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Magnesium: effects on reperfusion arrhythmias and membrane potential in isolated rat hearts.

The effects of Mg2+ concentration (Mg2+o, 0, 1.2, 2.4, and 4.8 mM) on the incidence of reperfusion arrhythmias and on the cellular electrical activity were studied in spontaneously beating rat hearts. The surface electrogram and the membrane potential were recorded in control conditions, during 10 min of regional ischemia (ligature of the left anterior descending coronary artery), and on reflow. Changes in Mg2+o did not alter action potential morphology but the depolarization induced by ischemia decreased with increasing Mg2+o. In hearts perfused with Mg2+ free solution or 1.2 mM subthreshold delayed afterdepolarizations (DADs) were often detected during ischemia. Moreover, DADs could be identified as initial events in the production of extrabeats or tachycardia appearing on reperfusion under these conditions. Chaotic electrical activity during fibrillation precluded the observation of DADs. The overall incidence (100%) and severity of ventricular tachyarrhythmias (80% tachycardia and fibrillation) was similar in both groups. At high Mg2+o, subthreshold DADs were occasionally observed during ischemia and often on reperfusion where they did not lead to the development of overt arrhythmias. Consequently, the incidence, severity, and duration of arrhythmic episodes on reflow was markedly reduced. Raising Mg2+ only on reperfusion did not prevent the development of arrhythmias, whose morphology in the intracellular recordings was similar to that found in hearts perfused without Mg2+ or with 1.2 mM. The recovery of sinus rhythm after 10 min of reperfusion was linearly related to Mg2+o. Our data strengthen the view that reperfusion arrhythmias belong to the Ca2+ mediated non reentrant type and suggest that Mg2+ counteracts these arrhythmias by depressing cytosolic Ca2+ oscillations. Besides, it appears that raising Mg2+o reduces ischemic K+o accumulation. The resulting changes in resting potential could contribute to lower DADs amplitude and thus decrease the arrhythmogenic potential of the Ca2+i oscillations induced by reperfusion.

Modulation of the electrophysiological effects of ischemia reperfusion by methylisobutyl amiloride.

We studied the effect of the Na+/H+ exchanger inhibitor methylisobutyl amiloride (MIA, 1 microM) on action potential characteristics and arrhythmias induced by: (a) reperfusion following regional ischemia in rat hearts and (b) realkalization after lactate acidosis in rabbit hearts. We also determined the effect of MIA on the incidence of transient inward currents (ITIs) induced by acidosis-realkalization in rabbit cardiocytes. Ligation of the LAD coronary artery for 10 min depolarized the resting potential from -78 +/- 1.9 mV to -66.9 +/- 1.0 mV and depressed the action potential but did not induce overt arrhythmias. Delayed afterdepolarizations were observed during ischemia in 50% of untreated hearts whereas reperfusion produced severe ventricular tachyarrhythmias in all of them. MIA reduced the incidence of arrhythmias to 27% and their duration to less than 1 min. MIA increased action potential duration by 38 +/- 4.1%. BaCl2 produced a similar APD lengthening and had an antifibrillatory effect. Acidic reperfusion induced bradycardia and reduced severity of arrhythmias. In rabbit hearts, MIA increased the action potential duration by 61 +/- 4.3% and abolished arrhythmias on realkalization. Eleven out of 18 cells developed transient inward currents during acidosis-realkalization and seven of them underwent irreversible injury. MIA prevented the appearance of ITIs, had no effect on ICa,L but decreased the outward component of IK1 by 50%. Our results suggest that the protective effect of MIA is in part due to changes in cellular electrical activity that modulate Na+ and Ca2+ entry via different pathways.

Early action potential shortening in hypoxic hearts: role of chloride current(s) mediated by catecholamine release.

We tested the hypothesis that the early action potential shortening induced by hypoxia in perfused hearts is attributable to chloride currents activated or modulated by endogenous catecholamine release. Rabbit hearts perfused at 33 degrees C and paced at 2.5-2.8 Hz were used for membrane potential recordings with microelectrodes. Catecholamine depletion was induced with reserpine treatment. The effects of nadolol (10 microM), the stilbenedisulfonic acid derivatives DIDS (10 microM) and SITS (1 mM), and diphenylamine-2 carboxylate (DPC, 100 microM) on action potential characteristics were determined at different times during hypoxia. The effect of chloride transport blockers on the outward currents induced by 200 nM carbonyl cyanide (CCCP) or by 1 microM isoproterenol in isolated cells was also tested. In control hearts, action potential duration (APD) at 25 and 95% repolarization decreased by 50 +/- 9% and 32 +/- 7% respectively after 5 min of hypoxia. This effect was fully antagonized by reserpine pretreatment, by respiratory acidosis, and by nadolol when present from the beginning of hypoxia. None of these agents affected action potential characteristics in normoxia and nadolol had no effect when added after 15 min of hypoxia. Lowering the chloride concentration to 17.5 mM reproduced the effects of nadolol and reserpine. DIDS and SITS lengthened APD in normoxia and attenuated the early APD shortening in hypoxia. DPC had no effect in normoxia but fully counteracted APD shortening produced by isoproterenol or early hypoxia. In isolated cells, DIDS did not affect the glibenclamide sensitive outward current induced by CCCP and DPC blocked the isoproterenol induced current. The data suggest that in whole hearts, chloride currents mediated by endogenous catecholamine release are involved in the early action potential shortening induced by hypoxia with preservation of glycolysis.

A modified technique for canine sinoatrial node autoperfusion.

