In vitro assessment of cardiac performance after irradiation using an isolated working rat heart preparation

Alteration of renal Na,K-ATPase in rats following the mediastinal γ-irradiation

Na,K-ATPase represents the key enzyme that maintains the homeostasis of sodium and potassium ions in the cells. It was documented that in directly irradiated organs the activity of this enzyme is decreased. The aim of present study was to clarify the remote effect of irradiation in mediastinal area on the activity of the Na,K-ATPase in kidneys in rats. Ionizing radiation in single dose 25 Gy resulted in consequent decrease of the body weight gain as well as the size of kidneys in Wistar rats. In addition, radiation induced alterations in the oxidative status of blood plasma. Irradiation also decreased the activity of renal Na,K-ATPase. Measurements of enzyme kinetics that were dependent on the concentration of energy substrate ATP or cofactor Na+ indicated that the lowered enzyme activity is probably a consequence of decreased number of active molecules of the enzyme, as suggested by lowered Vmax values. Immunoblot analysis confirmed the lowered expression of the catalytic alpha subunit together with decreased content of the glycosylated form of beta subunit in the renal tissue of irradiated rats. The ability of the enzyme to bind the substrate ATP, as well as Na+ was not affected, as shown by unaltered values of Km and KNa . Irradiation of the body in the mediastinal area despite protection of kidneys by lead plates during application of X-ray was followed by significant decline of activity of the renal Na,K-ATPase, what may result in deteriorated homeostasis in the organism.

Potential markers and metabolic processes involved in the mechanism of radiation-induced heart injury

Irradiation of normal tissues leads to acute increase in reactive oxygen/nitrogen species that serve as intra- and inter-cellular signaling to alter cell and tissue function. In the case of chest irradiation, it can affect the heart, blood vessels, and lungs, with consequent tissue remodelation and adverse side effects and symptoms. This complex process is orchestrated by a large number of interacting molecular signals, including cytokines, chemokines, and growth factors. Inflammation, endothelial cell dysfunction, thrombogenesis, organ dysfunction, and ultimate failing of the heart occur as a pathological entity - "radiation-induced heart disease" (RIHD) that is major source of morbidity and mortality. The purpose of this review is to bring insights into the basic mechanisms of RIHD that may lead to the identification of targets for intervention in the radiotherapy side effect. Studies of authors also provide knowledge about how to select targeted drugs or biological molecules to modify the progression of radiation damage in the heart. New prospective studies are needed to validate that assessed factors and changes are useful as early markers of cardiac damage.

A novel technique for image-guided local heart irradiation in the rat

In radiotherapy treatment of thoracic, breast and chest wall tumors, the heart may be included (partially or fully) in the radiation field. As a result, patients may develop radiation-induced heart disease (RIHD) several years after exposure to radiation. There are few methods available to prevent or reverse RIHD and the biological mechanisms remain poorly understood. In order to further study the effects of radiation on the heart, we developed a model of local heart irradiation in rats using an image-guided small animal conformal radiation therapy device (SACRTD) developed at our institution. First, Monte Carlo based simulations were used to design an appropriate collimator. EBT-2 films were used to measure relative dosimetry, and the absolute dose rate at the isocenter was measured using the AAPM protocol TG-61. The hearts of adult male Sprague-Dawley rats were irradiated with a total dose of 21 Gy. For this purpose, rats were anesthetized with isoflurane and placed in a custom-made vertical rat holder. Each heart was irradiated with a 3-beam technique (one AP field and 2 lateral fields), with each beam delivering 7 Gy. For each field, the heart was visualized with a digital flat panel X-ray imager and placed at the isocenter of the 1.8 cm diameter beam. In biological analysis of radiation exposure, immunohistochemistry showed γH2Ax foci and nitrotyrosine throughout the irradiated hearts but not in the lungs. Long-term follow-up of animals revealed histopathological manifestations of RIHD, including myocardial degeneration and fibrosis. The results demonstrate that the rat heart irradiation technique using the SACRTD was successful and that surrounding untargeted tissues were spared, making this approach a powerful tool for in vivo radiobiological studies of RIHD. Functional and structural changes in the rat heart after local irradiation are ongoing.

