Cardiovascular adaptation to environmental stress: Its role in the development of hypertension, responsible mechanisms, and hypotheses

Behavioral and physiological responses of guide dogs to a situation of emotional distress

The aim of this study was to analyze the behavioral and physiological reactions of guide dogs in a distressing situation which promotes attachment behaviors towards their blind owners, and to compare such reactions with those of untrained or trainee dogs. The subjects were 57 adult Labrador and Golden retriever dogs (14 males, 43 females) belonging to four different groups: 19 Custody dogs, 13 Apprentice dogs, 10 Guide dogs and 15 Pet dogs. Dogs were tested using the Strange Situation Test, consisting in seven 3-minute episodes in which the dogs were placed in an unfamiliar environment, introduced to an unfamiliar woman and subjected to separation from their human companion. Tests were video-recorded and behaviors were scored using a 5-second point sampling method. Polar Vantage telemetric system was used to record cardiac activity. ANOVAs for repeated measures with groups and breeds as independent variables, showed a more anxious reaction in pet dogs, which revealed a high degree of proximity seeking behavior. Cardiac activity increased during episodes characterized by the exclusive presence of the stranger, but this increase was more conspicuous in guide dogs than in custody and apprentice dogs. Golden retrievers showed more behaviors suggesting distress compared to Labrador retrievers. This study showed that guide dogs, when separated from their blind owner, reveal a controlled behavioral reaction that is however accompanied by a stronger cardiac activation.

Hemodynamic response patterns to acute behavioral stressors resemble those to cocaine

Hemodynamic responses to cocaine vary greatly between animals, and the variability is related to the incidence of cocaine-induced cardiomyopathies and hypertension. The variability in cardiac output and systemic vascular resistance responses to cocaine in individuals is correlated with the responses to acute startle (air jet). This experiment was designed to determine whether responses to cocaine and to air jet are related to those evoked by a conditioned stimulus (tone preceding foot shock) and to an unconditioned stimulus (cold water). We verified the relationship in hemodynamic response patterns between cocaine and cold stress using selective receptor antagonists. Rats were instrumented with a pulsed Doppler flow probe on the ascending aorta for determination of cardiac output and with an arterial cannula for recording arterial pressure and heart rate. After recovery, some rats were tested multiple times with four different stimuli: air jet (6 trials), 15-s tone preceding 1-s foot shock (12 trials), cold water exposure (1 cm deep for 1 min, 4-12 trials), and cocaine (5 mg/kg iv, 4-6 trials) while hemodynamic parameters were recorded. Each stimulus was capable of eliciting a pressor response that was associated with variable changes in cardiac output. The cardiac output response to cocaine was correlated with the initial responses to each stressor in individual rats. Responses evoked by cold stress were most similar to those elicited by cocaine. Furthermore, nicardipine (25 microg/kg iv) or atropine methylbromide (0.5 mg/kg iv) pretreatment prevented the cardiac output differences to acute cold stress, as noted after cocaine administration. On the other hand, propranolol (1 mg/kg iv) exacerbated both the decrease in cardiac output and the stress-induced increase in systemic vascular resistance as previously reported with cocaine. Therefore, the initial response to cold water exposure is a reliable method of evoking characteristic hemodynamic response patterns that, as seen with cocaine, may provide a suitable model for identifying the causes for predilection to stress-induced cardiovascular disease.

Blood pressure hyperreactivity in non-human primates during dietary sodium combined with behavioral stress

The potential for behavioral stress alone or combined with dietary salt to augment pressor reactivity to the onset of daily experimental sessions was examined in normotensive, intact baboons over the course of four months. During twice daily experimental sessions, adult male baboons experienced food/shock conflict such that lever pulling not only served to earn food, but was also occasionally punished with cued mild electric shock. Blood pressure and heart rate were measured during a baseline period of fixed-ratio food reinforcement (3 weeks), during conflict stress (2 weeks), and after dietary salt was added to the daily conflict protocol (CONFLICT + SODIUM, 3 weeks). Reactivity, i.e., acute changes in blood pressure and heart rate to the daily experimental sessions, was not evident during food reinforcement sessions nor during the CONFLICT stress alone condition. The addition of a high salt diet virtually doubled blood pressure increases and heart rate decreases to the onset of experimental sessions. Average reactivities during CONFLICT + SODIUM periods were 11.2/7.9% delta for SBP/DBP (systolic/diastolic blood pressure, mmHg), and -5.65% delta for HR (heart rate, BPM). Neither atenolol nor hydrochlorothiazide diuretic significantly altered cardiovascular reactivity during CONFLICT + SODIUM in comparison to a preceding non-drug CONFLICT + SODIUM period. When atenolol and diuretic effects were directly compared, atenolol mildly augmented, while diuretic mildly decreased DBP but not SBP reactivity during CONFLICT + SODIUM. Reactivity was eliminated after salt loading and behavioral sessions were terminated. These findings provide evidence that enhanced salt ingestion may synergistically act with behavioral stress to produce pressor hyperresponsiveness to otherwise benign environmental events.

