Impaired function of inhibitory G proteins during acute myocardial ischemia of canine hearts and its reversal during reperfusion and a second period of ischemia. Possible implications for the protective mechanism of ischemic preconditioning

The effects of supplementation with omega-3 polyunsaturated Fatty acids on cardiac rhythm: anti-arrhythmic, pro-arrhythmic, both or neither? It depends…

Supplementation of omega-3 fatty acids (Ω-3) has been associated with a decreased cardiovascular risk, thereby concentrating attention on a potentially preventive effect regarding tachyarrhythmias and sudden cardiac death. However, recent randomized controlled trials challenge the efficacy of the additional application of Ω-3 and its anti-arrhythmic effect under certain clinical conditions. The present paper reflects the results of earlier and recent clinical studies with respect to the individual background conditions that may determine the clinical outcome of Ω-3 supplementation and thereby explain apparently conflicting clinical results. It is concluded that the efficacy of Ω-3 supplementation to prevent cardiac arrhythmias strongly depends on the underlying clinical and pharmacological conditions, a hypothesis that also is supported by data from experimental animal studies and by molecular interactions of Ω-3 at the cellular level.

Role of M3 receptor in aconitine/barium-chloride-induced preconditioning against arrhythmias in rats

We demonstrated here that an initial treatment with aconitine- or barium-chloride-induced arrhythmias and resulted in reduced susceptibility of the heart to the induction of arrhythmias by a repeated drug treatment 24 h after the initial one, a delayed preconditioning cardioprotection. This delayed preconditioning was accompanied by enhanced expression of cardiac muscarinic M(3) receptor and abolished by M(3)-selective antagonist. We conclude that muscarinic M(3) receptors might play an important role in conferring the pharmacological preconditioning against arrhythmias. This study thus expands our understanding of the cellular function and pathophysiological roles of muscarinic M(3) receptor and reconsolidates our view of cardioprotective effects of muscarinic M(3) receptor on myocardium.

Effects of neuropeptide Y2 receptor blockade on ventricular arrhythmias in rats with acute myocardial infarction

Excessive sympathetic activity is believed to be the key arrhythmogenic factor both in the setting of acute myocardial infarction and during chronic heart failure. The aim of this study was to evaluate the effect of neuropeptide Y2 blockade on malignant ventricular arrhythmias in rats with acute myocardial infarction. Vagotonic dose-finding study for neuropeptide Y2 receptor antagonist, (S)-N2-[2-[4-[(R,S)-5,11-dihydro-6(6h)-oxodibenz[b,e]azepin-11-yl]-1-piperazinyl]-2-oxoethyl] cylopentyl] acetyl]-N-[2-[1,2-dihydro-3,5 (4H)-dioxo-1,2-diphenyl-3H-1,2,4-triazol-4-yl]ethyl]-argininamid (AR-H05359) was conducted in guinea pigs (n=50) and rats (n=3). Induction of postinfarction arrhythmias was conducted in Sprague-Dawley rats that were randomized into 3 groups. Neuropeptide Y2 antagonist treated rats (n=7), placebo group (n=10) and amiodarone treated rats (n=8). Myocardial infarction was induced by ligation of the left coronary artery. Computerized telemetric ECG tracings were obtained continuously before induction of myocardial infarction and up to 120 min postinfarction. Occurrence of ventricular arrhythmias was analyzed according to a 10-point arrhythmia score. There was no difference in the arrhythmia scores between the neuropeptide Y2 and the saline group. The amiodarone treated animals had lower score compared to the neuropeptide Y2 and the placebo group (p<0.05). The blockade of receptors does not reduce ventricular arrhythmias in the rats with acute myocardial infarction. Further studies are needed to evaluate whether increasing vagal tonus during sympathetic activation may be valuable anti-arrhythmic strategy to prevent sudden death in patients with myocardial infarction and heart failure.

