Clinical and Experimental Pharmacology and Physiology (1992) 19, 187-192
CARDIOVASCULAR HABITUATION TO EMOTIONAL STRESS IN LYON HYPERTENSIVE RATS Claude Julien, Joelle Sacquet, Patrice Kandza, Ding-Feng Su*, Madeleine Vincent and Christian Bards Dkpartement de Physiologie et Pharrnacologie Clinique, Centre National de la Recherche Scientifique, Unite de Recherche AssociCe 606, Facultb de Pharmacie, Lyon, France (Received 11 September 1991; revision received 24 October 1991)
SUMMARY 1. Intra-aortic blood pressure was recorded continuously in freely moving genetically hypertensive (LH), normotensive (LN) and low blood pressure (LL) rats of the Lyon strain during two 11 h periods (08:OO-19:OO h). During the first period (control), the animals were left undisturbed and during the second period (stress), a jet of air was applied for 20 min every hour. Urine was collected simultaneously and analysed for its content in norepinephrine and epinephrine. 2. The first exposure to the stressor induced larger increases in blood pressure and heart rate in LH than in LN and LL rats. However blood pressure and heart rate responses to the 10 following stressors decreased in LH rats while they remained stable in LN and LL animals. 3. Repeated stress exposure induced significant increases in epinephrine excretion in both LN and LL but not in LH rats. 4. It is concluded that LH rats exhibit marked cardiovascular habituation to repeated stress. Taken together with the lack of stress-induced sympathoadrenal activation, this suggests a reduced level of emotional responsiveness in Lyon hypertensive rats. Key words: baroreflex sensitivity, blood pressure, hypertension, rat, stress, urinary catecholamines.
INTRODUCTION The demonstration of cardiovascular hyper-responsiveness to emotional stress in essential hypertensives (Falkner et al. 1979) as well as in spontaneously hypertensive rats (SHR; Hallback & Folkow 1974; Lundin et al. 1984; Lawler et al. 1985; Yamamoto et al. 1987; Kirby et al. 1989) has led to the hypothesis that exaggerated pressor responses to stressful events could participate in the genesis and/or the maintenance of hypertension, mainly by triggering and/ or amplifying adaptive structural changes at the vascular wall level (Folkow 1982).
However laboratory stress testing generally involves single trials which do not mimic the daily-life situations in which repeated exposure to the same stressor frequently occurs. Reactivity studies in laboratory animals, mainly rats, have focused on either acute responses to a single stressor (Hallback & Folkow . Kirby et 1974; Lundin et al. 1984; Lawler eb ~ 11985; al. 1989) or long-term effects of chronic stress, usually one daily exposure to the stressor for various durations (Lawler et al. 1981). Short-term habituation of cardiovascular responses to repeated stress has not been
Correspondence: Dr C. Julien, URA CNRS 606, FacultC de Pharmacie, 8 avenue Rockefeller, 69008 Lyon, France. *Present address: Department of Pharmacology, Second Military Medical University, 594 Xiang Yin Road, 200433 Shanghai, China.
188
C. Julien et al.
systematically evaluated in rats with genetically different blood pressure (BP) levels. In a previous study (Julien et al. 1988), it was reported that young and adult genetically hypertensive rats of the Lyon strain (LH) had larger BP responses than age-matched normotensive controls when exposed for the first time to a mild emotional stressor (ajet of air). The aim of this study was to characterize the cardiovascular responses to repeated stress exposure in LH and their normotensive (LN) and low blood pressure (LL) controls. In addition, the urinary excretion of norepinephrine (NE) and epinephrine (EPI) was measured as it has been shown previously to provide reliable indexes of the overall sympathetic nervous system activity and adrenal medullary secretion, respectively (Julien et al. 1989).
METHODS
lowing parameters: systolic (SBP, mmHg), diastolic (DBP, mmHg) BP and heart rate (HR, beatslmin). The values were stored on hard disks for off-line graphical treatment and statistical analysis (Cerutti et al. 1985).