Experimental radiation-induced heart disease: past, present, and future

Radiation-induced heart disease (RIHD) is a serious side effect of radiotherapy for intrathoracic and chest wall tumors. The threshold dose for development of clinically significant RIHD is believed to be lower than previously assumed. Therefore, research into mechanisms of RIHD has gained substantial momentum. RIHD becomes clinically apparent ten to fifteen years after radiation exposure. Chronic manifestations of RIHD include accelerated atherosclerosis, cardiomyopathy, and valve abnormalities. Reducing exposure of the heart during radiotherapy is the only known method of preventing RIHD, and there are no approaches to reverse RIHD once it occurs. We use a combination of pharmacological and genetic animal models to determine biological mechanisms of RIHD. Major technological advances in small animal research have made this type of study more valuable. The long-term goal of this work is to identify targets for intervention in RIHD, thereby enhancing the efficacy and safety of thoracic radiotherapy.

Radiation as a risk factor for cardiovascular disease

Abstract population are ubiquitous background radiation and medical exposure of patients. From the early 1980s to 2006, the average dose per individual in the United States for all sources of radiation increased by a factor of 1.7-6.2 mSv, with this increase due to the growth of medical imaging procedures. Radiation can place individuals at an increased risk of developing cardiovascular disease. Excess risk of cardiovascular disease occurs a long time after exposure to lower doses of radiation as demonstrated in Japanese atomic bomb survivors. This review examines sources of radiation (atomic bombs, radiation accidents, radiological terrorism, cancer treatment, space exploration, radiosurgery for cardiac arrhythmia, and computed tomography) and the risk for developing cardiovascular disease. The evidence presented suggests an association between cardiovascular disease and exposure to low-to-moderate levels of radiation, as well as the well-known association at high doses. Studies are needed to define the extent that diagnostic and therapeutic radiation results in increased risk factors for cardiovascular disease, to understand the mechanisms involved, and to develop strategies to mitigate or treat radiation-induced cardiovascular disease.

Potential targets for intervention in radiation-induced heart disease

Radiotherapy of thoracic and chest wall tumors, if all or part of the heart was included in the radiation field, can lead to radiation-induced heart disease (RIHD), a late and potentially severe side effect. RIHD presents clinically several years after irradiation and manifestations include accelerated atherosclerosis, pericardial and myocardial fibrosis, conduction abnormalities, and injury to cardiac valves. The pathogenesis of RIHD is largely unknown, and a treatment is not available. Hence, ongoing pre-clinical studies aim to elucidate molecular and cellular mechanisms of RIHD. Here, an overview of recent pre-clinical studies is given, and based on the results of these studies, potential targets for intervention in RIHD are discussed.

10 Gy total body irradiation increases risk of coronary sclerosis, degeneration of heart structure and function in a rat model

PURPOSE

To determine the impact of 10 Gy total body irradiation (TBI) or local thorax irradiation, a dose relevant to a radiological terrorist threat, on lipid and liver profile, coronary microvasculature and ventricular function.

MATERIALS AND METHODS

WAG/RijCmcr rats received 10 Gy TBI followed by bone marrow transplantation, or 10 Gy local thorax irradiation. Age-matched, non-irradiated rats served as controls. The lipid profile and liver enzymes, coronary vessel morphology, nitric oxide synthase (NOS) isoforms, protease activated receptor (PAR)-1 expression and fibrinogen levels were compared. Two-dimensional strain echocardiography assessed global radial and circumferential strain on the heart.

