Doxorubicin (adriamycin), is one of the anthracycline group of drugs, with a potent anticancer activity against a wide range of tumors (Blum and Carter, 1974; Rosenberg et al., 1983; Torti et al., 1983; Ogawa, 1999). Unfortunately the clinical use of this drug is limited due to its cardiotoxicity (Olson and Mushlin, 1990; Paglia and Radliffe, 2000). Two approaches look to be logical in facing this problem.
(i) Search of the new analogues with low or without cardiotoxic tendency (Shenkenberg and Von Hoff, 1986; Shenkenberg et al., 2000).
(ii) Search of the adjuvant compounds, or drugs, which can be combined with adriamycin to reduce its cardiotoxicity (AI-Nasser, 1998; Kunisada et al., 2000).
With the view of second approach, numerous models have been developed for the early and rapid evaluation of cardiotoxicity using myocardial cells (Jiang et al., 1994), papillary muscle (Lee et al., 1991) mitochondria (Solem and Wallace, 1993) or isolated heart perfused with anthracycline (Del Tecca et al., 1987; Platel et al., 1999). A number of theories have been presented in order to explain the mechanism of cardiotoxicity by doxorubicin including effects on nucleic acid and on protein synthesis (Buja et al., 1973), damage to mitochondria due to excess calcium influx (Oslen et al., 1974), free radical generation and subsequent lipid peroxidation (Bachur, 1975; Myers et al., 1977), release of histamine and catecholamines with resultant myocardial damage (Bristow et al., 1981), effect of adriamycin on various membrane systems including Na+- Ca++ exchange (Caroni et al., 1981), role of electrolytes like Ca++, K+ and Mg++ (Akimoto et al., 1993). But the exact mechanism is still unknown. This study was planned to look the involvement of different electrolytes in cardiotoxicity of doxorubicin. Isolated Rabbit heart is used for experimentation. Since the inotropic and chronotropic activities of heart are the major indicators of proper cardiac functioning, therefore we selected these parameters in our experiment. The whole study contained three areas.
i) Interactions of doxorubicin, with Ca++ and K+ with or without verapamil were noted in acute form, we found following by using isolated heart technique.
An irregular positive inotropism and negative chronotropism were noted at different doses (p<0.0l at 0.75, 1.25, 2.50 and 2.75mg of doxorubicin). The maximum changes in the two effects were not observed simultaneously (inotropic effect is maximum at 1.25 while chronotropic effect is highest at 2.50mg doxorubicin) showing the presence of two different mechanism for both the changes and sensitively of both the mechanism at different levels. The changes in inotropic effects were more pronounced than in chronotropic effects. Both the changes looked to reflect arrhythmias and cardiomyopathy observed during clinical use. However the heart seems to be resistant in case of inotropic profile rather than chronotropic profile for cardiotoxicity.
Positive inotropic and chronotropic effects were observed by Ca++ and K+. However highly significant negative effects (p<0.0l at the dose of 0.010, 0.025, 0.050, 0.075 and 0.100mg of doxorubicin) were observed in case of Ca++ or K+ with doxorubicin.
Negative inotropic and chronotropic effects were observed by verapamil. These effects were potentiated with doxorubicin (p
Ca++ could not produced the same degree of effects after verapamil as it produced alone, showing that verapamil decreased the rate of recovery of Ca++ channel.
(ii) Interactions of doxorubicin with Ca++ and K+ are noted in chronic form. We found,
No change in the chronotropic or inotropic effects of Ca++ produced if the heart of the animals were used which received a certain dose of doxorubicin for a certain period. This shows transient nature of toxicity induced by doxorubicin, if the drug is used for short period.
Significantly decreased (p<0.05 for 0.0l0mg while p<0.0l for 0.025, 0.050, 0.075 and 0.l00mg of Ca++ if heart has been treated with 4.54mg of doxorubicin for 24hrs before experiment) chronotropic or inotropic effects of Ca++ were observed, if the heart of the animals were used which received a fixed dose of doxorubicin for relatively longer period. This shows permanent nature of toxicity of doxorubicin, if it is used for longer period.
Animals received different doses of K+ on isolated heart of animal, which received a fixed dose of doxorubicin in planned fractions for longer period, before experiment. Unipolar, regular negative highly significant inotropic and chronotropic effects (p<0.0l for lxl0-3 and 2xl0-3 mg of K+ in case of inotropic effects while p<0.0l for lxl0-3, 2xl0-3, 3xl0-3 and 4xl0-3 mg of K+ in case of chronotropic effects) were observed showing marked inhibition of cardiac activity.
(iii) Evaluation of interactions of doxorubicin and Ca++ were carried out in another group, in which isolated heart of animal was used which received Ca++ for longer period before experiment. We observed highly significant regular, unipolar, negative inotropic (p<0.01 for 0.75, 1.25, 2.50 and 3.75mg of doxorubicin) and chronotropic (p<0.01 for 1.25 and 3.75mg of doxorubicin) effects. We noted that as the dose of doxorubicin was increased the inotropic and chronotropic effects were decreased regularly accordingly.
From all these results it is concluded that;
1. Effect of doxorubicin dominates on Ca++, if given concomitantly and acutely.
2. Doxorubicin has receptor-mediated activity. The phenomena can be supported by indirect evidences.
3. Doxorubicin alters the myocardium, if given for prolonged period, so the actual inotropic effects of Ca++ are not achieved.
4. Doxorubicin changes the permeability of K+ on myocardium, so it means the effect of K+ dominates in the presence of doxorubicin.
5. Verapamil potentiates the cardiotoxicity induced by doxorubicin.