Comparison of the electrophysiological effect of amiodarone, lidocaine and quinidine on rat ventricular cells.

The effects of 20 microM each of amiodarone, lidocaine and quinidine on action potential and membrane currents were studied in rat ventricular cells. At a stimulation frequency of 0.1 Hz, quinidine prolonged the action potential duration (APD50) from 120 +/- 26 to 660 +/- 8 msec and increased the time to peak (Tp) amplitude from 7 +/- 1 msec to 32 +/- 6 msec. Lidocaine shortened APD50 from 123 +/- 15 to 83 +/- 6 msec without altering Tp. Amiodarone changed neither APD50 nor Tp. Voltage clamp study revealed that quinidine inhibited sodium inward current (INa) even when this current was elicited by depolarizing pulses at 0.1 Hz from a holding potential of -90 mV. For amiodarone and lidocaine, the inhibition was observed when INa was elicited from a holding potential of -70 mV. A frequency-dependent inhibition of INa by amiodarone and lidocaine was observed at frequencies higher than 1 Hz. Quinidine showed this inhibition even at 1 Hz. In correlation with the stronger frequency dependent inhibition of INa, a greater delay of the recovery and increase of the non-recovery fraction of INa was induced by quinidine. For lidocaine and amiodarone, only the recovery time constant was delayed. In cells treated with sea anemone toxin (ATX, 0.2 microM), APD50 was prolonged to 4-5 sec in 5 min. Quinidine, but not amiodarone, completely reversed the effect of ATX. Quinidine showed use-dependent inhibition of INa in these ATX-treated cells. Amiodarone, however, did not show this inhibition. It is likely that amiodarone suppresses INa by delaying the recovery of INa instead of blocking the open-state Na(+)-channels.(ABSTRACT TRUNCATED AT 250 WORDS)     

Repeated amiodarone exposure in the rat: toxicity and effects on hepatic and extrahepatic monooxygenase activities

Amiodarone is a potent and efficacious antiarrhythmic agent, yet associated with its use are life-threatening pulmonary fibrosis and hepatotoxicity. We have investigated the susceptibility of the male Sprague-Dawley rat to pulmonary and hepatic toxicity after repeated exposure to amiodarone and the effects of such exposure on hepatic and extrahepatic drug metabolizing enzymes. Animals received amiodarone (200 mg.kg-1.day-1 i.p., 5 days/week) for 1 week followed by 150 mg.kg-1.day-1 (5 days/week) for 3 additional weeks. No signs of pulmonary fibrosis or hepatotoxicity were observed, based on histological examination, lung hydroxyproline content, and plasma alanine aminotransferase activity. Analysis of tissues revealed extensive accumulation of amiodarone and desethylamiodarone in lung and liver, but concentrations were significantly lower in animals treated for 4 weeks than for 1 week. In a separate experiment, rats received amiodarone 150 mg.kg-1.day-1 i.p. (5 days/week) for 1 or 4 weeks. No differences in tissue concentrations of amiodarone and desethylamiodarone were detected between animals treated for 1 or 4 weeks. This regimen did not affect hepatic or extrahepatic monooxygenase activities. These results indicate that, in the male Sprague-Dawley rat, there is no observable pulmonary or hepatic toxicity following short-term amiodarone exposure, and there is enhanced elimination of amiodarone and desethylamiodarone when the daily dose of amiodarone is decreased after 1 week from 200 to 150 mg/kg.     

Comparison of the effects of neuropeptide Y (NPY) and 4-norleucine-NPY on isolated perfused rat hearts; effects of NPY on atrial and ventricular strips of rat heart and on rabbit heart mitochondria

Isolated perfused rat hearts were used to compare the effects of the synthetic neuropeptide Y (NPY) and 4-norleucine-NPY on cardiac function. Each peptide exhibited both negative inotropic and chronotropic effects, and also caused coronary vasoconstriction leading to a reduction in coronary flow. A comparison of the IC50 values from dose-response curves using 10(-14) to 10(-7) M peptides (IC50 is the peptide concentration that produced a 50% decrease of the maximal effect) indicated that NPY was more potent as inhibitor of contractility and less potently inhibited coronary flow and heart rate, whereas 4-norleucine-NPY had more inhibitory influence on coronary flow and heart rate and less on cardiac contractility. This difference in potencies suggests that the inhibitory effects of NPY on contractility, coronary flow and heart rate may be independent of each other. Since NPY also decreased the contractile force of isolated left atrial and right ventricular strips of the rat heart, the coronary flow decrease cannot be the cause of the negative inotropy of isolated heart. Pretreatment of atrial and ventricular strips with NPY did not influence the positive inotropic effect produced by the cardiac glycoside ouabain indicating that sarcolemmal Na+, K+-ATPase was not involved in the inhibitory inotropic effect of NPY. Further studies towards elucidating the mechanism of the negative inotropy of cardiac muscles using isolated heart mitochondria revealed that NPY uncoupled oxidative phosphorylation and blocked mitochondrial calcium uptake; the former event fosters negative inotropy. Since these effects on mitochondria occurred at concentrations 100-fold higher than those required for negative inotropy, the two effects of NPY may not be related.     

