Effect of transmural versus nontransmural myocardial infarction on inducibility of ventricular arrhythmias during sympathetic stimulation in dogs

Effect of proton pump inhibitors on sympathetic hyperinnervation in infarcted rats: Role of magnesium

The long-term use of proton pump inhibitors (PPIs) has been shown to increase the risk of cardiovascular mortality, however the molecular mechanisms are unknown. Superoxide has been implicated in the regulation of nerve growth factor (NGF), a mediator of sympathetic innervation. The purpose of this study was to determine whether PPIs increase ventricular arrhythmias through magnesium-mediated superoxide production in infarcted rats. Male Wistar rats were randomly assigned to receive vehicle, omeprazole, omeprazole + magnesium sulfate, or famotidine treatment for 4 weeks starting 24 hours after the induction of myocardial infarction by ligating the coronary artery. Increased myocardial superoxide and nitrotyrosine levels were noted post-infarction, in addition to a significant upregulation of NGF expression on mRNA and protein levels. Sympathetic hyperinnervation after infarction was confirmed by measuring myocardial norepinephrine and immunofluorescent analysis. Compared with the vehicle, omeprazole-treated infarcted rats had significantly reduced myocardial magnesium content, increased oxidant production, and increased sympathetic innervation, which in turn increased ventricular arrhythmias. These effects were prevented by the coadministration of magnesium sulfate. In an in vivo study, an omeprazole-induced increase in NGF was associated with a superoxide pathway, which was further confirmed by an ex vivo study showing the attenuation of NGF levels after coadministration of the superoxide scavenger Tiron. Magnesium sulfate did not further attenuate NGF levels compared with omeprazole + Tiron. Our results indicate that the long-term administration of PPIs was associated with reduced tissue magnesium content and increased myocardial superoxide production, which exacerbated ventricular arrhythmias after infarction. Magnesium may be a potential target for PPI-related arrhythmias after infarction.

Inhibition of infarction-induced sympathetic innervation with endothelin receptor antagonism via a PI3K/GSK-3β-dependent pathway

Although endothelin (ET)-1 has been shown to upregulate nerve growth factor (NGF) expression, the molecular mechanisms are largely unknown. Phosphatidylinositol 3-kinase (PI3K)/Akt/glycogen synthase kinase (GSK)-3β signal has been implicated in the regulation of NGF. We investigated whether selective ET receptor blockers attenuated cardiac sympathetic reinnervation through restoring PI3K/Akt/GSK-3β activity. After ligation of the left anterior descending artery, male Wistar rats were randomized to either vehicle, atrasentan (an ET_A receptor antagonist) or A-192621 (an ET_B receptor antagonist) for 4 weeks. Sympathetic hyperinnervation after infarction was confirmed by myocardial norepinephrine measurement and immunofluorescent analysis. Post infarction was associated with increased reactive oxygen species (ROS), as measured by myocardial superoxide levels and dihydroethidine fluorescence staining. This was paralleled by a significant upregulation of NGF expression on mRNA and protein levels in the vehicle-treated rats, which reduced after administering atrasentan, not A-192621. Arrhythmic scores in the vehicle-treated rats were significantly higher than those treated with atrasentan. In an in vivo study atrasentan-induced decreased NGF was associated with activation of PI3K/Akt signaling pathway, which was further confirmed by the ex vivo study showing the restoration of NGF levels after coadministration of PI3K inhibitors (wortmannin and LY294002). Lithium chloride, an inhibitor of GSK-3β, did not provide additional attenuated NGF levels compared with atrasentan alone. Finally, atrasentan-attenuated NGF levels were reversed in the presence of peroxynitrite generator. ET_A receptor antagonism is a mediator to attenuate sympathetic hyperinnervation probably through restoration of PI3K/Akt/GSK-3β/ROS signaling pathway, a potential pharmacological target for arrhythmias after infarction.

