Effect of renal denervation on elevation of blood pressure in cold-exposed rats

Temperature, cardiovascular mortality, and the role of hypertension and renin-angiotensin-aldosterone axis in seasonal adversity: a narrative review

Environmental temperature is now well known to have a U-shaped relationship with cardiovascular (CV) and all-cause mortality. Both heat and cold above and below an optimum temperature, respectively, are associated with adverse outcomes. However, cold in general and moderate cold specifically is predominantly responsible for much of temperature-attributable adversity. Importantly, hypertension-the most important CV risk factor-has seasonal variation such that BP is significantly higher in winter. Besides worsening BP control in established hypertensives, cold-induced BP increase also contributes to long-term BP variability among normotensive and pre-hypertensive patients, also a known CV risk factor. Disappointingly, despite the now well-stablished impact of temperature on BP and on CV mortality separately, direct linkage between seasonal BP change and CV outcomes remains preliminary. Proving or disproving this link is of immense clinical and public health importance because if seasonal BP variation contributes to seasonal adversity, this should be a modifiable risk. Mechanistically, existing evidence strongly suggests a central role of the sympathetic nervous system (SNS), and secondarily, the renin-angiotensin-aldosterone axis (RAAS) in mediating cold-induced BP increase. Though numerous other inflammatory, metabolic, and vascular perturbations likely also contribute, these may also well be secondary to cold-induced SNS/RAAS activation. This review aims to summarize the current evidence linking temperature, BP and CV outcomes. We also examine underlying mechanisms especially in regard to the SNS/RAAS axis, and highlight possible mitigation measures for clinicians.

The influence of a moderate temperature drift on thermal physiology and perception

Humans spend approximately 80-90% of their time indoors. In current practice, indoor temperatures in many buildings are controlled very tightly. However, allowing more variation in indoor temperature results in more energy-efficient buildings and could potentially improve human metabolic and cardiovascular health. Therefore, this study aimed to evaluate the effect of a drifting ambient temperature versus a fixed ambient temperature on thermal physiological parameters and subjective perception. A cross-over intervention design was conducted in 16 healthy men (age 26 ± 4 y; BMI 23.0 ± 1.7 kg/m²) between July 2018 and May 2019. All participants underwent two whole-day (8:30-17:00) experimental sessions, during which they were exposed to a drifting (17-25°C with a morning ramp of 2.58°C/h and afternoon ramp of -2.58°C/h) or constant ambient temperature (21°C) in randomized order. The experiments took place in respiratory chambers, which simulated a typical office environment and in which temperature conditions can be controlled accurately. Throughout the experimental sessions core and skin temperature, heart rate, blood pressure, energy expenditure as well as activity levels were measured. Subjective thermal perception, such as thermal comfort and sensation, was assessed by questionnaires every 30 min. Results reveal that energy expenditure was higher in the morning during the drifting session, which was accompanied by an increase in activity levels. Both drifting and fixed sessions were judged as comfortable although during the drift thermal comfort was lower in the morning and afternoon and higher during midday. The results indicate that a drifting ambient temperature can be applied in practice, and as such, can contribute to a healthier and more sustainable built environment. More research is needed to understand the role of a drifting temperature on the long term.

What is cold-related mortality? A multi-disciplinary perspective to inform climate change impact assessments

BACKGROUND

There is a growing discussion regarding the mortality burdens of hot and cold weather and how the balance between these may alter as a result of climate change. Net effects of climate change are often presented, and in some settings these may suggest that reductions in cold-related mortality will outweigh increases in heat-related mortality. However, key to these discussions is that the magnitude of temperature-related mortality is wholly sensitive to the placement of the temperature threshold above or below which effects are modelled. For cold exposure especially, where threshold effects are often ill-defined, choices in threshold placement have varied widely between published studies, even within the same location. Despite this, there is little discussion around appropriate threshold selection and whether reported associations reflect true causal relationships - i.e. whether all deaths occurring below a given temperature threshold can be regarded as cold-related and are therefore likely to decrease as climate warms.

OBJECTIVES

Our objectives are to initiate a discussion around the importance of threshold placement and examine evidence for causality across the full range of temperatures used to quantify cold-related mortality. We examine whether understanding causal mechanisms can inform threshold selection, the interpretation of current and future cold-related health burdens and their use in policy formation.

METHODS

Using Greater London data as an example, we first illustrate the sensitivity of cold related mortality to threshold selection. Using the Bradford Hill criteria as a framework, we then integrate knowledge and evidence from multiple disciplines and areas- including animal and human physiology, epidemiology, biomarker studies and population level studies. This allows for discussion of several possible direct and indirect causal mechanisms operating across the range of 'cold' temperatures and lag periods used in health impact studies, and whether this in turn can inform appropriate threshold placement.

RESULTS

Evidence from a range of disciplines appears to support a causal relationship for cold across a range of temperatures and lag periods, although there is more consistent evidence for a causal effect at more extreme temperatures. It is plausible that 'direct' mechanisms for cold mortality are likely to occur at lower temperatures and 'indirect' mechanisms (e.g. via increased spread of infection) may occur at milder temperatures.

CONCLUSIONS

Separating the effects of 'extreme' and 'moderate' cold (e.g. temperatures between approximately 8-9 °C and 18 °C in the UK) could help the interpretation of studies quoting attributable mortality burdens. However there remains the general dilemma of whether it is better to use a lower cold threshold below which we are more certain of a causal relationship, but at the risk of under-estimating deaths attributable to cold.

Changes in ambient temperature elicit divergent control of metabolic and cardiovascular actions by leptin

Interactions of hypothalamic signaling pathways that control body temperature (BT), blood pressure (BP), and energy balance are poorly understood. We investigated whether the chronic BP and metabolic actions of leptin are differentially modulated by changes in ambient temperature (T_A ). Mean arterial pressure (MAP), heart rate (HR), BT, motor activity (MA), and oxygen consumption (Vo₂) were measured 24 h/d at normal laboratory T_A (23°C), at thermoneutral zone (TNZ, 30°C) for mice or during cold exposure (15°C) in male wild-type mice. After control measurements, leptin (4 μg/kg/min) or saline vehicle was infused for 7 d. At TNZ, leptin reduced food intake (-11.0 ± 0.5 g cumulative deficit) and body weight by 6% but caused no changes in MAP or HR. At 15°C, leptin infusion did not alter food intake but increased MAP and HR (8 ± 1 mmHg and 33 ± 7 bpm), while Vo₂ increased by ∼10%. Leptin reduced plasma glucose and insulin levels at 15°C but not at 30°C. These results demonstrate that the chronic anorexic effects of leptin are enhanced at TNZ, while its effects on insulin and glucose levels are attenuated and its effects on BP and HR are abolished. Conversely, cold T_A caused resistance to leptin's anorexic effects but amplified its effects to raise BP and reduce insulin and glucose levels. Thus, the brain circuits by which leptin regulates food intake and cardiovascular function are differentially influenced by changes in T_A -Do Carmo, J. M., da Silva, A. A., Romero, D. G., Hall, J. E. Changes in ambient temperature elicit divergent control of metabolic and cardiovascular actions by leptin.

