Akihiko Sawamura

Adriamycin cardiotoxicity: Possible pathogenic mechanisms

Abstract The antitumor agent, adriamycin, causes in humans a cardiomyopathy associated with elevated tissue Ca 2+ . Hence, adriamycin was tested for an ability to affect the Ca 2+ influx mediated by the slow channels in ventricular cells of isolated perfused chick hearts. The fast Na + channels were blocked by tetrodotoxin or voltage inactivated by elevated (25 m m ) K + , thus rendering the hearts inexcitable. Low concentrations of adriamycin (0.01 to 0.05 mg/ml) restored excitability in the form of slow action potentials (APs), and enhanced the maximum upstroke velocity ( + V max ) of slow APs induced by isoproterenol (10 −6 m ). Higher concentrations (0.1 to 0.5 mg/ml) did not induce slow APs, and actually depressed or blocked the isoproterenol-induced slow APs. On the contractions recorded from hearts perfused with normal Tyrode solution, adriamycin also had a dual effect: at low concentrations, it had a positive inotropic action, whereas at higher concentrations, it had a negative inotropic action. Adriamycin (0.05 mg/ml) caused the cyclic AMP level to increase by about 50% over the control within 15 min, thus suggesting that this might be responsible for its positive inotropism. Higher concentrations (0.3 mg/ml) also raised the cyclic AMP, but the ATP level was depressed. In isolated mitochondria, adriamycin (0.5 mg/ml) depressed ADP-stimulated respiration, suggesting that impaired mitochondrial function could cause the depressed ATP levels. The results indicate that low concentrations of adriamycin augment the slow current, possibly by an increase in cyclic AMP level, whereas high concentration (0.5 mg/ml) depresses the slow current, perhaps due to lowered ATP levels. The enhanced Ca 2+ influx (via stimulation of the slow channels) could be a factor in the Ca 2+ overload associated with the adriamycin-induced cardiomyopathy.

Plasma endothelin levels during myocardial ischemia and reperfusion

Endothelin, an endothelium-derived vasoconstrictive peptide, has a strong potency of coronary artery constriction. However, the role of endogeneous endothelin under pathophysiological conditions has not yet been known. In this study, we examined plasma endothelin concentration in dogs with myocardial ischemia and reperfusion. Anesthetized open-chest dogs underwent either 45 minutes occlusion of the left anterior descending coronary artery followed by 3 hours reperfusion, or 4-10 hours of continuous occlusion. Plasma concentration of endothelin from the central vein was measured by the highly sensitive enzyme-immunoassay. Plasma endothelin concentration increased 2.2-fold with the peak level at 60 minutes after release of the ligated artery, but occlusion per se caused no remarkable change. These data suggest that reperfusion of the occluded artery might be needed to increase the plasma concentration of endothelin in case of myocardial infarction.

Taurine modulates ion influx through cardiac Ca2+ channels

The effects of taurine on the inward Ca2+ current (ICa) were investigated by means of the whole-cell voltage-clamp technique in isolated single guinea pig ventricular myocytes. ICa were elicited by 200-ms test pulses from a conditioning holding potential of -45 mV to various test potentials at a rate of 0.5 Hz. Taurine (10-20 mM) had different effects on ICa, depending on the extracellular Ca2+ concentration [( Ca]o). A small stimulatory effect of taurine was found in low [Ca]o (0.8 mM), and a small inhibitory effect was found in high [Ca]o (3.6 mM). Taurine had no significant effect on ICa in normal [Ca]o (1.8 mM). Such dual effects on ICa may explain the various effects reported for taurine especially its dual inotropic actions on cardiac muscle depending upon [Ca]o. Thus, taurine acts in a manner to keep ICa relatively constant. Taurine increased the resting potential irrespective of [Ca]o, suggesting that, in addition, taurine increased K+ conductance and/or ion exchange systems such as the Na/Ca and Na/K exchange.

Calcium Overload-Induced Myocardial Damage Caused by Isoproterenol and by Adriamycin: Possible Role of Taurine in its Prevention

Calcium ion (Ca2+) is essential for excitation-contraction coupling and for maintenance of cell integrity in the myocardium. On the other hand, it is clear that cytosolic Ca2+ loading may be the first event leading to cell death in certain forms of myocytic injury, such as Ca2+ paradox and isoproterenol (ISO) toxicity. The Ca2+ overload injury is characterized by an exhaustion of tissue high-energy phosphate, massive release of enzymes and extensive ultrastructural damage, as well as excessive influx of Ca2+ into the myocardial cells.

Cyclic adenosine monophosphate modulation of contractility via slow Ca2+ channels in chick heart

Abstract The catecholamines exert a positive inotropic effect associated with elevated tissue cyclic AMP levels and possibly with increase in the number of membrane slow cationic channels available for voltage activation. In the present study, catecholamines (isoproterenol, dopamine and dobutamine) were tested for their ability to affect the maximum upstroke velocity (+ V max ) of the slow action potentials, the first derivative ( d T d t ) of developed tension accompanying the slow responses, and the tissue cyclic AMP levels in the ventricular myocardium of isolated perfused chick hearts. To study the slow channels exclusively, the fast Na + channels were voltage inactivated by elevated (25 m m ) K + . In this condition of functional removal of the fast channels, the heart could not be excited by intense electrical stimulation. It was found that these catecholamines induced slow action potentials accompanied by contractions. Elevation of the concentration of these agents produced increases in + V max , d T d t , and cyclic AMP in a dose-dependent fashion; a close correlation was obtained between the cyclic AMP level, + V max and d T d t . These results support the hypothesis that the increases in + V max of the slow action potentials and in contraction are explained by increase in the number of available slow channels mediated by intracellular cyclic AMP levels, and the resulting increase in the Ca 2+ influx.