Lesions of the anteroventral third ventricle and development of stress-induce hypertension in the borderline hypertensive rat

The anteroventral third ventricle (AV3V) region plays a critical role in the pathogenesis of many forms of experimental hypertension. The present study sought to determine whether the integrity of this area was necessary for the development of stress-induced hypertension in the borderline hypertensive rat (BHR). Male BHRs were assigned to three groups at 8 weeks of age: 1) AV3V lesion, 2) sham lesion, and 3) maturation control. BHRs with AV3V and sham lesions were exposed to 12 weeks of conflict stress (2 hr/day, 5 days/wk). At the end of the conflict protocol period, direct measurement of resting mean arterial pressure indicated that BHRs with sham lesions had significantly higher blood pressure (153 +/- 2.9 mm Hg) than rats with AV3V lesions (126 +/- 5.2 mm Hg) and maturation control rats (133 +/- 4.3 mm Hg). Although AV3V lesions prevented stress-induced hypertension in BHRs, these rats were still capable of transiently raising blood pressure. Specifically, the results also indicate that BHRs with AV3V lesions showed greater increase in blood pressure in response to an electric foot-shock paradigm. This study suggests a critical role for this forebrain region in the production of stress-induced hypertension in genetically predisposed animals.

Open-field behaviour and blood pressure in spontaneously hypertensive rats

The relation between the development of hypertension and changes in behaviour was investigated. Open-field activity of male and female Spontaneously Hypertensive Rats (SHR) and Wistar-Kyoto controls (WKY) was scored at 4, 6 or 10 weeks of age. SHR generally showed higher locomotor activity and exploratory rearing behaviour, but lower grooming activity and defecation. These changes were found for rearing (3-5 fold increase) and grooming scores at all ages, ambulation at 4 weeks and 10 weeks (ambulation-inner) and defecation at 6 and 10 weeks of age. Differences were generally more pronounced in female rats. SHR showed less habituation than WKY. Already at the age of 4 and 6 weeks blood pressure was increased in SHR compared with WKY (approximately 120 mm Hg vs. 100 mm Hg). Between 6 and 10 weeks of age blood pressure increased rapidly in SHR, leading to a marked difference at the latter age (about 40 mm Hg), in both male and female rats. These experiments show that already at a young age, when blood pressure differences with WKY are small, marked behavioural changes are present in SHR. The altered behaviour could play a role in the development of hypertension in SHR.

Summation of baroreflex and classically conditioned heart rate responses in dogs

Following instrumentation with pneumatic cuffs around the inferior vena cava and the descending aorta, dogs were studied either following differential classical conditioning or in a control state. The cuffs functioned to raise and lower blood pressure for the construction of baroreflex curves for heart rate. Conditioned dogs received 8 trials each day with each CS+ (tone paired with flank shock) and CS- (a different tone without shock). Curves were constructed from cuff inflations timed to coincide with the maximum conditioned heart rate response. These curves were constructed from data acquired during infusion of saline, methyl atropine, or propranolol. Comparison of these curves revealed that the CS+ shifted the curves toward higher heart rates while the CS- curve was shifted toward lower heart rates without a change in gain. The amount of shift was comparable to that of the conditioned heart rate response. This suggested that the responses were independent and additive. Neither propranolol nor atropine eliminated this separation between the curves induced by the conditioning. These observations lead us to conclude that classically conditioned stress and baroreceptor stimulation exert independent control over heart rate that are mediated by both sympathetic and parasympathetic influences.