Inhibition of glycogen synthase kinase-3β prevents sympathetic hyperinnervation in infarcted rats

We have demonstrated that nerve growth factor (NGF) expression in the myocardium is selectively increased during chronic stage of myocardial infarction, resulting in sympathetic hyperinnervation. Glycogen synthase kinase-3 (GSK-3) signal has been shown to play key roles in the regulation of cytoskeletal assembly during axon regeneration. We assessed whether lithium, a GSK-3 inhibitor, attenuates cardiac sympathetic reinnervation after myocardial infarction through attenuated NGF expression and Tau expression. Twenty-four hours after ligation of the anterior descending artery, male Wistar rats were randomized to either LiCl or SB216763, chemically unrelated inhibitors of GSK-3β, a combination of LiCl and SB216763, or vehicle for four weeks. Myocardial norepinephrine levels revealed a significant elevation in vehicle-treated rats compared with sham-operated rats, consistent with excessive sympathetic reinnervation after infarction. Immunohistochemical analysis for sympathetic nerve also confirmed the change of myocardial norepinephrine. This was paralleled by a significant upregulation of NGF protein and mRNA in the vehicle-treated rats, which was reduced after administering either LiCl, SB216763, or combination. Arrhythmic scores during programmed stimulation in the vehicle-treated rats were significantly higher than those treated with GSK-3 inhibitors. Addition of SB216763 did not have additional beneficial effects compared with those seen in rats treated with LiCl alone. Furthermore, lithium treatment increased Tau1 and decreased AT8 and AT180 levels. Chronic use of lithium after infarction, resulting in attenuated sympathetic reinnervation by GSK-3 inhibition, may modify the arrhythmogenic response to programmed electrical stimulation.

Antiarrhythmic effect of lithium in rats after myocardial infarction by activation of Nrf2/HO-1 signaling

Glycogen synthase kinase-3 (GSK-3) signaling has been shown to play a role in the regulation of nuclear factor erythroid-2-related factor 2 (Nrf2), a master regulator of antioxidant genes, including heme oxygenase-1 (HO-1). We assessed whether lithium, a GSK-3 inhibitor, attenuates cardiac sympathetic reinnervation after myocardial infarction, a status of high reactive oxygen species (ROS), by attenuating nerve growth factor (NGF) expression and whether Nrf2/HO-1 signaling is involved in the protection. Twenty-four hours after ligation of the left anterior descending artery, male Wistar rats were treated for 4 weeks. The postinfarction period was associated with increased oxidative-nitrosative stress, as measured by myocardial superoxide, nitrotyrosine, and dihydroethidium fluorescent staining. In concert, myocardial norepinephrine levels and immunohistochemical analysis of sympathetic nerve revealed a significant increase in innervation in vehicle-treated rats compared with sham-operated rats. Arrhythmic scores during programmed stimulation in the vehicle-treated rats were significantly higher than those in sham. This was paralleled by a significant upregulation of NGF protein and mRNA in the vehicle-treated rats, which was reduced after administration of LiCl. LiCl stimulated the nuclear translocation of Nrf2 and the transactivation of the Nrf2 target gene HO-1. Inhibition of phosphoinositide 3-kinase by wortmannin reduced the increase in Nrf2 nucleus translocation and HO-1 expression compared with lithium alone. In addition, the lithium-attenuated NGF levels were reversed in the presence of the Nrf2 inhibitor trigonelline, HO-1 inhibitor SnPP, and peroxynitrite generator SIN-1, indicating the role of Nrf2/HO-1/ROS. In conclusion, lithium protects against ventricular arrhythmias by attenuating NGF-induced sympathetic innervation via antioxidant activation of the Nrf2/HO-1 axis.

