Young Adult Spontaneously Hypertensive Rats Research Articles

Early structural alterations of intrinsic cardiac ganglionated plexus in spontaneously hypertensive rats.

Persistent arterial hypertension leads to structural and functional remodeling of the heart resulting in myocardial ischemia, fibrosis, hypertrophy, and eventually heart failure. Previous studies have shown that individual neurons composing the intracardiac ganglia are hypertrophied in the failing human, dog, and rat hearts, indicating that this process involves changes in cardiac innervation. However, despite a wealth of data on changes in intrinsic cardiac ganglionated plexus (GP) in late-stage disease models, little is known about the effects of hypertension on cardiac innervation during the early onset of heart failure development. Thus, we examined the impact of early hypertension on the structural organization of the intrinsic cardiac ganglionated plexus in juvenile (8-9 weeks) and adult (12-18 weeks) spontaneously hypertensive (SH) and age-matched Wistar-Kyoto (WKY) rats. GP was studied using a combination of immunofluorescence confocal microscopy and transmission electron microscopy in whole-mount preparations and tissue sections. Here, we report intrinsic cardiac GP of SH rats to display multiple structural alterations: (i) a decrease in the intracardiac neuronal number, (ii) a marked reduction in axonal diameters and their proportion within intracardiac nerves, (iii) an increased density of myocardial nerve fibers, and (iv) neuropathic abnormalities in cardiac glial cells. These findings represent early neurological changes of the intrinsic ganglionated plexus of the heart introduced by early-onset arterial hypertension in young adult SH rats.

Reduced carotid artery blood flow of spontaneously hypertensive rats is unaffected by aging from 20 weeks (young adulthood) to 65 weeks (upper middle age) (LB690)

The objective of this study was to quantify blood flow (using a perivascular flowprobe) and pressure (using a catheter‐tip transducer) in the common carotid artery (CCA) of anesthetized young adult (~20 week old) and upper middle aged (~65 week old) Wistar Kyoto rats (WKY) and Spontaneously Hypertensive rats (SHR). In young adult SHR vs. WKY, mean (172±10 vs. 82±2 mmHg), systolic (198±13 vs. 92±4), diastolic (151±9 vs. 72±5), and pulse pressure (48±5 vs. 20±2), and heart rate (371±12 vs. 277±19 beats/min), were each elevated (p<0.01). In SHR, aging had no effect on these pressure parameters, or heart rate; whereas, in WKY, aging was associated with a significant increase in pulse pressure (31±1 mmHg; p=0.04) but no other pressure parameter. Mean (4.94±0.29 vs. 3.80±0.54 ml/min), maximum systolic (11.58±1.08 vs. 19.00±2.18), and minimum diastolic blood flow (0.62±0.73 vs. 1.90±0.34) were each reduced in young adult SHR vs. WKY (p<0.05), while flow pulsatility was no different (3.23±0.73 vs. 3.43±0.44). Aging had no effect on these flow parameters in either SHR or WKY. Together, these data demonstrate that CCA blood flow is >20% reduced in young adult SHR, and that this reduction is neither exacerbated nor attenuated at upper middle age. Given the important contribution of the internal branches of the CCAs to the total perfusion of the brain, these findings may be of pathophysiological and clinical relevance.Grant Funding Source: Supported by NSERC Canada

I030 Young spontaneously hypertensive rats (SHR) with chronic inhibition of nitric oxide exhibit similar aortic stiffness to old SHR