Effects of Simulated Heat Waves with Strong Sudden Cooling Weather on ApoE Knockout Mice

This study analyzes the mechanism of influence of heat waves with strong sudden cooling on cardiovascular diseases (CVD) in ApoE−/− mice. The process of heat waves with strong sudden cooling was simulated with a TEM1880 meteorological-environment simulation chamber according to the data obtained at 5 a.m. of 19 June 2006 to 11 p.m. of 22 June 2006. Forty-eight ApoE−/− mice were divided into six blocks based on their weight. Two mice from each block were randomly assigned to control, heat wave, temperature drop, and rewarming temperature groups. The experimental groups were transferred into the climate simulator chamber for exposure to the simulated heat wave process with strong sudden temperature drop. After 55, 59, and 75 h of exposure, the experimental groups were successively removed from the chamber to monitor physiological indicators. Blood samples were collected by decollation, and the hearts were harvested in all groups. The levels of heat stress factors (HSP60, SOD, TNF, sICAM-1, HIF-1α), cold stress factors (NE, EPI), vasoconstrictor factors (ANGII, ET-1, NO), and four items of blood lipid (TC, TG, HDL-C, and LDL-C) were measured in each ApoE−/− mouse. Results showed that the heat waves increased the levels of heat stress factors except SOD decreased, and decreased the levels of vasoconstrictor factors and blood lipid factors except TC increased. The strong sudden temperature drop in the heat wave process increased the levels of cold stress factors, vasoconstrictor factors and four blood lipid items (except the level of HDL-C which decreased) and decreased the levels of heat stress factors (except the level of SOD which increased). The analysis showed that heat waves could enhance atherosclerosis of ApoE−/− mice. The strong sudden temperature drop during the heat wave process increased the plasma concentrations of NE and ANGII, which indicates SNS activation, and resulted in increased blood pressure. NE and ANGII are vasoconstrictors involved in systemic vasoconstriction especially in the superficial areas of the body and conducive to increased blood pressure. The increase in the blood lipid levels of TG, LDL-C, TC, and LDL-C/HDL-C further aggravated CVD. This paper explored the influence mechanism of the heat waves with sudden cooling on CVD in ApoE−/− mice.

Seasonal Variation in Paroxysmal Atrial Fibrillation: A Systematic Review

INTRODUCTION

A variety of cardiovascular diseases have been demonstrated to have seasonal variations with peaks in the winter and troughs in the summer. Studies regarding atrial fibrillation (AF) have had varying results and this review describes the current data regarding the seasonal variation of AF and mechanisms mediating this seasonal fluctuation.

METHODS

A systematic review was conducted of PubMed, EBSCO and OVID for manuscripts describing the association between seasonal variation and the occurrence of AF. Studies meeting eligibility criteria were assessed for quality and reporting bias. Data was extracted in regards to the following associations: seasonal variation and AF paroxysms, temperature and AFparoxysms, duration of daylight and AF paroxysms, barometric pressure and AF paroxysms, alcohol and AF paroxysms, as well as seasonal variation and AF related stroke.

RESULTS

A total of 15 studies were identified for inclusion. Of these, 11 studies assessed seasonal variation and the remaining 4 studies assessed seasonal variation in AF related stroke. AF paroxysms peaked in winter with a trough in summer. There was an inverse correlation between temperatures as well barometric pressure and the occurrence of AF paroxysms and a positive correlation with duration of daylight.

CONCLUSIONS

The rate of occurrence of paroxysmal AF varies by seasons and is greatest during winter and least in summer.

Gastrointestinal intervention ameliorates high blood pressure through antagonizing overdrive of the sympathetic nerve in hypertensive patients and rats

Background

We investigated the hypothesis that the favorable effects of gastrointestinal (GI) intervention on hypertension (HTN) and cardiovascular (CV) disturbances are mediated by antagonizing overdrive of the sympathetic nervous system (SNS).

Methods and Results

Hypertensive patients with metabolic disturbances underwent laparoscopic Roux‐en‐Y gastric bypass surgery, and spontaneously hypertensive rats (SHRs) underwent RYGB or sham surgery. Blood pressure (BP), heart rate (HR), endothelium‐dependent flow‐mediated dilation, and anthropometric as well as laboratory parameters were measured at baseline and during follow‐up. Changes of BP and HR in response to cold stress, renal sympathetic nervous activity (RSNA), vasoconstriction induced by electrical field stimulation, microinjection of nucleus of the solitary tract (NTS), and CV function and structure were examined in SHRs with or without surgery. Compared with baseline, BP and HR were significantly reduced in both hypertensive patients with type 2 diabetes and rats. Impaired endothelial‐dependent vasodilatation and metabolic disturbances in hypertensive patients were also ameliorated after surgery. CV disturbances were reversed by surgery in SHRs. Under acute cold exposure, the variations in BP and HR were smaller in surgically treated SHRs, compared to sham SHRs. RSNA and vasoconstriction induced by perivascular nerve stimulation as well as NTS‐mediated changes of BP were decreased in surgically treated SHRs, compared to sham SHR. Weight loss did not affect BP and RSNA in sham SHRs.

Conclusions

GI intervention ameliorates HTN in both hypertensive patients and rats by inhibiting overdrive of the SNS. Therefore, targeting gastrointestine could be a novel strategy to treat HTN with metabolic disturbances.

Analysis of differentially expressed genes in cold-exposed mice to investigate the potential causes of cold-induced hypertension

Cold exposure is considered to be an important contributing factor to the high morbidity of hypertension. In order to elucidate the cause and mechanism of cold-induced hypertension (CIH), gene expression analysis was performed on microarray data for two groups of cold-exposed mice (4°C for 1 week and 4°C for 5 weeks, three replicates per group) and their respective control groups maintained at 30°C. Analysis results indicated that the differentially expressed genes with the most significance were associated with adaptive thermogenesis, fatty acid metabolism and energy metabolism. The expected marked increase in metabolism during cold exposure caused tissue hypoxia. Genes involved in the hypoxia-inducible factor signaling pathway were activated. In addition, genes associated with oxidative stress were significantly upregulated, including superoxide dismutase 2 (SOD2) and epoxide hydrolase 2 (EPHX2). The majority of genes involved in inflammation-associated pathways were shown to be downregulated in the 4°C 5-week group. Therefore, the results of the present study indicate that tissue hypoxia and increased oxidative stress may play important roles in the process of CIH.