Protective effect of taurine on the irregular beating pattern of cultured myocardial cells induced by high and low extracellular calcium ion

Abnormal beating patterns were induced in spontaneously contracting cultured embryonic mouse myocardial cells either by elevating or by lowering extracellular calcium. At low calcium (0.4 mM), the number of beating cells and beating rate decreased while the number of arrhythmic cells increased. By contrast, at high calcium (20 mM), the number of beating cells decreased while beating rate and the number of arrhythmic cells increased. Addition of taurine (20 mM) to the medium attenuated the response to varying calcium; the taurine effect appeared to be specific since neither taurine analog tested (beta-alanine nor glycine) provided much protection against these abnormalities. The protective effect of taurine also appeared to differ from that of verapamil, which was effective only in decreasing beating rate in the high calcium condition. Uptake of 14C-taurine by the cells was higher at both low and high extracellular calcium when compared to the normal calcium (2 mM) concentration. The results raise the possibility that the protective effect of taurine on beating abnormalities caused by low or high calcium is related to taurine uptake.

Mechanism of direct cardiostimulating actions of hydralazine.

The vasodilator, hydralazine, was reported to also exert a direct positive inotropic effect on the myocardium at high concentrations. In the present study we investigated the mechanism of this positive inotropic action by using the ventricular myocardium of isolated perfused chick hearts. Hydralazine (10(-3) M) enhanced contractile force and heart rate, and elevated the myocardial cyclic AMP level. To study the Ca2+-dependent slow action potentials, the fast N+ channels were voltage-inactivated with elevated K+ (25 mM), resulting in a loss of electrical excitability. Hydralazine (10(-4) M) rapidly (less than 3 min) allowed the generation of slow action potentials and accompanying contractions by electrical stimulation. These effects of hydralazine were only partially prevented by propranolol. The results suggest that the increase of myocardial contractility produced by hydralazine is the result, at least in part, of a direct effect on the myocardium to increase Ca2+ inflow. The increased Ca2+ influx and inward slow current is due partly to activation of beta-adrenoceptors, with resultant elevation of cyclic AMP, and partly to another mechanism.

Beneficial effect of coenzyme Q on myocardial slow action potentials in hearts subjected to decreased perfusion pressure—hypoxia—substrate-free perfusion

SummaryCoenzyme Q, an important component of the electron transfer system in mitochondria, plays a central role in energy production aerobically. The effect of pretreatment with coenzyme Q10 (Co Q) on myocardial slow action potentials (APs) and accompanying contractions and on myocardial high energy phosphate content was studied in perfused hearts subjected to decreased perfusion pressure—hypoxia—substrate-free. Post-hatched chicks were treated i.p. with 10 mg/kg of Co Q daily for 5 days. To study the slow APs exclusively, the fast Na+ channels were voltage-inactivated by elevated K+ (25 mM) Tyrode solution. The Ca++-dependent slow APs were induced by elevating [Ca]0 to 5.4 mM; hearts were paced at a rate of 40 per min. Hearts which had been pretreated with Co Q were protected against the deleterious effect of decreased perfusion pressure—hypoxia—substrate-free perfusion on mechanical performance accompanying the slow Ca++−Na+ APs. The slow APs in hearts pretreated with Co Q were also less affected than were non-treated hearts. However, the myocardial ATP and total adenine nucleotides were not affected by exogenous Co Q. It was suggested that exogenous Co Q could protect against the decline of cardiac contractions via improved availability of slow APs during decreased perfusion pressure—hypoxia—substrate-free, independently of the cellular high energy phosphate level.

Enhanced suppression of myocardial slow action potentials during hypoxia by free fatty acids

Effects of free fatty acids (palmitate and linoleate) on myocardial contractility and slow action potentials (APs) were examined in Langendorff-perfused chick hearts. To study the slow APs exclusively, the fast Na+ channels were voltage-inactivated in elevated K+ (25 mM), and the concentration of Ca2+ ion was increased to 5.4 mM in order to induce slow APs. Palmitate (0.18, 0.54 or 0.72 mM) along with albumin (0.12 mM) was added to the perfusate. Albumin by itself did not affect contractility or the slow APs during normoxia and hypoxia. Under well oxygenated conditions, palmitate had no effect on contractility or the slow APs. However, palmitate accelerated the decline of contractility during hypoxia in a dose-dependent fashion. Hypoxia suppressed the slow APs, and palmitate and linoleate further exacerbated the suppression of slow APs produced by hypoxia. Nevertheless, palmitate and linoleate did not enhance the hypoxic reduction of the tissue high energy phosphate level. The present results suggest that free fatty acids elicit cardio-depressant effects on hearts through their direct action on the myocardial cell membrane (slow channels) rather than through any metabolic effects.

Cellular oncogene expression in the idiopathic cardiomyopathic hamster heart during the growing process

The expression of various proto-oncogenes was evaluated in the Syrian hamster with hereditary idiopathic cardiomyopathy. mRNA from the heart and aorta of controls (BIO-RB) and cardiomyopathic hamsters (UM-X7.1 strain, BIO 14.6 line) was tested using RNA hybridization techniques. Of the 19 different v-oncogene probes used in the study, only the v-myc probe revealed a substantially greater expression of oncogene in the 30th day of cardiomyopathic hamster heart than in control hamster heart. The amplified expression of c-myc was also observed in the heart of 1-year-old, but not of 7-day-old cardiomyopathic hamster. Overexpression of c-myc, otherwise associated with the regulation of cell differentiation or rapid growth, may relate to the pathological state or pathogenesis of the hereditary cardiomyopathy.