RESULTS

TBI resulted in a sustained increase in total and low density lipoprotein (LDL) cholesterol (190 +/- 8 vs. 58 +/- 6; 82 +/- 8 vs. 13 +/- 3 mg/dl, respectively). The density of small coronary arterioles was decreased by 32%. Histology revealed complete blockage of some vessels while cardiomyocytes remained normal. TBI resulted in cellular peri-arterial fibrosis whereas control hearts had symmetrical penetrating vessels with less collagen and fibroblasts. TBI resulted in a 32 +/- 4% and 28 +/- 3% decrease in endothelial NOS and inducible NOS protein, respectively, and a 21 +/- 4% and 35 +/- 5% increase in fibrinogen and PAR-1 protein respectively, after 120 days. TBI reduced radial strain (19 +/- 8 vs. 46 +/- 7%) and circumferential strain (-8 +/- 3 vs. -15 +/- 3%) compared to controls. Thorax-only irradiation produced no changes over the same time frame.

CONCLUSIONS

TBI with 10 Gy, a dose relevant to radiological terrorist threats, worsened lipid profile, injured coronary microvasculature, altered endothelial physiology and myocardial mechanics. These changes were not manifest with local thorax irradiation. Non-thoracic circulating factors may be promoting radiation-induced injury to the heart.

Effects of adenovirus-mediated delivery of the human hepatocyte growth factor gene in experimental radiation-induced heart disease

PURPOSE

Irradiation to the heart may lead to late cardiovascular complications. The purpose of this study was to investigate whether adenovirus-mediated delivery of the human hepatocyte growth factor gene could reduce post-irradiation damage of the rat heart and improve heart function.

METHODS AND MATERIALS

Twenty rats received single-dose irradiation of 20 Gy gamma ray locally to the heart and were randomized into two groups. Two weeks after irradiation, these two groups of rats received Ad-HGF or mock adenovirus vector intramyocardial injection, respectively. Another 10 rats served as sham-irradiated controls. At post-irradiation Day 120, myocardial perfusion was tested by myocardial contrast echocardiography with contrast agent injected intravenously. At post-irradiation Day 180, cardiac function was assessed using the Langendorff technique with an isolated working heart model, after which heart samples were collected for histological evaluation.

RESULTS

Myocardial blood flow was significantly improved in HGF-treated animals as measured by myocardial contrast echocardiography at post-irradiation Day 120 . At post-irradiation Day 180, cardiac function was significantly improved in the HGF group compared with mock vector group, as measured by left ventricular peak systolic pressure (58.80 +/- 9.01 vs. 41.94 +/- 6.65 mm Hg, p < 0.05), the maximum dP/dt (5634 +/- 1303 vs. 1667 +/- 304 mm Hg/s, p < 0.01), and the minimum dP/dt (3477 +/- 1084 vs. 1566 +/- 499 mm Hg/s, p < 0.05). Picrosirius red staining analysis also revealed a significant reduction of fibrosis in the HGF group.

CONCLUSION

Based on the study findings, hepatocyte growth factor gene transfer can attenuate radiation-induced cardiac injury and can preserve cardiac function.

Hepatocyte growth factor protects endothelial cells against gamma ray irradiation-induced damage

AIM

To investigate the effect of HGF on proliferation, apoptosis and migratory ability of human vascular endothelial cells against gamma ray irradiation.

METHODS

ECV304 cells derived from adult human umbilical vein endothelial cells (HUVEC) were irradiated with a single gamma ray dose of 20 Gy. Immunocytochemistry and Western blot analysis were used to detect c-Met protein expression and HGF/c-Met signal pathway. In the HGF-treated groups, ECV304 cells were incubated with HGF (20 or 40 ng/mL) 3 h prior to irradiation. At 48 h post-irradiation, the proliferation of ECV304 cells was measured by MTT assay, the apoptosis was assessed by flow cytometry, and the migratory ability of ECV304 cells was measured by transwell chamber assay.