Effects of different cold-air exposure intensities on the risk of cardiovascular disease in healthy and hypertensive rats

Ten-week-old male Wistar rats (systolic blood pressure, 106-116 mmHg; body weight, 300-320 g) and spontaneously hypertensive rats (systolic blood pressure, 160-176 mmHg; body weight, 210.9-244.9 g) were used as healthy and hypertensive subjects to determine the effects of varying degrees of cold-air exposure in a climate chamber box. The three cold-air ranks were cold air I [minimum temperature (TMIN) 6.4 °C, ↓∆T48 8.6 °C], cold air II (TMIN 3.8 °C, ↓∆T48 11.2 °C), and cold air III (TMIN -0.3 °C, ↓∆T48 15.3 °C), as established from the cold-air data of Zhangye City, China. Each cold-air rank consisted of a temperature drop and a temperature increase with the same initial and terminal temperatures (15 °C). After cold-air exposure, the risk factors for cardiovascular disease (CVD) such as systolic blood pressure, whole blood viscosity (10/s and 150/s), plasma fibrinogen, and blood lipids of the rats were determined. The results indicated that the CVD risk factors of the healthy and hypertensive rats increased significantly with cold-air exposure intensities. The increase in systolic blood pressure was greater during temperature drops, whereas the increases in whole blood viscosity and plasma fibrinogen were greater after cold-air exposure. The effects of cold-air exposure on the CVD risk factors of healthy rats, particularly the systolic blood pressure, whole blood viscosity (150/s), and LDL/HDL, were greater than those in hypertensive rats. In conclusion, CVD risk may increase with cold-air ranks. Blood pressure-induced CVD risk may be greater during cold-air temperature drop, whereas atherosclerosis-induced CVD risk may be greater after cold-air exposure. The effect of cold air on the CVD risk factors in healthy subjects may be more significant than those in hypertensive subjects.

Higher frequency of atrial fibrillation linked to colder seasons and air temperature on the day of ischemic stroke onset

BACKGROUND

Whether a seasonal variation of atrial fibrillation among acute ischemic stroke (AIS) patients occurs is unknown. We studied the distribution of atrial fibrillation across seasons and air temperatures in a cohort of AIS patients.

METHODS

We selected 899 AIS patients from the Argentinean Stroke Registry (ReNACer), who were admitted to 43 centers in the Province of Buenos Aires. We recorded the minimum and maximum temperatures at local weather centers on the day and the city where each stroke occurred. We used the goodness-of-fit χ(2) test to assess the distribution of atrial fibrillation across seasons and air temperatures and the Pearson correlation coefficient to assess the relationship between these variables. We developed a regression model for testing the association between seasons and atrial fibrillation.

RESULTS

We found a seasonal variation in the occurrence of atrial fibrillation, with a peak in winter and a valley in summer (23.1% versus 14.0%, P < .001). The semester comprised by autumn and winter was associated with atrial fibrillation (Pearson P < .001). Atrial fibrillation showed a nonhomogeneous distribution across ranges of temperature (P < .001, goodness-of-fit test), with a peak between 5°C and 9°C, and was associated with minimum (Pearson P = .042) and maximum (Pearson P = .002) air temperature. After adjusting for significant covariates, there was a 2-fold risk of atrial fibrillation during autumn and winter.

CONCLUSIONS

In this cohort of AIS patients, atrial fibrillation showed a seasonal variation and a nonhomogeneous distribution across air temperatures, with peaks in cold seasons and low temperatures on the day of stroke onset.

AAV-based RNAi silencing of NADPH oxidase gp91(phox) attenuates cold-induced cardiovascular dysfunction

Clinical observations and epidemiological surveys indicated that the prevalence of hypertension and heart diseases is increased in cold regions or during winter. Cold exposure increased NADPH oxidase gp91(phox) protein expression in heart, kidneys, and aorta in rats. The aim of this study was to investigate if RNA interference (RNAi) silencing of gp91(phox) would attenuate cold-induced hypertension and cardiovascular and renal damage. The recombinant adeno-associated virus serotype 2 (AAV-2) vector carrying gp91(phox)-shRNA (gp91-shRNA) was constructed for inhibiting gp91(phox) protein expression in cold-exposed rats. Blood pressure (BP) was monitored using a telemetry system. BP was increased in the Control-shRNA and PBS groups within 1 week of exposure to moderate cold (5°C) and reached a plateau after 7 weeks. The cold-induced increase in BP was attenuated significantly by intravenous delivery of gp91-shRNA (1.25×10(10) particles/rat, 0.5 mL). One single dose of gp91-shRNA controlled hypertension for up to 10 weeks. In addition, gp91-shRNA reversed cold-induced vascular dysfunction. gp91-shRNA abolished the cold-induced up-regulation of gp91(phox) protein expression in heart, kidneys, and aorta, confirming effective silencing of gp91(phox). The cold-induced increases in NADPH oxidase activity and superoxide production were eliminated by silencing of gp91(phox), suggesting that the cold-induced up-regulation of NADPH oxidase activity may be attributed to the increased gp91(phox) protein expression. RNAi silencing of gp91(phox) abolished cold-induced cardiac and renal hypertrophy and attenuated aortic, coronary, and renal remodeling. The up-regulation of gp91(phox) may play a critical role in cold-induced cardiovascular dysfunction and organ damage. AAV delivery of gp91-shRNA may be a new and effective therapeutic approach for cold-related cardiovascular disorders.

Effects of cold air on cardiovascular disease risk factors in rat

The purpose of this study is to explore possible potential implications of cold air in cardiovascular disease (CVD) risk in rats. Healthy Wistar rats were exposed to artificial cold air under laboratory conditions, and their systolic blood pressure, heart rate, vasoconstriction, CVD risk factors, and myocardial damage indicators after cold air exposure were determined and evaluated. Systolic blood pressure, whole blood viscosity, and plasma level of norepinephrine, angiotensinⅡ, low density lipoprotein, total cholesterol, and fibrinogen in treatment groups increased significantly compared with control groups. No significant variations were found in plasma Mb and cTnT and myocardial tissue between the treatment and control groups. Results indicate that: (1) higher levels of SBP, WBV and LDL/HDL, total cholesterol (TC), and FG in blood may indicate higher CVD risks during cold air exposure; (2) cold air may exert continuous impacts on SBP and other CVD risk factors.

Ribonucleic acid interference knockdown of interleukin 6 attenuates cold-induced hypertension

The purpose of this study was to determine the role of the proinflammatory cytokine interleukin (IL) 6 in cold-induced hypertension. Four groups of male Sprague-Dawley rats were used (6 rats per group). After blood pressure was stabilized, 3 groups received intravenous delivery of adenoassociated virus carrying IL-6 small hairpin RNA (shRNA), adenoassociated virus carrying scrambled shRNA, and PBS, respectively, before exposure to a cold environment (5 degrees C). The last group received PBS and was kept at room temperature (25 degrees C, warm) as a control. Adenoassociated virus delivery of IL-6 shRNA significantly attenuated cold-induced elevation of systolic blood pressure and kept it at the control level for < or =7 weeks (length of the study). Chronic exposure to cold upregulated IL-6 expression in aorta, heart, and kidneys and increased macrophage and T-cell infiltration in kidneys, suggesting that cold exposure increases inflammation. IL-6 shRNA delivery abolished the cold-induced upregulation of IL-6, indicating effective silence of IL-6. Interestingly, RNA interference knockdown of IL-6 prevented cold-induced inflammation, as evidenced by a complete inhibition of tumor necrosis factor-alpha expression and leukocyte infiltration by IL-6 shRNA. RNA interference knockdown of IL-6 significantly decreased the cold-induced increase in vascular superoxide production. It is noted that IL-6 shRNA abolished the cold-induced increase in collagen deposition in the heart, suggesting that inflammation is involved in cold-induced cardiac remodeling. Cold exposure caused glomerular collapses, which could be prevented by knockdown of IL-6, suggesting an important role of inflammation in cold-induced renal damage. In conclusion, cold exposure increased IL-6 expression and inflammation, which play critical roles in the pathogenesis of cold-induced hypertension and cardiac and renal damage.