Modulation of L-type Ca2+ channels in neonatal rat heart by a novel Ca2+ channel agonist

L-type Ca2+ channels are essential in triggering the intracellular Ca2+ release and contraction in heart cells. In this study, we used patch clamp technique to compare the effect of two pure enantiomers of L-type Ca2+ channel agonists: (+)-CGP 48506 and the dihydropyridine (+)-SDZ-202 791 in cardiomyocytes from rats 2-5 days old. The predominant Ca2+ current activated by standard step pulses in these myocytes was L-type Ca2+ current. The dihydropyridine antagonist (+)-PN200-110 (5 microM) blocked over 90% of Ca2+ currents in most cells tested. CGP 48506 lead to a maximum of 200% increase in currents. The threshold concentration for the CGP effect was at 1 microM and the maximum was reached at 20 microM. SDZ-202 791 had effects in nanomolar concentrations and a maximum effect at about 2 microM. The maximal effect of (+)-SDZ-202 791 was a 400% increase in the amplitude of Ca2+ currents and was accompanied by a 10-15 mV leftward shift in the voltage dependence of activation. CGP 48506 increased the currents equally at all voltages tested. Both compounds slowed the deactivation of tail currents and lead to the appearance of slowly activating and slowly deactivating current components. However, SDZ-202 791 had larger effects on deactivation and CGP 48506 had larger effect on the rate of Ca2+ current activation. The effect of SDZ-202 791 was fully additive to that of CGP 48506 even after maximum concentrations of CGP. This observation suggests that the two Ca2+ channel agonists may act at two different sites on the L-type Ca2+ channel. We suggest that CGP 48506 would be a potential cardiotonic agent without the deleterious proarrhythmic effects attributable to the dihydropyridine agonists.

Differential effects of acidosis, high potassium concentrations, and metabolic inhibition on noradrenaline release and its presynaptic muscarinic regulation

It was the aim of the present study to characterize the effect of single components of ischaemia, such as inhibition of aerobic and anaerobic energy production by combined anoxic and glucose-free perfusion (metabolic inhibition), high extracellular potassium concentrations (hyperkalaemia), and acidosis, on (1). the stimulated release of noradrenaline from the in situ perfused guinea-pig heart and (2). its presynaptic modulation by the muscarinic agonist carbachol. The release of endogenous noradrenaline from efferent cardiac sympathetic nerve endings was induced by electrical stimulation of the left stellate ganglion (1 min, 5 V, 12 Hz) and quantified in the coronary venous effluent by high-performance liquid chromatography. Under control conditions, two consecutive electrical stimulations (S1, S2) elicited a similar noradrenaline overflow (S2/S1: 0.98 plus minus 0.05). After 10 min of global myocardial ischaemia overflow of endogenous noradrenaline was significantly reduced (S2/S1: 0.18 plus minus 0.03; P< 0.05). When studied separately, metabolic inhibition, hyperkalaemia (16 mM), and acidosis (pH 6.0) each markedly attenuated stimulated noradrenaline overflow (S2/S1: 0.65 plus minus 0.05, 0.43 plus minus 0.14, and 0.37 plus minus 0.09, respectively; P< 0.05). The muscarinic agonist carbachol (10 microM) inhibited stimulated noradrenaline release under normoxic conditions (S2/S1: 0.41 plus minus 0.07; P< 0.05). However, after 10 min of global myocardial ischaemia the inhibitory effect of carbachol on noradrenaline overflow was completely lost. Single components of ischaemia had a differential effect on presynaptic muscarinic modulation. Whereas hyperkalaemia (8-16 mM) did not affect muscarinic inhibition of noradrenaline release, carbachol lost its inhibitory effect during acidosis and metabolic inhibition. In conclusion, hyperkalaemia, metabolic inhibition, and severe acidosis each contribute to reduced overflow of noradrenaline after 10 min of myocardial ischaemia. However, presynaptic muscarinic inhibition of noradrenaline release was not affected by hyperkalaemia, but was sensitive to metabolic inhibition and low degrees of acidosis.

