Time-related increases in cardiac concentrations of doxorubicinol could interact with doxorubicin to depress myocardial contractile function.

1. The present study evaluated the time-dependency of acute anthracycline cardiotoxicity by varying the duration of exposure of rabbit isolated atria to doxorubicin and determining changes (1) in contraction and relaxation and (2) in atrial concentrations of doxorubicin and its C-13 hydroxy metabolite, doxorubicinol. 2. Following addition of doxorubicin (175 microM) to atria, contractility (dF/dt), muscle stiffness (resting force, RF) and relaxation (90% relaxation time, 90% RT) were monitored for a 3.5 h period. 3. Doxorubicin (175 microM) progressively diminished mechanical function (decreased dF/dt, increased RF and prolonged 90% RT) over 3 h. Doxorubicinol (1.8 microM), however, failed to produce time-related cardiac dysfunction; it depressed contractile function and increased muscle stiffness during the first 30 min without causing additional cardiac dysfunction during the remaining 3 h of observation. Doxorubicinol had no effect on 90% RT. 4. During treatment with doxorubicin, atria contained considerably more doxorubicin than doxorubicinol (ratio of doxorubicin to doxorubicinol ranged from 778 to 74 at 0.5 and 3 h, respectively). Elevations of doxorubicin and doxorubicinol in atria paralleled the degree of dysfunction of both contraction and relaxation; increases in muscle stiffness, however, were more closely associated with increases of doxorubicinol than doxorubicin. 5. To probe the relation between cardiac doxorubicinol and myocardial dysfunction further, without confounding effects of cardiac doxorubicin, concentration-response experiments with doxorubicinol (0.9-7.2 microM) were conducted. 6. Plots of doxorubicinol concentrations in atria vs contractility indicated that the cardiac concentration of doxorubicinol, at which contractility is reduced by 50%, is five fold lower in doxorubicin-treated than in doxorubicinol-treated preparations. Thus, doxorubicin and doxorubicinol appear to interact to depress contractile function.7. Cardiac concentrations of both doxorubicin and doxorubicinol, as observed in these studies, were found to stimulate markedly Ca2+ release from isolated SR vesicles, but 3 microM doxorubicinol promoted a 15 fold greater release rate than 3 microM doxorubicin.8. Our observations coupled with the previously reported finding that doxorubicinol inhibits Ca2+loading of SR, suggests that doxorubicinol accumulation in heart contributes to the time-dependent component of doxorubicin cardiotoxicity, through a mechanism that could involve perturbations of Ca2+ homeostasis.

Aging alters the force-frequency relationship and toxicity of oxidative stress in rabbit heart.

Adult (6 months) and senescent (greater than 5 years) rabbit atria were studied under conditions known to increase cytoplasmic calcium (increased frequency of contraction and oxidative stress). At a contraction frequency of 1/sec, cardiac relaxation (90% relaxation time) was similar in senescent and adult atria but at a frequency of 2 or 3/sec, relaxation was significantly slower in senescent preparations (P less than 0.05). Additional experiments indicated that H2O2 (500 microM), a powerful oxidant, increased resting force and decreased developed force (DF) much more rapidly in senescent than adult atria; the maximum decrease in DF, however, was less in senescent preparations (adult = 81 +/- 6% and senescent = 42 +/- 27% of pre-H2O2 values; P less than 0.05). Age-related differences in effects of H2O2 did not result simply from a decreased ability of senescent hearts to detoxify an oxidative stress by the glutathione pathway. Both basal glutathione (GSH) concentrations and the H2O2-mediated decreases in GSH were similar in adult and senescent ventricular preparations, as were activities of glutathione peroxidase and glutathione reductase. These observations suggest that interventions known to increase cytoplasmic calcium can amplify age-related impairments of cardiac relaxation through mechanisms that may be independent of the glutathione pathway.

End-systolic elastance as an evaluation of myocardial function in shock.

The response of the heart during sepsis has been studied in human and animal models with disparate results. Because sepsis induces marked peripheral vascular changes, to accurately determine the cardiac response, one must use indices of cardiac performance that are independent of loading conditions and heart rate. The slope of the end-systolic pressure-diameter relationship (ESPDR) has been proposed to have these properties. Pigs were equipped with transducers to measure left ventricular pressure, internal short axis diameter (D), and pulmonary and coronary artery blood flows. After 7-10 days of basal observations, an endotoxin-loaded osmotic pump delivering endotoxin at 10 micrograms/kg/hr was implanted into each pig. Fourteen pigs were so treated, and 4 expired before 24 hr of endotoxin challenge. In the surviving pigs, cardiac output, heart rate, dP/dtmax, and peak systolic pressures were elevated. However, both ESPDR and % D shortening were both significantly depressed. These data suggest that the cardiac response to chronic endotoxin challenge includes a loss of inotropic state as indicated by the load-insensitive indicator, ESPDR, and confirmed by depressed % D shortening. One possible mechanism for reduced inotropic state during endotoxin challenge could be the loss of coronary perfusion. The surviving endotoxin-challenged pigs demonstrated a significant increase in coronary perfusion while stroke work remained unchanged, suggesting that depressed cardiac inotropic state during endotoxin challenge was not caused by reduced coronary blood flow. Rather, the myocardium was relatively overperfused. Another possible mechanism for the loss of cardiac inotropism during endotoxin challenge may be endotoxin-induced generation of reactive oxygen free radicals. This possibility was tested by measuring total cardiac gluthathione, a cellular component depleted by oxidant stress. Endotoxemia reduced these levels 50%. These results suggest that cardiac injury may be mediated by the generation of reactive oxygen free radicals. Further study will determine if this mechanism participates in the loss of cardiac inotropism during endotoxin challenge.

Deleterious effects of buthionine sulfoximine on cardiac function during continuous endotoxemia.

Sepsis has been associated with reversible cardiac injury. To determine whether this injury is mediated by generation of reactive oxidants, tissue glutathione (GSH)--the major intracellular antioxidant--was depleted before endotoxemia. Basal values of cardiac contractile function, perfusion, and cardiac output were measured 5-7 days postsurgery. Salmonella enteritidis endotoxin was continuously infused at 3 micrograms/kg/hr iv via an osmotic pump (Alzet Corp). Endotoxemia significantly reduced myocardial glutathione content (394 +/- 46) to 206 +/- 9 micrograms/g), indicating oxidant stress during endotoxemia. Buthionine sulfoximine (BSO) pretreatment significantly reduced cardiac glutathione in sham pigs from 394 +/- 46 to 199 +/- 26 micrograms/g; and in endotoxemic pigs, BSO pretreatment significantly reduced cardiac glutathione to 106 +/- 18 micrograms/g. Vehicle- and BSO-treated endotoxemic groups demonstrated similar cardiovascular responses to endotoxin challenge. Heart rate increases (122 +/- 15 to 140 +/- 17 bpm) and cardiac outputs decreases (1.50 +/- 0.24 to 1.11 +/- 0.35 l/min) were similar, indicating similar cardiovascular insults induced by endotoxemia. Percent short axis shortening and end-systolic pressure-diameter relation (ESPDR) were significantly reduced in BSO pretreated compared with vehicle-treated endotoxemic pigs. Results support a conclusion that endotoxemia-induced cardiac injury is mediated, in part, by free radical injury. This conclusion is based upon the finding that endogenous myocardial glutathione was depleted by continuous endotoxin infusion and that prior depletion of myocardial glutathione by buthionine sulfoximine exacerbated cardiac injury.