Effect of glucose and of fatty acid on rhythm,enzyme release and oxygen uptake in isolated perfused working rat heart with coronary artery ligation

The effects of exogenous substrates on post-occlusion arrhythmias remain controversial because of the difficulty in controlling circulating substrate concentrations in vivo. Isolated working rat hearts, with or without coronary ligation, were perfused in a non-recirculation system with glucose or palmitate as fatty acid. The medium K⁺ was 3.0 mm. Ectopic activity and tachyarrhythmias, chiefly ventricular tachycardia or fibrillation, were quantified as ventricular premature systoles/min or as the duration of ventricular tachycardia or fibrillation (s/min). Hearts were perfused with glucose (5.5 mm) alone, with glucose plus “low” palmitate (0.5 mm bound to albumin 0.25 mm), with glucose plus “high” palmitate (1.5 mm, albumin 0.25 mm) or with “high” palmitate without glucose. Effects on arrhythmias were compared with those on release of lactate dehydrogenase, cardiac output, oxygen uptake, efficiency of work, and the cardiac content of noradrenaline. The striking feature was that hearts perfused with high palmitate without glucose had a severity of ventricular arrhythmias and of depletion of cardiac noradrenaline no greater than hearts perfused with glucose alone. Although glucose-perfused coronary ligated hearts had lower rates of release of enzyme and a more favourable index of efficiency than hearts perfused with palmitate alone in high concentration, the incidence of arrhythmias in these two situations was approximately equal. In this model, palmitate in high concentration is not a potent arrhythmogenic stimulus but markedly increases enzyme release and decreases efficiency of work.     

Arrhythmogenic action of alpha 1-adrenoceptor stimulation in normoxic rat ventricular myocardium: influence of nisoldipine, reduced extracellular Ca2+ and ryanodine

This study examines the arrhythmogenic action of alpha 1 and alpha 2-adrenoceptor stimulation in the isolated perfused rat heart. The alpha 1-agonist methoxamine in the presence of the beta 1-antagonist atenolol 10(-6) M decreased the ventricular fibrillation threshold in the normoxic rat ventricular myocardium: VFT values (mA): Control 11.2 +/- 0.5; methoxamine 10(-6) M 4.9 +/- 0.9 (P less than 0.01 vs control); methoxamine 10(-5) M 3.5 +/- 0.5 (P less than 0.01 vs control). The alpha 1-antagonist prazosin 10(-8) M prevented the methoxamine-induced fall in ventricular fibrillation threshold. The alpha 2-agonist BHT 933 (azepexole) in the presence of atenolol 10(-6) M produced no alteration in the ventricular fibrillation threshold. Methoxamine 10(-6) M to 10(-5) M had a positive inotropic effect with increased left ventricular pressure development, myocardial oxygen consumption and QT-interval; however, tissue levels of cyclic AMP remained unchanged. Methoxamine 10(-6) M did not alter heart rate, coronary flow rate or deplete tissue levels of adenosine triphosphate, phosphocreatine or glycogen. The enhanced vulnerability to ventricular fibrillation induced by methoxamine could be demonstrated only at supraphysiological extracellular calcium concentrations (2.5 mM) but not at physiological calcium concentrations (1.25 mM). The arrhythmogenic and inotropic effect of methoxamine 10(-6) M was prevented by inhibition of transsarcolemmal Ca2+ ion influx by nisoldipine 10(-8) M or by inhibition of release of Ca2+ from sarcoplasmic reticulum by ryanodine 10(-9) M to 10(-8) M. Thus in isolated normoxic rat heart preparations, activity of the alpha 1-receptor appears to mediate ventricular arrhythmogenesis but only in the setting of myocardial calcium overload. The arrhythmogenic effect of alpha 1-stimulation may be due to increased transsarcolemmal calcium influx and enhanced release of calcium from the sarcoplasmic reticulum; increased myocardial oxygen consumption secondary to greater left ventricular pressure development may contribute in part.     

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.     

Antidysrhythmic actions of meobentine sulfate

The antiarrhythmic efficacy of meobentine sulfate, a bethanidine derivative lacking inhibitory effects on adrenergic neuronal function, was assessed in three canine models. Intravenous meobentine sulfate, administered in dosages of 5.0, 10,0, and 20.0 mg/kg, produced a dose-related increase in the ventricular fibrillation threshold (VFT) under nonischemic conditions (7.6 +/- 1.8 mA vs 37.8 +/- 8.6 mA) (20 mg/kg; p less than 0.05) and during regional myocardial ischemia (5.6 +/- 1.5 mA vs 41.8 +/- 9.1 mA) (20 mg/kg; p less than 0.05). The VFT was also increased in the presence of chronic ischemic injury (6.4 +/- 1 mA to 31 +/- 10 mA) (20 mg/kg; p 0.05). In the conscious dog, 4 days after an anterior myocardial infarction, programmed electrical stimulation (PES) produced nonsustained ventricular tachycardia (VT) in five dogs. After meobentine sulfate administration, eight of nine animals had sustained VT and one animal developed ventricular fibrillation (VF). At a dose of 20 mg/kg, there was prolongation of the cycle length of the VT (169 +/- 11 msec to 237 +/- 20 msec), prolongation of the QRS duration (58 +/- 2.6 msec to 71 +/- 3.7 msec), and prolongation of the delay in epicardial activation. There was an enhanced potential after meobentine administration for programmed stimulation to produce ventricular arrhythmias with the introduction of fewer premature impulses. In the third canine model, conscious dogs with a previous anterior myocardial infarction developed VF in response to electrically induced left circumflex coronary artery injury. Meobentine (20 mg/kg) failed to prevent VF in eight of eight dogs. These results suggest that while meobentine sulfate significantly increases the electrical VFT, it does not protect the conscious canine from the induction of ventricular tachyarrhythmias in response to PES, and it does not prevent VF in a conscious canine model of sudden coronary death.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of magnesium administered during postischemic reperfusion on myocardial oxidative metabolism in isolated rat hearts