The antiarrhythmic effects of ischaemic preconditioning in anaesthetized dogs are prevented by atropine; role of changes in baroreceptor reflex sensitivity

1. Dogs, anaesthetized with chloralose and urethane, were subjected to a 25 min occlusion of the left anterior descending coronary artery. This resulted in ventricular ectopic activity, a reduction in baroreflex sensitivity (BRS, measured following the intravenous administration of phenylephrine), elevations in the epicardial ST-segment and increases in the degree of inhomogeneity of electrical activation, both measured from the ischaemic region of the left ventricular wall. 2. These changes were markedly reduced when the 25 min occlusion was preceded, 20 min earlier, by a 5 min (preconditioning) occlusion of the same coronary artery (e.g. VF during ischaemia reduced from 40% in the controls to 0%; P<0.05; BRS increased from 1.22+/-0.23 pre-occlusion to 1.61+/-0.25 mmHg ms(-1) post-occlusion in preconditioned dogs; cf. 1.28+/-0.29 to 0.45+/-0.12 mmHg ms(-1) respectively in the controls, P<0.05). 3. These beneficial effects of preconditioning were prevented by the administration, 10 min prior to the 25 min coronary artery occlusion, of atropine (1 mg kg(-1) i.v. followed by a continuous infusion of 0.04 mg kg(-1) h(-1)). For example, VF during occlusion was increased from 0% in the preconditioned dogs to 40% (P<0.05) in the presence of atropine and BRS was again reduced during occlusion (from 1.75+/-0.29 to 0.30+/-0.08 mmHg ms(-1); P<0.05). 4. We conclude that preconditioning reduces arrhythmia severity during ischaemia by favourably modifying cardiac autonomic receptor mechanism through enhancing vagal influences.

Effect of repeated regional myocardial ischemia in the rat heart on reperfusion arrhythmias and release of norepinephrine

We tested the hypothesis that repetitive regional myocardial ischemia in the rat could decrease reperfusion ventricular arrhythmias, possibly acting by diminished release of norepinephrine. Isolated perfused working rat hearts were pre-labeled with tritiated norepinephrine (NE3H). The efflux of 3H-labeled compounds was measured in the effluent coronary flow. Each heart was subjected to two consecutive periods of regional myocardial ischemia induced by ligature of the left coronary artery. The duration of the first ischemic period was 5 or 10 min and that of the second was 10 min. Serious rhythm disturbances did not occur during the first period of ischemia but did after reperfusion. The amount of NE3H liberated during the reperfusion period was more marked after an initial ischemic period of 10 min than after 5 min of ischemia. Reperfusion arrhythmias were of little importance after 5 min of ischemia but developed in a sustained pattern when reperfusion followed 10 min of ischemia. After 5 min of ischemia, the mean duration of reperfusion arrhythmias was 12.8 +/- 10.4 s during the first 3 min of reperfusion, but after 10 min of ischemia the mean duration of serious rhythm disturbances was 149.7 +/- 16.7 s. Reperfusion after the second 10-min occlusion increased the release of NE3H. In series 5-10, the percentage of NE3H compared with the total radioactivity was a mean of 71.4 +/- 3.3% during the 5 min of ligature, 79.0 +/- 5.3% during the first 3 min of reperfusion. During the 10-10 series in which the ligature was maintained for 10 min, the percentage of NE3H compared with the total radioactivity was 70.6 +/- 5.1%, 81.1 +/- 8.7% during the first 3 min of reperfusion. These results show no reduction of any catecholamine release or of reperfusion arrhythmias by repetitive regional ischemia and provide no evidence for any preconditioning effect after short periods of regional ischemia. The antiarrhythmic effects of repetitive myocardial ischemia such as preconditioning previously reported may depend on the exact protocols used.

Low-dose dipyridamole infusion acutely increases exercise capacity in angina pectoris: a double-blind, placebo controlled crossover stress echocardiographic study

OBJECTIVES

The aim of this study was to assess whether endogenous accumulation of adenosine, induced by low-dose dipyridamole infusion, protects from exercise-induced ischemia.

BACKGROUND

Adenosine is a recognized mediator of ischemic preconditioning in experimental settings.