RESULTS

c-Met protein is expressed in ECV304 cells and can be activated by HGF. Gamma ray irradiation inhibits proliferation and migration of ECV304 cells in a dose-dependent manner. HGF significantly promoted the proliferation of ECV304 cells, and flow cytometry revealed that HGF can inhibit apoptosis of ECV304 cells. Transwell chamber assay also showed that HGF increases migration activity of endothelial cells.

CONCLUSION

HGF may afford protection to vascular endothelial cells against gamma ray irradiation-induced damage.

Influence of endothelin 1 receptor inhibition on functional, structural and molecular changes in the rat heart after irradiation

Radiation-induced heart disease is a severe side effect of thoracic radiotherapy. Studies suggest that mast cells play a protective role in radiation-induced heart disease and that the endothelin (ET) system mediates protective effects of mast cells in other disorders. This study examined whether mast cells modulate the cardiac ET system and examined the effects of ET receptor inhibition in a rat model of radiation-induced heart disease. Mast cell-deficient (Ws/Ws), mast cell-competent (+/+) and Sprague-Dawley rats received 18 Gy irradiation to the heart. Left ventricular mRNA of ET1 and its receptors (ETA and ETB) was measured in Ws/Ws and +/+ rats at 1 week and 3 months. Sprague-Dawley rats were treated with the ETA/ETB antagonist bosentan, and at 6 months cardiac changes were assessed using the Langendorff perfused rat heart preparation, immunohistochemistry and real-time PCR. Ws/Ws and +/+ rat hearts did not differ in baseline mRNA. In contrast, +/+ rats hearts exhibited up-regulation of ET1 after irradiation, whereas Ws/Ws rats hearts did not, suggesting the possibility of interactions between mast cells and the cardiac ET system. Bosentan induced reductions in left ventricular systolic pressure, developed pressure and +dP/dtmax but did not affect fibrosis. Because of the known opposing effects of ETA and ETB, studies with selective antagonists may clarify the role of each receptor.

Influence of Mast Cells on Structural and Functional Manifestations of Radiation-Induced Heart Disease

Radiation-induced heart disease (RIHD), characterized by accelerated atherosclerosis and adverse tissue remodeling, is a serious sequelae after radiotherapy of thoracic and chest wall tumors. Adverse cardiac remodeling in RIHD and other cardiac disorders is frequently accompanied by mast cell hyperplasia, suggesting that mast cells may affect the development of cardiac fibrosis. This study used a mast cell-deficient rat model to define the role of mast cells in RIHD. Mast cell-deficient rats (Ws/Ws) and mast cell-competent littermate controls (+/+) were exposed to 18 Gy localized single-dose irradiation of the heart. Six months after irradiation, cardiac function was examined by echocardiography and Langendorff-perfused isolated heart preparation, whereas structural changes were assessed using quantitative histology and immunohistochemical analysis. Mast cell-deficient rats exhibited more severe postradiation changes than mast cell-competent littermates. Hence, mast cell-deficient rats exhibited a greater upward/leftward shift in the left ventricular (LV) diastolic pressure-volume relationship (P = 0.001), a greater reduction in in vivo LV diastolic area (from 0.50 +/- 0.024 cm in age-matched controls to 0.24 +/- 0.032 cm after irradiation; P = 0.006), and a greater increase in LV posterior wall thickness (from 0.13 +/- 0.003 cm in age-matched controls to 0.15 +/- 0.003 cm after irradiation; P = 0.04). Structural analysis revealed more pronounced postradiation accumulation of interstitial collagen III but less myocardial degeneration in hearts from mast cell-deficient rats. These data show that the absence of mast cells accelerates the development of functional changes in the irradiated heart, particularly diastolic dysfunction, and suggest that, in contrast to what has been the prevailing assumption, the role of mast cells in RIHD is predominantly protective.

