Effect of age on coronary circulation after imposition of pressure-overload in rats

Cardiac recovery from pressure overload is not altered by thyroid hormone status in old mice

Introduction

Thyroid hormones (THs) are known to have various effects on the cardiovascular system. However, the impact of TH levels on preexisting cardiac diseases is still unclear. Pressure overload due to arterial hypertension or aortic stenosis and aging are major risk factors for the development of structural and functional abnormalities and subsequent heart failure. Here, we assessed the sensitivity to altered TH levels in aged mice with maladaptive cardiac hypertrophy and cardiac dysfunction induced by transverse aortic constriction (TAC).

Methods

Mice at the age of 12 months underwent TAC and received T4 or anti-thyroid medication in drinking water over the course of 4 weeks after induction of left ventricular pressure overload.

Results

T4 excess or deprivation in older mice had no or only very little impact on cardiac function (fractional shortening), cardiac remodeling (cardiac wall thickness, heart weight, cardiomyocyte size, apoptosis, and interstitial fibrosis), and mortality. This is surprising because T4 excess or deprivation had significantly changed the outcome after TAC in young 8-week-old mice. Comparing the gene expression of deiodinases (Dio) 2 and 3 and TH receptor alpha (TRα) 1 and the dominant-negative acting isoform TRα2 between young and aged mice revealed that aged mice exhibited a higher expression of TRα2 and Dio3, while expression of Dio2 was reduced compared with young mice. These changes in Dio2 and 3 expressions might lead to reduced TH availability in the hearts of 12-month-old mice accompanied by reduced TRα action due to higher TRα2.

Discussion

In summary, our study shows that low and high TH availability have little impact on cardiac function and remodeling in older mice with preexisting pressure-induced cardiac damage. This observation seems to be the result of an altered expression of deiodinases and TRα isoforms, thus suggesting that even though cardiovascular risk is increasing with age, the response to TH stress may be dampened in certain conditions.

Angiotensin II type 1 receptor blockade restores angiotensin-(1–7)-induced coronary vasodilation in hypertrophic rat hearts

The aim of the present study was to investigate the coronary effects of Ang-(1–7) [angiotensin-(1–7)] in hypertrophic rat hearts. Heart hypertrophy was induced by abdominal aorta CoA (coarctation). Ang-(1–7) and AVE 0991, a non-peptide Mas-receptor agonist, at picomolar concentration, induced a significant vasodilation in hearts from sham-operated rats. These effects were blocked by the Mas receptor antagonist A-779. Pre-treatment with L-NAME (NG-nitro-L-arginine methyl ester) or ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinozalin-1-one) [NOS (NO synthase) and soluble guanylate cyclase inhibitors respectively] also abolished the effect of Ang-(1–7) in control hearts. The coronary vasodilation produced by Ang-(1–7) and AVE 0991 was completely blunted in hypertrophic hearts. Chronic oral administration of losartan in CoA rats restored the coronary vasodilation effect of Ang-(1–7). This effect was blocked by A-779 and AT2 receptor (angiotensin II type 2 receptor) antagonist PD123319. Acute pre-incubation with losartan also restored the Ang-(1–7)-induced, but not BK (bradykinin)-induced, coronary vasodilation in hypertrophic hearts. This effect was inhibited by A-779, PD123319 and L-NAME. Chronic treatment with losartan did not change the protein expression of Mas and AT2 receptor and ACE (angiotensin-converting enzyme) and ACE2 in coronary arteries from CoA rats, but induced a slight increase in AT2 receptor in aorta of these animals. Ang-(1–7)-induced relaxation in aortas from sham-operated rats was absent in aortas from CoA rats. In vitro pre-treatment with losartan restored the Ang-(1–7)-induced relaxation in aortic rings of CoA rats, which was blocked by the Mas antagonist A-779 and L-NAME. These data demonstrate that Mas is strongly involved in coronary vasodilation and that AT1 receptor (angiotensin II type 1 receptor) blockade potentiates the vasodilatory effects of Ang-(1–7) in the coronary beds of pressure-overloaded rat hearts through NO-related AT2- and Mas-receptor-dependent mechanisms. These data suggest the association of Ang-(1–7) and AT1 receptor antagonists as a potential therapeutic avenue for coronary artery diseases.