Stress procedure Aversive sessions utilized a continuous blast of air directed at the rat during 20 min at a constant 100 kPa pressure (Lundin et al. 1984; Julien et al. 1988). Two days after surgery, the rats were placed in individual metabolic cages and connected to the recording system. After 14-16 h of acclimatization, BP was recorded continuously during two 11 h periods (08:OO-19:OO h). During the first period (control), the animals were left undisturbed. During the second period (stress), the stressor was applied for 20 min at the beginning of each hour by means of an electropneumatic valve under microprocessor control.
Animals Fourteen week old male LH, LN and LL rats were used. Animals were housed under a 12 h (0890-2O:OO h) light-dark cycle and provided with standard rat chow and water ad libitum. Two days before and during the study, the rats were fed a cereal-free diet (No 902487, ICN, Cleveland, OH, USA), in order to avoid dietary influences on the urinary excretion of catecholamines (Cottet-Emard et al. 1980).
Blood pressure recordings Direct blood pressure was measured in conscious freely moving rats using a method described previously (Su et al. 1986). Under halothane anaesthesia (2% in oxygen), a floating polyethylene catheter was inserted into the lower abdominal aorta. The signal was transmitted to a pressure transducer, digitized and processed by a minicomputer (DIGITAL MINC DECLAB 11/ 23) which calculated on-line the fol-
Baroreflex testing Cardiac baroreflex sensitivity (BRS, ms/ mmHg) was measured after 2-3 injections of phenylephrine (3 pg/ kg i.v.) and was given by the computed slope of the change in heart period (60000/HR, ms) as a function of increasing SBP (Su et al. 1986; Lo et al. 1989). BRS measurements took place between 19:OO and 20:OO h at the end of the control period and 24 h after the end of stress period.
Urinary catecholamines measurement Urine was collected during the first (control) and the second (stress) periods over NazEDTA (5 mg) and sodium metabisulfite (1 mg) as preservatives, Unconjugated NE and EPI were measured by high-perform a n e liquid chromatography with electrochemical detection (Julien el al. 1989). The excretion rates of
Table 1. Characteristics of LH, LN and LL rats
LH (n = 8) Bodyweight (g) SBP (mmHg) DBP (mmHg) HR (beatslmin) BRS (ms/mmHg) 1 1 h urinary NE (nmol/ mmol creatinine) 1 1 h urinary EPI (nmol/ mmol creatinine)
*P
CARDIOVASCULAR HABITUATION TO EMOTIONAL STRESS IN LYON HYPERTENSIVE RATS Claude Julien, Joelle Sacquet, Patrice Kandza, Ding-Feng Su*, Madeleine Vincent and Christian Bards Dkpartement de Physiologie et Pharrnacologie Clinique, Centre National de la Recherche Scientifique, Unite de Recherche AssociCe 606, Facultb de Pharmacie, Lyon, France (Received 11 September 1991; revision received 24 October 1991)
SUMMARY 1. Intra-aortic blood pressure was recorded continuously in freely moving genetically hypertensive (LH), normotensive (LN) and low blood pressure (LL) rats of the Lyon strain during two 11 h periods (08:OO-19:OO h). During the first period (control), the animals were left undisturbed and during the second period (stress), a jet of air was applied for 20 min every hour. Urine was collected simultaneously and analysed for its content in norepinephrine and epinephrine. 2. The first exposure to the stressor induced larger increases in blood pressure and heart rate in LH than in LN and LL rats. However blood pressure and heart rate responses to the 10 following stressors decreased in LH rats while they remained stable in LN and LL animals. 3. Repeated stress exposure induced significant increases in epinephrine excretion in both LN and LL but not in LH rats. 4. It is concluded that LH rats exhibit marked cardiovascular habituation to repeated stress. Taken together with the lack of stress-induced sympathoadrenal activation, this suggests a reduced level of emotional responsiveness in Lyon hypertensive rats. Key words: baroreflex sensitivity, blood pressure, hypertension, rat, stress, urinary catecholamines.
INTRODUCTION The demonstration of cardiovascular hyper-responsiveness to emotional stress in essential hypertensives (Falkner et al. 1979) as well as in spontaneously hypertensive rats (SHR; Hallback & Folkow 1974; Lundin et al. 1984; Lawler et al. 1985; Yamamoto et al. 1987; Kirby et al. 1989) has led to the hypothesis that exaggerated pressor responses to stressful events could participate in the genesis and/or the maintenance of hypertension, mainly by triggering and/ or amplifying adaptive structural changes at the vascular wall level (Folkow 1982).
