| Abstract
| - 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 = ll) ν 13.6(0.5)%(n = 14), p<0.02; or with isoprenaline, at 11.6(0.6)% shortening ν 13.1(0.4)%, p<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 106 cells was lower in preparations from treated animals (p<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<0.00l). 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 β adrenoceptor sensitivity following adriamycin treatment. To confirm that it was possible to desensitise rabbit cardiac β 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<0.05), and increased the EC50 from 7.9 to 224 nM (p<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 p adrenoceptor desensitisation.
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