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

Forgotten Circulation: Reduced Mesenteric Venous Capacitance in Hypertensive Rats Is Improved by Decreasing Sympathetic Activity

BACKGROUND

The mesenteric venous reservoir plays a vital role in mediating blood volume and pressure changes and is richly innervated by sympathetic nerves; however, the precise nature of venous sympathetic regulation and its role during hypertension remains unclear. We hypothesized that sympathetic drive to mesenteric veins in spontaneously hypertensive (SH) rats is raised, increasing mean circulatory filling pressure (MCFP), and impairing mesenteric capacitance.

METHODS

Arterial pressure, central venous pressure, mesenteric arterial, and venous blood flow were measured simultaneously in conscious male Wistar and SH rats. MCFP was assessed using an intraatrial balloon. Hemodynamic responses to volume changes (±20%) were measured before and after ganglionic blockade and carotid body denervation. Sympathetic venoconstrictor activity was measured in situ.

RESULTS

MCFP in vivo (10.8±1.6 versus 8.0±2.1 mm Hg; P=0.0005) and sympathetic venoconstrictor drive in situ (18±1 versus 10±2 µV; P<0.0001) were higher in SH rats; MCFP decreased in SH rats after hexamethonium and carotid body denervation (7.6±1.4; P<0.0001 and 8.5±1.0 mm Hg; P=0.0045). During volume changes, arterial pressure remained stable. With blood loss, net efflux of blood from the mesenteric bed was measured in both strains. However, during volume infusion, we observed net influx in Wistar (+2.3±2.6 mL/min) but efflux in SH rats (-1.0±1.0 mL/min; P=0.0032); this counterintuitive efflux was abolished by hexamethonium and carotid body denervation (+0.3±1.7 and 0.5±1.6 mL/min, respectively).

CONCLUSIONS

In SH rats, excessive sympathetic venoconstriction elevates MCFP and reduces capacitance, impairing volume buffering by mesenteric veins. We propose selective targeting of mesenteric veins through sympathetic drive reduction as a novel therapeutic opportunity for hypertension.

Role of Brain Endothelin Receptor Type B (ETB) in the Regulation of Tyrosine Hydroxylase in the Olfactory Bulb of DOCA-Salt Hypertensive Rats

BACKGROUND

We previously reported that endothelins (ETs) regulate tyrosine hydroxylase (TH) activity and expression in the olfactory bulb (OB) of normotensive and hypertensive animals. Applying an ET receptor type A (ETA) antagonist to the brain suggested that endogenous ETs bind to ET receptor type B (ETB) to elicit effects.

OBJECTIVE

The aim of the present work was to evaluate the role of central ETB stimulation on the regulation of blood pressure (BP) and the catecholaminergic system in the OB of deoxycorticosterone acetate (DOCA)-salt hypertensive rats.

METHODS

DOCA-salt hypertensive rats were infused for 7 days with cerebrospinal fluid or IRL-1620 (ETB receptor agonist) through a cannula placed in the lateral brain ventricle. Systolic BP (SBP) and heart rate were recorded by plethysmography. The expression of TH and its phosphorylated forms in the OB were determined by immunoblotting, TH activity by a radioenzymatic assay, and TH mRNA by quantitative real-time polymerase chain reaction.

RESULTS

Chronic administration of IRL-1620 decreased SBP in hypertensive rats but not in normotensive animals. Furthermore, the blockade of ETB receptors also decreased TH-mRNA in DOCA-salt rats, but it did not modify TH activity or protein expression.

CONCLUSION

These findings suggest that brain ETs through the activation of ETB receptors contribute to SBP regulation in DOCA-salt hypertension. However, the catecholaminergic system in the OB does not appear to be conclusively involved although mRNA TH was reduced. Present and previous findings suggest that in this salt-sensitive animal model of hypertension, the OB contributes to chronic BP elevation.

Venous endothelial function in cardiovascular disease

The essential role of the endothelium in vascular homeostasis is associated with the release of endothelium-dependent relaxing and contractile factors (EDRF and EDCF, respectively). Different from arteries, where these factors are widely studied, the vasoactive factors derived from the venous endothelium have been given less attention. There is evidence for a role of the nitric oxide (NO), endothelium-dependent hyperpolarization (EDH) mechanism, and cyclooxygenase (COX)-derived metabolites as EDRFs; while the EDCFs need to be better evaluated since no consensus has been reached about their identity in venous vessels. The imbalance between the synthesis, bioavailability, and/or action of EDRFs and/or EDCFs results in a pathological process known as endothelial dysfunction, which leads to reduced vasodilation and/or increased vasoconstriction. In the venous system, endothelial dysfunction is relevant since reduced venodilation may increase venous tone and decrease venous compliance, thus enhancing mean circulatory filling pressure, which maintains or modify cardiac workload contributing to the etiology of cardiovascular diseases. Interestingly, some alterations in venous function appear at the early stages (or even before) the establishment of these diseases. However, if the venous endothelium dysfunction is involved in these alterations is not yet fully understood and requires further studies. In this sense, the present study aims to review the current knowledge on venous endothelial function and dysfunction, and the general state of the venous tone in two important cardiovascular diseases of high incidence and morbimortality worldwide: hypertension and heart failure.

Acute dietary nitrate supplementation does not change venous volume and compliance in healthy young adults

In this randomized single-blind, placebo-controlled, crossover study, we investigated the influence of inorganic nitrate (NO₃^-) supplementation on venous volume and compliance in the resting forearm and calf. Twenty healthy young adults were assigned to receive an NO₃^--rich beverage (beetroot juice [BRJ]: 140 mL; ~8 mmol NO₃^-) or an NO₃^¯-depleted control beverage (prune juice [CON]: 166 mL; < 0.01 mmol NO₃^-). Two hours after consuming the allocated beverage, each participant rested in the supine position for 20 min. Cuffs were then placed around the right upper arm and right thigh, inflated to 60 mmHg for 8 min, and then decreased to 0 mmHg at a rate of 1 mmHg/s. During inflation and deflation of cuff pressure, changes in venous volume in the forearm and calf were measured by venous occlusion plethysmography. Venous compliance was calculated as the numerical derivative of the cuff pressure‒venous volume curve in the limbs. The plasma NO₃^- concentration was elevated by intake of BRJ (before, 15.5 ± 5.8 µM; after, 572.0 ± 116.1 µM, P < 0.05) but not by CON (before, 14.8 ± 7.2 µM; after, 15.3 ± 7.4 µM, P > 0.05). On the other hand, there was no significant difference in venous volume or compliance in the forearm or calf between BRJ and CON. These findings suggest that although acute inorganic NO₃^- supplementation may enhance the activity of nitric oxide (NO) via NO₃^- → nitrite → NO pathway, it does not influence venous volume or compliance in the limbs in healthy young adults.

Venous volume and compliance in the calf and forearm does not change after acute endurance exercise performed at continuous or interval workloads

Short-term endurance exercise training for 6-8 weeks leads to increases in venous volume and compliance in the limbs. However, it is not known whether these venous vascular properties are improved by acute endurance exercise. We examined the effects of acute endurance exercise involving continuous or interval workloads on venous volume and compliance in the exercising (calf) and non-exercising (forearm) limbs. Sixteen healthy young volunteers performed cycling exercise involving a continuous workload of 60% heart rate (HR) reserve or an interval workload of 40% HRreserve and 80% HRreserve, alternating every 2 min, for a total of 32 min each. Before and 60 min after acute cycling exercise, venous volume in the calf and forearm was measured by venous occlusion plethysmography during a cuff-deflation protocol with a venous collecting cuff wrapped to the thigh and upper arm and strain gauges attached to the calf and forearm. The cuff pressure was maintained at 60 mmHg for 8 min and was then deflated to 0 mmHg at a rate of 1 mmHg/s. Venous compliance was calculated as the numerical derivative of the cuff pressure-limb venous volume curve. In both the calf and forearm, the cuff pressure-venous volume curve and the cuff pressure-venous compliance relationship did not differ between before and 60 min after exercise involving continuous or interval workloads. These results suggest that acute exercise does not improve venous volume and compliance in both the exercising and non-exercising limbs.