RNAi silencing of brain klotho potentiates cold-induced elevation of blood pressure via the endothelin pathway

Klotho is a recently identified antiaging gene. Brain endothelin-1 (ET1) is important in the regulation of blood pressure (BP). We hypothesized that silence of brain klotho potentiates cold-induced elevation of BP via the endothelin pathway. To silence brain klotho, we constructed adeno-associated virus (AAV) carrying rat klotho small interference hairpin RNA (KL-shRNA). AAV carrying ET1-shRNA was used to silence brain ET1. Scrambled shRNA was used as Control-shRNA. Three groups of male Sprague-Dawley rats (6 rats/group) received KL-shRNA, KL-shRNA plus ET1-shRNA, and Control-shRNA, respectively, via intracerebroventricular injection. BP was monitored daily using a telemetry system. All animals were exposed to a moderate cold environment (5°C) at 12 days after gene delivery. KL-shRNA significantly increased BP by 9 days of exposure to cold, while BP in the Control-shRNA group remained unchanged. ET1-shRNA abolished KL-shRNA-induced elevation of BP during cold exposure. Interestingly, KL-shRNA increased brain ET1 expression and plasma norepinephrine level, suggesting that silencing of brain klotho increased ET1 production and the sympathetic nervous activity. The KL-shRNA-induced increase in sympathetic nervous activity was mediated by ET1 because it could be abolished by silencing of ET1. These results demonstrated that silencing of brain klotho potentiated and expedited cold-induced elevation of BP by upregulation of ET1 and the subsequent activation of the sympathetic nervous system.

Cardiovascular responses to cold exposure

The prevalence of hypertension is increased in winter and in cold regions of the world. Cold temperatures make hypertension worse and trigger cardiovascular complications (stroke, myocardial infarction, heart failure, etc.). Chronic or intermittent exposure to cold causes hypertension and cardiac hypertrophy in animals. The purpose of this review is to provide the recent advances in the mechanistic investigation of cold-induced hypertension (CIH). Cold temperatures increase the activities of the sympathetic nervous system (SNS) and the renin-angiotensin system (RAS). The SNS initiates CIH via the RAS. Cold exposure suppresses the expression of eNOS and formation of NO, increases the production of endothelin-1 (ET-1), up-regulates ETA receptors, but down-regulates ETB receptors. The roles of these factors and their relations in CIH will be reviewed.

Modification of atrial natriuretic peptide system in cold-induced hypertensive rats

Cold exposure induces hypertension and cardiac hypertrophy via sympathetic activation. The sympathetic nervous system is fundamentally important for the regulation of cardiac atrial natriuretic peptide (ANP) secretion. The present study aimed to define changes in ANP level with renal functions during cold exposure of rats. We also measured the direct effects of adrenergic stimulation on ANP secretion in cold-induced hypertensive rat atria. Sustained elevation of blood pressure and tachycardia were observed by 2-wk cold exposure. Cold exposure increased urine volume, UNaV, UKV and positive water balance. Atrial ANP content, its mRNA level, and plasma ANP concentration increased. Plasma norepinephrine level was increased but both alpha(1A)- and beta(1)-adrenoceptor (AR) mRNA levels in atrium were decreased. In isolated perfused atria from cold-exposed rats, basal ANP secretion increased and pulse pressure decreased. Phenylephrine (alpha(1)-AR agonist)-induced stimulation of ANP secretion, and isoproterenol (beta-AR agonist)-induced suppression of ANP secretion were significantly attenuated. These results suggest that an increased plasma and atrial ANP level by cold exposure may be a compensatory response to changes in hemodynamics and body fluid balance. The phenylephrine- and isoproterenol-induced attenuation of ANP secretion in cold-exposed rat atria may be due to the downregulation of alpha(1A)- and beta(1)-adrenoceptors mRNA levels.

RNAi inhibition of mineralocorticoid receptors prevents the development of cold-induced hypertension

The objective was to determine whether the mineralocorticoid receptor (MR) plays a role in the initiation and development of cold-induced hypertension (CIH) by testing the hypothesis that the RNA interference (RNAi) inhibition of the MR attenuates CIH. The recombinant adeno-associated virus (AAV) carrying a short-hairpin small-interference RNA for MR (MRshRNA) or a scrambled sequence (ControlshRNA) was constructed. Six groups of albino mice were used (6 mice/group). Three groups were exposed to cold (6.7°C), whereas the remaining three groups were kept at room temperature (RT; warm) as controls. In each temperature condition, three groups received an intravenous injection of MRshRNA, ControlshRNA, or virus-free PBS, respectively, before exposure to cold. The viral complexes (0.35 × 1011 particles/mouse, 0.5 ml) or PBS (0.5 ml) was delivered into the circulation via the tail vein. The blood pressure (BP) of the mice treated with ControlshRNA or PBS increased significantly during exposure to cold, whereas the BP of the cold-exposed MRshRNA-treated mice did not increase and remained at the level of the control group kept at RT. Thus AAV delivery of MRshRNA prevented the initiation of CIH. MRshRNA significantly attenuated cardiac and renal hypertrophy. MRshRNA decreased the cold-induced increase in MR protein expression to the control level in the hypothalamus, kidneys, and heart, indicating an effective prevention of the cold-induced upregulation of MR. RNAi inhibition of MR resulted in significant decreases in the plasma level of norepinephrine, plasma renin activity, and plasma level of aldosterone in cold-exposed mice. MR played a critical role in the initiation and development of CIH. AAV delivery of MRshRNA may serve as a new approach for the prevention of cold-induced hypertension.

Heart-specific inhibition of protooncogene c-myc attenuates cold-induced cardiac hypertrophy

The protooncogene c-myc is involved in the regulation of cell growth. Although increased c-Myc expression is found in hypertrophied hearts, the role of c-Myc in the development of cardiac hypertrophy (CH) has never been determined. The aim of this study was to test the effect of heart-specific inhibition of c-Myc expression on the development of cold-induced cardiac hypertrophy (CICH). We hypothesized that heart-specific inhibition of c-Myc expression attenuates CICH. We constructed c-Myc antisense (c-MycAS) plasmid and green fluorescent protein (GFP) plasmid driven by a heart-specific promoter, alpha-myosin heavy chain (MHC). The cell culture study indicated that c-MycAS can effectively inhibit c-Myc expression and that GFP can express in the rat heart cells. Four groups of rats were used to test the effect of in vivo inhibition of cardiac c-Myc expression on the development of CICH. Three groups received an intravenous injection of c-MycAS, GFP and buffer, respectively, at the beginning of exposure to moderate cold (6.7 degrees C), while the last group received buffer and was kept at room temperature (25 degrees C) to serve as a control. Blood pressure (BP) of the cold-exposed groups receiving buffer or GFP increased significantly, whereas BP of the c-MycAS group did not increase until 28 days after exposure to cold. Thus, c-MycAS delayed and attenuated cold-induced hypertension (CIH). The antihypertensive effect of c-MycAS was probably due to the decreased cardiac output. Magnetic resonance imaging (MRI) showed that the in vivo left ventricle wall thickness of cold-exposed rats was decreased significantly by c-MycAS. Consistently, the cold-induced increase in heart weight was attenuated by inhibition of cardiac c-Myc expression. The heart specificity of alpha-MHC promoter was confirmed by the selective inhibition of c-Myc expression in the heart and by the selective expression of both GFP mRNA and GFP protein in the heart. Heart-specific inhibition of c-Myc expression attenuated the development of CICH. The increased c-Myc expression may play a critical role in the pathogenesis of CICH. Thus, heart-specific inhibition of c-Myc expression may be a new and effective approach for the control of CH.

