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Adriamycin cardiomyopathy in the rabbit: alterations in contractile proteins and myocyte function
S M Jones1, M S Kirby, S E Harding
1Department of Cardiac Medicine, National Heart and Lung Institute, London.
Study Objective:
The aim was to determine the mechanism of the cardiotoxic effect of adriamycin, particularly at the level of the function of the cardiac myocyte.
Design:
After chronic exposure to adriamycin, the contractile responses of single isolated cardiac myocytes to increasing calcium and isoprenaline were measured, as well as oxygen consumption of myocyte suspensions. Creatine kinase and myosin isoforms were investigated in whole ventricle. The degree of fibrosis of the ventricle was quantified using histological methods.
Subjects:
24 white male New Zealand rabbits were treated with adriamycin (1 mg.kg-1) twice a week for eight weeks, and allowed to recover for two weeks. There were 29 untreated controls. Six further rabbits were implanted with mini osmotic pumps delivering a constant infusion of isoprenaline for one week; five controls had pumps containing saline.
Measurements And Main Results:
Cardiac myocytes were enzymatically isolated, and their contraction amplitude and velocity monitored. Cells isolated from adriamycin treated rabbits had a lower contraction amplitude than those from controls when maximally activated with calcium, at 11.1(0.9)% shortening (n = 11) v 13.6(0.5)% (n = 14), p less than 0.02; or with isoprenaline, at 11.6(0.6)% shortening v 13.1(0.4)%, p less than 0.05. Contraction, but not relaxation, velocity in maximum calcium or isoprenaline was also significantly lower in cells from adriamycin treated animals. Oxygen consumption per 10(6) cells was lower in preparations from treated animals (p less than 0.05), but the relative effects of glucose, acetate, 2,4-DNP, and cyanide were unaffected. There was no significant change in creatine kinase or myosin isozyme composition or in amounts of fibrosis following adriamycin treatment. However, the quantity of myosin per g wet weight of tissue was significantly reduced from 8.04(0.45) mg.g-1 wet tissue, n = 4, in controls to 5.76(1.55) mg.g-1, n = 6, in adriamycin treated animals (p less than 0.001). The EC50 for isoprenaline was unchanged in cells from treated animals. Together with the unaltered maximum isoprenaline/calcium ratio, this implies that there is no change in beta adrenoceptor sensitivity following adriamycin treatment. To confirm that it was possible to desensitise rabbit cardiac beta adrenoceptors, and to detect changes in sensitivity on single cells, rabbits were treated with isoprenaline for one week. Such treatment decreased the maximum isoprenaline/calcium contraction amplitude ratio from 0.97(0.15), n = 5, to 0.47(0.12), n = 6 (p less than 0.05), and increased the EC50 from 7.9 to 224 nM (p less than 0.05).
Conclusions:
Single cardiac myocytes isolated from the hearts of adriamycin treated rabbits show a decrease in contraction amplitude, velocity, and oxygen consumption compared to controls. The decreased contractility of individual myocytes may relate to their low myosin content, and could contribute to the reduced cardiac output produced by adriamycin treatment. Heart failure induced by adriamycin in the rabbit is not accompanied by beta adrenoceptor desensitisation.
Insights
Adriamycin treatment reduces cardiac myocyte contractility and oxygen consumption in rabbits. This decreased function may be linked to lower myosin content, contributing to adriamycin-induced heart failure without beta-adrenoceptor desensitization.
Area of Science:
- Cardiology
- Pharmacology
- Cell Biology
Background:
- Adriamycin (doxorubicin) is a widely used chemotherapy agent with known cardiotoxic effects.
- The precise mechanisms underlying adriamycin-induced cardiotoxicity at the cellular level require further elucidation.
- Understanding these mechanisms is crucial for developing strategies to mitigate cardiac damage during chemotherapy.
Purpose of the Study:
- To investigate the functional consequences of chronic adriamycin exposure on individual cardiac myocytes.
- To determine the impact of adriamycin on myocyte contractility, oxygen consumption, and related molecular components.
- To assess whether adriamycin-induced cardiotoxicity involves alterations in beta-adrenoceptor sensitivity.
Main Methods:
- Chronic adriamycin administration to New Zealand rabbits, followed by myocyte isolation and functional assessment.
- Measurement of contractile responses to calcium and isoprenaline in isolated cardiac myocytes.
- Quantification of myocyte oxygen consumption, and analysis of cardiac myosin content and fibrosis.
- Assessment of beta-adrenoceptor sensitivity using isoprenaline challenge in separate rabbit groups.
Main Results:
- Adriamycin-treated rabbit cardiac myocytes exhibited significantly reduced contraction amplitude and velocity compared to controls.
- Myocyte oxygen consumption was also decreased in adriamycin-exposed animals.
- A significant reduction in cardiac myosin content per gram of ventricular tissue was observed, but fibrosis and creatine kinase levels remained unchanged.
- Beta-adrenoceptor sensitivity, assessed by isoprenaline response, was not altered by adriamycin treatment.
Conclusions:
- Adriamycin treatment impairs cardiac myocyte function, characterized by decreased contractility and oxygen consumption.
- The reduction in myosin content is a potential contributor to the observed myocyte dysfunction and adriamycin-induced cardiotoxicity.
- The findings suggest that adriamycin-induced heart failure in rabbits does not involve beta-adrenoceptor desensitization.