Development of cardiac sympathetic and adrenal-medullary responses in borderline hypertensive rats

Borderline hypertensive (BHR) rats are the first generation offspring of a cross of spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive rats. In adulthood, BHRs have systolic blood pressures in the 140-160 mm Hg range. If subjected to chronic stress paradigms, however, BHRs develop sustained and permanent elevations in systolic blood pressure (180-200 mm Hg). In the present study, we examined the functional development of cardiac and adrenal medullary responses to reflex activation of the sympathetic nervous system in preweanling BHR and WKY rats. Pups of the two groups were injected with insulin or saline at 4, 8, 12, or 16 days of age and sacrificed 3 h later. Insulin produces an acute lowering of blood glucose which is attended by a centrally mediated increase in sympathetic activity. The induction of ornithine decarboxylase (ODC) activity in heart and the depletion of epinephrine from the adrenal medulla were biochemical indicators of functional sympathetic neurotransmission. WKY and BHR pups had similar levels of cardiac ODC activity under basal conditions and following administration of insulin. In contrast, BHRs had higher amounts of adrenal norepinephrine and epinephrine from 4 to 16 days of age and greater depletion of adrenal epinephrine following insulin administration at 8, 12 and 16 days of age. These findings indicate that BHRs have a greater capacity for catecholamine biosynthesis, storage and release in the adrenal medulla during the preweanling period compared to age-matched normotensive WKY controls. This alteration in the adrenal medulla during the preweanling period may contribute to the susceptibility of adult BHR rats to stress-induced hypertension.

Changes in blood pressure, heart rate and blood constituents during heat exposure in men with elevated blood pressure

Although the vascular volume response of hypertensive men during exercise has been rather well characterized, the effect of resting heat exposure in this patient population has not been examined. This was done in the present report in seven men with high blood pressure (BP) (i.e., diastolic pressure greater than 12 kPa (90 mmHg) upon initial interview) and 5 normotensive control subjects. 50 min after each subject had consumed an amount of water equal to 1% of his body weight, he reclined on a cot. 10 min later the subject was carried into an environmental chamber equilibrated at Tdb = 45 degrees C, Twb = 28 degrees C. Free-flowing venous blood samples were obtained from a cubital vein, and BP and heart rate were measured, before the heat exposure and at 15 min intervals during the experiment. Within 30 min systolic, diastolic and mean BP of the high BP subjects had decreased to normal levels; no BP changes were detected in normotensive subjects. Accompanying this depressor response was an exaggerated elevation in plasma glucose concentration. No alterations were found with haematocrit, plasma osmolality or electrolytes, or total protein and albumin. The data suggest that heat exposure may have been more stressful for the subjects with high BP than for their controls. This finding implies that phasic depressor responses may be as important as phasic pressor episodes in the aetiology of established essential hypertension.

The borderline hypertensive rat (BHR): a new model for the study of environmental factors in the development of hypertension

While many studies have attempted to produce hypertension through the use of various environmental stressors, few have succeeded in producing chronic elevations in blood pressure beyond levels considered to be borderline hypertensive (140-160 mm Hg systolic). The problem with most studies stems from the use of genetically normotensive animals and the selection of stressors to which animals readily adapt. A new approach is suggested, which recognizes the role of genetics in human essential hypertension. Animals with one hypertensive parent do not develop spontaneous hypertension but show a more sensitive cardiovascular response to environmental stressors than animals with normotensive parents. Preliminary studies revealed that animals with a mixed genetic history of hypertension develop spontaneous borderline hypertension. When subjected to shock-shock conflict, these borderline hypertensive rats (BHR) developed permanent hypertension that failed to abate even after a ten-week, shock-free recovery period. The hypertension was accompanied by elevated heart weight to body weight ratios and by significant cardiac pathology. Subsequent work has demonstrated that these animals also become hypertensive when fed a high-sodium diet. Finally, in a series of exercise studies, we found that BHRs subjected to a shock stressor were protected against stress-induced hypertension if they exercised daily. The potential of this model for studies of the mechanisms by which environmental variables produce permanent hypertension is discussed.

Endocrine influence on disease outcome: experimental findings and implications

The psychosocial environment may impose stressor effects on animal and man. Adaptation to these environmental changes requires behavioural, autonomic, neuroendocrine, metabolic, etc. processes. The neuroendocrine system plays a key role in the integration of these processes. Experimental evidence obtained in the rat suggests that neuropeptides related to ACTH, endogenous opioids and their fragments, vasopressin, etc., but also oestrogens may selectively influence the form and magnitude of acute cardiac response to emotional stressors. Dichotomies between the behavioural and cardiac responses may occur too. It is suggested that neuroendocrine action on brain mechanisms that are involved in the organization of behavioural and bodily responses to stressors are important in physiological adaptation. Neuroendocrine disturbances (two much or too little neuropeptides and other hormones) may thereby contribute to the outcome of psychosomatic diseases.