Effects of chronic cold exposure on the endothelin system

Cold temperatures have adverse effects on the human cardiovascular system. Endothelin (ET)-1 is a potent vasoconstrictor. We hypothesized that cold exposure increases ET-1 production and upregulates ET type A (ETA) receptors. The aim of this study was to determine the effect of cold exposure on regulation of the ET system. Four groups of rats (6–7 rats/group) were used: three groups were exposed to moderate cold (6.7 ± 2°C) for 1, 3, and 5 wk, respectively, and the remaining group was maintained at room temperature (25°C) and served as control. Cold exposure significantly increased ET-1 levels in the heart, mesenteric arteries, renal cortex, and renal medulla. Cold exposure increased ETA receptor protein expression in the heart and renal cortex. ET type B (ETB) receptor expression, however, was decreased significantly in the heart and renal medulla of cold-exposed rats. Cold exposure significantly increased the ratio of ETA to ETB receptors in the heart. An additional four groups of rats (3 rats/group) were used to localize changes in ETA and ETB receptors at 1, 3, and 5 wk of cold exposure. Immunohistochemical analysis showed an increase in ETA, but a decrease in ETB, receptor immunoreactivity in cardiomyocytes of cold-exposed rats. Increased ETA receptor immunoreactivity was also found in vascular smooth muscle cells of cold-exposed rats. Cold exposure increased ETA receptor immunoreactivity in tubule epithelial cells in the renal cortex but decreased ETB receptor immunoreactivity in tubule epithelial cells in the renal medulla. Therefore, cold exposure increased ET-1 production, upregulated ETA receptors, and downregulated ETB receptors.

AAV delivery of mineralocorticoid receptor shRNA prevents progression of cold-induced hypertension and attenuates renal damage

The aim of this study was to determine the effect of RNA interference inhibition of mineralocorticoid receptor (MR) on cold-induced hypertension (CIH) and renal damage. Recombinant adeno-associated virus (AAV) carrying short hairpin small interference (si)RNA for MR (AAV.MR-shRNA) was constructed and tested for the ability to inhibit renal MR and to control CIH. Three groups of rats with CIH received AAV.MR-shRNA (1.25 x 10(9) particles/rat, intravenous), AAV carrying scrambled shRNA (AAV.Control-shRNA) (1.25 x 10(9) particles/rat, intravenous) and phosphate buffer solution (PBS), respectively. All rats were kept in a cold chamber (6.7 degrees C) throughout the experiment. Adeno-associated virus delivery of MR-shRNA prevented progression of CIH. Blood pressure (BP) of the AAV.MR-shRNA-treated group did not increase and remained at 145+/-3 mm Hg, whereas BP of the AAV.Control-shRNA-treated and PBS-treated group increased to 167+/-4 and 161+/-3 mm Hg, respectively, at 3 weeks after gene delivery. Thus, the antihypertensive effect of a single injection of AAV.MR-shRNA lasted for at least 3 weeks (length of the study). Adeno-associated virus carrying short hairpin siRNA for MR significantly increased urinary sodium excretion and decreased proteinuria. It also decreased serum creatinine and blood urea nitrogen, suggesting enhanced renal function. Both Western blot and immunohistochemical analysis showed that MR expression was decreased significantly in the kidney in the AAV.MR-shRNA-treated rats, confirming that renal MR is effectively inhibited by AAV.MR-shRNA. Adeno-associated virus carrying short hairpin siRNA for MR also significantly attenuated renal hypertrophy. In addition, AAV delivery of MR-shRNA prevented atrophy and dilation of renal tubules and abolished tubular deposition of proteinaceous material seen in CIH rats.

CONCLUSIONS

(1) AAV delivery of MR-shRNA effectively silenced MR in vivo. (2) RNA interference inhibition of MR may open a new avenue for the long-term control of hypertension and renal damage.

Overview of recent advances in molecular cardiology

Molecular cardiology is a new and fast-growing area of cardiovascular medicine that aims to apply molecular biology techniques for the mechanistic investigation, diagnosis, prevention and treatment of cardiovascular disease. As an emerging discipline, it has changed conceptual thinking of cardiovascular development, disease etiology and pathophysiology. Although molecular cardiology is still at a very early stage, it has opened a promising avenue for understanding and controlling cardiovascular disease. With the rapid development and application of molecular biology techniques, scientists and clinicians are closer to curing heart diseases that were thought to be incurable 20 years ago. There clearly is a need for a more thorough understanding of the molecular mechanisms of cardiovascular diseases to promote the advancement of stem cell therapy and gene therapy for heart diseases. The present paper briefly reviews the state-of-the-art techniques in the following areas of molecular cardiology: gene analysis in the diseased heart; transgenic techniques in cardiac research; gene transfer and gene therapy for cardiovascular disease; and stem cell therapy for cardiovascular disease.

Essential hypertension seems to result from melatonin-induced epigenetic modifications in area postrema

Essential hypertension is a complex multifactorial disorder with epigenetic and environmental factors contributing to its prevalence. Epigenetic system is a genetic regulatory mechanism that allows humans to maintain extraordinarily stable patterns of gene expression over many generations. Sympathetic nervous system plays a major role in the maintenance of hypertension and the rostral ventrolateral medulla is the main source of this sympathetic activation. A possible mechanism to explain the sympathetic hyperactivity in the rostral ventrolateral medulla is an action of the area postrema. Area postrema seems to be the region where a shift of the set-point to a higher operating pressure occurs resulting in hypertension. But, how can a shift occur in the area postrema. We propose that melatonin-induced epigenetic modifications in the neurons of area postrema plays a role in this shift. Area postrema is reported to contain high levels of melatonin receptors that play a role in the epigenetic modifications in certain cells. Environmental stressors cause epigenetic modifications in the neurons of area postrema via the pineal hormone melatonin and these changes lead to a shift in the set-point to a higher operating pressure. This signal is then sent via efferent projections to key medullary sympathetic nuclei in rostral ventrolateral medulla resulting in increases in sympathetic nerve activity. This model may explain the long-term alterations in sympathetic activity in essential hypertension.

