To study further the pathophysiology of radiation-induced cardiomyopathy, we investigated resting hemodynamics, myocardial catecholamine synthesis and storage, and β-adrenoceptor density after local heart irradiation. In Wistar rats, a radiation dose of 20 Gy eventually leads to compromised myocardial function which is characterized by a reduction in cardiac output to 43 ± 11% and in the left ventricular ejection fraction to 66 ± 7.5%, and an increase in the left ventricular end-diastolic volume to 187 ± 17% of control values. This reduction in function is correlated with focal degeneration of 23 ± 4% of the myocardium. Measurement of tyrosine hydroxylase activity and catecholamine content revealed that catecholamine biosynthesis is unchanged in the adrenals but is significantly reduced in the hearts of irradiated animals, while cardiac β-adrenoceptor density is increased to about 140% of that in age-matched controls. This is in contrast to findings in dilated or ischemic cardiomyopathy. Time-course studies showed that the development of myocardial degeneration starts simultaneously with the decrease in cardiac output and ejection fraction and the increase in β-adrenoceptors at 50-80 days postirradiation. Myocardial degeneration is maximal in extent and severity at 100 days and does not progress thereafter. Cardiac output decreases at 80-100 days postirradiation to 60 ± 7% of control values. A significant further decrease is seen only when congestive heart failure becomes manifest at 249 ± 21 days after 20 Gy. Thus there is a delay between structural myocardial injury and hemodynamic deterioration which could be due to a compensatory increase in β-adrenoceptor density during the initial stages of the cardiomyopathy. A comparison of two strains of rat shows that survival time correlates with the time course as well as the severity of hemodynamic and morphological changes.

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