Isolated mouse atrium as a model to study anthracycline cardiotoxicity: The role of the β-adrenoceptor system and reactive oxygen species

Cancer chemotherapy with anthracyclines, of which doxorubicin (DX²) is the main representative, is limited by cardiomyopathy developing in animals and patients after cumulative dosing. The toxicity is probably related to free radical formation by the anthracycline as well as its metabolites with concomitant O₂·^- and ·OH generation resulting in lipid peroxidation and subsequent membrane damage. An in vitro model is required to investigate the individual contribution of each metabolic to cardiotoxicity. For in vivo studies, the species of choice is the mouse because it lacks the DX-induced nephrotic syndrome seen for instance in rats and rabbits. Thus, isolated mouse heart muscle was chosen as an in vitro model. To characterize the model, we used l-isoprenaline/dl-propranolol and metacholine/atropine to measure the β-adrenergic and the muscarinic responses of (spontaneously beating) right and (paced) left atrium. Dose response curves (n≥4) were highly reproducible: pD(2,iso) = 8.0 ± 0.3 (left) and 8.5 ± 0.4 (right); pD(2,met) = 6.7 ± 0.1 (left) and 6.2 ± 0.3 (right). Propranolol as well as atropine behaved as competitive antagonists, with pA₂-values of 8.4 ± 0.2/8.5 ± 0.2 (l/r) and 9.1 ± 0.1/9.1 ± 0.2 (l/r), respectively. These values corresponded to those obtained with other organ preparations. We tested the effect of DX in two ways: a) by measuring the direct inotropic and chronotropic effect during 60 minutes of incubation with 10-100 μM DX in the organ bath, and b) by determining the remaining β-adrenergic response to l-isoprenaline after the incubation period. Both variables turned out to be equally affected. For paced left atria an IC₅₀ (causing 50% depression of contractile force) of 35 μM was determined. Right atria stopped beating at concentrations above 50 μM, thus hampering IC₅₀ determination. The results indicate that anthracyclines exert an effect nor related to receptor integrity, but directly to the functionality of heart muscle. To check whether radical stress can be involved in the observed negative inotropic effect, incubations with xanthine/xanthine oxidase (to produce reactive oxygen species) were performed. A pronounced negative effect on mouse atrial contraction was indeed observed. However, initially a positive inotropic effect accompanied by an increased resting tension were seen. It can be concluded that mouse atrium can be used as a model to compare anthracyclines and their metabolites with regard to their acute cardiotoxic effects.