Oscillations of membrane current and excitability driven by metabolic oscillations in heart cells

Potent antifibrillatory effect of combined blockade of calcium channels and 5-HT2 receptors with nexopamil during myocardial ischemia and reperfusion in dogs: comparison to diltiazem

The need for novel antifibrillatory therapy is underscored by clinical trials indicating that the incidence of sudden cardiac death is increased by sodium or potassium channel blockade and is only partially reduced by beta-blockade. We examined the efficacy of nexopamil, which possesses the unique combination of calcium channel and 5-HT2 receptor blockade, in preventing ventricular tachycardia (VT) and fibrillation (VF) and reducing T-wave alternans magnitude during coronary artery occlusion and abrupt reperfusion in dogs. The results were compared with L-type calcium channel blockade alone with diltiazem. The effect of nexopamil was tested during a 10-min period of left anterior descending (LAD) coronary artery occlusion and release in chloralose-anesthetized dogs. T-wave alternans magnitude was assessed by complex demodulation. The drug reduced the incidence of VT during occlusion (from 5 of 6 to 0 of 6, p < 0.03) and VT/VF during abrupt reperfusion (from 5 of 6 to 0 of 6, p < 0.03) and suppressed the T-wave alternans magnitude increase induced by occlusion (from 14.62 +/- 3.96 to 1.39 +/- 0.34 mV x ms, p < 0.01) and reperfusion (from 17.33 +/- 4.67 to 2.34 +/- 0.77 mV x ms, p < 0.01). When 30-s left stellate ganglion stimulation (10 V, 5-ms pulses, 10 Hz) was superimposed on occlusion, nexopamil reduced the VT/VF incidence (from 8 of 11 to 4 of 11, p < 0.05) and T-wave alternans magnitude (from 24.80 +/- 5.05 to 15.81 +/- 5.09 mV x ms, p < 0.05). Calcium channel blockade alone with diltiazem decreased the incidence of ventricular tachyarrhythmias (from 5 of 10 to 1 of 10, p < 0.05) and T-wave alternans magnitude (from 16.75 +/- 3.06 to 2.87 +/- 1.23 mV x ms, p < 0.05) during coronary artery occlusion. During reperfusion, diltiazem's reduction in arrhythmia incidence (from 5 of 8 to 2 of 8) was not statistically significant, although the decrease in T-wave alternans (from 28.60 +/- 3.43 to 8.27 +/- 3.73 mV x ms, p < 0.05) was significant. Therefore, nexopamil was superior to diltiazem in protecting against reperfusion-induced arrhythmias. Nexopamil's significant antifibrillatory effect during both coronary artery occlusion and abrupt reperfusion is reliably tracked by T-wave alternans magnitude. Because the major component of the protection could be reproduced by blockade of the L-type calcium channel with diltiazem, nexopamil's antiarrhythmic action appears to be due mainly to blockade of this channel. Nexopamil's antiplatelet action through blockade of 5-HT2 receptors may confer additional protection against reperfusion arrhythmias.

Oscillations in KATP channel activity promote oscillations in cytoplasmic free Ca2+ concentration in the pancreatic beta cell

Pancreatic beta cells exhibit oscillations in electrical activity, cytoplasmic free Ca2+ concentration ([Ca2+](i)), and insulin release upon glucose stimulation. The mechanism by which these oscillations are generated is not known. Here we demonstrate fluctuations in the activity of the ATP-dependent K+ channels (K(ATP) channels) in single beta cells subject to glucose stimulation or to stimulation with low concentrations of tolbutamide. During stimulation with glucose or low concentrations of tolbutamide, K(ATP) channel activity decreased and action potentials ensued. After 2-3 min, despite continuous stimulation, action potentials subsided and openings of K(ATP) channels could again be observed. Transient suppression of metabolism by azide in glucose-stimulated beta cells caused reversible termination of electrical activity, mimicking the spontaneous changes observed with continuous glucose stimulation. Thus, oscillations in K(ATP) channel activity during continuous glucose stimulation result in oscillations in electrical activity and [Ca2+](i).