The old spontaneously hypertensive rat (SHR) is known to be a good experimental model that is highly analogous to essential hypertension in man but is costly and time consuming. Chronic inhibition of nitric oxide (NO) production in young adult SHR is an experimental model of malignant hypertension and chronic renal disease, very similar to old SHR1. Considering that acute or chronic decreases in NO production in normotensive rats increase arterial stiffness, we aimed to compare the aortic stiffness between SHR/L-NAME and old SHR. Male SHR (18 weeks old) received N-Nitro-L-Arginine Methyl Ester (L-NAME) 6 mg/kg/d in drinking water (SHR/L-NAME) for 2 weeks and were compared to SHR and Wistar Kyoto rats (WKY). Moreover, two groups are composed of 55 weeks aged SHR (Old SHR) and WKY (Old WKY). Under anesthesia, two catheters were introduced in the left common carotid artery and the left femoral artery to measure central and peripheral blood pressure and assess aortic stiffness with pulse wave velocity (PWV= distance between the tips of the two catheters / transit time), β-index (2.11×(PWV2/Diastolic blood pressure)), amplification and pulse pressure (PP). At 20 weeks, although SHR have higher central systolic blood pressure (SBP) than WKY (201±7 mmHg vs. 154±4 mmHg), no significant difference in PWV, PP, amplification and β-index was observed between these two groups. With age, old WKY have higher SBP (181±8 mmHg) and higher PWV (6.7±0.5 m/s) and β-index (0.70±0.08) than young WKY. Old SHR exhibit higher SBP (252±13 mmHg), PWV (9.3±0.7 m/s), PP (71±7 mmHg), β-index (1.08±0.16) than old WKY and young SHR (PWV=6.3±0.2 m/s, PP=39±3 mmHg and β-index=0.50±0.03). Young SHR, after 2 weeks of L-NAME administration, have higher SBP (238±10 mmHg) and aortic stiffness (PWV=8.6±0.6 m/s, PP=54±6 mmHg and β-index=0.89±0.14) than young SHR and possess the aortic stiffness observed in old SHR. In conclusion, our results indicate a close analogy in arterial stiffness between SHR/L-NAME and old SHR. Thus, this experimental model shows similarities to hypertensive humans and appears relevant to test new anti-hypertensive treatments.

Endothelium‐dependent contractions in the carotid artery of young adult spontaneously hypertensive rats are mediated by cyclooxygenase and markedly augmented by nitric oxide inhibition

We have recently shown that ACh‐stimulated vasomotor activity in isolated PE‐precontracted common carotid artery (CCA) rings from young adult spontaneously hypertensive rats (SHR) involves a dose‐dependent relaxation phase from −9 to −7 LogM followed by a ‘re’‐contraction phase through to −4 LogM, whereas in normotensive Wistar Kyoto rats (WKY) only a robust relaxation phase existed. The objective of this study was to test the hypothesis that [ACh] >−7 LogM stimulates contractions in quiescent CCA from SHR but not WKY. Reference KCl contractions in CCA were similar between strains and across all drug conditions (0.9−1.0g). [ACh] from −9 to −4 did not elicit vasomotor activity in WKY CCA, nor did [ACh] elicit vasomotor activity in SHR CCA from −9 to −7; however, [ACh] >−7 stimulated a dose‐dependent contraction in SHR CCA reaching a plateau at −4 (0.16±0.03g). Endothelium denudation completely abolished these ACh‐stimulated contractions as did cyclooxygenase (COX) inhibition, whereas blocking endothelial nitric oxide (NO) synthase markedly increased the amplitude (0.61±0.08g) but not the threshold or sensitivity of the contractile response. These data demonstrate that only [ACh] >−7 LogM can stimulate contractions in CCA from young adult SHR (but not WKY) that are endothelium‐dependent, COX‐mediated, and powerfully modulated by the availability of NO.Research Support: HSFO and NSERC

Effect of repeated nicotine exposure on high-affinity nicotinic acetylcholine receptor density in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHRs) are often used as a model of attention deficit hyperactivity disorder (ADHD) and to investigate the effects of hypertension on cognitive function. Further, they appear to have reduced numbers of central nicotinic acetylcholine receptors (nAChRs) and, therefore, may be useful to model certain aspects of Alzheimer's disease (AD) and other forms of dementia given that a decrease in nAChRs is thought to contribute to cognitive decline in these disorders. In the present study, based on reports that chronic nicotine exposure increases nAChRs in several mammalian models, we tested the hypothesis that repeated exposures to a relatively low dose of the alkaloid would ameliorate the receptor deficits in SHR. Thus, young-adult SHRs and age-matched Wistar–Kyoto (WKY) control rats were treated with either saline or nicotine twice a day for 14 days (total daily dose = 0.7 mg/kg nicotine base) and then sacrificed. Quantitative receptor autoradiography with [ 125I]-IPH, an epibatidine analog, revealed: (1) that high-affinity nAChRs were higher in saline-treated WKY (control) rats compared to saline-treated SHRs in 18 of the 19 brain region measured, although statistically different only in the mediodorsal thalamic nuclei, (2) that nicotine significantly increased nAChR binding in WKY rats in six brain areas including cortical regions and the anterior thalamic nucleus, (3) that there were no cases where nicotine significantly increased nAChR binding in SHRs. These results indicate that subjects deficient in nAChRs may be less sensitive to nAChR upregulation with nicotine than normal subjects and require higher doses or longer periods of exposure.