High dietary salt amplifies osmoresponsiveness in vasopressin-releasing neurons

High dietary salt increases arterial pressure partly through activation of magnocellular neurosecretory cells (MNCVP) that secrete the antidiuretic and vasoconstrictor hormone vasopressin (VP) into the circulation. Here, we show that the intrinsic and synaptic excitation of MNCVP caused by hypertonicity are differentially potentiated in two models of salt-dependent hypertension in rats. One model combined salty chow with a chronic subpressor dose of angiotensin II (AngII-salt), the other involved replacing drinking water with 2% NaCl (salt loading, SL). In both models, we observed a significant increase in the quantal amplitude of EPSCs on MNCVP. However, model-specific changes were also observed. AngII-salt increased the probability of glutamate release by osmoreceptor afferents and increased overall excitatory network drive. In contrast, SL specifically increased membrane stiffness and the intrinsic osmosensitivity of MNCVP. These results reveal that dietary salt increases the excitability of MNCVP through effects on the cell-autonomous and synaptic osmoresponsiveness of MNCVP.

Association between vegetable consumption and calf venous compliance in healthy young adults

BACKGROUND

Venous compliance decreases with aging and/or physical inactivity, which is thought to be involved partly in the pathogenesis of cardiovascular disease such as hypertension. This suggests that it is important to maintain high venous compliance from a young age in order to prevent cardiovascular disease. Both nutrient and exercise could play an important role in the improvement and maintenance of vascular health. Indeed, habitual endurance exercise is known to improve the venous compliance, although little is known about the effect of diet on venous compliance. Considering that higher consumption of vegetables could contribute to the arterial vascular health and the decreased blood pressure, it is hypothesized that venous compliance may be greater as vegetable intake is higher. Thus, the purpose of this study was to clarify the association between vegetable intake and venous compliance in healthy young adults.

METHODS

Dietary intake was assessed in 94 subjects (male: n = 44, female: n = 50) using a self-administered diet history questionnaire (DHQ). Intakes of nutrients and food groups that were obtained from the DHQ were adjusted according to total energy intake using the residual method. Based on the adjusted intake of food groups, total vegetable intake was calculated as the sum of green/yellow and white vegetables consumed. Calf volume was measured using venous occlusion plethysmography with a cuff deflation protocol. Calf venous compliance was calculated as the numerical derivative of the cuff pressure-calf volume curve. In addition, circulatory responses (heart rate and systolic and diastolic blood pressure) at resting and maximal oxygen uptake were assessed in all subjects.

RESULTS

Mean value of total vegetables intake was 162.2 ± 98.2 g/day. Simple linear regression analysis showed that greater venous compliance was significantly associated with higher total vegetable consumption (r = 0.260, P = 0.011) and green/yellow vegetable intake (r = 0.351, P = 0.001) but not white vegetable intake (r = 0.013, P = 0.902). These significant associations did not change in the multivariate linear regression models which were adjusted by sex and maximal oxygen uptake.

CONCLUSION

These findings suggest that higher consumption of vegetables, especially of the green/yellow vegetables, may be associated with greater venous compliance in young healthy adults.

Adipose Mesenchymal Cells-Derived EVs Alleviate DOCA-Salt-Induced Hypertension by Promoting Cardio-Renal Protection

Hypertension is a long-term condition that can increase organ susceptibility to insults and lead to severe complications such as chronic kidney disease (CKD). Extracellular vesicles (EVs) are cell-derived membrane structures that participate in cell-cell communication by exporting encapsulated molecules to target cells, regulating physiological and pathological processes. We here demonstrate that multiple administration of EVs from adipose-derived mesenchymal stromal cells (ASC-EVs) in deoxycorticosterone acetate (DOCA)-salt hypertensive model can protect renal tissue by maintaining its filtration capacity. Indeed, ASC-EVs downregulated the pro-inflammatory molecules monocyte chemoattracting protein-1 (MCP-1) and plasminogen activating inhibitor-1 (PAI1) and reduced recruitment of macrophages in the kidney. Moreover, ASC-EVs prevented cardiac tissue fibrosis and maintained blood pressure within normal levels, thus demonstrating their multiple favorable effects in different organs. By applying microRNA (miRNA) microarray profile of the kidney of DOCA-salt rats, we identified a selective miRNA signature associated with epithelial-mesenchymal transition (EMT). One of the key pathways found was the axis miR-200-TGF-β, that was significantly altered by EV administration, thereby affecting the EMT signaling and preventing renal inflammatory response and fibrosis development. Our results indicate that EVs can be a potent therapeutic tool for the treatment of hypertension-induced CKD in cardio-renal syndrome.

Activation of bradykinin-sensitive pericardial afferents increases systemic venous tone in conscious rats

Our understanding of reflex regulation of veins lags behind that of the arterial system. While the cardiac sympathetic afferent reflex (CSAR) exerts control over sympathetic outflow, its effect on venous tone is not known. We tested the hypothesis that activation of pericardial bradykinin sensitive afferents elicits systemic venoconstriction. Male and female Sprague Dawley rats were chronically instrumented for measurement of arterial pressure and mean circulatory filling pressure, an index of venous tone, and with an indwelling pericardial catheter. Mean arterial pressure, heart rate and mean circulatory filling pressure responses were assessed in conscious rats in response to graded pericardial injections of bradykinin (1.5-20 μg/kg) before and after ganglionic blockade, and to intravenous norepinephrine (0.05-0.8 μg/kg). Bradykinin B2 receptor was assessed by Western blot. Pericardial bradykinin injections caused graded increases in mean arterial pressure, heart rate and mean circulatory filling pressure. These responses were markedly attenuated after autonomic blockade. The increments in mean circulatory filling pressure were attenuated in female rats. There were no differences in the venoconstrictor responses to norepinephrine or ventricular bradykinin receptor expression between male and females. We interpret these findings to indicate that activation of bradykinin sensitive pericardial afferents elicits a sexually dimorphic, autonomically mediated systemic venoconstrictor response. Differences in venous smooth muscle responses to norepinephrine or ventricular bradykinin receptor expression do not account for the sexual dimorphism. We conclude that systemic venoconstriction contributes to the overall hemodynamic response to activation of the cardiac sympathetic afferent reflex and that this effect is sexually dimorphic.

Effects of high-fat diet on sympathetic neurotransmission in mesenteric arteries from Dahl salt-sensitive rat

Obesity hypertension is driven by sympathetic neurotransmission to the heart and blood vessels. We tested the hypothesis that high-fat diet (HFD)-induced hypertension is driven by sympathetic neurotransmission to mesenteric arteries (MA) in male but not female Dahl salt-sensitive (Dahl ss) rat. Rats were fed a control diet (CD; 10 kcal% from fat) or HFD (60 kcal% from fat) beginning at 3 weeks (wk) of age; measurements were made at 10-, 17- and 24-wk. Body weight increased with HFD, age and sex. Mean arterial pressure (MAP) was higher in HFD versus CD rats from both sexes at 17- and 24-wk. MA constriction measured using pressure myography, and electrical field stimulation (EFS, 0.2-30 Hz) was greater in HFD versus CD in males at 17-wk; this was not due to changes in α2 autoreceptor or norepinephrine transporter (NET) function. Prazosin (α1-AR antagonist) and suramin (P2 receptor antagonist) inhibited neurogenic MA constriction equally in all groups. Arterial reactivity to exogenous norepinephrine (NE; 10^-8 - 10^-5 M) was lower in HFD versus CD at 10-wk in males. Female MA reactivity to exogenous ATP was lower at 24-weeks compared to earlier time points. HFD did not affect tyrosine hydroxylase (TH) or the vesicular nucleotide transporter (VNUT) nerve density in MA from both sexes. NE content was lower in MA but higher in plasma at 24-wk compared to 10- and 17-wk in both sexes. In conclusion, HFD-induced hypertension is not driven by increased sympathetic neurotransmission to MA in male and female Dahl ss rats.

Lesion of the OVLT markedly attenuates chronic DOCA-salt hypertension in rats

Lesions of the anteroventral third ventricle (AV3V region) are known to prevent many forms of experimental hypertension, including mineralocorticoid [deoxycorticosterone acetate (DOCA)-salt] hypertension in the rat. However, AV3V lesions include the organum vasculosum of the lamina terminalis (OVLT), portions of the median preoptic nucleus, and efferent fibers from the subfornical organ (SFO), thereby limiting the ability to define the individual contribution of these structures to the prevention of experimental hypertension. Having previously reported that the SFO does not play a significant role in the development of DOCA-salt hypertension, the present study was designed to test the hypothesis that the OVLT is necessary for DOCA-salt hypertension in the rat. In uninephrectomized OVLT-lesioned (OVLTx; n = 6) and sham-operated ( n = 4) Sprague-Dawley rats consuming a 0.1% NaCl diet and 0.9% NaCl drinking solution, 24-h mean arterial pressure (MAP) was recorded telemetrically 5 days before and 21 days after DOCA implantation (100 mg sc per rat). No differences in control MAP were observed between groups. The chronic pressor response to DOCA was attenuated in OVLTx rats such that MAP increased to 133 ± 3 mmHg in sham-operated rats by day 21 of DOCA compared with 120 ± 4 mmHg (means ± SE) in OVLTx rats. These results support the hypothesis that the OVLT is an important brain site of action for the pathogenesis of DOCA-salt hypertension in the rat.