Nitric oxide and coronary vascular endothelium adaptations in hypertension

This review highlights a number of nitric oxide (NO)-related mechanisms that contribute to coronary vascular function and that are likely affected by hypertension and thus become important clinically as potential considerations in prevention, diagnosis, and treatment of coronary complications of hypertension. Coronary vascular resistance is elevated in hypertension in part due to impaired endothelium-dependent function of coronary arteries. Several lines of evidence suggest that other NO synthase isoforms and dilators other than NO may compensate for impairments in endothelial NO synthase (eNOS) to protect coronary artery function, and that NO-dependent function of coronary blood vessels depends on the position of the vessel in the vascular tree. Adaptations in NOS isoforms in the coronary circulation to hypertension are not well described so the compensatory relationship between these and eNOS in hypertensive vessels is not clear. It is important to understand potential functional consequences of these adaptations as they will impact the efficacy of treatments designed to control hypertension and coronary vascular disease. Polymorphisms of the eNOS gene result in significant associations with incidence of hypertension, although mechanistic details linking the polymorphisms with alterations in coronary vasomotor responses and adaptations to hypertension are not established. This understanding should be developed in order to better predict those individuals at the highest risk for coronary vascular complications of hypertension. Greater endothelium-dependent dilation observed in female coronary arteries is likely related to endothelial Ca(2+) control and eNOS expression and activity. In hypertension models, the coronary vasculature has not been studied extensively to establish mechanisms for sex differences in NO-dependent function. Genomic and nongenomic effects of estrogen on eNOS and direct and indirect antioxidant activities of estrogen are discussed as potential mechanisms of interest in coronary circulation that could have implications for sex- and estrogen status-dependent therapy for hypertension and coronary dysfunction. The current review identifies some important basic knowledge gaps and speculates on the potential clinical relevance of hypertension adaptations in factors regulating coronary NO function.

Chronic inhibition of nitric oxide synthase induces hypertension and cardiomyocyte mitochondrial and myocardial collagen remodelling in the absence of hypertrophy

OBJECTIVES

To evaluate the morphometric and ultrastructural alterations of the heart produced by long-term inhibition of nitric oxide (NO) synthase with N(omega)-nitro-l-arginine methyl ester (l-NAME) and to examine whether the changes are caused by l-NAME-induced hypertension or a lack of NO.

METHODS

Male Wistar rats were divided randomly into three sets: control group, standard diet/l-NAME-treated group, and standard diet/l-NAME + captopril-treated group.

RESULTS

Chronic inhibition of NO synthesis with l-NAME given for 4 weeks promoted a time-dependent hypertensive response which was not accompanied by an increase in cardiac mass, myocellular hypertrophy or other evidence of myocyte damage. However, this response was associated with left ventricular cardiomyocyte mitochondrial remodelling and discrete interstitial fibrosis in both the left and right ventricles. The remodelling was characterized by an increase in the number and size of mitochondria. Importantly, systolic pressure overload did not result in left ventricle decompensation. The concomitant treatment with l-NAME and captopril abolished the development of hypertension and left ventricular cardiomyocyte subcellular remodelling, but not the discrete interstitial fibrosis in the left and right ventricle.

CONCLUSIONS

The present study suggests that, in the l-NAME model of hypertension, decreased NO production could be an important means of controlling cardiovascular hypertensive stress by regulating mitochondrial biogenesis and function in the tissue. On the other hand, discrete interstitial ventricular myocardial fibrosis observed in l-NAME-treated rats, either hypertensive or rendered normotensive with captopril, clearly indicates that this response depends on a process associated with NO insufficiency.

Acute and chronic adaptation to hemodynamic overload and ischemia in the aged heart

Congestive heart failure occurs more frequently in older individuals. This higher incidence of heart failure may be caused by the diminished capacity of aged hearts to adapt to increased hemodynamic overload and ischemia which are the most important triggers for heart failure in the aged. In the immediate early phase after the imposition of ascending aortic banding, the mRNA expression of proto-oncogenes (c-fos, c-jun and c-myc) was diminished in aged rat hearts compared with young adult hearts. In the later phase, the pattern of expression of contractile protein genes in aged hearts differed quantitatively from that in adult hearts. The hypertrophic responses to the imposition of not only pressure-overload but also volume-overload were also diminished at the organ and cellular levels. In addition, this diminution was observed both in the left and right ventricles. Against ischemic insults, aged hearts responded with a diminished expression of proto-oncogenes and heat shock proteins. Thus, aged hearts are characterized by poor adaptation to hemodynamic overload and by a poor self-protective mechanism against cell death through necrosis and apoptosis. Of interest, more severe hemodynamic overload elevated the diminished responses to a level similar to that in adult hearts, suggesting that the threshold for the heart to respond to hemodynamic overload or ischemia is elevated in aged hearts. In addition, even in aged hearts ischemic preconditioning upregulated the diminished gene expression in a gene-dependent manner. Thus, the capacity for adaptation to hemodynamic overload and ischemia is diminished in aged hearts, but aged hearts preserve the ability to respond to these under some conditions.