Review: Promise of prevention and reversal of target organ involvement in hypertension.

JRAAS 2001;2 (suppl 1):S4-S9 Introduction The benefits derived from antihypertensive therapy over the past four decades have been substantial. Clinical outcome studies in the US have demonstrated a progressive reduction of death and morbidity from strokes as well as from coronary heart disease (CHD) in the order of approximately 70% and 55%, respectively. Reduction in deaths from stroke has been demonstrated consistently in most prospective double-blind, placebocontrolled studies, probably because the cerebral circulation seems to be responsive to therapy that reduces arterial pressure. However, the initial review of the earlier multicentre trials demonstrated a significant reduction in deaths from CHD, but this was not as great as had been predicted. This led some observers to ascribe this finding to an atherogenic effect of diuretics mediated by elevated blood lipid levels. However, when the recommended doses of the thiazide diuretics were reduced drastically from those used in the earlier trials (e.g., from 100–150 mg hydrochlorothiazide to 12.5–25 mg daily) the same initially-projected reduction in deaths from CHD was confirmed. A subsequent meta-analysis involving elderly hypertensive patients with systolic and diastolic pressure elevation showed a 26% reduction in CHD deaths. Thus, reports from many national and international hypertension advisory panels continue to extol the benefits of the initial antihypertensive therapy in its ability to prevent strokes and CHD. Unfortunately, this confidence has been challenged over recent years. Worldwide efforts to treat patients with hypertension have become less successful. In many countries, the numbers of patients with hypertension detected, evaluated, treated and controlled are significantly less today than in the early 1970s when the national education and control efforts in the US were begun. Secondly, despite the continued reduction in stroke and CHD mortality, there has been a continuous and steady increase in the numbers of patients with hypertension who have progressed to end-stage renal disease (ESRD) or congestive heart failure (CHF). Further,although the greatest numbers of patients with ESRD have diabetes mellitus, patients with hypertension are the second most common group. Many of those patients with diabetes mellitus also have co-existent hypertension. Furthermore, as will be discussed below, while hypertension was the most common diagnostic group of patients with CHF in the pre-antihypertensive drug therapy era, its nature is very different today although continues to be the most common cause of CHF. This paper summarises studies from our laboratory in rats with CHD, ESRD in elderly spontaneously hypertensive rats (SHR) and young adult SHRs with exacerbated ESRD resulting from induced endothelial dysfunction. The beneficial effects on SHR target organs of antihypertensive therapy will be highlighted. These observations in the SHR will be supported by recent clinical experiences in patients with essential hypertension that underscore our present thesis that there is a real potential for prevention and reversal of target organ involvement in hypertension.

Decreased local cerebral blood flow in young and aged spontaneously hypertensive rats.

Local cerebral blood flow (LCBF) was measured using the [14C] iodoantipyrine method in 76 discrete brain regions in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) of 4 months old (young-adult) and 16-17 months old (aged). Compared with the young-adult WKY, in both the young-adult and aged SHR, LCBF was reduced in scattered brain regions, in more wide-spread regions in the aged SHR, while LCBF was not significantly decreased in the aged WKY. The results indicate that, firstly, cerebral hypoperfusion is already established at the age of 4 months by persistent hypertension in SHR, but age-related decline of LCBF thereafter is not remarkable, and secondly, cerebral perfusion is preserved in normotensive WKY even in their senescence. Combined with the present results and the previous study that local cerebral glucose utilization (LCGU) reduced in the aged groups of WKY and SHR in scattered brain regions, more intensively and more extensively in the aged SHR, but did not in the young-adult SHR, the followings are suggested: 1) in the young-adult SHR, LCBF is decreased but brain metabolism is preserved in a state of misery perfusion, 2) in the normotensive aged WKY, LCBF is preserved but brain metabolism is reduced implying "primary hypometabolism during senescence", and 3) in the aged SHR, LCBF is decreased and brain metabolism is more severely reduced, probably resulting from a combination of the primary hypometabolism of the brain by aging and a secondary hypometabolism due to the long-standing hypertension related hypoperfusion.

Adrenomedullin gene delivery reduces blood pressure in spontaneously hypertensive rats.

