(Hypertension. 1997;30:301-304.)
© 1997 American Heart Association, Inc.
Articles |
Initial Characterization of Hamsters With Spontaneous Hypertension
From the Biologic Resources Laboratory (C.L.T., J.E.A., R.M.B.) and Department of Medicine (H.S., X.-p.G., I.R.), University of Illinois at Chicago, and Canadian Hybrid Farms, Halls Harbour, Nova Scotia, Canada (E.W., A.S.).
Correspondence to Dr Israel Rubinstein, Department of Medicine (M/C 787), University of Illinois at Chicago, 840 S Wood St, Chicago, IL 60612-7323. E-mail IRubinst{at}uic.edu
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Abstract |
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Abstract The purpose of this study was to begin to characterize a new inbred strain of adult male hamsters with established spontaneous hypertension along with their genetically/age-matched normotensive controls. We found that mean arterial pressure was 162±3 mm Hg in hypertensive hamsters and 94±4 mm Hg in controls (mean±SEM; P<.05). Body weight was significantly lower in hypertensive hamsters relative to normotensive hamsters (P<.05). Hypertension was associated with a significant increase in heart weight, thickness of the left ventricular wall, and amplitude of the QRS complex in standard electrocardiographic leads I and aVR (P<.05). No gross or microscopic abnormalities were observed in other organs. Plasma renin activity and the number of circulating neutrophils were significantly increased in hypertensive hamsters relative to controls (P<.05). Serum concentrations of creatinine, blood urea nitrogen, sodium, potassium, and calcium as well as urinalysis were similar in both groups. Overall, these data suggest that the spontaneously hypertensive hamster could be a suitable model for the study of spontaneous hypertension.
Key Words: blood pressure • hypertension, essential • hypertrophy, left ventricular • leukocytes • plasma renin activity • hamsters
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Introduction |
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Hypertension remains a major public health concern in the United States because it is associated with a relatively high morbidity and mortality rate and increased medical expenditure.1 Clinical and epidemiological studies suggest that genetic factors could play a role in the pathophysiology of essential hypertension.1 2 For instance, the salutary effects of antihypertensive medications differ considerably between whites and blacks in the United States.2 Hence, genetic factors should be taken into account when new drugs for the treatment of hypertension are being developed and tested.
Angiotensin I–converting enzyme (ACE) inhibitors have been shown to elicit a significant reduction in systemic arterial pressure in spontaneously hypertensive rats (SHR), a genetic model of hypertension, but are relatively ineffective in rats with deoxycorticosterone-salt–induced hypertension.3 4 5 6 7 By contrast, inhibitors of neutral endopeptidase (NEP), a proposed new class of antihypertensive agents, elicit a significant decrease in systemic arterial pressure in deoxycorticosterone-salt hypertensive rats, whereas they are less effective in SHR.4 5 8 9 10 11 12 It is conceivable that the salutary effects of experimental antihypertensive drugs depend in part on the animal model used and that the SHR may not be an appropriate model for testing all of them.3 4 5 13
The purpose of this study was to address this issue by beginning to characterize a new inbred strain of adult male hamsters with spontaneous hypertension along with their genetically/age-matched normotensive controls.
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Methods |
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Derivation of the Halls Harbour Hypertensive Hamster (H4)
A colony of inbred albino hamsters (CHF 148) was established 13 years ago at the Canadian Hybrid Farms (Halls Harbour, Nova Scotia, Canada) as a result of experimental breeding between an established inbred cardiomyopathic hamster strain (CHF 146) and a golden Syrian hamster outbred strain (CHF GS).14 15 16 17 18 19 20 The original intent was to develop cardiomyopathic and normal sibling hamsters from a single litter. However, several years later, few offspring exhibited sustained elevations in systolic arterial pressure ranging from 150 to 200 mm Hg. With selective breeding of subsequent generations, 100% of hamsters per litter exhibited spontaneous hypertension. Hypertension is first manifested in these animals at 8 weeks of age and is fully established by 10 to 12 weeks (E.W, A.S., unpublished observations, 1996). The control group consists of normotensive albino hamsters (CHF 148).16 17
Characterization of the Model
Experiments were conducted in accordance with the National
Institutes of Health Guide for the Care and Use of Laboratory
Animals and were approved by the Animal Care Committee of the
University of Illinois at Chicago. Eight-month-old male hamsters with
established spontaneous hypertension (n=15) and genetically/age-matched
normotensive hamsters (n=16) were housed in a controlled environment
and maintained on a standard pellet diet with free access to water.