Genetic AVP deficiency abolishes cold-induced diuresis but does not attenuate cold-induced hypertension

Chronic cold exposure causes hypertension and diuresis. The aim of this study was to determine whether vasopressin (AVP) plays a role in cold-induced hypertension and diuresis. Two groups of Long-Evans (LE) and two groups of homozygous AVP-deficient Brattleboro (VD) rats were used. Blood pressure (BP) was not different among the four groups during a 2-wk control period at room temperature (25 degrees C, warm). After the control period, one LE group and one VD group were exposed to cold (5 degrees C); the remaining groups were kept at room temperature. BP and body weight were measured weekly during exposure to cold. Food intake, water intake, urine output, and urine osmolality were measured during weeks 1, 3, and 5 of cold exposure. At the end of week 5, all animals were killed and blood was collected for measurement of plasma AVP. Kidneys were removed for measurement of renal medulla V2 receptor mRNA and aquaporin-2 (AQP-2) protein expression. BP of LE and VD rats increased significantly by week 2 of cold exposure and reached a high level by week 5. BP elevations developed at approximately the same rate and to the same degree in LE and VD rats. AVP deficiency significantly increased urine output and solute-free water clearance and decreased urine osmolality. Chronic cold exposure increased urine output and solute-free water clearance and decreased urine osmolality in LE rats, indicating that cold exposure caused diuresis in LE rats. Cold exposure failed to affect these parameters in VD rats, suggesting that the AVP system is responsible for cold-induced diuresis. Cold exposure did not alter plasma AVP in LE rats. Renal medulla V2 receptor mRNA and AQP-2 protein expression levels were decreased significantly in the cold-exposed LE rats, suggesting that cold exposure inhibited renal V2 receptors and AVP-inducible AQP-2 water channels. We conclude that 1) AVP may not be involved in the pathogenesis of cold-induced hypertension, 2) the AVP system plays a critical role in cold-induced diuresis, and 3) cold-induced diuresis is due to suppression of renal V2 receptors and the associated AQP-2 water channels, rather than inhibition of AVP release.

Prolonged attenuation of cold-induced hypertension by adenoviral delivery of renin antisense

BACKGROUND

Renin has been linked to the pathogenesis of some forms of hypertension, including cold-induced hypertension (CIH). Although several antihypertensive drugs that inhibit angiotensin-converting enzyme (ACE) and angiotensin II (Ang II) type 1 (AT(1)) receptors are available, they are short-lasting and have side effects. Inhibition of renin [the first and rate-limiting step of the renin-angiotensin system (RAS)] would provide an inhibition of the entire RAS. Thus, we developed an antisense approach for specific inhibition of renin based on the genetic design. The objective of this study was to test our hypothesis that adenoviral delivery of renin antisense inhibits renin and attenuates CIH.

METHODS

Recombinant adenoviruses carrying rat renin antisense (rAdv.RRA) and LacZ reporter gene (rAdv.LacZ) were constructed and used for in vivo gene transfer via intravenous injection. Four groups of rats were used (six rats/group). Blood pressure did not differ among the four groups during the control period at room temperature (25 degrees C). Two groups of rats received rAdv.RRA (2.5 x 10(9) pfu/rat, intravenously), while the other two groups received the same dose of rAdv.LacZ and served as controls. After gene delivery, one rAdv.LacZ-treated and one rAdv.RRA-treated group were exposed to cold (5 degrees C), while the remaining groups were kept at 25 degrees C. Blood pressure was monitored weekly during cold exposure. A 24-hour urine sample was collected during weeks 1, 3, and 5 for measuring urinary aldosterone excretion. At the end of week 5, all animals were killed and blood was collected for measurement of plasma renin activity (PRA), total plasma renin, plasma active renin, and plasma aldosterone. Vascular Ang II contents were measured in all rats.

RESULTS

Blood pressure of the rAdv.LacZ-treated group rose significantly within 2 weeks of exposure to cold and reached 158.2 +/- 6.4 mm Hg by week 5. In contrast, blood pressure (117.1 +/- 5.3 mm Hg) of the cold-exposed group treated with rAdv.RRA did not increase until 5 weeks after exposure to cold. Thus, a single dose of rAdv.RRA prevented CIH for at least 5 weeks. rAdv.RRA abolished the cold-induced increases in PRA, total plasma renin, plasma active renin, vascular Ang II, and plasma and urine aldosterone, indicating effective inhibition of the entire RAS.

CONCLUSION

rAdv.RRA effectively inhibited the entire RAS and produced prolonged attenuation of CIH. Antisense inhibition of renin may be a novel and ideal approach for long-term control of hypertension.

Human eNOS gene delivery attenuates cold-induced elevation of blood pressure in rats

We previously showed that chronic cold exposure inhibits endothelial nitric oxide synthase (eNOS) expression and decreases nitric oxide (NO) production. The aim of the present study was to evaluate the possible role of the NO system in the development of cold-induced hypertension (CIH) by testing the hypothesis that adenoviral delivery of human eNOS gene increases NO production and attenuates CIH in rats. The effect of in vivo delivery of adenovirus carrying human eNOS full-length cDNA (rAdv.heNOS) on CIH was tested using four groups of Sprague-Dawley rats (6 rats/group). Blood pressure (BP) did not differ among the four groups during the control period at room temperature (24 degrees C). Two groups of rats received intravenous injection of rAdv.heNOS (1 x 10(9) plaque-forming units/rat), and the other two groups received the same dose of rAdv.LacZ to serve as controls. After gene delivery, one rAdv.heNOS-treated group and one rAdv.LacZ-treated group were exposed to cold (6 degrees C) while the remaining groups were kept at 24 degrees C. We found that the BP of the rAdv.LacZ group increased significantly within 1 wk of exposure to cold and reached a peak level at week 5 (152.2 +/- 6.4 mmHg). In contrast, BP (118.7 +/- 8.4 mmHg) of the cold-exposed rAdv.heNOS group did not increase until 5 wk after exposure to cold. The rAdv.heNOS increased plasma and urine levels of NO significantly in cold-exposed rats, which indicates that eNOS gene transfer increased NO production. Notably, rAdv.heNOS decreased plasma levels of norepinephrine and plasma renin activity in cold-exposed rats, which suggests that eNOS gene transfer may decrease the activities of the sympathetic nervous system and the renin-angiotensin system. Immunohistochemical analysis showed that the transferred human eNOS was expressed in both endothelium and adventitia of mesenteric arteries. We conclude that 1) eNOS gene transfer attenuates CIH by increasing NO production and inhibiting the sympathetic nervous system and the renin-angiotensin system; and 2) the NO system appears to mediate this nongenetic, nonpharmacological, nonsurgical model of hypertension.

Historic perspectives and recent advances in major animal models of hypertension

Hypertension and related cardiovascular diseases are the leading causes of death in many countries. The etiology of human essential hypertension is largely unknown. It is highly likely that hypertension is a complex and multifactorial disease resulting from the interaction of multiple genetic and environmental factors. Animal models of hypertension have been proved to be useful to study the pathogenesis of, and to find a new therapy for, hypertension. The aim of this article is to briefly review the most widely used rodent models of experimental hypertension, including history and recent advances. These models are classified as genetically-induced, environmentally-induced, pharmacologically-induced, and renal-induced hypertension according to the way of induction; the typical representatives of each of these major types of experimental hypertension are spontaneous hypertension, cold-induced hypertension, DOCA-salt-induced hypertension, and renal-induced hypertension, respectively. The processes of induction of hypertension, possible pathogenesis, characteristics, advantages, and limitations of these animal models are reviewed. In addition, the clinical implications of the above experimental models of hypertension are addressed.