METHODS

Ten patients (all men: mean age 63.4 +/- 7.3 years) with chronic stable angina, angiographically assessed coronary artery disease (n = 7) or previous myocardial infarction (n = 3) and exercise-induced ischemia underwent on different days two exercise-stress echo tests after premedication with placebo or dipyridamole (15 mg in 30 min, stopped 5 min before testing) in a double-blind, placebo controlled, randomized crossover design.

RESULTS

In comparison with placebo, dipyridamole less frequently induced chest pain (20% vs. 100%, p = 0.001) and >0.1 mV ST segment depression (50% vs. 100%, p < 0.05). Wall motion abnormalities during exercise-stress test were less frequent (placebo = 100% vs. dipyridamole = 70%, p = ns) and significantly less severe (wall motion score index at peak stress: placebo = 1.55 +/- 0.17 vs. dipyridamole = 1.27 +/- 0.2, p < 0.01) following dipyridamole, which also determined an increase in exercise time up to echocardiographic positivity (placebo = 385.9 +/- 51.4 vs. dipyridamole = 594.4 +/- 156.9 s, p < 0.01).

CONCLUSIONS

Low-dose dipyridamole infusion increases exercise tolerance in chronic stable angina, possibly by endogenous adenosine accumulation acting on high affinity A1 myocardial receptors involved in preconditioning or positively modulating coronary flow through collaterals.

Pertussis toxin abolishes the cardioprotective effect of ischemic preconditioning in intact rat heart

It has been previously demonstrated that Gi/o proteins are involved in ischemic preconditioning (IPC) in rabbits and dogs; however, there has been controversy as to the role of Gi/o proteins in IPC in in vivo rat infarct models. Therefore, the role of G proteins in the cardioprotective effect of IPC in a rat infarct model was reevaluated. Cardioprotection as indicated by infarct size (IS) as a percentage of the area at risk (IS/AAR) was determined by triphenyltetrazolium stain. The control group, which was subjected to 30 min of occlusion (Occ) and 2 h of reperfusion (Rep), had an IS/AAR of 46 +/- 6%. A single 5-min Occ followed by 10 min of Rep (1x PC) as well as three 5-min Occ periods interspersed with 5 min of Rep (3x PC) markedly reduced IS/AAR (6 +/- 1 and 8 +/- 1%, respectively). Pretreatment with pertussis toxin (10 microg/kg ip) for 48 h before 1x PC or 3x PC completely abolished their cardioprotective effects (46 +/- 5 and 38 +/- 4%, respectively). Pertussis toxin had no effect on IS/AAR and did not inactivate Gi/o proteins as assessed by an in vitro ADP-ribosylation assay of heart homogenates. These results demonstrate that the cardioprotective effect of IPC is mediated by a small subpopulation of Gi/o proteins in the intact rat heart.

Ischemic preconditioning: exploring the paradox

Brief transient episodes of nonlethal myocardial ischemia protect or "precondition" the heart and render the myocardium resistant to a subsequent more sustained ischemic insult. The hallmark of this phenomenon--documented in virtually all species and experimental models evaluated to date in countless laboratories worldwide--is the profound reduction in infarct size seen in preconditioned groups versus time-matched controls. Efforts to identify the cellular mechanisms responsible for this paradoxical ischemia-induced cardioprotection, to expand the definition of ischemic preconditioning beyond infarct size reduction, and, perhaps most importantly, to evaluate the efficacy of preconditioning in disease models and in the clinical setting, are all topics of intensive ongoing investigation.

Differential presynaptic modulation of noradrenaline release in human atrial tissue in normoxia and anoxia