DOCA-Salt Hypertension: an Update

Hypertension is a multifaceted disease that is involved in ∼40% of cardiovascular mortalities and is the result of both genetic and environmental factors. Because of its complexity, hypertension has been studied by using various models and approaches, each of which tends to focus on individual organs or tissues to isolate the most critical and treatable causes of hypertension and the related damage to end-organs. Animal models of hypertension have ranged from Goldblatt's kidney clip models in which the origin of the disease is clearly renal to animals that spontaneously develop hypertension either through targeted genetic manipulations, such as the TGR(mRen2)27, or selective breeding resulting in more enigmatic origins, as exemplified by the spontaneously hypertensive rat (SHR). These two genetically derived models simulate the less-common human primary hypertension in which research has been able to define a Mendelian linkage. Several models are more neurogenic or endocrine in nature and illustrate that crosstalk between the nervous system and hormones can cause a significant rise in blood pressure (BP). This review will examine one of these neurogenic models of hypertension, i.e., the deoxycorticosterone acetate (DOCA), reduced renal mass, and high-salt diet (DOCA-salt) rodent model, one of the most common experimental models used today. Although the DOCA-salt model is mainly believed to be neurogenic and has been shown to impact the central and peripheral nervous systems, it also significantly involves many other body organs.

Chronic Blockade of Brain Endothelin Receptor Type-A (ETA) Reduces Blood Pressure and Prevents Catecholaminergic Overactivity in the Right Olfactory Bulb of DOCA-Salt Hypertensive Rats

Overactivity of the sympathetic nervous system and central endothelins (ETs) are involved in the development of hypertension. Besides the well-known brain structures involved in the regulation of blood pressure like the hypothalamus or locus coeruleus, evidence suggests that the olfactory bulb (OB) also modulates cardiovascular function. In the present study, we evaluated the interaction between the endothelinergic and catecholaminergic systems in the OB of deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Following brain ET receptor type A (ET_A) blockade by BQ610 (selective antagonist), transcriptional, traductional, and post-traductional changes in tyrosine hydroxylase (TH) were assessed in the OB of normotensive and DOCA-salt hypertensive rats. Time course variations in systolic blood pressure and heart rate were also registered. Results showed that ET_A blockade dose dependently reduced blood pressure in hypertensive rats, but it did not change heart rate. It also prevented the increase in TH activity and expression (mRNA and protein) in the right OB of hypertensive animals. However, ET_A blockade did not affect hemodynamics or TH in normotensive animals. Present results support that brain ET_A are not involved in blood pressure regulation in normal rats, but they significantly contribute to chronic blood pressure elevation in hypertensive animals. Changes in TH activity and expression were observed in the right but not in the left OB, supporting functional asymmetry, in line with previous studies regarding cardiovascular regulation. Present findings provide further evidence on the role of ETs in the regulation of catecholaminergic activity and the contribution of the right OB to DOCA-salt hypertension.

Macrophage-dependent impairment of α2-adrenergic autoreceptor inhibition of Ca2+ channels in sympathetic neurons from DOCA-salt but not high-fat diet-induced hypertensive rats

DOCA-salt and obesity-related hypertension are associated with inflammation and sympathetic nervous system hyperactivity. Prejunctional α₂-adrenergic receptors (α₂ARs) provide negative feedback to norepinephrine release from sympathetic nerves through inhibition of N-type Ca²⁺ channels. Increased neuronal norepinephrine release in DOCA-salt and obesity-related hypertension occurs through impaired α₂AR signaling; however, the mechanisms involved are unclear. Mesenteric arteries are resistance arteries that receive sympathetic innervation from the superior mesenteric and celiac ganglia (SMCG). We tested the hypothesis that macrophages impair α₂AR-mediated inhibition of Ca²⁺ channels in SMCG neurons from DOCA-salt and high-fat diet (HFD)-induced hypertensive rats. Whole cell patch-clamp methods were used to record Ca²⁺ currents from SMCG neurons maintained in primary culture. We found that DOCA-salt, but not HFD-induced, hypertension caused macrophage accumulation in mesenteric arteries, increased SMCG mRNA levels of monocyte chemoattractant protein-1 and tumor necrosis factor-α, and impaired α₂AR-mediated inhibition of Ca²⁺ currents in SMCG neurons. α₂AR dysfunction did not involve changes in α₂AR expression, desensitization, or downstream signaling factors. Oxidative stress impaired α₂AR-mediated inhibition of Ca²⁺ currents in SMCG neurons and resulted in receptor internalization in human embryonic kidney-293T cells. Systemic clodronate-induced macrophage depletion preserved α₂AR function and lowered blood pressure in DOCA-salt rats. HFD caused hypertension without obesity in Sprague-Dawley rats and hypertension with obesity in Dahl salt-sensitive rats. HFD-induced hypertension was not associated with inflammation in SMCG and mesenteric arteries or α₂AR dysfunction in SMCG neurons. These results suggest that macrophage-mediated α₂AR dysfunction in the mesenteric circulation may only be relevant to mineralocorticoid-salt excess. NEW & NOTEWORTHY Here, we identify a contribution of macrophages to hypertension development through impaired α₂-adrenergic receptor (α₂AR)-mediated inhibition of sympathetic nerve terminal Ca²⁺ channels in DOCA-salt hypertensive rats. Impaired α₂AR function may involve oxidative stress-induced receptor internalization. α₂AR dysfunction may be unique to mineralocorticoid-salt excess, as it does not occur in obesity-related hypertension.

Emerging Roles of the Mineralocorticoid Receptor in Pathology: Toward New Paradigms in Clinical Pharmacology

The mineralocorticoid receptor (MR) and its ligand aldosterone are the principal modulators of hormone-regulated renal sodium reabsorption. In addition to the kidney, there are several other cells and organs expressing MR, in which its activation mediates pathologic changes, indicating potential therapeutic applications of pharmacological MR antagonism. Steroidal MR antagonists have been used for decades to fight hypertension and more recently heart failure. New therapeutic indications are now arising, and nonsteroidal MR antagonists are currently under development. This review is focused on nonclassic MR targets in cardiac, vascular, renal, metabolic, ocular, and cutaneous diseases. The MR, associated with other risk factors, is involved in organ fibrosis, inflammation, oxidative stress, and aging; for example, in the kidney and heart MR mediates hormonal tissue-specific ion channel regulation. Genetic and epigenetic modifications of MR expression/activity that have been documented in hypertension may also present significant risk factors in other diseases and be susceptible to MR antagonism. Excess mineralocorticoid signaling, mediated by aldosterone or glucocorticoids binding, now appears deleterious in the progression of pathologies that may lead to end-stage organ failure and could therefore benefit from the repositioning of pharmacological MR antagonists.

Serial Measurements of Splanchnic Vein Diameters in Rats Using High-Frequency Ultrasound

The purpose of this study was to investigate serial ultrasound imaging in rats as a fully non-invasive method to (1) quantify the diameters of splanchnic veins in real time as an indirect surrogate for the capacitance function of those veins, and (2) assess the effects of drugs on venous dimensions. A 21 MHz probe was used on anesthetized male Sprague–Dawley rats to collect images containing the portal vein (PV), superior mesenteric vein (SMV), abdominal inferior vena cava (IVC), and splenic vein (SpV; used as a landmark in timed studies) and the abdominal aorta (AA). Stable landmarks were established that allowed reproducible quantification of cross-sectional diameters within an animal. The average diameters of vessels measured every 5 min over 45 min remained within 0.75 ± 0.15% (PV), 0.2 ± 0.09% (SMV), 0.5 ± 0.12% (IVC), and 0.38 ± 0.06% (AA) of baseline (PV: 2.0 ± 0.12 mm; SMV: 1.7 ± 0.04 mm; IVC: 3.2 ± 0.1 mm; AA: 2.3 ± 0.14 mm). The maximal effects of the vasodilator sodium nitroprusside (SNP; 2 mg/kg, i.v. bolus) on venous diameters were determined 5 min post SNP bolus; the diameters of all noted veins were significantly increased by SNP, while mean arterial pressure (MAP) decreased 29 ± 4 mmHg. By contrast, administration of the venoconstrictor sarafotoxin (S6c; 5 ng/kg, i.v. bolus) significantly decreased PV and SpV, but not IVC, SMV, or AA, diameters 5 min post S6c bolus; MAP increased by 6 ± 2 mmHg. In order to determine if resting splanchnic vein diameters were stable over much longer periods of time, vessel diameters were measured every 2 weeks for 8 weeks. Measurements were found to be highly reproducible within animals over this time period. Finally, to evaluate the utility of vein imaging in a chronic condition, images were acquired from 4-week deoxycorticosterone acetate salt (DOCA-salt) hypertensive and normotensive (SHAM) control rats. All vessel diameters increased from baseline while MAP increased (67 ± 4 mmHg) in DOCA-salt rats compared to SHAM at 4 weeks after pellet implantation. Vessel diameters remained unchanged in SHAM animals. Together, these results support serial ultrasound imaging as a non-invasive, reliable technique able to measure acute and chronic changes in the diameter of splanchnic veins in intact rats.