However laboratory stress testing generally involves single trials which do not mimic the daily-life situations in which repeated exposure to the same stressor frequently occurs. Reactivity studies in laboratory animals, mainly rats, have focused on either acute responses to a single stressor (Hallback & Folkow . Kirby et 1974; Lundin et al. 1984; Lawler eb ~ 11985; al. 1989) or long-term effects of chronic stress, usually one daily exposure to the stressor for various durations (Lawler et al. 1981). Short-term habituation of cardiovascular responses to repeated stress has not been
Correspondence: Dr C. Julien, URA CNRS 606, FacultC de Pharmacie, 8 avenue Rockefeller, 69008 Lyon, France. *Present address: Department of Pharmacology, Second Military Medical University, 594 Xiang Yin Road, 200433 Shanghai, China.
188
C. Julien et al.
systematically evaluated in rats with genetically different blood pressure (BP) levels. In a previous study (Julien et al. 1988), it was reported that young and adult genetically hypertensive rats of the Lyon strain (LH) had larger BP responses than age-matched normotensive controls when exposed for the first time to a mild emotional stressor (ajet of air). The aim of this study was to characterize the cardiovascular responses to repeated stress exposure in LH and their normotensive (LN) and low blood pressure (LL) controls. In addition, the urinary excretion of norepinephrine (NE) and epinephrine (EPI) was measured as it has been shown previously to provide reliable indexes of the overall sympathetic nervous system activity and adrenal medullary secretion, respectively (Julien et al. 1989).
METHODS
lowing parameters: systolic (SBP, mmHg), diastolic (DBP, mmHg) BP and heart rate (HR, beatslmin). The values were stored on hard disks for off-line graphical treatment and statistical analysis (Cerutti et al. 1985).
Stress procedure Aversive sessions utilized a continuous blast of air directed at the rat during 20 min at a constant 100 kPa pressure (Lundin et al. 1984; Julien et al. 1988). Two days after surgery, the rats were placed in individual metabolic cages and connected to the recording system. After 14-16 h of acclimatization, BP was recorded continuously during two 11 h periods (08:OO-19:OO h). During the first period (control), the animals were left undisturbed. During the second period (stress), the stressor was applied for 20 min at the beginning of each hour by means of an electropneumatic valve under microprocessor control.
Animals Fourteen week old male LH, LN and LL rats were used. Animals were housed under a 12 h (0890-2O:OO h) light-dark cycle and provided with standard rat chow and water ad libitum. Two days before and during the study, the rats were fed a cereal-free diet (No 902487, ICN, Cleveland, OH, USA), in order to avoid dietary influences on the urinary excretion of catecholamines (Cottet-Emard et al. 1980).
Blood pressure recordings Direct blood pressure was measured in conscious freely moving rats using a method described previously (Su et al. 1986). Under halothane anaesthesia (2% in oxygen), a floating polyethylene catheter was inserted into the lower abdominal aorta. The signal was transmitted to a pressure transducer, digitized and processed by a minicomputer (DIGITAL MINC DECLAB 11/ 23) which calculated on-line the fol-
Baroreflex testing Cardiac baroreflex sensitivity (BRS, ms/ mmHg) was measured after 2-3 injections of phenylephrine (3 pg/ kg i.v.) and was given by the computed slope of the change in heart period (60000/HR, ms) as a function of increasing SBP (Su et al. 1986; Lo et al. 1989). BRS measurements took place between 19:OO and 20:OO h at the end of the control period and 24 h after the end of stress period.
Urinary catecholamines measurement Urine was collected during the first (control) and the second (stress) periods over NazEDTA (5 mg) and sodium metabisulfite (1 mg) as preservatives, Unconjugated NE and EPI were measured by high-perform a n e liquid chromatography with electrochemical detection (Julien el al. 1989). The excretion rates of
Table 1. Characteristics of LH, LN and LL rats
LH (n = 8) Bodyweight (g) SBP (mmHg) DBP (mmHg) HR (beatslmin) BRS (ms/mmHg) 1 1 h urinary NE (nmol/ mmol creatinine) 1 1 h urinary EPI (nmol/ mmol creatinine)
*P