They were anesthetized with sodium pentobarbital (6 mg/100 g
body wt IP). Body temperature was monitored and maintained constant
(37°C to 38°C) throughout the experiment with a heating pad.
Animals were placed supine on a Plexiglas stage, and a polyethylene
catheter containing 0.9% saline was inserted into a femoral artery and
positioned in the abdominal aorta for continuous measurement and
recording of mean arterial pressure with a pressure
transducer (Single Line Monitoring Kit, Abbott Critical Care System,
Abbott Laboratories) and a strip-chart recorder (Brush 2000,
Gould), respectively. Six standard and one anterior chest wall
electrocardiographic (ECG) leads were placed with the use of pediatric
ECG electrodes (type SP-00-S, Medicotest) for continuous monitoring and
recording of heart rate and ECG (model 2000RS, Datascope). In
each ECG tracing, the average amplitude of 10 consecutive QRS complexes
in each lead was measured in millimeters with a caliper and expressed
as millivolts.
At the end of the experiment, each animal was exsanguinated by rapidly withdrawing with a syringe a total of 7 to 8 mL blood from a femoral artery into prechilled test tubes for further analysis.21 In addition, a urine sample (0.5 mL) was aspirated from the urinary bladder for urinalysis. Then, the kidneys, adrenals, brain, pituitary, lungs, liver, and spleen were removed, weighed, and immersed in 10% neutral buffered formalin at room temperature. The heart was also removed in diastole and placed in formalin within 7 to 10 minutes after death in the absence of rigor mortis. After fixation, the heart was bisected longitudinally through the left and right atria and ventricles. One half was further sectioned coronally through the ventricle two thirds of the distance proximal to the apex. The longitudinal and two cross sections of the heart were then processed, cut surface down. All tissues were embedded in paraffin, cut into 5- to 7-µm sections, placed on glass slides, and stained with hematoxylin and eosin for light microscopic examination. Tissue sections were examined by a veterinary pathologist (R.M.B.) unaware of systemic arterial pressure data using an Olympus microscope (Leco Corp). The thicknesses of the left and right cardiac ventricular walls and interventricular septa were determined in five randomly selected microscopic fields using a micrometer built into the eyepiece at a magnification of x40. These data were expressed as millimeters per 100 g body wt.
Complete blood count was performed with an electric resistance detection system (Sysmex K-1000 Automated Quantitative Hematology Analyzer, Baxter Diagnostics, Inc). Plasma renin activity was determined with the Renin GammaCoat 125I Plasma Renin Activity Radioimmunoassay Kit (Incstar) according to the manufacturer's instructions. Serum concentrations of creatinine, blood urea nitrogen, sodium, potassium, and calcium were determined by spectrophotometry (model 550, Clinical Chemistry Analyzer, Ciba-Corning). Urinalysis was performed with the reagent multiparameter test stick method (Chemstrip 9, Boehringer Mannheim Corp).
Statistical Analysis
Data are expressed as mean±SEM. Statistical analysis
was performed with two-way ANOVA and the Newman-Keuls multiple range
test. Cardiac wall thickness data were analyzed with the
Wilcoxon signed rank test. A value of P<.05 was
considered statistically significant.
