Adriamycin cardiomyopathy in the rabbit: alterations in contractile proteins and myocyte function.
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.


METHODS
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.


METHODS
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.


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.
Jones
S M
SM
Department of Cardiac Medicine, National Heart and Lung Institute, London.
Kirby
M S
MS
Harding
S E
SE
Vescova
G
G
Wanless
R B
RB
Dalla Libera
L D
LD
Poole-Wilson
P A
PA
D000818:Animals; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D009202:Cardiomyopathies / Q000139:chemically induced* / Q000378:metabolism / Q000503:physiopathology; D003094:Collagen / Q000378:metabolism; D003402:Creatine Kinase / Q000378:metabolism; D004317:Doxorubicin / Q000633:toxicity*; D004591:Electrophoresis, Polyacrylamide Gel; D006321:Heart / Q000187:drug effects; D066298:In Vitro Techniques; D007527:Isoenzymes; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism; D010101:Oxygen Consumption; D011817:Rabbits
D007527:Isoenzymes; D004317:Doxorubicin; D003094:Collagen; D003402:Creatine Kinase; D009218:Myosins; D000252:Calcium-Transporting ATPases; D007545:Isoproterenol