Summary To determine the effect of magnesium on myocardial function and oxidative metabolism after reperfusion, isolated rat hearts perfused retrogradely with erythrocyte-enriched medium (0.4 mM palmitate bound to 0.4 mM albumin, 11 mM glucose) were subjected to 60 minutes of no-flow ischemia followed by 60 minutes of reperfusion. Untreated postischemic hearts exhibited after 15 minutes of reperfusion recovery of myocardial oxygen consumption to 65% of the preischemic value despite persistent depression of left ventricular isovolumic pressure development to 21%. Magnesium (15 mM) administered during the initial 30 minutes of reperfusion reduced myocardial oxygen consumption of reperfuse myocardium by 35%. Oxidation of [1-¹⁴C]palmitate was slightly more reduced (–55%) than oxidation of [U-¹⁴C]glucose(–42%). Magnesium did not influence ultimate recovery of contractile function and cumulative myocardial release of creatine kinase. Thus, 15 mM magnesium administered during reperfusion elicited a reduction of oxidative metabolism. However, magnesium did not modify myocardial injury.Supported by the Swiss National Science Foundation grant 32-26373.89 and the Swiss Foundation of Cardiology     

Cellular electrophysiologic changes and "arrhythmias" during experimental ischemia and reperfusion in isolated cat ventricular myocardium

The cellular electrophysiologic consequences of both regional and global experimental ischemia and reperfusion were studied in the isolated cat myocardium, using conventional microelectrode techniques. Oxygenated Tyrode's solution was perfused through the left anterior descending and circumflex coronary arteries, while the preparation was superfused with Tyrode's solution gassed with 95% nitrogen and 5% carbon dioxide. Electrophysiologic characteristics of endocardial muscle cells were normal during coronary perfusion. When perfusion was discontinued for 30 minutes, resting membrane potential was decreased by 21.6 +/- 4.1%, action potential amplitude was decreased by 29.1 +/- 8.6% and action potential duration was decreased by 54.1 +/- 12.5% (p less than 0.001). Ectopic activity occurred after 5 to 10 minutes of ischemia and was more frequent in regional than in global ischemia (p less than 0.05). Rapid ventricular activity was observed in only 5 (17%) of 29 preparations during ischemia, whereas it occurred in 24 (83%) of 29 preparations during reperfusion. Rapid ventricular activity began 5 to 40 seconds (mean 19) after the start of reperfusion, stopped spontaneously after a mean of 113 +/- 211 seconds and occurred after both regional and global ischemia. The cellular electrophysiologic changes induced by ischemia returned to baseline values within the next 5 minutes. Repeated ischemia and reperfusion runs reproduced the same electrophysiologic changes and rapid ventricular activity. Coronary perfusion with procainamide (20 mg/liter) aggravated the ischemic depressions of action potential amplitude and action potential duration and increased conduction delay during ischemia, but it did not prevent rapid ventricular activity induced by reperfusion. In contrast, verapamil (1 mg/liter) perfusion did not affect the changes in action potential variables during ischemia but prevented reperfusion-induced rapid ventricular activity. Perfusion with calcium ion (Ca2+)-free Tyrode's solution just before ischemia and during reperfusion slowed or prevented reperfusion-induced rapid ventricular activity, without affecting the action potential changes during ischemia. It is concluded that, in these isolated perfused ventricular muscle preparations, different mechanisms may be operative in ischemic and reperfusion arrhythmias and Ca2+ may play an important role in the development of arrhythmias during the reperfusion phase of ischemia/reperfusion sequences.     

The effects of coronary artery disease on the ventricular fibrillation threshold in man.

The ventricular fibrillation threshold (VFT) was measured in 28 patients at the time of cardiac surgery. The VFT was measured with a 100 Hz train of 24 rectangular pulses positioned across the ST segment and T wave. Current was applied to the epicardial surface of either ventricle with a bipolar electrode probe. In six patients, the normal right VFT was 24.3 +/- 5.2 mA, and in 10 patients the normal left VFT was 33.6 +/- mA (p less than 0.05). In 12 patients with greater than or equal to 75% obstruction of the left anterior descending coronary artery, the left VFT was 18.6 +/- 6.9 mA. This value was significantly less than the left VFT in patients without coronary artery disease (p less than 0.001). This study shows that the VFT can be measured in man and that coronary artery disease reduces this parameter.     