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Results |
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The anatomic and physiological characteristics of adult male hamsters with spontaneous hypertension and genetically/age-matched normotensive control hamsters are summarized in Table 1
. Mean arterial pressure was
162±3 mm Hg in hypertensive hamsters and 94±4 mm Hg in
controls (P<.05). Heart rate did not differ significantly
between the groups (Table 1, P>.5). In addition, mean
arterial pressure and heart rate did not change
significantly in the groups during the experiments (P>.5).
The body weight of hypertensive hamsters was significantly lower than
that of normotensive controls (P<.05). Heart weight,
expressed as a percentage of body weight, was significantly higher in
hypertensive hamsters relative to normotensive controls
(P<.05). Because of their small size, it was not feasible
to dissect separately the four cardiac chambers. The weight of other
organs, normalized for body weight, did not differ significantly
between the groups (Table 1, P>.5).
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Data on complete blood count in hamsters with and without hypertension
are summarized in Table 2. The number of circulating
leukocytes in hamsters with spontaneous hypertension relative to
normotensive hamsters increased by approximately 51%
(P<.05). This increase was attributed to a significant
increase in the number of circulating neutrophils (Table 2,
P<.05). Hemoglobin concentration, hematocrit, and
platelet count were similar in the groups (Table 2,
P>.5).
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Data on plasma chemistry in hamsters with and without hypertension are
summarized in Table 3. Plasma renin activity was
significantly elevated by almost threefold in hamsters with spontaneous
hypertension relative to normotensive hamsters (P<.05).
Serum creatinine, blood urea nitrogen, sodium, potassium,
and calcium concentrations were similar in the groups (Table 3,
P>.5). Urinalysis was normal in hamsters with and without
hypertension (data not shown).
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The thickness of the left ventricular wall was
significantly increased in hamsters with spontaneous hypertension
relative to normotensive hamsters (Table 4,
P<.05). Interventricular septal
and right ventricular wall thicknesses did not differ
significantly between the groups (Table 4, P>.5).
Microscopic examination of the hearts of hamsters with and without
hypertension revealed occasional small aggregates of neutrophils in the
atrioventricular valve leaflets, subendocardial
connective tissue, and perivascular connective tissue in the
myocardium. In addition, minimal pavementing of the
endothelium by neutrophils was observed over the
atrioventricular valve leaflets, within the atria, and
within the media of large blood vessels. There was no evidence of
altered myofibrillar morphology indicative of so-called myofibrillar
disarray, necrosis, and fibrosis in the myocardium of
hamsters with spontaneous hypertension. In addition, there was no
evidence of vascular hypertrophy in the coronary
arteries of these hamsters. Gross and microscopic examination of other
organs revealed no obvious abnormalities in hamsters with spontaneous
hypertension and normotensive controls. Specifically, there was no
evidence of renal vascular hypertrophy in hypertensive
hamsters (data not shown).
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ECG data on the amplitude of the QRS complex recorded from hamsters
with and without hypertension are summarized in Table 5.
The amplitude of the QRS complex in standard leads I and aVR was
increased significantly in hamsters with spontaneous hypertension
relative to normotensive hamsters (P<.05).
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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In this study, we began to characterize a new inbred strain of hamsters with spontaneous hypertension and their genetically/age-matched normotensive controls. We found that the magnitude of systemic arterial pressure in these animals was substantial and associated with a significant decrease in body weight, left ventricular hypertrophy, hyperreninemia, and neutrophilia. No gross or microscopic abnormalities were observed in other organs. In addition, serum concentrations of creatinine, blood urea nitrogen, sodium, potassium, and calcium as well as urinalysis were similar in hamsters with spontaneous hypertension and controls. Overall, these data suggest that the spontaneously hypertensive hamster could be a suitable model for the study of spontaneous hypertension.