Gender-dependent effects of selenite on the perfused rat heart

Gender differences are related to the manner in which the heart responds to chronic and acute stress conditions of physiological and pathological nature. Depending on dose, sodium selenite acts as an antioxidant proven to have beneficial effects in several pathological conditions G. Drasch, J. Schopfer, and G. N. Schrauzer, Selenium/cadmium ratios in human prostates: indicators of prostate cancer risk of smokers and non-smokers, and relevance to the cancer protective effects of selenium,Biol. Trace Element Res. 103(2), 103–107 (2005); R. G. Kasseroller and G. N. Schrauzer, Treatment of secondary lymphedema of the arm with physical decongestive therapy and sodium selenite: a review,Am. J. Ther. 7(4), 273–279 (2000); G. N. Schrauzer, Anticarcinogenic effects of selenium,Cell. Mol. Life Sci. 57(13–14), 1864–1873 (2000); I. S. Palmer and O. E. Olson, Relative toxicities of selenite and selenate in the drinking water of rats,J. Nutr. 104(3), 306–314 (1974). To date, little is known about the gender-dependent direct effects of toxic doses of selenite on electrophysiology of the cardiovascular system H. A. Schroeder and M. Mitchener, Selenium and tellurium in rats: effect on growth, survival and tumors,J. Nutr. 101(11), 1531–1540 (1971); G. N. Schrauzer, The nutritional significance, metabolism and toxicology of selenomethionine,Adv. Food Nutr. Res. 47, 73–112 (2003). In the present study, the effects of in vitro toxic concentrations of sodium selenite ranging from 10^-6 M to 10^-3 M were tested on both male and female rat heart preparations. The toxic effects seen in an electrocardiogram and left ventricular pressure were dose and sex dependent at most of the tested concentrations. The present study clearly shows that at toxic doses, stress conditions are induced by selenite, resulting in gender-dependent modifications of the heart function. This modification is more pronounced in the contraction cascade of female rats. Males, on the other hand, had been much more affected in excitation-related parameters.     

Some effects of tetracaine on rat heart mitochondria

The effect of the local anesthetic tetracaine on some functions of rat heart mitochondria was studied, and the results show that this tertiary amine inhibits the oxidation of NAD-dependent substrates by affecting the mitochondrial membrane in a site located between the NADH dehydrogenase and Co Q span through a process which is not affected by K⁺, Ca2⁺, or Mg2⁺. Also the results show that tetracaine induces sodium uptake probably by a H⁺/Na⁺ exchange reaction.     

The effects of glycerol treatment on the isolated rat heart

Summary Isolated rat hearts were perfused with a balanced electrolyte solution containing 1000mM glycerol for 15min and then perfused with normal electrolyte solution for up to 32 min. The perfusion with hypertonic glycerol solution and subsequent washout is termed glycerol treatment. Initially, glycerol removal causes swelling and rupture of the T-system in ventricular myocardial cells which correlates temporally with a period of cardiac arrest. Contractility returns during further glycerol removal and concomitant recovery of the T-system is observed. Atomic absorption spectometry and neutron activation analysis were used to measure ventricular sodium, potassium and calcium ion content. There is no apparent correlation between changes in ion content and cardiac arrest or recovery. The water movements were calculated from wet weight, dry weight and inulin space, and confirmed by morphometric analysis of extracellular and intracellular space. It is suggested that the swelling and rupture of the T-system is due to the rapid water movements that were observed during the onset of glycerol removal. Ultrastructural analysis of glycerol-treated atrium from the same hearts shows damage of mitochondria and of the L-system and intracellular edema. The structural changes are correlated with a loss of atrial contraction. As in ventricular myocardium, resumption of contraction is associated with an almost complete recovery from ultrastructural damage.The studies were supported by the German Research Foundation within the SFB 90 Cardiovasculäres System     

Effect of insulin replacement on streptozotocin-induced effects in the rat heart

Streptozotocin (65 mg/kg) was used to induce diabetes in male Sprague-Dawley rats. Isolated cardiac tissue exhibited a systematic depression in atrial pacemaker function and an enhancement in ventricular function accompanied by a supersensitivity to calcium relative to control animals. β-Adrenoceptor density was found to be significantly lowered in the treated animals. However, no change in responsiveness of the tissues to isoproterenol was observed. The systematic changes in atria and ventricle were found to be completely and partially reversed respectively, by daily administration of 4–5 units of Ultralente (U-100) insulin, whereas the decrease in β-adrenoceptor number and supersensitivity to calcium were completely reversed. These results suggest that STZ by itself might not have toxic effects in the heart and that its effects may be overcome by chronic insulin-replacement.     