1. Presynaptic modulation of noradrenaline release in human atrial tissue specimens was investigated under normoxic and anoxic conditions. 2. Noradrenaline release was induced by electrical stimulation and release during experimental intervention (S2) was compared with release during a preceding control stimulation (S1). The results were expressed as the geometric means and 95% confidence intervals of the S2/S1 ratio. 3. The alpha 2-adrenoceptor agonist, UK 14304 (0.1 mumol-1) significantly inhibited noradrenaline release, resulting in a S2/S1 ratio of 0.49 (0.40-0.59), and the a 2-adrenoceptor antagonist, yohimbine (1 mumol l-1) increased noradrenaline release (S2/S1 1.83 [1.43-2.35]) during normoxia. Both compounds were ineffective during anoxia. 4. Adenosine (30 mumol-1) inhibited noradrenaline release with a S2/S1 ratio of 0.54 (0.42-0.66). The adenosine antagonist, 8-phenyltheophylline, alone had no effect during normoxia. During anoxia, neither adenosine nor 8-phenyltheophylline altered noradrenaline release. 5. The beta 2-adrenoceptor agonist, terbutaline (1 mumol l-1) increased (1.53 [1.14-2.01]) and the beta-adrenoceptor antagonist, pindolol (1 mumol l-1) suppressed noradrenaline release (0.62 [0.49-0.79]) under normoxic conditions. During anoxia, pindolol significantly inhibited noradrenaline release with a S2/S1 ratio of 0.66 (0.51-0.85), whereas terbutaline did not influence noradrenaline release. 6. Angiotensin II (0.1 mumol l-1 enhanced noradrenaline release resulting in a S2/S1 ratio of 1.44 (1.34-1.54), while the angiotensin II antagonist, losartan (1 mumol l-1) had no effect on noradrenaline release during normoxia. Conversely, angiotensin II did not increase noradrenaline release and losartan significantly inhibited noradrenaline release to a S2/S1 ratio of 0.60 (0.46-0.77) during anoxia. 7. In conclusion, human cardiac tissue possesses presynaptic inhibitory alpha 2-adrenoceptors and adenosine receptors, as well as facilitatory beta 2-adrenoceptors and angiotensin II receptors regulating noradrenaline release under normoxic conditions. During anoxia the responses to alpha 2-adrenoceptors and adenosine receptor stimulation are lost, whereas facilitatory responses to beta 2-adrenoceptors and adenosine II receptor stimulation are maintained and these receptors appear to be maximally stimulated. This differential presynaptic modulation in anoxia may contribute to enhanced sympathetic activity in ischaemia.

Signal transduction in myocardial ischaemia and reperfusion

Recent studies in the non-ischaemic myocardium indicated that drugs stimulating cAMP formation inhibit alpha 1-mediated inositol phosphate generation, while alpha 1-adrenergic stimulation lowered tissue cAMP levels, implicating cross-talk between alpha 1- and beta-adrenergic signalling pathways in normal physiological conditions. Massive amounts of endogenous catecholamines, predominantly noradrenaline, are released during myocardial ischaemia and reperfusion, causing stimulation of both alpha 1- and beta-adrenergic receptors which, in turn, may contribute to intracellular Ca2+ overload and subsequent cell damage. Since no information is available regarding cross-talk in pathophysiological conditions, the aim of this study was to evaluate the interactions between alpha 1- and beta-adrenergic signalling pathways during different periods of ischaemia and reperfusion. Isolated rat hearts were perfused retrogradely for 30 min before being subjected to (i) 5-25 min global ischaemia and (ii) 1-5 min of reperfusion after 20 min global ischaemia. Drugs (prazosin, 10(-7) M; propranolol, 10(-6) M; phenylephrine 3 x 10(-5) M; isoproterenol 10(-9) M) were added 10 min before the onset of ischaemia and were present during reperfusion. Increasing periods of ischaemia caused an immediate rise and progressive lowering in tissue cAMP and Ins(1,4,5)P3 levels respectively. In contrast, reperfusion caused an elevation in Ins(1,4,5)P3 levels and reduced cAMP. Prazosin elevated cAMP levels during both ischaemia and reperfusion, while propranolol had no effects on tissue Ins(1,4,5)P3. The activity of the alpha 1-adrenergic signal transduction pathway appears to have an inhibitory effect on the activity of the beta-adrenergic system during ischaemia and reperfusion.