Macrophage depletion lowers blood pressure and restores sympathetic nerve α2-adrenergic receptor function in mesenteric arteries of DOCA-salt hypertensive rats

We tested the hypothesis that vascular macrophage infiltration and O2 (-) release impairs sympathetic nerve α2-adrenergic autoreceptor (α2AR) function in mesenteric arteries (MAs) of DOCA-salt hypertensive rats. Male rats were uninephrectomized or sham operated (sham). DOCA pellets were implanted subcutaneously in uninephrectomized rats who were provided high-salt drinking water or high-salt water with apocynin. Sham rats received tap water. Blood pressure was measured using radiotelemetry. Treatment of sham and DOCA-salt rats with liposome-encapsulated clodronate was used to deplete macrophages. After 3-5, 10-13, and 18-21 days of DOCA-salt treatment, MAs and peritoneal fluid were harvested from euthanized rats. Norepinephrine (NE) release from periarterial sympathetic nerves was measured in vitro using amperometry with microelectrodes. Macrophage infiltration into MAs as well as TNF-α and p22(phox) were measured using immunohistochemistry. Peritoneal macrophage activation was measured by flow cytometry. O2 (-) was measured using dihydroethidium staining. Hypertension developed over 28 days, and apocynin reduced blood pressure on days 18-21. O2 (-) and macrophage infiltration were greater in DOCA-salt MAs compared with sham MAs after day 10. Peritoneal macrophage activation occurred after day 10 in DOCA-salt rats. Macrophages expressing TNF-α and p22(phox) were localized near sympathetic nerves. Impaired α2AR function and increased NE release from sympathetic nerves occurred in MAs from DOCA-salt rats after day 18. Macrophage depletion reduced blood pressure and vascular O2 (-) while restoring α2AR function in DOCA-salt rats. Macrophage infiltration into the vascular adventitia contributes to increased blood pressure in DOCA-salt rats by releasing O2 (-), which disrupts α2AR function, causing enhanced NE release from sympathetic nerves.

Enhanced function of inhibitory presynaptic cannabinoid CB1 receptors on sympathetic nerves of DOCA-salt hypertensive rats

AIMS

This study was performed to examine whether hypertension affects the sympathetic transmission to resistance vessels of pithed rats via inhibitory presynaptic cannabinoid CB1 receptors and whether endocannabinoids are involved in this response.

MATERIALS AND METHODS

We compared uninephrectomised rats rendered hypertensive by high salt diet and deoxycorticosterone acetate (DOCA) injections with normotensive animals (uninephrectomy only). Experiments were performed on vagotomised and pithed animals. Increases in diastolic blood pressure (DBP) were induced four times (S1-S4) by electrical stimulation or phenylephrine injection.

KEY FINDINGS

Electrical stimulation (0.75Hz, 1ms, 50V, 5 impulses) of the preganglionic sympathetic nerve fibres innervating the blood vessels more strongly increased DBP in normotensive than in DOCA-salt rats. Phenylephrine (0.01μmol/kg) induced similar increases in DBP in both groups. The cannabinoid receptor agonist CP55940 (0.01-1μmol/kg) did not modify the rises in DBP induced by phenylephrine. However, it inhibited the electrically stimulated increases in DBP, more strongly in DOCA-salt than in normotensive animals (maximally by 50 and 30%, respectively). The effect of CP55940 was attenuated by the CB1 antagonist AM251 (3μmol/kg). AM251 enhanced the neurogenic vasopressor response during S4 by itself in hypertensive rats only. URB597 (3μmol/kg), which inhibits degradation of the endocannabinoid anandamide, did not modify the electrically stimulated increases in DBP.

SIGNIFICANCE

The function of inhibitory presynaptic CB1 receptors on sympathetic nerves is enhanced in DOCA-salt hypertensive rats. Thus, the CB1 receptor-mediated inhibition of noradrenaline release from the sympathetic nerve fibres innervating the resistance vessels might play a protective role in hypertension.

The pathophysiological role of interstitial sodium in heart failure

The current understanding of heart failure (HF) does not fully explain the spectrum of HF symptoms. Most HF hospitalizations are related to sodium (Na(+)) and fluid retention resulting from neurohumoral up-regulation. Recent insights suggest that Na(+) is not distributed in the body solely as a free cation, but that it is also bound to large interstitial glycosaminoglycan (GAG) networks in different tissues, which have an important regulatory function. In HF, high Na(+) intake and neurohumoral alterations disrupt GAG structure, leading to loss of the interstitial buffer capacity and disproportionate interstitial fluid accumulation. Moreover, a diminished endothelial GAG network (the endothelial glycocalyx) results in increased vascular resistance and disturbed endothelial nitric oxide production. New imaging modalities can help evaluate interstitial Na(+) and endothelial glycocalyx integrity. Furthermore, several therapies have been proven to stabilize interstitial GAG networks. Hence, a better appreciation of this new Na(+) "compartment" might improve current management of HF.

A review of deoxycorticosterone acetate-salt hypertension and its relevance for cardiovascular physiotherapy research

[Purpose] The purpose of this review was to elucidate the deoxycorticosterone acetate (DOCA)-salt-related hypertensive mechanism and to contribute to future studies of cardiovascular physiotherapy. [Methods] This paper focuses on the signal transductions that control hypertension and its mechanisms. We include results reported by our laboratory in a literature review. [Results] Our results and the literature show the various mechanisms of DOCA-salt hypertension. [Conclusion] In this review paper, we carefully discuss the signal transduction in hypertension based on our studies and with reference to cardiovascular physiotherapy research.

Perioperative corticosteroid management for patients with inflammatory bowel disease

Guidelines on the appropriate use of perioperative steroids in patients with inflammatory bowel disease (IBD) are lacking. As a result, corticosteroid supplementation during and after colorectal surgery procedures has been shown to be highly variable. A clearer understanding of the indications for perioperative corticosteroid administration relative to preoperative corticosteroid dosing and duration of therapy is essential. In this review, we outline the basic tenets of the hypothalamic-pituitary-adrenal (HPA) axis and its normal response to stress, describe how corticosteroid use is thought to affect this system, and provide an overview of the currently available data on perioperative corticosteroid supplementation including the limited evidence pertaining to patients with inflammatory bowel disease. Based on currently existing data, we define "adrenal suppression," and propose a patient-based approach to perioperative corticosteroid management in the inflammatory bowel disease population based on an individual's historical use of corticosteroids, the type of surgery they are undergoing, and HPA axis testing when applicable. Patients without adrenal suppression (<5 mg prednisone per day) do not require extra corticosteroid supplementation in the perioperative period; patients with adrenal suppression (>20 mg prednisone per day) should be treated with additional perioperative corticosteroid coverage above their baseline home regimen; and patients with unclear HPA axis function (>5 and <20 mg prednisone per day) should undergo preoperative HPA axis testing to determine the best management practices. The proposed management algorithm attempts to balance the risks of adrenal insufficiency and immunosuppression.

Divergent signaling mechanisms for venous versus arterial contraction as revealed by endothelin-1

OBJECTIVE

Venous function is underappreciated in its role in blood pressure determination, a physiologic parameter normally ascribed to changes in arterial function. Significant evidence points to the hormone endothelin-1 (ET-1) as being important to venous contributions to blood pressure. We hypothesized that the artery and vein should similarly depend on the signaling pathways stimulated by ET-1, specifically phospholipase C (PLC) activation. This produces two functional arms of signaling: diacylglycerol (DAG; protein kinase C [PKC] activation) and inositol trisphosphate (IP3) production (intracellular calcium release).

METHODS

The model was the male Sprague-Dawley rat. Isolated tissue baths were used to measure isometric contraction. Western blot and immunocytochemical analyses measured the magnitude of expression and site of expression, respectively, of IP3 receptors in smooth muscle/tissue. Pharmacologic methods were used to modify PLC activity and signaling elements downstream of PLC (IP3 receptors, PKC).