Norepinephrine amplifies effects of vasopressin on the isolated rat heart.

We compared effects of perfusion of norepinephrine (NE, 10(-9) mol l-1 and of unchanged Krebs-Henseleit solution on the cardiac response to bolus injection of arginine vasopressin (AVP, 2 ng). 14 isolated rat working heart preparations were used in a balanced cross-over design. Coronary flow, oxygen consumption and extraction, heart rate and total flow were continuously recorded. The concentration of NE was below that exerting per se systematic influences on cardiac activity. However, NE changed the cardiac response to AVP: (1) the AVP-induced reduction in coronary flow was greater during NE (mean: 41.7%) than vehicle perfusion (30.5%, P less than 0.005. (2) The AVP-induced decrease in oxygen consumption was stronger on top of the NE (41.5%) than vehicle perfusion (33.6%, P less than 0.005). (3) Following AVP, oxygen extraction during NE was increased compared to oxygen extraction during vehicle perfusion (3.61 +/- 0.03 vs. 3.46 +/- 0.02 microliters O2 ml-1 g-1, P less than 0.005). Results support the view of a potentiating role of catecholamines for direct cardiovascular effects of AVP.     

Inhibitory effects of quinidine on rat heart muscarinic receptors

Quinidine inhibited binding of the labelled agonist [3H]oxotremorine M [( 3H]Oxo-M) and the labelled antagonist [3H]N-methylscopolamine [( 3H]NMS) to rat heart muscarinic receptors. Kinetic studies demonstrated that quinidine decreased the association rates (I50: 4 and 7.5 microM) and dissociation rates (I50: 100 and 68 microM) of [3H]Oxo-M and [3H]NMS, with different potencies. These cooperative effects explained the low Hill coefficients and apparent selectivity of quinidine competition curves.     

Comparison of adrenergic agonist and insulin effects on 3-O-methyl-D-glucose efflux and sarcolemmal cytochalasin B binding by perfused rat heart

1. alpha- and beta-Adrenergic agonists as well as insulin stimulate 3-O-methyl-D-glucose efflux by the perfused rat heart and increase D-glucose inhibitable cytochalasin B binding by isolated sarcolemma. 2. alpha- and beta-Agonists like insulin increase Vmax for 3-O-methyl-D-glucose efflux and increase Bmax for cytochalasin B binding. 3. The effects of alpha- and beta-agonists are totally Ca2+-dependent whilst those of insulin appear to be only partly Ca2+-dependent.     

Influence of fasting on the effects of diazoxide in the ischemic-reperfused rat heart

This investigation aimed to assess whether the mitochondrial ATP-sensitive potassium channel opener diazoxide could reproduce the protection conferred by ischemic preconditioning and to ascertain whether its effects are associated with changes in glycogen breakdown and glycolytic activity. Hearts of fed and 24-h fasted rats were perfused with 10 mM glucose containing medium and exposed to 25 min no-flow ischemia plus 30 min reperfusion. Diazoxide (10 microM) perfusion was begun 10 min before ischemia and continued throughout the experiment. Fasting accelerated reperfusion recovery of contraction, reduced the post-ischemic contracture and decreased lactate accumulation during ischemia but had no effects on glycogen levels and cellular viability. Diazoxide, did not affect glycogen catabolism but improved reperfusion recovery of contraction. Furthermore, diazoxide reduced ischemic lactate accumulation and contracture amplitude only in the fed group whereas it improved cell viability in the fed and fasted groups. These data indicate that: 1) reduced lactate production which may attenuate myocyte acidification might explain, at least in part, the beneficial effects of diazoxide on mechanical function, although data obtained with the fasted rat hearts indicate that other mechanisms must be involved as well; 2) the reduction of lactate production occurring in the fed group, does not seem to be related to glycogenolysis; and 3) since diazoxide improved cell viability in the fasted rat group where it did not reduce glycolytic activity, other mechanisms may be responsible for this cytoprotective effect.     

Comparison of the precursor and mature forms of rat heart mitochondrial malate dehydrogenase

The complete amino acid sequence of mitochondrial malate dehydrogenase from rat heart has been determined by chemical methods. Peptides used in this study were purified after digestions with cyanogen bromide, trypsin, endoproteinase Lys C, and staphylococcal protease V-8. The amino acid sequence of this mature enzyme is compared with that of the precursor form, which includes the primary structure of the transit peptide. The transit peptide is required for incorporation into mitochondria and appears to be homologous to the NH2-terminal arm of a related cytoplasmic enzyme, pig heart lactate dehydrogenase. The amino acid differences between the rat heart and pig heart mitochondrial malate dehydrogenases are analyzed in terms of the three-dimensional structure of the latter. Only 12/314 differences are found; most are conservative changes, and all are on or near the surface of the enzyme. We propose that the transit peptide is located on the surface of the mitochondrial malate dehydrogenase precursor.