Sitagliptin attenuates sympathetic innervation via modulating reactive oxygen species and interstitial adenosine in infarcted rat hearts

We investigated whether sitagliptin, a dipeptidyl peptidase-4 (DPP-4) inhibitor, attenuates arrhythmias through inhibiting nerve growth factor (NGF) expression in post-infarcted normoglycemic rats, focusing on adenosine and reactive oxygen species production. DPP-4 bound adenosine deaminase has been shown to catalyse extracellular adenosine to inosine. DPP-4 inhibitors increased adenosine levels by inhibiting the complex formation. Normoglycemic male Wistar rats were subjected to coronary ligation and then randomized to either saline or sitagliptin in in vivo and ex vivo studies. Post-infarction was associated with increased oxidative stress, as measured by myocardial superoxide, nitrotyrosine and dihydroethidium fluorescent staining. Measurement of myocardial norepinephrine levels revealed a significant elevation in vehicle-treated infarcted rats compared with sham. Compared with vehicle, infarcted rats treated with sitagliptin significantly increased interstitial adenosine levels and attenuated oxidative stress. Sympathetic hyperinnervation was blunted after administering sitagliptin, as assessed by immunofluorescent analysis and western blotting and real-time quantitative RT-PCR of NGF. Arrhythmic scores in the sitagliptin-treated infarcted rats were significantly lower than those in vehicle. Ex vivo studies showed a similar effect of erythro-9-(2-hydroxy-3-nonyl) adenine (an adenosine deaminase inhibitor) to sitagliptin on attenuated levels of superoxide and NGF. Furthermore, the beneficial effects of sitagliptin on superoxide anion production and NGF levels can be reversed by 8-cyclopentyl-1,3-dipropulxanthine (adenosine A₁ receptor antagonist) and exogenous hypoxanthine. Sitagliptin protects ventricular arrhythmias by attenuating sympathetic innervation via adenosine A₁ receptor and xanthine oxidase-dependent pathways, which converge through the attenuated formation of superoxide in the non-diabetic infarcted rats.

Both PKA and Epac pathways mediate N-acetylcysteine-induced Connexin43 preservation in rats with myocardial infarction

Cardiac remodeling was shown to be associated with reduced gap junction expression after myocardial infarction. A reduction in gap junctional proteins between myocytes may trigger ventricular arrhythmia. Therefore, we investigated whether N-acetylcysteine exerted antiarrhythmic effect by preserving connexin43 expression in postinfarcted rats, focusing on cAMP downstream molecules such as protein kinase A (PKA) and exchange protein directly activated by cAMP (Epac). Male Wistar rats after ligating coronary artery were randomized to either vehicle, or N-acetylcysteine for 4 weeks starting 24 hours after operation. Infarct size was similar between two groups. Compared with vehicle, cAMP levels were increased by N-acetylcysteine treatment after infarction. Myocardial connexin43 expression was significantly decreased in vehicle-treated infarcted rats compared with sham operated rats. Attenuated connexin43 expression and function were blunted after administering N-acetylcysteine, assessed by immunofluorescent analysis, dye coupling, Western blotting, and real-time quantitative RT-PCR of connexin43. Arrhythmic scores during programmed stimulation in the N-acetylcysteine-treated rats were significantly lower than those treated with vehicle. In an ex vivo study, enhanced connexin43 levels afforded by N-acetylcysteine were partially blocked by either H-89 (a PKA inhibitor) or brefeldin A (an Epac-signaling inhibitor) and completely blocked when H-89 and brefeldin A were given in combination. Addition of either the PKA specific activator N6Bz or Epac specific activator 8-CPT did not have additional increased connexin43 levels compared with rats treated with lithium chloride alone. These findings suggest that N-acetylcysteine protects ventricular arrhythmias by attenuating reduced connexin43 expression and function via both PKA- and Epac-dependent pathways, which converge through the inactivation of glycogen synthase kinase-3β.

K ATP channel agonists preserve connexin43 protein in infarcted rats by a protein kinase C-dependent pathway