Genetic AT1Areceptor deficiency attenuates cold-induced hypertension

The aim of this study was to test our hypotheses that AT1Areceptors play a role in the pathogenesis of cold-induced hypertension (CIH) and in the cold-induced increase in drinking responses to ANG II. Two groups of wild-type (WT) and two groups of AT1Areceptor gene knockout (AT1A-KO) mice were used (6/group). Blood pressures (BP) of the four groups were similar during the control period at room temperature (25°C). After the control period, one group of WT and one group of AT1A-KO mice were exposed to cold (5°C), while the remaining groups were kept at 25°C. BP of the cold-exposed WT group elevated significantly within 1 wk of exposure to cold and increased gradually to a maximum level by week 5. However, there was only a slight increase in BP of the cold-exposed AT1A-KO group. The maximal cold-induced increase in BP (ΔBP) is significantly less in AT1A-KO group (11 ± 3 mmHg) than in WT group (49 ± 6 mmHg), indicating that AT1Areceptor deficiency attenuates cold-induced elevation of BP. Interestingly, both WT and AT1A-KO mice developed cardiac and renal hypertrophy to the same extent. AT1A-KO caused a significant increase in urine and plasma levels of nitric oxide (NO), indicating that the renin-angiotensin system inhibits NO formation probably via AT1Areceptors. Cold exposure inhibited endothelial NO synthase protein expressions and decreased urine and plasma levels of NO, which may be mediated partially by AT1Areceptors. AT1A-KO completely abolished the cold-induced increase in drinking responses to ANG II. We conclude that 1) AT1Areceptors play an essential role in the pathogenesis of CIH but not cardiac hypertrophy; 2) the role of AT1Areceptors in CIH may be mediated partially by its inhibitory effect on the NO system; and 3) cold-induced increase in drinking response to ANG II is mediated by AT1Areceptors.

Angiotensinogen gene knockout delays and attenuates cold-induced hypertension

The aim of the present study was to assess our hypothesis that the renin-angiotensin system (RAS) is responsible for cold-induced hypertension and cardiac hypertrophy. Two groups of wild-type (WT) mice and 2 groups of angiotensinogen gene knockout (Agt-KO) mice (6 per group) were used. After blood pressures (BP) of the four groups were measured 3 times at room temperature (25 degrees C), 1 WT and 1 Agt-KO group were exposed to cold (5 degrees C). The remaining groups were kept at 25 degrees C. BP of the cold-exposed WT group increased significantly in 1 week of cold exposure and rose gradually to 168+/-7 mm Hg by week 5, whereas the BP of the Agt-KO group did not increase until week 3. The cold-induced increase in BP (DeltaBP) was decreased significantly in the Agt-KO mice (19+/-3 mm Hg) compared with that of the WT mice (61+/-5 mm Hg) by 5 weeks of exposure to cold. Both WT and Agt-KO groups had cardiac hypertrophy in cold to the same extent. Agt-KO caused a significant increase in nitric oxide (NO) production. Thus, the RAS may inhibit NO formation. Chronic cold exposure decreased NO production, which may be mediated partially by activation of the RAS. These results strongly support that the RAS plays a critical role in the development of cold-induced hypertension but not cardiac hypertrophy. Moreover, the role of the RAS in cold-induced hypertension may be mediated in part by its inhibition on NO production. The findings also reveal the possible relation between the RAS and NO in cardiovascular regulation.

Renal responses to chronic cold exposure

The aim of this study was to assess our hypothesis that the release of antidiuretic hormone (ADH), the renal concentrating response to ADH, or both is decreased by prolonged cold exposure. Six groups (n = 6/group) of rats were used. Three groups were exposed to cold (5 degrees C), whilethe remaining three groups were kept at room temperature (25 degrees C). It was found that urine osmolality decreased significantly and serum osmolality increased significantly during cold exposure. The ratio of water/food intake was not affected by prolonged cold exposure. However, prolonged cold exposure increased the ratio of urine output/food intake in the cold-exposed rats, indicating that more urine flow is required by the cold-exposed rats to excrete the osmotic substance at a given food intake. The difference between water intake and urine output decreased significantly in the cold-exposed rats. Thus, prolonged cold exposure increases water loss from excretion. Renal concentrating responses to 24-h dehydration and Pitressin were decreased significantly in the cold-exposed rats. Plasma ADH levels remained unchanged, but renal ADH receptor (V2 receptor) mRNA was decreased significantly in the cold-exposed rats. The results strongly support the conclusion that cold exposure increases excretive water loss, and this may be due to suppression of renal V2 receptors rather than inhibition of ADH release.

Role of central angiotensin II receptors in cold-induced hypertension

BACKGROUND

Earlier studies from this laboratory showed that central angiotensin II (AngII) receptors are upregulated by chronic cold exposure. The purpose of this study was to determine whether central AngII receptors may play a role in the development of cold-induced hypertension.

METHODS

Four groups of rats (six rats each) were used. Two groups were exposed to cold (5 degrees C) and the other two groups were kept at 25 degrees C. One cold-exposed and one warm-adapted group were treated chronically, via osmotic minipumps, with AngII type 1 (AT1) receptor blocker (losartan, 6.0 microg/2.5 microL/h, intracerebroventricularly) at the beginning of cold exposure.

RESULTS

Systolic blood pressure (BP) of the cold-exposed untreated group increased during week 1 of cold exposure and rose to 160+/-4 mm Hg by week 4, whereas BP of the losartan-treated group in cold did not increase and remained at 121+/-3 mm Hg. Cold-induced increases in drinking response to AngII, plasma renin activity, and urine norepinephrine output disappeared in the treated rats, indicating blockade of central AngII receptors. Withdrawal of losartan at 4 weeks resulted in an increase in BP of this group to the cold-exposed untreated level, which was accompanied by an increase in the above parameters. Significant increases in AngII-induced drinking response and hypothalamic AT1 receptor mRNA content of the cold-exposed rats indicate upregulation of AngII receptors during chronic cold exposure. Hypothalamic AngII level was not affected by cold exposure.

CONCLUSION

Upregulation of brain AT1 receptors plays a role in the development of cold-induced hypertension.

Structural vascular changes in hypertension: role of angiotensin II, dietary sodium supplementation, and sympathetic stimulation, alone and in combination in rats