RESULTS

ET-1-induced contraction was PLC dependent in both tissues as the PLC inhibitor U-73122 significantly reduced contraction in aorta (86% ± 4% of control; P < .05) and vena cava (49% ± 11% of control; P < .05). However, ET-1-induced contraction was not significantly inhibited by the IP3 receptor inhibitor 2-aminoethoxydiphenylborane (100 μM) in vena cava (82% ± 8% of control; P = .23) but was in the aorta (55% ± 4% of control; P < .05). All three IP3 receptor isoforms were located in venous smooth muscle. IP3 receptors were functional in both tissues as the novel membrane-permeable IP3 analogue (Bt-IP3; 10 μM) contracted aorta and vena cava. Similarly, whereas the PKC inhibitor chelerythrine (10 μM) attenuated ET-1-induced contraction in vena cava and aorta (5% ± 2% and 50% ± 5% of control, respectively; P < .05), only the vena cava contracted to the DAG analogue 1-oleoyl-2-acetyl-sn-glycerol.

CONCLUSIONS

These findings suggest that ET-1 activates PLC in aorta and vena cava, but vena cava contraction to ET-1 may be largely IP3 independent. Rather, DAG—not IP3—may contribute to contraction to ET-1 in vena cava, in part by activation of PKC. These studies outline a fundamental difference between venous and arterial smooth muscle and further reinforce a heterogeneity of vascular smooth muscle function that could be taken advantage of for therapeutic development.

Increased catecholamine secretion from single adrenal chromaffin cells in DOCA-salt hypertension is associated with potassium channel dysfunction

The mechanism of catecholamine release from single adrenal chromaffin cells isolated from normotensive and DOCA-salt hypertensive rats was investigated. These cells were used as a model for sympathetic nerves to better understand how exocytotic release of catecholamines is altered in this model of hypertension. Catecholamine secretion was evoked by local application of acetylcholine (1 mM) or high K+ (70 mM), and continuous amperometry was used to monitor catecholamine secretion as an oxidative current. The total number of catecholamine molecules secreted from a vesicle, the total number of vesicles fusing and secreting, and the duration of secretion in response to a stimulus were all significantly greater for chromaffin cells from hypertensive rats as compared to normotensive controls. The greater catecholamine secretion from DOCA-salt cells results, at least in part, from functionally impaired large conductance, Ca2+-activated (BK) and ATP-sensitive K+ channels. This work reveals that there is altered vesicular release of catecholamines from these cells (and possibly from perivascular sympathetic nerves) and this may contribute to increased vasomotor tone in DOCA-salt hypertension.

Enhanced assymetrical noradrenergic transmission in the olfactory bulb of deoxycorticosterone acetate-salt hypertensive rats

The ablation of olfactory bulb induces critical changes in dopamine, and monoamine oxidase activity in the brain stem. Growing evidence supports the participation of this telencephalic region in the regulation blood pressure and cardiovascular activity but little is known about its contribution to hypertension. We have previously reported that in the olfactory bulb of normotensive rats endothelins enhance noradrenergic activity by increasing tyrosine hydroxylase activity and norepinephrine release. In the present study we sought to establish the status of noradrenergic activity in the olfactory bulb of deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Different steps in norepinephrine transmission including tyrosine hydroxylase activity, neuronal norepinephrine release and uptake were assessed in the left and right olfactory bulb of DOCA-salt hypertensive rats. Increased tyrosine hydroxylase activity, and decreased neuronal norepinephrine uptake were observed in the olfactory bulb of DOCA-salt hypertensive rats. Furthermore the expression of tyrosine hydroxylase and its phosphorylated forms were also augmented. Intriguingly, asymmetrical responses between the right and left olfactory bulb of normotensive and hypertensive rats were observed. Neuronal norepinephrine release was increased in the right but not in the left olfactory bulb of DOCA-salt hypertensive rats, whereas non asymmetrical differences were observed in normotensive animals. Present findings indicate that the olfactory bulb of hypertensive rats show an asymmetrical increase in norepinephrine activity. The observed changes in noradrenergic transmission may likely contribute to the onset and/or progression of hypertension in this animal model.

Angiotensin type 1a receptors in the subfornical organ are required for deoxycorticosterone acetate-salt hypertension

Although elevated renin-angiotensin system activity and angiotensinergic signaling within the brain are required for hypertension, polydipsia, and increased metabolic rate induced by deoxycorticosterone acetate (DOCA)-salt, the contribution of specific receptor subtypes and brain nuclei mediating these responses remains poorly defined. We hypothesized that angiotensin type 1a receptors (AT(1a)R) within the subfornical organ (SFO) mediate these responses. Transgenic mice carrying a conditional allele of the endogenous AT(1a)R (AT(1a)R(flox)) were administered an adenovirus encoding Cre-recombinase and enhanced green fluorescent protein (eGFP) or adenovirus encoding eGFP alone into the lateral cerebral ventricle. Adenovirus encoding Cre-recombinase reduced AT(1a)R mRNA and induced recombination in AT(1a)R(flox) genomic DNA specifically in the SFO, without significant effect in the paraventricular or arcuate nuclei, and also induced SFO-specific recombination in ROSA(TdTomato) reporter mice. The effect of SFO-targeted ablation of endogenous AT(1a)R was evaluated in AT(1a)R(flox) mice at 3 time points: (1) baseline, (2) 1 week after virus injection but before DOCA-salt, and (3) after 3 weeks of DOCA-salt. DOCA-salt-treated mice with deletion of AT(1a)R in SFO exhibited a blunted increase in arterial pressure. Increased sympathetic cardiac modulation and urine copeptin, a marker of vasopressin release, were both significantly reduced in DOCA-salt mice when AT(1a)R was deleted in the SFO. Additionally, deletion of AT(1a)R in the SFO significantly attenuated the polydipsia, polyuria, and sodium intake in response to DOCA-salt. Together, these data highlight the contribution of AT(1a)R in the SFO to arterial pressure regulation potentially through changes on sympathetic cardiac modulation, vasopressin release, and hydromineral balance in the DOCA-salt model of hypertension.

Ryanodine receptors are uncoupled from contraction in rat vena cava

Ryanodine receptors (RyR) are Ca(2+)-sensitive ion channels in the sarcoplasmic reticulum (SR) membrane, and are important effectors of SR Ca(2+) release and smooth muscle excitation-contraction coupling. While the relationship between RyR activation and contraction is well characterized in arteries, little is known about the role of RyR in excitation-contraction coupling in veins. We hypothesized that RyR are present and directly coupled to contraction in rat aorta (RA) and vena cava (RVC). RA and RVC expressed mRNA for all 3 RyR subtypes, and immunofluorescence showed RyR protein was present in RA and RVC smooth muscle cells. RA and RVC rings contracted when Ca(2+) was re-introduced after stores depletion with thapsigargin (1μM), indicating both tissues contained intracellular Ca(2+) stores. To assess RyR function, contraction was then measured in RA and RVC exposed to the RyR activator caffeine (20mM). In RA, caffeine caused contraction that was attenuated by the RyR antagonists ryanodine (10μM) and tetracaine (100μM). However, caffeine (20mM) did not contract RVC. We next measured contraction and intracellular Ca(2+) (Ca(2+)(i)) simultaneously in RA and RVC exposed to caffeine. While caffeine increased Ca(2+)(i) and contracted RA, it had no significant effect on Ca(2+)(i) or contraction in RVC. These data suggest that ryanodine receptors, while present in both RA and RVC, are inactive and uncoupled from Ca(2+) release and contraction in RVC.

Requirement for functional BK channels in maintaining oscillation in venomotor tone revealed by species differences in expression of the β1 accessory subunits

We determined the possible role of large-conductance Ca2+-activated K (BK) channels in regulation of venous tone in small capacitance veins and blood pressure. In rat mesenteric venous smooth muscle cells (MV SMC), BK channel α- and β1-subunits were coexpressed, unitary BK currents were detected, and single-channel currents were sensitive to voltage and [Ca2+]i. Rat MV SMCs displayed Ca sparks and iberiotoxin-sensitive spontaneous transient outward currents. Under resting conditions in vitro, rat MV exhibited nifedipine-sensitive spontaneous oscillatory constrictions. Blockade of BK channels by paxilline and Ca2+ sparks by ryanodine constricted rat MV. Nifedipine caused venodilation and blocked paxilline-induced, KCl-induced (20 mM), and BayK8644-induced contraction. Acute inhibition of BK channels with iberiotoxin in vivo increased blood pressure and reduced venous capacitance, measured as an increase in mean circulatory filling pressure in conscious rats. BK channel α-subunits and L-type Ca2+ channel α1-C subunits are expressed in murine MV. However, these channels are not functional because murine MV lack nifedipine-sensitive basal tone and rhythmic constrictions. Murine MV were also insensitive to paxilline, ryanodine, KCl, and BayK8644, consistent with our previous studies showing that murine MV do not have BK β1-subunits. These data show that not only there are species-dependent properties in ion channel control of venomotor tone but also BK channels are required for rhythmic oscillations in venous tone.