Glucose is arrhythmogenic in the anoxic-reoxygenated embryonic chick heart

Unlike in adult heart, embryonic myocardium works at low PO2 and depends preferentially on glucose. Therefore, activity of the embryonic heart during anoxia and reoxygenation should be particularly affected by changes in glucose availability. Hearts excised from 4-d-old chick embryos were submitted in vitro to strictly controlled anoxia-reoxygenation transitions at glucose concentrations varying from 0 to 20 mmol/L. Spontaneous and regular heart contractions were detected optically as movements of the ventricle wall and instantaneous heart rate, amplitude of contraction, and velocities of contraction and relaxation were determined. Anoxia induced transient tachycardia and rapidly depressed contractile activity, whereas reoxygenation provoked a temporary and complete cardioplegia (oxygen paradox). In the presence of glucose, atrial rhythm became irregular during anoxia and chaotic-periodic during reoxygenation. The incidence of these arrhythmias depended on duration of anoxia, and no ventricular ectopic beats were observed. Removal of glucose or blockade of glycolysis suppressed arrhythmias. These results show similarities but also differences with respect to the adult heart. Indeed, glucose 1) delayed and anoxic contractile failure, shortened the reoxygenation-induced cardiac arrest, and improved the recovery of contractile activity; 2) attenuated stunning at 20 mmol/L but worsened it at 8 mmol/L; and 3) paradoxically, was arrhythmogenic during anoxia and reoxygenation, especially when present at the physiologic concentration of 8 mmol/L. The last named phenomenon seems to be characteristic of the young embryonic heart, and our findings underscore that fluctuations of glycolytic activity may play a role in the reactivity of the embryonic myocardium to anoxiareoxygenation transitions.

Traveling NADH and proton waves during oscillatory glycolysis in vitro

Propagation and mutual annihilation of circular and spiral NADH and proton waves were detected by spatially resolved spectrophotometry and fluorescent proton indicators in a biological in vitro system: an organelle-free yeast extract. Spontaneous wave generation during glycolytic sugar degradation is established after an induction period of about 1 h. Controlled wave initiation could be performed by local injection of the strong activator of phosphofructokinase, fructose 2,6-bisphosphate. A crucial role for wave initiation and control of pattern dynamics is attributed to the key enzyme of glycolysis, the allosterically regulated phosphofructokinase. An overall increase in the concentration of its positive effector AMP leads to the formation of rotating spirals. The dynamics of the observed wave patterns resemble that of self-organized calcium waves as recently found in frog eggs and heart cells.

Electrophysiologic changes in ischemic ventricular myocardium: I. Influence of ionic, metabolic, and energetic changes

Myocardial ischemia leads to significant changes in the intracellular and extracellular ionic milieu, high-energy phosphate compounds, and accumulation of metabolic by-products. Changes are measured in extracellular pH and K+, and intracellular pH, Ca2+, Na+, Mg2+, ATP, ADP, and inorganic phosphate. Alterations of membrane currents occur as a consequence of these ionic changes, adrenergic receptor stimulation, and accumulation of lactate, amphipathic compounds, and adenosine. Changes in the volume of the extracellular and intracellular spaces contribute further to the ultimate perturbations of active and passive membrane properties that underlie alterations in excitability, abnormal automaticity, refractoriness, and conduction. These characteristic changes of electrophysiologic properties culminate in loss of excitability and failure of impulse propagation and form the substrate for ventricular arrhythmias mediated through abnormal impulse formation and reentry. The ability to detail the changes in ions, metabolites, and high-energy phosphate compounds in both the extracellular and intracellular spaces and to correlate them directly with the simultaneously occurring electrophysiologic changes have greatly enhanced our understanding of the electrical events that characterize the ischemic process and hold promise for permitting studies aimed at developing interventions that may lessen the lethal consequences of ischemia.

Metabolic oscillations in heart cells

Oscillatory rhythms underlie biological processes as diverse and fundamental as neuronal firing, secretion, and muscle contraction. We have detected periodic changes in membrane ionic current driven by intrinsic oscillations of energy metabolism in guinea pig heart cells. Withdrawal of exogenous substrates initiated oscillatory activation of ATP-sensitive potassium current and cyclical suppression of depolarization-evoked intracellular calcium transients. The oscillations in membrane current were not driven by pacemaker currents or by alterations in intracellular calcium and thus represent a novel cytoplasmic cardiac oscillator. The linkage to energy metabolism was demonstrated by monitoring oscillations in the oxidation state of pyridine nucleotides. Interventions which altered the rate of glucose metabolism modulated the oscillations, suggesting that the rhythms originated at the level of glycolysis. The metabolic oscillations produced cyclical changes in electrical excitability, underscoring the potential importance of this intrinsic oscillator in the genesis of cardiac arrhythmias.