Elevated circulating angiotensin (Ang) II levels, dietary sodium, and sympathetic stimulation are recurrent themes of hypertension research, but their in vivo interaction in physiologically meaningful doses has not been adequately investigated. In this study, the interaction of a subpressor dose of Ang II (50 ng. kg(-1). min(-1) SC), 2% NaCl diet, and sympathetic stimulation in the form of overnight cold exposure was investigated in the development of hypertension and of structural vascular changes in male Sprague-Dawley rats. There were 8 experimental groups: sham operation and treatment (control), Ang II, 2% NaCl diet, cold exposure (5 degrees C), Ang II plus 2% NaCl diet, Ang II plus cold exposure, cold exposure plus 2% NaCl diet, and Ang II plus 2% NaCl diet plus cold exposure (triple treatment). For each group, the duration of treatment was 12 weeks. Morphometric measurements of maximally dilated, in situ fixed, second-order (250 to 320 microm OD), intermediate-size (100 to 150 microm OD), and small (50 to 100 microm OD) mesenteric arteries were performed, and wall-to-lumen ratios (W/L) were calculated. During the 12-week study, the blood pressure (BP) load (the area under the systolic BP curve) of rats receiving the combined treatment of Ang II and 2% NaCl diet was increased (P:<0.05), and that of rats receiving the combined treatment of cold exposure and 2% NaCl diet was decreased (P:<0.05); there were no BP changes in the remaining groups of rats. The most pronounced changes among groups occurred in W/L of small resistance arteries. The W/L of small arteries increased in Ang II-treated (P:<0.01) and in cold-stressed rats (P:<0.01). The effect of Ang II was potentiated by the addition of a 2% NaCl diet. In contrast, the addition of 2% NaCl diet to cold stress reduced the W/L of small arteries (P:<0.01). No other positive or negative synergism occurred among groups, including the rats receiving triple treatment. The findings confirm the potentiation of the hypertensinogenic and vascular trophic effects of Ang II by a high-sodium diet but do not provide evidence for synergism between Ang II and sympathetic stimulation. The finding of hypotension and reduced W/L of small resistance arteries in rats receiving the combined treatment of cold stress and high-sodium diet is unique because there are few known nonpharmacological vascular "hypotrophic" stimuli. The ultimate test of the hypertensinogenic potential of pressor stimuli alone or in combination is their long-term administration in physiologically meaningful doses to experimental animals.

Increased tail artery vascular responsiveness to angiotensin II in cold-treated rats

Chronic exposure of rats to cold for 1-3 weeks results in a mild form of hypertension. The renin-angiotensin system (RAS) has been implicated in this model of cold-induced hypertension. Previously we have characterized the vascular responsiveness in cold-acclimated animals, using aortic tissue, and recent studies have focused on the thermoregulatory responses of angiotensin II (AngII), utilizing the tail artery of the rat. Therefore in the current study we evaluated the vascular responsiveness of cold-treated rats to AngII in both aorta and tail artery at 2 and 4 weeks of cold exposure (5 ± 2°C). Systolic blood pressures were significantly elevated in cold-treated animals compared with control animals at both 2 and 4 weeks of cold exposure. At both of these time points body weights were reduced and ventricular weights were increased in cold-treated animals. After 2 weeks of cold exposure the vascular responsiveness of the aorta to AngII was significantly lower than that of controls. This vascular responsiveness to AngII was elevated and returned to control levels after 5 weeks of cold exposure. However, this pattern was not observed in the tail artery. The vascular responsiveness of tail artery rings from cold-treated rats to AngII was significantly greater than that of control animals during both 2 and 5 weeks of exposure to cold. The vascular contractile responses of both the aorta and tail artery to KCl in the cold-treated animals was not different from that of the control animals maintained at ambient room temperature, suggesting that the vascular smooth muscle contractile components were not altered by the cold exposure. Thus, the in vitro vascular reactivity to the receptor-mediated vasoconstrictor AngII was decreased in the sparsely innervated aorta and increased in the more densely innervated tail artery of the cold-treated animals when compared with controls. These results suggest that the increased responsiveness of AngII on the smooth muscle of the tail artery may play a role in adaptation to the cold and the maintenance of cold-induced hypertension.Key words: cold exposure, hypertension, renin-angiotensin system, vascular responsiveness, angiotensin II.

Body fluid distribution in rats with cold-induced hypertension

The purpose of this experiment was to determine body fluid distribution during chronic cold exposure and to further understand the mechanism of cold-induced hypertension. Blood pressures, hematocrit, and the plasma, blood, and extracellular fluid volumes were measured in rats at intervals of 1, 3, and 5 weeks after exposure to cold (5 degrees C). Resting systolic, diastolic, and mean blood pressures measured by direct arterial cannula were significantly elevated in a time-dependent manner over the duration of cold exposure. The increase in diastolic blood pressure, which reflects the peripheral vascular resistance, exceeded that of systolic blood pressure after both 3 and 5 weeks of exposure to cold. Pulse pressure was significantly decreased by 3 and 5 weeks of cold exposure. The plasma, blood, and extracellular fluid volumes were significantly increased after both 1 and 3 weeks of exposure to cold, but had returned to control levels by 5 weeks of cold exposure. Cold exposure, however, did not affect the hematocrit. The 2-h water intake after the cold-exposed rats were returned to warm (25 degrees C) (thermogenic drinking) was significantly increased compared to that of warm-acclimated rats during the first, third, and fifth week of exposure to cold. The present results suggest that the development of cold-induced hypertension is associated with blood volume expansion, and that the elevated blood pressure is maintained by increased peripheral vascular resistance without blood volume expansion. The results also imply that exposure to cold induces a dehydration in rats.

Cold-induced hypertension. A model of mineralocorticoid-induced hypertension

Hypertension, tachycardia and cardiac hypertrophy develop in rats exposed to mild cold (5 degrees C, 41 degrees F) for 1 to 3 weeks. Elevation of blood pressure (BP) during cold exposure is sodium dependent, although the rats still have an elevation of BP with a minimum of NaCl in their diet. Drugs that interfere with the renin-angiotensin-aldosterone (RAA) system at various levels (propranolol, clonidine, captopril, losartan and spironolactone) are able to prevent the development of cold-induced hypertension (CIH). Plasma renin activity (PRA) increases during the first 3 weeks of exposure to cold and then gradually decreases toward control level. Increased blood pressure and dipsogenic sensitivity to administration of angiotensin II (Ang II) have been demonstrated during the first 3 weeks of exposure to cold suggesting an upregulation of Ang II receptors when PRA is elevated. Additional studies have shown greater Fos-like immunoreactivity in the diencephalon of cold-exposed compared to warm-acclimated rats after 1 hr i.v. infusion of Ang II (333 ng/kg/min). Thus, most characteristics of cold-induced hypertension mimic those of hypertension induced experimentally by chronic administration of large doses of deoxycorticosterone acetate (DOCA) and salt. The results suggest that CIH is a mineralocorticoid-induced hypertension and that various levels of the RAA system contribute.

Comparison of changes in blood pressure and dipsogenic responsiveness to angiotensin II in male and female rats chronically exposed to cold

In most forms of experimentally induced hypertension in rats, females develop a less severe form of the disease than males. The objective of the present study was to compare the two genders with respect to the development of cold-induced hypertension. The results of the study indicate that both males and females develop comparable elevations of blood pressure and at approximately the same rate. Thus, the blood pressures of both groups increased significantly within 2 weeks of exposure to cold and reached similar maximal levels by the seventh week. The dipsogenic responsiveness of both groups of cold-exposed rats to acute administration of the peptide hormone, angiotensin II (AngII), was increased to approximately the same extent above that of warm-adapted counterparts, suggesting an increase in the responsiveness to AngII in the brain. To assess this possibility, the induction of the oncogene, cFos, was studied in brain following IV infusion of AngII (333 ng/kg/min). Fos-like immunoreactivity (FLI) was greater (p < 0.01) in subfornical organ, supraoptic and paraventricular hypothalamic nuclei of both cold-exposed groups compared to warm-adapted controls. Thus, both male and female rats have similar elevations of blood pressure as well as increased dipsogenic and FLI responsiveness to administration of AngII during chronic exposure to cold.