Radiation-induced cardiomyopathy as a function of radiation beam gating to the cardiac cycle

Portions of the heart are often unavoidably included in the primary treatment volume during thoracic radiotherapy, and radiation-induced heart disease has been observed as a treatment-related complication. Such complications have been observed in humans following radiation therapy for Hodgkin's disease and treatment of the left breast for carcinoma. Recent attempts have been made to prevent re-stenosis following angioplasty procedures using external beam irradiation. These attempts were not successful, however, due to the large volume of heart included in the treatment field and subsequent cardiac morbidity. We suggest a mechanism for sparing the heart from radiation damage by synchronizing the radiation beam with the cardiac cycle and delivering radiation only when the heart is in a relatively hypoxic state. We present data from a rat model testing this hypothesis and show that radiation damage to the heart can be altered by synchronizing the radiation beam with the cardiac cycle. This technique may be useful in reducing radiation damage to the heart secondary to treatment for diseases such as Hodgkin's disease and breast cancer.

Structural changes in the auricles of the rat heart after local ionizing irradiation

BACKGROUND AND PURPOSE

Irradiation of the heart may lead to late cardiovascular complications and depending on the dose to cardiac-related death. There is increasing evidence that left atrial appendages play an important role in left ventricular filling especially in cardiac disease. The aim of the present study was to investigate the radiation response of the atria of the rat heart (auricles in particular) at morphological, histological and transcriptional level.

MATERIAL AND METHODS

Sprague-Dawley rats were irradiated with a single dose locally on the heart (0-22.5 Gy). End-diastolic diameters of left auricles were measured during evaluation of cardiac function. Histopathological evaluations were performed at various time points up to 16 months post irradiation. Changes in mRNA expression of procollagen types I and III and pro-fibrogenic cytokines (TGF-beta1 and IL-1beta) were investigated using competitive PCR.

RESULTS

Irradiation leads to a dose-dependent decrease in end-diastolic diameter of the left auricles. This decrease was observed at 4 months post-irradiation, where no gross damage of the ventricle has been reported. Histologically, epicardial fibrosis was found already 1 month post irradiation, and the frequency/severity of the structural changes appeared to be dose-dependent and progressive with time post irradiation. At 9 months, fibrosis was observed in all three layers (epicardium, myocardium and endocardium) of both auricles. On the level of gene expression, increases in procollagen types I and III were observed at 12 and 3 months post irradiation, respectively. Increases in IL-1beta and TGF-beta1, cytokines known to influence collagen deposition at different levels, preceded the upregulation of procollagen mRNA.

CONCLUSIONS

Auricles of the rat heart show a marked pathological response to ionizing radiation, characterized by generalized accumulation of collagen (fibrosis) and a reduction of end-diastolic diameter. The reduction of auricular volume and loss of elasticity will negatively contribute to the pump function of the irradiated ventricle.

Comparison of in vivo cardiac function with ex vivo cardiac performance of the rat heart after thoracic irradiation

The aim of the study was to compare in vivo cardiac function with ex vivo cardiac performance after local heart irradiation in the same rat. Left ventricular ejection fraction (LVEF) was measured in vivo by radionuclide ventriculography in Sprague-Dawley rats up to 16 months after a single dose of 20 Gy. Four days after in vivo measurements, cardiac performance was determined ex vivo, using the isolated working rat heart preparation. After irradiation, cardiac performance measured ex vivo deteriorated more rapidly than the in vivo measured LVEF. Within 4 months post-treatment, ex vivo cardiac output and stroke volume started to decrease and declined continuously throughout the observation period of 16 months. The reduction in stroke volume was already significant (p < 0.04) at 4 months post-treatment, whereas the decline in cardiac output was significant (p < 0.05) at 12 months post-treatment. In vivo, no change in LVEF was observed during the first 12 months post-treatment. Thereafter, LVEF decreased rapidly from 65 +/- 2% to 46 +/- 8% (p < 0.01), at 16 months post-treatment. Up to 12 months post-irradiation, LVEF was not correlated to ex vivo cardiac output. At 16 months post-treatment, when clinical symptoms of heart failure become evident, a positive relation between both parameters was found. The lack of correlation between the in vivo and ex vivo measurements of cardiac function during the first 12 months post-treatment might be explained by the involvement of compensatory mechanisms being operative in vivo to maintain sufficient cardiac output.