Heat shock protein expression in hearts hypertrophied by genetic and nongenetic hypertension

Genetically hypertensive animals are characterized by greater thermosensitivity and overexpression of heat shock proteins (HSP) upon thermal stimulation. We examined HSP72 expression under conditions of brief coronary occlusion or thermal stimulation, and the effects of the severity of these stimuli and of myocardial hypertrophy on the expression in hearts of spontaneously hypertensive rat (SHR) and Wistar Kyoto rat (WKY) groups, A snare was created around the left coronary artery in the SHR (n = 16) and WKY (n = 19) groups. In 7 WKY rats, the ascending aorta was banded and a snare was created simultaneously (WKY-AoB). By tying the snare, 4 weeks later, we applied 5- or 10-min coronary occlusion without opening the chest. For thermal stimulation, the SHR (n = 13) and WKY (n = 11) rats were placed in a 42 degrees C chamber for 15 or 40 min. The mRNA or protein level was estimated 1 or 24h after stimulation. In the SHR vs WKY groups, the mRNA and protein levels were higher after 5-min occlusion or 15-min thermal stimulation. After 10-min occlusion or 40-min thermal stimulation the difference was no longer observed. The overexpression was not observed in the WKY-AoB group despite the presence of hypertrophy similar to that seen in the SHR group (3.11+/-0.11 vs 3.20+/-0.06 mg/g in left ventricular weight/body weight). The HSP72 was overexpressed in hearts of genetically hypertensive animals after brief ischemia. Differential expression between the two groups was observed after mild stimuli, but not after more severe stimuli. Cardiac hypertrophy was not a major factor for determining the overexpression of HSP72.

Adverse influence of systemic vascular stiffening on cardiac dysfunction and adaptation to acute coronary occlusion

BACKGROUND

[corrected] Age is an independent risk factor for increased mortality from ischemic heart disease. Arterial stiffening with widening of the pulse pressure may contribute to this risk by exacerbating cardiac dysfunction after total coronary artery occlusion.

METHODS AND RESULTS

To test the above hypothesis, 14 open-chest dogs underwent surgery in which the intrathoracic aorta was bypassed with a stiff plastic tube. Directing ventricular outflow through the bypass widened the arterial pulse pressure from 41 to 115 mm Hg at similar mean pressure and flow. Hearts ejecting into the native aorta (NA) exhibited only modest dysfunction after two minutes of mid-left anterior descending coronary artery occlusion. However, the same occlusion applied during ejection into the bypass tube (BT) induced far more severe cardiodepression (ie, systolic pressure fell by -41+/-10 mm Hg for BT versus -15+/-3 mm Hg for NA, and end-systolic volume rose by 15+/-3 versus 6+/-2 mL), with a threefold greater decline in ejection fraction. This disparity was not due to higher baseline work loads because total pressure-volume area was similar in both cases. Furthermore, marked increases in basal work load and wall stress induced by angiotensin II infusion (in four additional studies) did not reproduce this behavior. Although peak systolic chamber stress was greater with the BT, this did not increase systolic dyskinesis as measured in the central ischemic zone. However, the total mass of myocardium that was rendered severely ischemic (ie, flow reduced by > or = 80%) was twice as large with BT ejection, likely expanding the region of dyskinesis. This disparity may relate to altered phasic coronary flow during BT ejection, which displays marked enhancement of systolic flow and renders the heart more vulnerable to diminished mean and systolic perfusion pressures.

CONCLUSIONS

Cardiac ejection into a stiff systemic vasculature augments cardiac dysfunction and ischemia due to coronary occlusion by tightening the link between cardiac systolic performance and myocardial perfusion. This may contribute to the higher mortality risk from ischemic heart disease due to age.