The guanine nucleotide-binding regulatory proteins (G proteins) in myocardium with ischemia

Guanine nucleotide-binding regulatory proteins (G proteins) play a major role in the regulation of a number of physiological processes, such as stimulation or Inhibition of adenylate cyclase activity or gaiting of ionic channels. Myocardial ischemia could induce the changes in receptor-G protein signal transduction system in the heart. Therefore, this article will focus on the role and alterations of G proteins (especially, Gs and Gi) in myocardial ischemia. The Gi protein rapidly loses functional activity during very early myocardial ischemia. In contrast to Gi protein, the function of Gs protein during this phase has not been evaluated. Moreover, the changes in Gs protein after 30 min of ischemia are contradictory. However, the sensitization of the adenylate cyclase activity in the very early phase of acute ischemia is gradually replaced by a decrease in adenylate cyclase activity with prolonged ischemia. The decrease in the function and amount of Gs protein may be one of the factors that induce these changes. The function of Gs protein was also decreased in the canine hearts with ischemia and reperfusion. In contrast to ischemia and reperfusion, there are no significant alterations in G proteins and modulation of adenylate cyclase in the stunned myocardium. It has become increasingly evident that Gi protein may play an important role in the cardioprotective effects of preconditioning. When beta-adrenoreceptor densities are reduced in chronic myocardial ischemia, decreased in the amount and function of Gi protein and increased amount of Gs protein may play the role in preservation of the adenylate cyclase activity. These alterations in G proteins may play the important role in the myocardial function during myocardial ischemia.

A comparison between ischemic preconditioning and anti-adrenergic interventions: cAMP, energy metabolism and functional recovery

OBJECTIVES

The postulate that ischemic preconditioning caused an attenuation in ischemia induced increases in tissue cAMP, and that this may pertain to the mechanism of ischemic preconditioning, was investigated in the isolated rat heart. A significant reduction in tissue cAMP in preconditioned hearts was observed for all time periods of global ischemia studied. The significance of this observation was evaluated by comparing the effect of anti-adrenergic interventions on energy metabolism and post-ischemic functional recovery of both non-preconditioned and preconditioned hearts.

METHODS

The isolated perfused rat heart was used as experimental model. Six groups were studied: Non-preconditioned rat hearts: i) untreated controls (Non-PC), ii) reserpinised (Non-PC Res), iii) propranolol treated (10(-7) M) (Non-PC Prop); Preconditioned rat hearts: iv) preconditioned controls (PC), v) reserpinised (PC Res) and vi) propranolol (10(-7) M) treated (PC Prop).

RESULTS

After 25 min global ischemia the concentration of cAMP was increased by 79.6% in the Non-PC group. This increase was attenuated in all of the treated groups, although in varying degrees. Energy utilization in these hearts also differed markedly between the groups. Functional recovery was however similar in all Non-PC and PC treated groups and significantly superior to that of Non-PC control hearts. Prior reserpinisation mimicked the protective effect of preconditioning on energy metabolism and functional recovery. To determine the significance of attenuation of the increase in cAMP in the protection conferred by preconditioning, hearts were pretreated with forskolin (10(-6) M). This caused an accumulation of tissue cAMP in preconditioned hearts to similar absolute values as seen in untreated non-preconditioned hearts during 25 min global ischemia. However, the percentage increase in forskolin-pretreated preconditioned hearts during sustained ischemia was only 50% vs. 71% in non-preconditioned hearts treated with forskolin, confirming an attenuated beta-response induced by preconditioning. Forskolin treatment of preconditioned hearts did not abolish the protective effect.

CONCLUSIONS

The findings suggest that the protection against ischemic damage conferred by preconditioning is associated with an attenuated beta-adrenergic response. However, whether the changes in cAMP occurring during sustained global ischemia is the cause of consequence of the elicited protection, remains to be established.

Stunned myocardium, an opinionated review

Brief coronary occlusions cause stunning and ischemic preconditioning, the molecular mechanisms of which are insufficiently known. A marked reprogramming of sarcolemmal functions (including those of the sarcoplasmic reticulum) seems to occur and may be the basis for many of the observed phenomena. A bewilderingly complex pattern of gene expression emerges from mRNA-studies with reperfused tissue which at present does not permit a focussed or coherent view of mechanisms.