Stress, predisposition and the onset of serious disease: implications about psychosomatic etiology

Based on the author's own work and a review of the literature, the hypothesis is made that potentially lethal disease does not usually occur in healthy animals or people but does so when covert or overt disease exists or when a predisposition for disease exists. The author supports this hypothesis in his assessment of the human literature on sudden death. Further support for the hypothesis is presented from 2 animal models being studied in his laboratory--stress-induced heart failure in the cardiomyopathic hamster and stress-induced sensitization of digitalis-toxic ventricular arrhythmias. This analysis suggests a different view from the classical one of what a psychosomatic disease might be.

Effects of stress on blood pressure and cardiac pathology in rats with borderline hypertension

Repeated attempts to produce hypertension (HT) through psychological stress have failed to elevate blood pressure (BP) to levels seen in chronic, untreated essential HT in humans. In general, these studies have two characteristics in common: they utilize the normotensive animal, with no genetic history of HT, and they involve stressors to which animals readily adapt. The present study utilized offspring with one HT parent. The male F1, offspring of SHR x WKY had borderline HT (-/x +/- SEM = 152.4 +/- 1.34 mm Hg). With a conflict paradigm used as the stressor, experimental animals eventually developed severe HT (188.3 +/- 2.70 mm Hg) compared to two non-stressed control groups (158.4 +/- 2.31 mm Hg and 151.9 +/- 2.25 mm Hg). After 15 weeks of stress for 2 hours daily, termination of conflict for 10 weeks did not reduce the HT in experimental animals. Subsequent analyses revealed that stressed animals, when compared to nonstressed controls, exhibited elevated heart-weight-to-body-weight ratios and significant cardiac pathology in the form of myofibrillar degeneration, accumulation of inflammatory cells, and fibrosis. The implications of using this model for the analysis of cardiovascular concomitants of stress-induced HT are discussed.

Essential hypertension: abnormal renal vascular and endocrine responses to a mild psychological stimulus

We have assessed the influence of a mild emotional stimulus on arterial blood pressure, heart rate, renal blood flow, plasma renin activity (PRA), and plasma aldosterone concentration in 24 normal subjects, eight of who had a parent with hypertension, and in 15 patients with essential hypertension. A nonverbal IQ test, Raven's Progressive Matrices, was employed as the stimulus. In 11 of the 15 hypertensives, arterial blood pressure rose transiently by 7 mm Hg or more, but in only three of 16 normal subjects (x2 = 7.23, p less than 0.01). Transient moderate increases in heart rate were also more common in the hypertensives (p less than 0.01). Renal blood flow rose in 11 of 16 normal subjects and fell in each of the 15 patients with essential hypertension (x2 = 15.1; p less than 0.005). As opposed to the transient changes in arterial pressure and heart rate, the fall in renal perfusion was sustained. The PRA fell in 10 of the 16 normal subjects with a negative family history and rose in 14 of 15 patients with essential hypertension (p less than 0.005). Changes in plasma angiotensin II concentration and in plasma aldosterone were in accord with the changes in PRA, but plasma cortisol did not change. Both the renal vascular response and the change in PRA were intermediate in normal subjects in whom family history was positive for hypertension. For the entire group of 39 subjects there was statistically significant agreement between the direction of the renal vascular response and the directional change in PRA: renal blood flow rose when PRA fell and fell when PRA rose (p less than 0.005). We conclude that there is an abnormality in the control of both the renal circulation and of renin release in patients with essential hypertension in response to psychological provocation, and that a similar process is present in some normotensive subjects whose parents have hypertension.

Behavioral hyperreactivity in the spontaneously hypertensive rat compared to its normotensive progenitor

The spontaneously hypertensive rat (SHR) is an excellent model of essential hypertensive disease. Hyperreactivity has been postulated as a contributing factor in the development of high blood pressure in the SHR and in man. In the maturing organism recurring hypertensive stesss responses may promote permanent vascular changes and result in a fixed hypertension. Simple behavioral activity and emotionality rating scales were used to compare a large number of SHR with equally large groups of closely-related and distantly-related normotensive rats. As predicted, the SHR were clearly more active and emotional than their ancestral Wistar Kyoto (WKY) strain. However, the distantly-related normotensive Wistars did not differ from the SHR in either activity or emotionality. These results indicate that behavior and hypertension are not necessarily related in the rat. Nonetheless, the behavioral differences between the SHR and their closest genetic match, the WKY, suggest that arousal and blood pressure levels may be causally linked in this case of naturally occurring hypertension.