Adrenomedullin (ADM) is a potent vasoactive peptide. In this study, we explored the effects of a continuous supply of ADM by somatic gene delivery on spontaneously hypertensive rats (SHR). DNA constructs containing the human ADM cDNA fused to either the cytomegalovirus promoter (CMV-cADM) or Rous Sarcoma virus 3'-long terminal repeat (RSV-cADM) were intravenously injected into SHR through the tail vein. Expression of human ADM in SHR was identified in the kidney, adrenal gland, heart, and lung by radioimmunoassay and reverse transcription-polymerase chain reaction followed by Southern blot analysis. A single injection of ADM plasmid DNA in young adult SHR (7 wk old) caused a significant reduction in systolic blood pressure for up to 5 wk (p < 0.05). A second injection of CMV-cADM 5 wk after the first delivery resulted in a further reduction in blood pressure for another 3 wk (p < 0.001). A maximal blood pressure reduction of 22 mmHg in SHR was observed 7 wk after injection of CMV-cADM plasmid DNA (185 +/- 1.7 mmHg, n = 6, p < 0.001), and a reduction of 15 mmHg was observed after injection of RSV-cADM (192 +/- 2.7 mmHg, n = 6, p < 0.001), as compared with control rats given vector DNA (207 +/- 2.4 mmHg, n = 6). Similarly, injection of CMV-cADM plasmid DNA in adult SHR (10 wk old) resulted in a significant reduction in blood pressure for up to 6 wk. Antibodies to either human ADM or its plasmid DNA were not detected in rat sera after the second injection. These studies indicate that intravenous injection of the human ADM gene in hypertensive rats results in expression of the foreign gene and induces a long-lasting reduction in blood pressure.

Smaller local brain volumes and cerebral atrophy in spontaneously hypertensive rats.

Spontaneously hypertensive rats (SHR) have enlarged cerebral ventricles from 8 weeks of age onward and smaller brains than age-matched, normotensive Wistar-Kyoto (WKY) rats (controls). At 6-7 months of age local cerebral glucose utilization is apparently lower in many brain areas of SHR relative to WKY rats. These observations led to the hypothesis that there are morphological differences between these two strains of rats in many, if not all, brain areas. This hypothesis was tested in 6-7-month-old SHR and WKY rats by quantitating 1) the volumes of the ventricular system, whole brain, six gray matter structures, and two white matter areas; 2) the thickness of two regions of the cerebral cortex; and 3) the frequency of neuronal nuclei (neuronal frequency) in nine brain areas. Ventricular volume was twofold greater in SHR than in control rats. The volumes of the entire brain and all six gray matter structures plus the thickness of the two cortical regions were 11-25% less in SHR. Neuronal frequency was, however, similar in the two rat strains. The latter finding coupled with the smaller regional tissue volumes indicates appreciably fewer neurons per brain structure in young adult SHR than in controls. These results indicate significant cerebral structural differences between young adult SHR and WKY rats and suggest that structure as well as metabolism are abnormal in the SHR brain.

Effect of swimming training on cardiac function and myosin ATPase activity in SHR.

Male spontaneously hypertensive rats (SHR) and Wistar-Kyoto normotensive rats (WKY) were subjected to swimming training 6 times/wk, commencing at 4 wk of age, to determine whether this type of endurance exercise might alter contractile proteins and cardiac function in young adult SHR. The total duration of exercise was 190 h. Myofibrillar adenosinetriphosphatase (ATPase) activity was assayed at various free [Ca2+] ranging from 10(-7) to 10(-5) M. Ca2+-stimulated ATPase activity of actomyosin and purified myosin was determined at various Ca2+ concentrations both in the low and high ionic strength buffers. Actin-activated myosin ATPase activity of purified myosin was assayed at several concentrations of actin purified from rabbit skeletal muscle. Under all these conditions the contractile protein ATPase activity was comparable between trained and untrained WKY and SHR. Analysis of myosin isoenzymes on pyrophosphate gels showed a single band corresponding to V1 isoenzyme, and there were no differences between swimming-trained and nontrained WKY and SHR. Ventricular performance was assessed by measuring cardiac output and stroke volume after rapid intravenous volume overloading. Both cardiac index and stroke index were comparable in nontrained WKY and SHR but were significantly increased in the trained groups compared with their respective nontrained controls. These results suggest that myosin ATPase activity and distribution of myosin isoenzymes are not altered in the moderately hypertrophied left ventricle whether the hypertrophy is due to genetic hypertension (SHR) or to exercise training (trained WKY). Moreover, the data indicate that SHR, despite the persistence of a pressure overload, undergo similar increases in left ventricular mass and peak cardiac index after training, as do normotensive WKY.