Chronic endothelin A receptor blockade attenuates contribution of sympathetic nervous system to salt hypertension development in adult but not in young Dahl rats

AIM

Endothelin-1 (ET-1) plays an important role in the pathogenesis of salt-dependent forms of hypertension in adult rats, but its participation in salt hypertension elicited in immature rats is still unknown. Therefore, we compared ET-1 role in the development or the maintenance of salt hypertension induced in young (4-week-old) or adult (12-week-old) Dahl rats.

METHODS

The contribution of pressor ET-1 effects to the maintenance of high blood pressure (BP) was studied using acute injection of ET(A) receptor antagonist ambrisentan (BSF 208075, 1 mg kg(-1) iv) to young or adult rats with established salt hypertension. Furthermore, using chronic ambrisentan treatment (30 mg kg(-1) day(-1) in the drinking fluid during 5 weeks of high salt intake), we investigated the age-dependent involvement of ET(A) receptors in salt hypertension development in these two age groups.

RESULTS

Acute ET(A) receptor blockade lowered BP in both age groups of salt hypertensive Dahl rats more than in rats fed a low-salt diet (but without any age-dependent difference). Chronic ET(A) receptor blockade strongly attenuated the development of salt hypertension and cardiac hypertrophy in adult rats, but it had no significant effects on salt hypertension in young animals. Pronounced BP reduction induced in adult salt hypertensive rats by chronic ambrisentan treatment was attributed to attenuated sympathetic BP component, without changes in nitric oxide (NO)-dependent BP regulation. In contrast, chronic ambrisentan treatment of young animals did not modify sympathetic BP component but substantially attenuated NO-dependent vasodilatation.

CONCLUSIONS

ET(A) receptor-mediated ET-1 effects play an important role in salt hypertension of adult but not young Dahl rats.

Efficacy of selective mineralocorticoid and glucocorticoid agonists in canine septic shock

OBJECTIVE

Corticosteroid regimens that stimulate both mineralocorticoid and glucocorticoid pathways consistently reverse vasopressor-dependent hypotension in septic shock but have variable effects on survival. The objective of this study was to determine whether exogenous mineralocorticoid and glucocorticoid treatments have distinct effects and whether the timing of administration alters their effects in septic shock. DESIGN, SETTING, SUBJECTS, AND INTERVENTIONS: Desoxycorticosterone, a selective mineralocorticoid agonist; dexamethasone, a selective glucocorticoid agonist; and placebo were administered either several days before (prophylactic) or immediately after (therapeutic) infectious challenge and continued for 96 hrs in 74 canines with staphylococcal pneumonia.

MEASUREMENTS AND MAIN RESULTS

Effects of desoxycorticosterone and dexamethasone were different and opposite depending on timing of administration for survival (p = .05); fluid requirements (p = .05); central venous pressures (p ≤ .007); indicators of hemoconcentration (i.e., sodium [p = .0004], albumin [p = .05], and platelet counts [p = .02]); interleukin-6 levels (p = .04); and cardiac dysfunction (p = .05). Prophylactic desoxycorticosterone treatment significantly improved survival, shock, and all the other outcomes stated, but therapeutic desoxycorticosterone did not. Conversely, prophylactic dexamethasone was much less effective for improving these outcomes compared with therapeutic dexamethasone with the exception of shock reversal. Prophylactic dexamethasone given before sepsis induction also significantly reduced serum aldosterone and cortisol levels and increased body temperature and lactate levels compared with therapeutic dexamethasone (p ≤ .05), consistent with adrenal suppression.

CONCLUSIONS

In septic shock, mineralocorticoids are only beneficial if given prophylactically, whereas glucocorticoids are most beneficial when given close to the onset of infection. Prophylactic mineralocorticoids should be further investigated in patients at high risk to develop sepsis, whereas glucocorticoids should only be administered therapeutically to prevent adrenal suppression and worse outcomes.

Allopurinol does not decrease blood pressure or prevent the development of hypertension in the deoxycorticosterone acetate-salt rat model

Reactive oxygen species play an important role in the pathogenesis of hypertension, disease in which reactive oxygen species levels and markers of oxidative stress are increased. Xanthine oxidase (XO) is a reactive oxygen species-producing enzyme the activity of which may increase during hypertension. Studies on XO inhibition effects on blood pressure have yielded controversial results. We hypothesized that XO inhibition would decrease blood pressure or attenuate the development of deoxycorticosterone acetate (DOCA)-salt hypertension. We administered the XO inhibitor, allopurinol (50 mg/kg per day, orally) or its vehicle to rats during the established or development stages of DOCA-salt hypertension. We validated XO inhibition by high-performance liquid chromatography measurements of XO metabolites in urine, serum, and tissues demonstrating a decrease in products, increase in substrates, and detection of the active metabolite of allopurinol, oxypurinol. We monitored blood pressure continuously through radiotelemetry and performed gross evaluations of target organs of hypertension. Allopurinol treatment did not impact the course of DOCA-salt hypertension regardless of the timing of administration. Aside from a significant decrease in pulse pressure in allopurinol-treated rats, no positive differences were observed between the allopurinol and the vehicle-treated rats. We conclude that XO does not play an important role in the development or maintenance of hypertension in the rat DOCA-salt hypertension model.

Splanchnic sympathetic nerves in the development of mild DOCA-salt hypertension

We previously reported that mild deoxycorticosterone acetate (DOCA)-salt hypertension develops in the absence of generalized sympathoexcitation. However, sympathetic nervous system activity (SNA) is regionally heterogeneous, so we began to investigate the role of sympathetic nerves to specific regions. Our first study on that possibility revealed no contribution of renal nerves to hypertension development. The splanchnic sympathetic nerves are implicated in blood pressure (BP) regulation because splanchnic denervation effectively lowers BP in human hypertension. Here we tested the hypothesis that splanchnic SNA contributes to the development of mild DOCA-salt hypertension. Splanchnic denervation was achieved by celiac ganglionectomy (CGX) in one group of rats while another group underwent sham surgery (SHAM-GX). After DOCA treatment (50 mg/kg) in rats with both kidneys intact, CGX rats exhibited a significantly attenuated increase in BP compared with SHAM-GX rats (15.6 ± 2.2 vs. 25.6 ± 2.2 mmHg, day 28 after DOCA treatment). In other rats, whole body norepinephrine (NE) spillover, measured to determine if CGX attenuated hypertension development by reducing global SNA, was not found to be different between SHAM-GX and CGX rats. In a third group, nonhepatic splanchnic NE spillover was measured as an index of splanchnic SNA, but this was not different between SHAM (non-DOCA-treated) and DOCA rats during hypertension development. In a final group, CGX effectively abolished nonhepatic splanchnic NE spillover. These data suggest that an intact splanchnic innervation is necessary for mild DOCA-salt hypertension development but not increased splanchnic SNA or NE release. Increased splanchnic vascular reactivity to NE during DOCA-salt treatment is one possible explanation.

Could mineralocorticoids play a role in the pathophysiology of open angle glaucoma?

Since the pathomechanisms of primary open angle glaucoma are still not defined, different aspects related to this topic have to be discussed and further investigated. Possible candidates are the mineralocorticoids, which are known to lower intraocular pressure. A data search and personal investigations assume a limited role of mineralocorticoids for the development of glaucoma. Specific experiments for a final conclusion are, however, not yet performed.