Reirradiation tolerance of the rat heart

PURPOSE

To investigate the influence of reirradiation on the tolerance of the heart after a previous irradiation treatment.

METHODS AND MATERIALS

Female Wistar rats were locally irradiated to the thorax. Development of cardiac function loss was studied with the ex vivo working rat heart preparation (20). To compare the retreatment experiments, initial, and reirradiation doses were expressed as the percentage of the extrapolated tolerance dose (ETD) (1).

RESULTS

Local heart irradiation with a single dose led to a dose-dependent and progressive decrease in cardiac function. The progressive nature of irradiation-induced heart disease is shown to affect the outcome of the retreatment, depending on both the time interval between subsequent doses and the size of the initial dose. The present data demonstrate that hearts are capable of repairing a large part of the initial dose of 10 Gy within the first 24 h. However, once biological damage as a result of the first treatment is fixed, the heart does not show any long-term recovery. At intervals up to 6 months between an initial treatment with 10 Gy and subsequent reirradiation, the reirradiation tolerance dose slightly decreased from 74% of the ETDref (at 24-h interval) to 68% of the ETDref (at 6-month interval). Between 6 and 9 months, reirradiation tolerance dose dropped more even to 43% of the ETDref. Treatment of the heart with an initial dose of 17.5 Gy, instead of 10 Gy, 6 months prior to reirradiation, also led to a further decrease of the reirradiation tolerance dose (< 38 vs. 68% of the ETDref).

CONCLUSIONS

The outcome of the present study shows a decreased tolerance of the heart to reirradiation at long time intervals (interval > 6 months). This has clinical implications for the estimation of reirradiation tolerance in patients whose mediastinum has to be reirradiated a long time after a first irradiation course.

Changes in cardiac performance and sympathetic stimulation during and after fractionated radiotherapy in a rat model

The consequences of fractionated irradiation on the number of cardiac alpha- and beta-adrenergic receptors, myocardial norepinephrine concentration and in vitro assessed heart function were studied in Sprague-Dawley rats. Animals were locally irradiated on the thorax with a total dose of 50 Gy, in 5 weeks, using two different fractionation schemes (5 x 2.0 Gy/week and 3 x 3.3 Gy/week). Functional and biochemical assays were performed during treatment and at 6 months after initiation of treatment. During fractionated irradiation, the numbers of alpha- and beta-adrenergic receptors tended to rise. During this period, myocardial norepinephrine concentration remained fairly constant and no decrease in cardiac output was observed. At 6 months, a significant increase of the numbers of alpha- and beta-adrenergic receptors was observed in the 3.3 Gy/fraction group compared to age-matched controls, p = 0.012 and p = 0.02, respectively. At this time point, the myocardial norepinephrine concentration had decreased below control levels (p = 0.008 for the 3.3. Gy/fraction schedule, and p = 0.03 for the 2.0 Gy/fraction schedule). At 6 months, the cardiac output declined to 61% (p = 0.009) and 69% (p = 0.04) of control values for the 3.3 and 2.0 Gy/fraction schedules, respectively. The present data clearly show development of late cardiac sequelae caused by fractionated thorax irradiation with a total dose of 50 Gy. Moreover, this study lends support to the importance of fraction size with regard to the severity of the radiation-induced cardiac damage.