Downward remodelling of gap junctional proteins between myocytes may trigger ventricular arrhythmia after myocardial infarction. We have demonstrated that ATP-sensitive potassium (K_ATP) channel agonists attenuated post-infarction arrhythmias. However, the involved mechanisms remain unclear. The purpose of this study was to determine whether K_ATP channel agonists can attenuate arrhythmias through preserving protein kinase C (PKC)-–dependent connexin43 level after myocardial infarction. Male Wistar rats after ligating coronary artery were randomized to either vehicle, nicorandil, pinacidil, glibenclamide or a combination of nicorandil and glibenclamide or pinacidil and glibenclamide for 4 weeks. To elucidate the role of PKC in the modulation of connexin43 level, carbachol and myristoylated PKC V1–2 peptide were also assessed. Myocardial connexin43 level was significantly decreased in vehicle-treated infarcted rats compared with sham. Attenuated connexin43 level was blunted after administering K_ATP channel agonists, assessed by immunofluorescent analysis, Western blotting, and real-time quantitative reverse transcription-PCR of connexin43. Arrhythmic scores during programmed stimulation in the K_ATP channel agonists-treated rats were significantly lower than those treated with vehicle. The beneficial effects of K_ATP channel agonists were blocked by either glibenclamide or 5-hydroxydecanoate. Addition of the PKC activator, phorbol 12-myristate 13-acetate and the specific PKC agonist, carbachol, blocked the effects of nicorandil on connexin43 phosphorylation and dye permeability. The specific PKC antagonist, myristoylated PKC V1–2 peptide, did not have additional beneficial effects on connexin43 phosphorylation compared with rats treated with nicorandil alone. Chronic use of K_ATP channel agonists after infarction, resulting in enhanced connexin43 level through a PKC-dependent pathway, may attenuate the arrhythmogenic response to programmed electrical stimulation.

The role of the autonomic nervous system in sudden cardiac death

The autonomic nervous system exerts a modulating effect on the risk of sudden cardiac death (SCD) in the setting of ischemic heart disease. The mechanism by which sympathetic tone increases the risk of ventricular arrhythmias is not known, though regional sympathetic denervation at and apical to the site of transmural infarction may result in regional supersensitivity to circulating catecholamines and play a role in ventricular arrhythmogenesis. [(123)I]MIBG scintigraphy enables noninvasive determination of regional cardiac denervation and may be a useful tool for probing the role of sympathetic nervous system in SCD. Increased vagal tone is generally protective against SCD. Newer tests such as baroreflex slope testing and various techniques for determination of heart rate variability, which provide indices of vagal tone, may have greater predictive value and are powerful tools in assessing the role of autonomic nervous system in SCD.

Differential effects of NADPH oxidase and xanthine oxidase inhibition on sympathetic reinnervation in postinfarct rat hearts

Superoxide has been shown to play a major role in ventricular remodeling and arrhythmias after myocardial infarction. However, the source of increased myocardial superoxide production and the role of superoxide in sympathetic innervation remain to be further characterized. Male Wistar rats, after coronary artery ligation, were randomized to vehicle, allopurinol, or apocynin for 4weeks. To determine the role of peroxynitrite in sympathetic reinnervation, we also used 3-morpholinosydnonimine (a peroxynitrite generator). The postinfarction period was associated with increased oxidative stress, as measured by myocardial superoxide, nitrotyrosine, xanthine oxidase activity, NADPH oxidase activity, and dihydroethidium fluorescent staining. Measurement of myocardial norepinephrine levels revealed a significant elevation in vehicle-treated infarcted rats compared with sham. Sympathetic hyperinnervation was blunted after administration of allopurinol. Arrhythmic scores in the allopurinol-treated infarcted rats were significantly lower than those in vehicle. For similar levels of ventricular remodeling, apocynin had no beneficial effects on oxidative stress, sympathetic hyperinnervation, or arrhythmia vulnerability. Allopurinol-treated hearts had significantly decreased nerve growth factor expression, which was substantially increased after coadministration of 3-morpholinosydnonimine. These results indicate that xanthine oxidase but not NADPH oxidase largely mediates superoxide production after myocardial infarction. Xanthine oxidase inhibition ameliorates sympathetic innervation and arrhythmias possibly via inhibition of the peroxynitrite-mediated nerve growth factor pathway.

