver injury after lipopolysaccharide administration.

The intravenous administration of lipopolysaccharide (LPS) to rats results in liver lesions characterized by the infiltration of both platelets and neutrophils into the liver and by midzonal hepatocellular necrosis. The liver injury is dependent on neutrophils, platelets and the coagulation system, as removal or inhibition of any of these components inhibits the development of hepatocellular necrosis. Platelet activating factor (PAF) and the cysteinyl leukotrienes (LTs) are potent inflammatory lipids that are critical for the development of some LPS-mediated alterations. To test the hypothesis that PAF, alone or in combination with LTs, contributes to the development of liver injury during LPS exposure, we conducted studies with the PAF receptor antagonist, WEB 2086, and the 5-lipoxygenase inhibitor, Zileuton. Female, Sprague-Dawley rats were pretreated with WEB 2086 (10 mg/kg, i.p.) alone or with Zileuton (40 mg/kg, p.o.) 1 h before the administration of LPS (4 mg/kg, i.v.) or its saline vehicle. Treatment with WEB 2086, alone or in combination with Zileuton, did not inhibit LPS-mediated hepatic neutrophil infiltration or liver injury, as assessed by histologic evaluation and increases in plasma alanine aminotransferase activity. Pretreatment with these agents also had no effect on the activation of the coagulation system or on the thrombocytopenia induced by LPS. These results suggest that PAF, alone or in combination with 5-lipoxygenase products, is not a critical mediator of LPS-induced hepatocellular injury in this model.

Long-term sympatho-excitatory effect of angiotensin II: a mechanism of spontaneous and renovascular hypertension.

1. The peptide hormone angiotensin II (AngII) is acknowledged to be an important factor in the pathophysiology of hypertension. This is particularly the case in hypertension caused by luminal narrowing of one renal artery, (i.e. renovascular hypertension). The primary mechanism by which AngII raises blood pressure, however, is disputed. Strong arguments can be made supporting either vascular contraction, effects on renal excretion of sodium and water, or trophic actions on cardiovascular structures as the key element. In this paper I review evidence that AngII influences blood pressure by modulating autonomic nervous system activity. Modulation can occur at both the peripheral and central aspects of the autonomic system, but I focus on brain pathways involved in determining sympathetic nervous system activity. 2. Experimental and clinical investigations are cited to support the hypothesis that sympathetically mediated pressor effects are increased by both circulating and brain-derived AngII in hypertension. Recent work points specifically to sympathetic pre-motor neurons in the rostral ventrolateral medulla (RVLM) as a critical site of action of brain AngII in normotensive and hypertensive animals. 3. This same set of neurons appears to be an important relay in the sympatho-excitatory response to circulating AngII initiated at circumventricular organs, particularly the area postrema. AngII has important effects on the baroreflex. These do not mediate the sympatho-excitation elicited by circulating AngII, but rather mask its expression. 4. Substantial data support the hypothesis that increased blood concentrations of AngII in renovascular hypertension elevate blood pressure by causing neurogenic vasoconstriction mediated through the area postrema and RVLM.