An experimental model for dog sinoatrial node autoperfusion is proposed. A by-pass was performed between right internal mammary artery and right atrial dorsal artery. A blood flow of 2.5 +/- 0.15 ml/min was calculated from blood forward velocity measurements. The injection of either isoproterenol or verapamil into the by-pass was followed by rapid heart rate modifications. These heart rate changes occurred without detectable modifications on the electrical behavior of the remaining myocardium, as assessed by electrocardiogram and endocavitary electrogram recordings. This model leaves right coronary circulation intact. The intrathoracic by-pass reduces heat-losses and lessens the probability of blood clotting. The model is suitable for chronic studies.

A modified technique for canine sinoatrial node autoperfusion.

An experimental model for dog sinoatrial node autoperfusion is proposed. A by-pass was performed between right internal mammary artery and right atrial dorsal artery. A blood flow of 2.5 +/- 0.15 ml/min was calculated from blood forward velocity measurements. The injection of either isoproterenol or verapamil into the by-pass was followed by rapid heart rate modifications. These heart rate changes occurred without detectable modifications on the electrical behavior of the remaining myocardium, as assessed by electrocardiogram and endocavitary electrogram recordings. This model leaves right coronary circulation intact. The intrathoracic by-pass reduces heat-losses and lessens the probability of blood clotting. The model is suitable for chronic studies.

Atrioventricular block in low potassium and K dependence of the nodal resting potential.

A progressive conduction block leading to atrioventricular dissociation develops in perfused rabbit hearts within 20-30 min of exposure to Krebs containing 0.5 mM potassium (low K). A decrease in potassium permeability resulting in membrane depolarization (as seen in Purkinje fibers) could be responsible for the loss of excitability in nodal cells. We investigated the K dependence of the resting potential and the long-term effects of low K perfusion on the resting and action potentials of nodal cells in rabbit hearts. The resting potential of atrial, atrionodal, and nodal cells varied by 52, 41, and 34 mV per decade of change in Ko within the range of 5-50 mM K. Hyperpolarization of the resting membrane, a progressive decline in action potential amplitude, and a decrease in maximum rate of rise were observed in nodal fibers when exposed to low K. Loss of propagated activity occurred in the middle node within 20-30 min while the cells remained hyperpolarized. There was no evidence of electrogenic Na extrusion and it seems that the low nodal resting potential results from a high resting PNa/PK permeability ratio. The early decrease in rate of rise in low K probably reflects an increase in K-dependent outward currents, whereas the progressive deterioration and final loss of conducted electrical activity may result from an accumulation of internal Na and Ca overload produced by low K inhibition of the Na pump.

Effects of hypoxia and altered K0 on the membrane potential of rabbit ventricle.

The upstroke of the ventricular action potential in the rabbit consists of two depolarizing components with different rates of rise. The effects of hypoxia on the resting potential (RP); the upstroke phases (I and II) and the maximum rate of rise of phase I (V max) were studied at different external K concentrations (K0). Perfused hearts were submitted to N2-equilibrated media containing 1.5 to 10 mM K0. Exposure of oxygenated hearts to different K0 changed the regenerative response from a fast rising action potential at 1.5 mM K0 to a depressed fast response at 7.5 and 10 mM K0. Hypoxia decreased the action potential amplitude (APA) at all K concentrations. In K0 less than or equal to 5 mM the reduction of APA was due to a decrease in the amplitude of phase II of the upstroke but the maximum rate of rise (V max) did not change. In contrast, phase I of the upstroke was markedly depressed by hypoxia in high K0, but phase II was unmodified and its V max compared well with values reported for other normoxic cardiac cells. Hyperkalemia per se did not slow conduction during normoxia but increased conduction time in hypoxia. The resting potential of hypoxic cells was closer to the K equilibrium potential than in the control. The RP v. Ko/Ki relation suggested that electrogenic Na extrusion persists in hypoxia. The electrogenic fraction of the resting potential as determined from pump inhibition with 10(-4) M ouabain amounted to -6 mV. Our results did not indicate whether the differential effects of hypoxia on the upstroke components were potential dependent or were related to direct effects of K+ on the ionic currents that determine the action potential. The persistence of phase II during hypoxia in partly depolarized cells may assure the maintenance of propagated electrical activity under conditions that are likely to be encountered in vivo during cardiac ischemia.

Experimental arrhythmia elicited by low K perfusion.

Isolated rabbit hearts undergo ventricular depolarization when exposed to 0.5 mM K--Krebs during 30 to 45 min. A ventricular tachyarrhythmia develops when these hearts are allowed to repolarize in 1.5 mM K--Krebs. Ventricular fibrillation usually follows. These arrhythmias do not cease spontaneously. Initially, perfusion with 0.5 mM K hyperpolarized the membrane to -100 +/- 1.2 mV (means +/- SEM), decreased the plateau level, greatly increased the action potential duration, and slowed down intraventricular conduction, despite the level of membrane polarization and a high rate of rise of the upstroke. After a delay of 15 to 30 min, a progressive depolarization occurred and a stable potential level of -61 +/- 1.9 mV was reached. The arrhythmia elicited by perfusion with 1.5 mM K was of ventricular origin but did not appear when the previous exposure to 0.5 mM K was reduced to 10 min. Addition of verapamil (1.0 microM) to the 1.5 mM K medium did not prevent the early appearance of the arrhythmia, but restored driven electrical activity after variable delays despite the persistence of alterations in the action potential configuration. Verapamil also abolished the slow depolarizing phase of the upstroke. It is proposed that the abnormal electrical activity arose in a partly depolarized Purkinje network and it may have been triggered by the electrical nonhomogeneity created by the repolarization of ventricular cells in an extracellular K+ concentration of 1.5 mM.