Age-related changes before and after imposition of hemodynamic stress in the mammalian heart

This review focusses on the following issues: how the mammalian heart grows and ages; age-related changes in the mammalian heart before and after imposition of hemodynamic stress; and antiaging modulation in the mammalian heart. The heart and other organs grow and age together in the whole body, and interactions occur between these organs. Therefore, the age-related changes at the molecular and cellular level in the in vivo heart are the summation of the changes of the heart per se and the effects of other organs or tissues on the heart. Furthermore, myocytes grow and age under the influence of age-related changes in other myocytes and other types of cells in the myocardial tissue through autocrine or paracrine mechanisms, because myocytes are exposed to many biologically active substances which are released from those cells. Since hypertension and ischemia are very common hemodynamic events in aged hearts, the characteristics in aged hearts are discussed in terms of responses to hypertension or ischemia. The induction of proto-oncogenes and heat shock protein genes in response to milder hemodynamic stress such as pressure-overload and ischemia is diminished in aged hearts. However, the aged heart can respond to more severe stress to a level similar to that of young-adult hearts. Therefore, the senescent heart is characterized by its attenuated adaptation to hemodynamic stress and by its ability to adapt to limited environmental changes. Several interventions have antiaging effects on the heart at the molecular and cellular level.

Effect of young age on coronary adaptations to left ventricular pressure overload hypertrophy in sheep

OBJECTIVES

This study attempted to determine the effect of young age on changes in coronary conductance and capillary density with left ventricular pressure overload hypertrophy. Mechanisms responsible for age differences in perfusion capacity were examined.

BACKGROUND

Hypertension in adults causes alterations in the coronary vasculature, resulting in diminished coronary perfusion capacity and myocardial ischemia. These processes are worsened in adults by advanced age. Young age may provide advantages in coronary adaptation to hypertension.

METHODS

Coronary conductance was examined in conscious chronically instrumented 10-week old lambs and adult sheep with progressive ascending aortic stenosis of 6-week's duration and age-matched control sheep by means of the microsphere technique and vasodilators. Capillary density was measured post-mortem.

RESULTS

Adult sheep with aortic stenosis had a decrease in left ventricular subendomyocardial capillary density by 17% and maximal coronary conductance with adenosine by 67%. In the nonhypertrophied right ventricle, maximal coronary conductance was depressed by 47%, whereas capillary density was normal, implying an effect of coronary hypertension on resistance vessels. In contrast, lambs with aortic stenosis maintained normal left ventricular capillary density, maximal coronary conductance and coronary reserve and had relatively little impairment of conductance in the right ventricular coronary bed (-15%, p = NS). Similar responses were found with other vasodilators, isoproterenol and chromonar.

CONCLUSIONS

Young age confers advantages to coronary adaptation to left ventricular pressure overload, including angiogenesis proportionate to hypertrophy, resulting in normal capillary density and coronary conductance. There is also less hypertension-induced impairment of coronary conductance distinct from the effects of hypertrophy.

Collagen deposition and the reversal of coronary reserve in cardiac hypertrophy

The aim of this study was to clarify how collagen deposition or medial hypertrophy of the vascular wall affects the coronary dilator reserve in pressure-overloaded hearts and whether inhibition of collagen deposition reverses the abnormalities after relief of pressure overload. We used ascending aortic banding and debanding methods and superimposed beta-aminopropionitrile in some of the banded rats (50 mg/kg i.p., twice a day). Ten weeks of banding increased in vivo peak systolic left ventricular pressure and produced medial hypertrophy, an increase in collagen deposition in the myocardial and perivascular tissues, and myocardial hypertrophy in the banded group without beta-aminopropionitrile treatment. Superimposition of beta-aminopropionitrile treatment on banding inhibited the increase in collagen deposition. In the groups debanded after the 10-week banding period, both with and without beta-amino-propionitrile treatment, medial and myocardial hypertrophy regressed 4 weeks after debanding. We estimated coronary dilator reserve in Langendorff preparations perfused with modified Tyrode's solution containing oxygenated bovine red blood cells and serum albumin. The ratio of reactive peak flow after brief ischemia-to-resting flow decreased in both of the banded groups. After debanding, the ratio remained lower in the banded group without beta-aminopropionitrile treatment than in the control group. However, debanding in the group with beta-aminopropionitrile treatment increased the ratio to a level similar to that of the control group. Thus, in pressure-overloaded cardiac hypertrophy with coronary hypertension, coronary reserve seems to be determined by medial hypertrophy independently of collagen deposition, but collagen deposition plays an important role in the reversal of vasodilator reserve after relief of the overload.