Effect of pravastatin on ventricular arrhythmias in infarcted rats: role of connexin43

Epidemiologic studies showed that men treated with statins appear to have a lower incidence of sudden death than men without statins. However, the specific factor for this remained disappointingly elusive. We assessed whether pravastatin enhanced connexin43 expression after myocardial infarction through attenuation of endothelin-1. Twenty-four hours after ligation of the anterior descending artery, male Wistar rats were randomized to vehicle, pravastatin, mevalonate, bosentan, or a combination of pravastatin and mevalonate or pravastatin and bosentan for 4 wk. Myocardial endothelin-1 levels were significantly elevated in vehicle-treated rats at the border zone compared with sham-operated rats. Myocardial connexin43 expression at the border zone was significantly decreased in vehicle-treated infarcted rats compared with sham-operated rats. Attenuated connexin43 expression was blunted after administration of pravastatin, as assessed by immunofluorescence analysis, Western blotting, and real-time quantitative RT-PCR of connexin43. Bosentan enhanced connexin43 amount in infarcted rats and did not have additional beneficial effects on pravastatin-treated rats. Arrhythmic scores during programmed stimulation in vehicle-treated rats were significantly higher than scores in those treated with pravastatin. In contrast, the beneficial effects of pravastatin-induced connexin43 were abolished by the addition of mevalonate and a protein kinase C inducer. In addition, the amount of connexin43 showed significant increase after addition of bisindolylmaleimide, implicating that protein kinase C is a relevant target in endothelin-1-mediated connexin43 expression. Thus chronic use of pravastatin after infarction, resulting in enhanced connexin43 amount by attenuation of mevalonate-dependent endothelin-1 through a protein kinase C-dependent pathway, may attenuate the arrhythmogenic response to programmed electrical stimulation.

17beta-Estradiol decreases vulnerability to ventricular arrhythmias by preserving connexin43 protein in infarcted rats

Epidemiological studies showed that a lower mortality rate of sudden cardiac death among women than among men may depend on the action of female sex hormones. This study assessed whether 17beta-estradiol exerts anti-arrhythmic effects through enhanced Connexin43 (Cx43) expression after infarction. Two weeks after ovariectomy, female Wistar rats were randomly assigned to coronary artery ligation or sham-operation. Twenty-four hours after coronary ligation, ovariectomized rats were randomized into vehicle, subcutaneous estradiol treatment, tamoxifen, or subcutaneous estradiol treatment+tamoxifen and followed for 4weeks. To verify the role of estradiol-related nitric oxide in modulating the expression of Cx43, N-nitro-L-arginine methyl ester was also assessed in an in vitro study. Myocardial Cx43 expression revealed a significant decrease in vehicle-treated infarcted rats compared with sham-operated rats at 24h and 4weeks after infarction. Attenuated Cx43 expression was blunted after administering estradiol, assessed by immunofluorescent analysis, Western blotting, and real-time quantitative RT-PCR of Cx43. The vulnerability for ventricular arrhythmia during programmed stimulation in estradiol-treated infarcted rats was significantly lower than in vehicle-treated infarcted rats. The beneficial effect of estradiol on Cx43 was abolished by tamoxifen. In addition, the invitro study demonstrated that the amount of Cx43 showed significant reduction after adding N-nitro-L-arginine methyl ester. Chronic administration of estradiol after infarction is associated with attenuated reduction of gap junction proteins probably through a nitric oxide-dependent pathway via the estrogen receptor and thus plays a critical role in the beneficial effect on arrhythmic vulnerability response to programmed electrical stimulation.

Granulocyte colony-stimulating factor increases sympathetic reinnervation and the arrhythmogenic response to programmed electrical stimulation after myocardial infarction in rats