Limb venous compliance responses to lower body negative pressure in humans with high blood pressure

This study tested the hypothesis that limb venous responses to baroreceptor unloading are altered in individuals with high blood pressure (HBP) compared with normotensive (NT) controls. Calf venous compliance was assessed in 20 subjects with prehypertension and stage-1 hypertension (mean arterial pressure, MAP: 104±1 mm Hg) and 13 NT controls (MAP: 86±2 mm Hg) at baseline and during lower body negative pressure (LBNP), using venous occlusion plethysmography. Baroreflex sensitivity (BRS) was measured using the sequence technique and total peripheral resistance (TPR) was estimated from finger plethysmography. Baseline venous compliance was not different between groups, but the HBP group had lower baseline lnBRS (2.22±0.14 vs 2.7±0.18 ms mm Hg(-1)) and greater baseline TPR (3828±138 vs 3250±111 dyn sec(-1) cm(-5) m(2), P<0.05). Calf venous compliance was reduced in response to LBNP only in the NT group (P<0.05). The HBP group had a greater increase in TPR (ΔTPR) compared with the NT group (+1649±335 vs +718±196 dyn sec(-1) cm(-5) m(2), P<0.05). In conclusion, the early stages of hypertension are characterized by an attenuated venoconstrictor response to baroreceptor unloading, which may compensate for an exaggerated vasoconstrictor response and protect against further increases in blood pressure.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Mild DOCA-salt hypertension: sympathetic system and role of renal nerves

Excess sympathetic nervous system activity (SNA) is linked to human essential and experimental hypertension. To test whether sympathetic activation is associated with a model of deoxycorticosterone acetate (DOCA)-salt hypertension featuring two kidneys and a moderate elevation of blood pressure, we measured whole body norepinephrine (NE) spillover as an index of global SNA. Studies were conducted in chronically catheterized male Sprague-Dawley rats drinking water containing 1% NaCl and 0.2% KCl. After a 7-day surgical recovery and a 3-day control period, a DOCA pellet (50 mg/kg) was implanted subcutaneously in one group of rats (DOCA), while the other group underwent sham implantation (Sham). NE spillover was measured on control day 2 and days 7 and 14 after DOCA administration or sham implantation. During the control period, mean arterial pressure (MAP) was similar in Sham and DOCA rats. MAP was significantly increased in the DOCA group compared with the Sham group after DOCA administration (day 14: Sham = 109 ± 5.3, DOCA = 128 ± 3.6 mmHg). However, plasma NE concentration, clearance, and spillover were not different in the two groups at any time. To determine whether selective sympathetic activation to the kidneys contributes to hypertension development, additional studies were performed in renal denervated (RDX) and sham-denervated (Sham-DX) rats. MAP, measured by radiotelemetry, was similar in both groups during the control and DOCA treatment periods. In conclusion, global SNA is not increased during the development of mild DOCA-salt hypertension, and fully intact renal nerves are not essential for hypertension development in this model.

Large-conductance Ca2+-activated K+ channel beta1-subunit knockout mice are not hypertensive

Large-conductance Ca2+-activated K+ (BK) channels are composed of pore-forming α-subunits and accessory β1-subunits that modulate Ca2+ sensitivity. BK channels regulate arterial myogenic tone and renal Na+ clearance/K+ reabsorption. Previous studies using indirect or short-term blood pressure measurements found that BK channel β1-subunit knockout (BK β1-KO) mice were hypertensive. We evaluated 24-h mean arterial pressure (MAP) and heart rate in BK β1-KO mice using radiotelemetry. BK β1-KO mice did not have a higher 24-h average MAP when compared with wild-type (WT) mice, although MAP was ∼10 mmHg higher at night. The dose-dependent peak declines in MAP by nifedipine were only slightly larger in BK β1-KO mice. In BK β1-KO mice, giving 1% NaCl to mice to drink for 7 days caused a transient (5 days) elevation of MAP (∼5 mmHg); MAP returned to pre-saline levels by day 6. BK β1-KO mesenteric arteries in vitro demonstrated diminished contractile responses to paxilline, increased reactivity to Bay K 8644 and norepinephrine (NE), and maintained relaxation to isoproterenol. Paxilline and Bay K 8644 did not constrict WT or BK β1-KO mesenteric veins (MV). BK β1-subunits are not expressed in MV. The results indicate that BK β1-KO mice are not hypertensive on normal or high-salt intake. BK channel deficiency increases arterial reactivity to NE and L-type Ca2+ channel function in vitro, but the L-type Ca2+ channel modulation of MAP is not altered in BK β1-KO mice. BK and L-type Ca(2+) channels do not modulate murine venous tone. It appears that selective loss of BK channel function in arteries only is not sufficient to cause sustained hypertension.

Adrenergic mechanisms do not contribute to age-related decreases in calf venous compliance

Limb venous compliance decreases with advancing age, even in healthy humans. To test the hypothesis that adrenergic mechanisms contribute to age-associated reductions in limb venous compliance, we measured calf venous compliance before and during acute systemic α- and β-adrenergic blockade in eight young (27 ± 1 yr old, mean ± SE) and eight older healthy men (67 ± 2 yr old). Calf venous compliance was determined in supine subjects by inflating a thigh-collecting cuff to 60 mmHg for 8 min and then decreasing it (1 mmHg/s) to 0 mmHg while calf volume was indexed with a strain gauge. The slope (·10⁻³) of the pressure-compliance relation (compliance= β₁ + 2·β₂·cuff pressure), which is the first derivative of the quadratic pressure-volume relation [(Δlimb volume) = β₀+ β₁·(cuff pressure) + β₂·(cuff pressure)²] during reductions in cuff pressure, was used to quantify calf venous compliance. Calf venous compliance was ∼30% lower (P < 0.01) in older compared with young men before adrenergic blockade. In response to adrenergic blockade calf venous compliance did not increase in young (-2.62 ± 0.14 and -2.29 ± 0.18 ml·dl⁻¹·mmHg⁻¹, before and during blockade, respectively) or older men (-1.78 ± 0.27 and -1.68 ± 0.21 ml·dl⁻¹ ·mmHg⁻¹). Moreover, during adrenergic blockade differences in calf venous compliance between young and older men observed before adrenergic blockade persisted. Collectively, these data strongly suggest that adrenergic mechanisms neither directly restrain calf venous compliance in young or older men nor do they contribute to age-associated reductions in calf venous compliance in healthy men.

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

Alterations in sympathetic neuroeffector transmission to mesenteric arteries but not veins in DOCA-salt hypertension

We studied hypertension-associated changes in prejunctional alpha(2) adrenergic receptor (alpha(2)-AR) function using amperometry to monitor in vitro norepinephrine (NE) measured as oxidation currents. Vasoconstriction was measured using video imaging. NE release was induced by electrical stimulation of sympathetic nerves associated with mesenteric arteries (MA) and veins (MV) of sham and DOCA-salt hypertensive rats. NE oxidation currents were larger in DOCA-salt compared to sham MA; there were no differences between currents in sham and DOCA-salt MV. Increases in NE oxidation currents followed a multi-exponential time course in sham MA. In DOCA-salt MA and sham and DOCA-salt MV, the time course was mono-exponential. Yohimbine (alpha(2)-AR antagonist, 1 microM), caused a mono-exponential increase in NE oxidation currents in sham and DOCA-salt MA. Yohimbine increased NE oxidation currents and constrictions more in sham compared to DOCA-salt MA and compared to MV. UK 14,304 (alpha(2)-AR agonist, 1.0 microM), reduced currents less in DOCA-salt MA and sham and DOCA-salt MV compared to sham MA. Prazosin (alpha(1)-AR antagonist, 0.1 microM) did not alter NE oxidation currents. Prazosin inhibited constrictions more in DOCA-salt compared to sham MA and almost completely blocked constrictions in sham and DOCA-salt MV. Prazosin-resistant constrictions in MA were blocked by the P2 receptor antagonist, PPADS (10 microM). Prejunctional alpha(2)-ARs modify NE concentrations near neuroeffector junctions in MA and MV. alpha(2)-AR function is most prominent in MA and is impaired in DOCA-salt MA but not MV. Purinergic transmission predominates in sham MA. NE is the dominant vasoconstrictor in DOCA-salt MA and sham and DOCA-salt MV.

Neurohumoral mechanisms in deoxycorticosterone acetate (DOCA)-salt hypertension in rats

This brief review describes the role of neural and non-neural mechanisms during different phases of deoxycorticosterone acetate (DOCA)-salt hypertension. There are contradictory data for and against a role of the sympathetic nervous system and neurohumoral agents, including endothelin and vasopressin. Elucidating the factors responsible for DOCA-salt hypertension will be helpful in understanding the causes of hypertension resulting from hypervolaemia, hyperaldosteronism and high salt intake.