Radiation-induced damage of the Wistar rat heart: biochemistry and function

A time sequence study was performed on Wistar rats to investigate the early effects of radiation on the mechanical function and energy metabolism of the heart. Two series of rats were exposed to 20 Gy electron irradiation to a field including the heart and approximately a third of the lungs. The hearts were excised at varying time intervals (8-180 days) post irradiation. In one series of hearts the mechanical function was measured using the isolated perfused working rat heart model. At the end of the perfusion the hearts were freeze-clamped for analysis of the high energy phosphate contents (ATP, ADP, AMP and creatine phosphate). In the second series, mitochondria were isolated and the oxidative phosphorylation function measured polarographically (substrate: glutamate). Maximal depression of mechanical function was observed at 60 days post irradiation. Thereafter the work performance of these hearts improved significantly, almost reaching control levels after 180 days. The mitochondrial oxidative phosphorylation function (as measured on the total mitochondrial population) was significantly depressed 30-120 days post irradiation. As in the case of the mechanical changes, the depression was transient and after 180 days post irradiation, values similar to those of controls were obtained. Myocardial high energy phosphates remained unaltered throughout the experiment.

Radiation-induced heart disease: review of experimental data on dose response and pathogenesis

Clinical and experimental heart irradiation can cause a variety of sequelae. A single dose of greater than or equal to 15 Gy leads to a reversible exudative pericarditis, occurring in dogs, rabbits or rats at around 100 days. Its time-course is very similar in all species investigated, but there are considerable species and strain differences in severity and incidence. After longer, dose-dependent latency times chronic congestive myocardial failure develops. At histological examination myocardial degeneration and necrosis is observed, which in some species is accompanied by a variable degree of interstitial fibrosis. In rabbits and rats, myocardial degeneration becomes apparent at around 70 days after 20 Gy and is preceded by a marked reduction in capillary density as well as ultrastructural endothelial cell degeneration. Simultaneously to structural capillary damage, a focal loss of the endothelial marker enzyme alkaline phosphatase was observed in rats in areas with subsequent myocardial degeneration. Cell kinetic studies in rabbits and rats revealed a radiation-induced wave of increased endothelial cell proliferation at 30-100 days postirradiation. In the rat it is exclusively seen in conjunction with alteration of endothelial cell marker enzymes. The temporal and spatial pattern of proliferative response exludes endothelial cell death in mitosis as the sole pathogenetic mechanism causing capillary loss and myocardial degeneration. Parallel to development of morphological damage, haemodynamic studies in various rats strains revealed a drop in cardiac output and left ventricular ejection fraction to about 64% of normal values after 20 Gy. In vivo, this slightly reduced cardiac function was then maintained in a steady state for many weeks, probably due to a compensatory up-regulation of cardiac beta-adrenergic receptors. In denervated working heart preparations in vitro, however, these compensatory mechanisms are not effective and stroke volume as well as cardiac contractility show a rapid and steady deterioration. In many respects radiation-induced heart disease conforms to radiobiological concepts of late-responding tissues, showing a chronic progressive time-course and a very pronounced fractionation effect. However, pathogenesis cannot be understood in terms of target cell depletion alone, and experimental evidence indicates the importance of alterations of regulatory mechanisms.

Time dependent changes in myocardial norepinephrine concentration and adrenergic receptor density following X-irradiation of the rat heart

The hearts of 9 to 12-weeks-old Sprague-Dawley rats were locally irradiated with a single dose of 20 Gy. The effects on myocardial norepinephrine concentrations and on alpha-adrenergic and beta-adrenergic receptor densities was examined up to 16 months post-treatment. Myocardial norepinephrine concentrations were reduced (to 50% of control values between 8 and 16 months) after irradiation. Receptor binding studies using radioactive ligands demonstrated that alpha-adrenergic receptor density was increased to maximally 210% of control values and that beta-adrenergic receptor density was increased to maximally 150% of control values, both measured at 8 months posttreatment. The affinities of both receptor types were not changed after irradiation. An inverse correlation was found between the myocardial norepinephrine concentration and the alpha-adrenergic receptor density. Myocardial norepinephrine concentration was not correlated to the beta-adrenergic receptor density. The changes in myocardial norepinephrine concentration and receptor density observed after irradiation suggest that even 16 months after irradiation overt cardiac failure was not occurring as the radiation-induced alterations differ considerably from those reported for failing hearts.