Effects of a thromboxane A2 synthetase inhibitor on ventricular fibrillation threshold during coronary artery occlusion and reperfusion

The effects of a new thromboxane A2 synthetase inhibitor (DP-1904) on electrical stability of the heart were tested in anesthetized, open chest dogs. The incidence of spontaneous ventricular arrhythmias, ventricular refractory period and ventricular fibrillation threshold (VFT) during ligation of the left anterior descending coronary artery (LAD) for 180 min and after reperfusion were measured as indices of stability. Ventricular fibrillation and ventricular tachycardia occurred spontaneously after ligation of LAD in 56% of 9 control dogs and 29% of 7 dogs which received intravenous DP-1904 (100 mg) before ligation of LAD (n.s.). In the control group, the ventricular refractory period decreased in the ischemic region; consequently, the difference in refractory period duration between the ischemic and non-ischemic regions (i.e., dispersion) increased 30 min after coronary ligation (7 +/- 9 ms vs 32 +/- 17 ms, p less than 0.05). The dispersion at 30 min after coronary ligation, though, was not affected in the DP-1904 treated group (2 +/- 4 ms vs 10 +/- 9 ms, n.s.). The VFT (determined with pulse trains) decreased from 28 +/- 5 mA to 15 +/- 11 mA (p less than 0.05) 30 min after coronary ligation in the control group, but was not affected (30 +/- 0 mA vs 27 +/- 4 mA) in the DP-1904 group. The plasma concentration of thromboxane B2 decreased after DP-1904 administration (baseline vs 30 min after coronary ligation: 475 +/- 165 pg/ml vs 165 +/- 74 pg/ml, n = 3, p less than 0.05), while the concentration of 6-keto-prostaglandin F1 alpha increased gradually. In conclusion, DP-1904 prevents a decline in electrical stability in the ischemic region of the canine heart during coronary occlusion.     

Effects of glucagon on free fatty acid metabolism in humans

To determine whether physiological changes in plasma glucagon concentrations are important in regulating basal adipose tissue lipolysis, FFA flux ([1-14C]palmitate) was measured in response to increases and decreases in plasma glucagon. Eight volunteers with insulin-dependent diabetes mellitus (IDDM) and nine healthy nondiabetic volunteers were studied using the pancreatic clamp technique to control plasma insulin, GH, and glucagon concentrations at desired levels. Palmitate flux at the chosen euglucagonemic hormone infusion rates was similar to baseline values (1.73 +/- 0.12 vs. 1.75 +/- 0.23 and 1.35 +/- 0.18 vs. 1.35 +/- 0.16 mumol/kg.min, respectively, in IDDM and nondiabetic subjects). No significant changes in palmitate flux occurred in response to glucagon withdrawal or mild (nondiabetic volunteers) or high physiological (IDDM volunteers) hyperglucagonemia. Thus, under conditions of normal FFA availability, changes in plasma glucagon concentrations within the physiological range have little or no effect on adipose tissue lipolysis.     

Hyperlipidemia with hypoglycemia reduces myocardial oxygen utilization efficiency but not contractile function during coronary hypoperfusion

This study was designed to determine changes in myocardial contractile function and fuel selection during moderate coronary hypoperfusion in the presence of elevated plasma free fatty acid (FFA) at normal and reduced blood glucose concentrations. Coronary perfusion pressure (CPP) was sequentially lowered from 100 to 60, 50, and 40 mmHg in the left anterior descending coronary artery (LAD) of anesthetized, open-chest dogs. Regional glucose uptake (GU), fatty acid uptake (FAU), percentage segment shortening (%SS), and oxygen consumption (MV O(2)) were determined with normal arterial plasma FFA concentrations (Group 1) or with elevated FFA concentrations (Groups 2 and 3). In Group 3, glucose in the coronary perfusate blood was reduced from 3.53+/-0.36 to 0.15+/-0.03 m M by hemodialysis. In Group 1, FAU fell by 85% as CPP was lowered to 60 mmHg and remained depressed as CPP was reduced further; GU did not fall significantly. Hyperlipidemia in Group 2 did not alter GU at any CPP, but maintained FAU at baseline levels until CPP was lowered to 40 mmHg. At 40 mmHg CPP, myocardial function and metabolic variables were similar in Groups 1 and 2. In Group 3 at 40 mmHg, FAU increased four-fold and MV O(2)doubled v Groups 1 and 2, and GU fell to zero. Despite these metabolic changes, %SS in Group 3 was unchanged relative to Group 2. Addition of glucose to the dialysate prevented the effects of dialysis on FAU, GU, and MV O(2). Thus, preferential glucose oxidation sustains myocardial oxygen utilization efficiency [(heart rate x %SS x maximum left ventricular pressure)/MV O(2)] during hypoperfusion. Blocking preferential glucose oxidation by combined hyperlipidemia and hypoglycemia lowers oxygen utilization efficiency, but does not compromise myocardial contractile function.