A comparison of reactive oxygen species metabolism in the rat aorta and vena cava: focus on xanthine oxidase

Reactive oxygen species (ROS) are important mediators in vascular biology. Venous function, although relevant to cardiovascular disease, is still understudied. We compared aspects of ROS metabolism between a major artery (the aorta) and a major vein (the vena cava, VC) of the rat, with the hypothesis that venous ROS metabolism would be overall increased compared with its arterial counterpart. Superoxide and hydrogen peroxide (H2O2) release in basal conditions was higher in VC compared with aorta. The antioxidant capacity for H2O2 was also higher in VC than in aorta. Exogenous superoxide induced a higher contraction in VC compared with aorta. Protein expression of three major ROS metabolizing enzymes, xanthine oxidase (XO), CuZn-SOD, and catalase, was higher in VC compared with aorta. Because XO seemed a likely source of the higher VC ROS levels, we examined it further and found higher mRNA expression and activity of XO in VC compared with aorta. We also investigated the impact of XO inhibition by allopurinol on aorta and VC functional responses to norepinephrine, ANG II, ET-1, and ACh. Maximal ET-1-mediated contraction was decreased by allopurinol in VC but not in the aorta. Our results suggest that there are overall differences in ROS metabolism between aorta and VC, with the latter operating normally at a higher set point, releasing but also being able to handle, higher ROS levels. We propose XO to be an important source for these differences. The result of this particular comparison may be reflective of a general arteriovenous contrast.

Limb venous tone and responsiveness in hypertensive humans

Hypertensive (HTN) animal models demonstrate lower venous compliance as well as increased venous tone and responsiveness compared with normotensive (NTN) controls. However, the extent to which findings in experimental animals can be extended to humans is unknown. Forearm and calf venous compliance were quantified in 9 NTN (23 +/- 1 yr) and 9 HTN (24 +/- 1 yr) men at baseline, after administration of nitroglycerin (NTG), during a cold pressor test (CP), and post-handgrip exercise ischemia (PEI). Individual pressure-volume relationships from a cuff deflation protocol (1 mmHg/s) were modeled with a quadratic regression. Regression parameters beta(1) and beta(2) were used to calculate compliance. A one-way ANOVA was used to compare the beta parameters and a repeated-measures ANOVA was used to compare volumes across all pressures (between groups at baseline and within groups during perturbations). Limb venous compliance was similar between groups (forearm: NTN beta(1) = 0.11 +/- 0.01 and beta(2) = -0.00097 +/- 0.0001, HTN beta(1) = 0.10 +/- 0.01 and beta(2) = -0.00088 +/- 0.0001; calf: NTN beta(1) = 0.12 +/- 0.01 and beta(2) = -0.00102 +/- 0.0001, HTN beta(1) = 0.11 +/- 0.01 and beta(2) = -0.00090 +/- 0.0001). However, at baseline, volume across all pressures (i.e., capacitance) was lower in the forearm (P < or = 0.01) and tended to be lower in the calf (P = 0.08) in HTN subjects. Venous compliance was not altered by any perturbation in either group. Forearm volume was increased during NTG in HTN subjects only. While venous compliance was similar between NTN and HTN adults, HTN adults have lower forearm venous capacitance (volume) which is increased with NTG. These data suggest that young HTN adults may have augmented venous smooth muscle tone compared with NTN controls.

A role for benzamil-sensitive proteins of the central nervous system in the pathogenesis of salt-dependent hypertension

1. Although increasing evidence suggests that salt-sensitive hypertension is a disorder of the central nervous system (CNS), little is known about the critical proteins (e.g. ion channels or exchangers) that play a role in the pathogenesis of the disease. 2. Central pathways involved in the regulation of arterial pressure have been investigated. In addition, systems such as the renin-angiotensin-aldosterone axis, initially characterized in the periphery, are present in the CNS and seem to play a role in the regulation of arterial pressure. 3. Central administration of amiloride, or its analogue benzamil hydrochloride, has been shown to attenuate several forms of salt-sensitive hypertension. In addition, intracerebroventricular (i.c.v.) benzamil effectively blocks pressor responses to acute osmotic stimuli, such as i.c.v. hypertonic saline. Amiloride or its analogues have been shown to interact with the brain renin-angiotensin-aldosterone system (RAAS) and to effect the expression of endogenous ouabain-like compounds. Alterations of brain RAAS function and/or endobain expression could play a role in the interaction between amiloride compounds and arterial pressure. Peripheral treatments with benzamil, even at higher doses than those given centrally, have little or no effect on arterial pressure. These data provide strong evidence that benzamil-sensitive proteins (BSPs) of the CNS play a role in cardiovascular responsiveness to sodium. 4. Mineralocorticoids have been linked to human hypertension; many patients with essential hypertension respond well to pharmacological agents antagonizing the mineralocorticoid receptor and certain genetic forms of hypertension are caused by chronically elevated levels of aldosterone. The deoxycorticosterone acetate (DOCA)-salt model of hypertension is a benzamil-sensitive model that incorporates several factors implicated in the aetiology of human disease, including mineralocorticoid action and increased dietary sodium. The DOCA-salt model is ideal for investigating the role of BSPs in the pathogenesis of hypertension, because mineralocorticoid action has been shown to modulate the activity of at least one benzamil-sensitive protein, namely the epithelial sodium channel. 5. Characterizing the BSPs involved in the pathogenesis of hypertension may provide a novel clinical target. Further studies are necessary to determine which BSPs are involved and where, in the nervous system, they are located.

A serotonergic system in veins: serotonin transporter-independent uptake

We hypothesized that the 5-hydroxytryptamine (5-HT; serotonin) system is present and functional in veins. In vena cava (VC), the presence of the 5-HT synthesis rate-limiting enzyme tryptophan hydroxylase-1 mRNA and accumulation of the 5-HT synthesis intermediate 5-hydroxytryptophan after incubation with tryptophan supported the ability of veins to synthesize 5-HT. The presence of 5-HT and its metabolite 5-hydroxyindole acetic acid was measured by high-performance liquid chromatography in VC and jugular vein (JV), and it was compared with similarly sized arteries aorta (RA) and carotid (CA), respectively. In rats treated with the monoamine oxidase-A (MAO-A) inhibitor pargyline to prevent 5-HT metabolism, basal 5-HT levels were higher in veins than in arteries. 5-HT uptake was observed after exposure to exogenous 5-HT in all vessels. The presence of MAO-A and the 5-HT transporter (SERT) in VC was observed by immunohistochemistry and Western analysis. However, 5-HT uptake was not inhibited by the SERT inhibitors fluoxetine and/or fluvoxamine in VC and JV, as opposed to the inhibition in RA and CA. Moreover, studies performed in VC from mutant rats lacking SERT showed no differences in 5-HT uptake compared with VC from wild type. These data suggest the SERT is not functional under physiological conditions in veins. The differences in 5-HT handling between veins and arteries may represent alternative avenues for targeting the 5-HT system in the peripheral circulation for controlling vascular tone.

Atrial natriuretic peptide behaviour and myocyte hypertrophic profile in combined pressure and volume-induced cardiac hypertrophy

OBJECTIVE

To investigate cardiomyocyte hypertrophy and hormonal profile in cardiac hypertrophy resulting from sequentially applied overloads.

METHODS

We studied Sprague-Dawley rats with renovascular hypertension (RV), where pressure overload predominates, or deoxycorticosterone acetate (DOCA)-salt (DS), where volume overload predominates, at 2 and 4 weeks of treatment, and the combination of both models in inverse sequence: RV 2 weeks/DS 2 weeks (RV2/DS2) and DS 2 weeks/RV 2 weeks (DS2/RV2), and their sham controls (Sh).

RESULTS

Blood pressure and cardiomyocyte diameter increased to a similar extent in RV and DS at 2 and 4 weeks and in combined models. Cardiomyocyte length increased remarkably in the DS4 group. Circulating atrial natriuretic peptide (ANP) was elevated in all hypertensive groups after 2 and 4 weeks. The RV2/DS2 group showed similar plasma ANP levels to RV4, but DS2/RV2 exhibited a three-fold increase in ANP levels (P<0.001 versus Sh4, DS2 and DS4). Atrial ANP mRNA remained unchanged in all groups. DS treatment alone or in combination with RV increased left ventricular ANP mRNA, meanwhile only RV treatment increased left ventricular B-type natriuretic peptide (BNP) mRNA. Ventricular ANP expression levels, but not circulating ANP, correlated with both cardiomyocyte diameter (r=0.859, P<0.01) and length (r=0.848, P<0.01). Renal expression of natriuretic peptide receptor C (NPR-C) was unchanged in RV4 but decreased to a similar extent in the DS4 group and both combined treatments.

CONCLUSION

Morphometric patterns seem to be more related to the paracrine function of the heart than to the secretion of ANP and the endocrine function. Plasma ANP in the DS2/RV2 group could indicate a different evolution of the remodelling process. ANP expression seems to be a more sensitive marker for volume than for pressure overload.