The hamster is an established model for investigating the mechanisms regulating vasomotor tone in the in situ peripheral microcirculation.14 16 18 19 20 For instance, Gao et al18 showed that vasodilation elicited by bradykinin, a potent vasoactive peptide thought to play a role in regulating renal blood flow,4 19 22 23 24 in the in situ cheek pouch of normotensive hamsters is modulated in part by ACE and NEP. Both membrane-bound peptidases are widely distributed in the peripheral microcirculation and cleave and inactivate bradykinin very effectively.8 22 23 In addition, Joyner et al20 showed that arteriolar pressure is elevated in the cheek pouch of hamsters with experimentally induced renovascular hypertension in the absence of vascular hypertrophy. Likewise, Suzuki et al16 showed recently that endothelium-dependent vasodilation is attenuated whereas endothelium-independent vasodilation is preserved in the in situ cheek pouch of adult male hamsters with spontaneous hypertension.
The results of the present study extend these observations by providing, for the first time, initial morphological, hematologic, and biochemical information on spontaneously hypertensive hamsters and their genetically/age-matched normotensive controls. Unlike SHR and their Wistar-Kyoto controls, spontaneously hypertensive hamsters and their normotensive controls are derived from the same ancestors.13 This may reduce genetic divergence and facilitate the study of essential hypertension. Nonetheless, the lower body weight and increase in circulating neutrophils in spontaneously hypertensive hamsters relative to controls are consistent with similar observations in SHR and may be partly related to activation of the sympathetic nervous system.25 26 27 28 To this end, Friedman et al29 showed that an initial increase in the number of circulating leukocytes is related to subsequent development of essential hypertension in humans. Whether circulating neutrophils play a role in the pathogenesis of hypertension in spontaneously hypertensive hamsters remains to be determined.
We found that plasma renin activity was increased in adult male hamsters with spontaneous hypertension relative to normotensive controls. This finding contrasts with normal circulating renin activity observed in SHR.3 30 31 However, these data should be interpreted with caution because renin activity was determined in the plasma of anesthetized hamsters. The presence of hyperreninemia and its relationship to the development of hypertension should therefore be established in conscious spontaneously hypertensive hamsters.
Tissue activity of ACE, a peptidase that produces angiotensin II, a potent vasoconstrictor, and cleaves and inactivates bradykinin, a potent vasodilator, is similar in normotensive and spontaneously hypertensive hamsters and SHR.3 17 30 31 By contrast, tissue NEP activity, a peptidase that also cleaves and inactivates bradykinin,8 22 is increased in spontaneously hypertensive hamsters but not in SHR (J.K. Vishwanatha, R.G. Davis, S. Blumberg, X.-p. Gao, I. Rubinstein, unpublished observations, 1996, and Reference 2323 ). It is conceivable that the increase in tissue NEP activity coupled with normal tissue ACE activity in spontaneously hypertensive hamsters may shift the balance of vasoregulatory mechanisms in the peripheral circulation toward vasoconstriction because bradykinin catabolism will be accelerated while angiotensin II production is unabated. Further studies are warranted to determine the relationship between tissue NEP activity and the development of hypertension in hamsters.
In summary, we began to characterize a new inbred strain of adult male hamsters with spontaneous hypertension and their genetically/age-matched normotensive controls. The increase in systemic arterial pressure in hamsters was associated with lower body weight, left ventricular hypertrophy, hyperreninemia, and increased circulating neutrophils relative to controls. Overall, these data suggest that the spontaneously hypertensive hamster could be a suitable model for the study of spontaneous hypertension.
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Acknowledgments |
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This study was supported in part by grants from the National Institutes of Health (DE10347); American Heart Association, Metropolitan Chicago Affiliate; and the Laerdal Foundation for Acute Medicine. Dr Rubinstein is a recipient of a Research Career Development Award from the National Institutes of Health (DE00386) and a University of Illinois Scholar Award. We thank Alicia Moore and Sergei Pakhlevaniants for technical assistance.
Received March 23, 1996; first decision April 15, 1996; accepted January 24, 1997.
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