Granulocyte colony-stimulating factor (G-CSF) has been used for the repair of infarcted myocardium, but concerns have been raised regarding its proarrhythmic potential. We analyzed the influence of G-CSF treatment on sympathetic nerve remodeling and the expression of nestin in a rat model of experimental myocardial infarction (MI). Twenty-four hours after ligation of the anterior descending artery, male Wistar rats were randomized to receive either saline (MI/C) or G-CSF (MI/G) for 5 days. At 56 days after infarction, MI/G rats had a significantly higher left ventricular ejection fraction accompanied by a significant decrease in the left ventricular end-diastolic dimension than the MI/C group. Myocardial norepinephrine levels revealed a significant elevation in MI/G rats in the border zone compared with MI/C rats. Immunohistochemical analysis for tyrosine hydroxylase, growth-associated protein 43, and neurofilament also confirmed the changes of myocardial norepinephrine. At 5 days after infarction, MI/G rats had increased numbers of tissue-infiltrated CD34(+) cells, although a similar increase in circulating neutrophil counts between sham-operated rats treated with G-CSF and MI/G rats was observed. Compared with MI/C rats, MI/G rats showed an increase of nestin and nerve growth factor expression, as assessed by protein expression and mRNA levels. The arrhythmia scores during programmed stimulation were significantly higher in MI/G rats than in MI/C rats, suggesting proarrhythmic potential. These findings suggest that, although G-CSF administration after infarction improved myocardial function, it resulted in the activation of nestin and nerve growth factor expression and increased sympathetic reinnervation, which may increase the arrhythmogenic response to programmed electrical stimulation.

Effect of ATP-sensitive potassium channel agonists on sympathetic hyperinnervation in postinfarcted rat hearts

Although the acute administration of ATP-sensitive potassium (KATP) channel agonists provides a neuroprotection, it is unclear whether similar benefits are found by modulating sympathetic innervation in chronic settings after myocardial infarction. We assessed whether KATP channel agonists can attenuate the sprouting of cardiac sympathetic nerves after infarction. Male Wistar rats after ligating coronary artery were randomized to either saline, nicorandil, pinacidil, glibenclamide, or a combination of 1) nicorandil and glibenclamide or 2) pinacidil and glibenclamide for 4 wk. To elucidate the role of mitochondrial KATP channels in modulating nerve growth factor, 5-hydroxydecanoate was assessed in an in vitro model. The measurement of myocardial norepinephrine levels revealed a significant elevation in saline-treated infarcted rats compared with sham-operated rats, consistent with excessive sympathetic innervation. Excessive sympathetic innervation was blunted after giving the rats either nicorandil or pinacidil, compared with saline, as assessed by the immunohistochemical analysis of tyrosine hydroxylase, growth associated protein-43, and neurofilament and Western blot analysis and real-time quantitative RT-PCR of nerve growth factor. The arrhythmic scores during programmed stimulation in the saline- or glibenclamide-treated infarcted rats were significantly higher than those of rats treated with KATP channel agonists. In contrast, the beneficial effects of nicorandil and pinacidil were abolished by administering either glibenclamide or 5-hydroxydecanoate. The sympathetic hyperinnervation after infarction is attenuated by the activation of mitochondrial KATP channels. The chronic use of mitochondrial KATP channel agonists after infarction may attenuate the arrhythmogenic response to programmed electrical stimulation.

Effect of endothelin receptor antagonists on ventricular susceptibility in postinfarcted rats

This study investigated whether selective endothelin (ET) type A (ETA) or nonselective ETA/ETB receptor blockade exerted antiarrhythmic effects through attenuated sympathetic reinnervation after infarction. Twenty-four hours after ligation of the left anterior descending artery, male Wistar rats received either vehicle, ABT-627 (selective ETA receptor antagonist), bosentan (nonselective ETA/ETB receptor antagonist), or hydralazine for 4 wk. The measurement of myocardial ET-1 levels at the remote zone revealed a significant increase in vehicle-treated infarcted rats compared with sham-operated rats, consistent with increased activities of ET-1 after infarction. Sympathetic nerve function changes assessed by the norepinephrine content of myocardium and the dialysate and plasma dihydroxyphenylglycol levels were parallel to ET-1 levels. Immunohistochemical analysis for tyrosine hydroxylase, growth-associated protein 43, and neurofilament also confirmed the change of nerve function. This was accompanied with a significant upregulation of nerve growth factor protein expression and mRNA in the vehicle-treated infarcted rats, which reduced after the administration of either ETA or ETA/ETB blockade to a similar extent. The beneficial effects of ET receptor antagonists on sympathetic nerve function and structures were dissociated from their blood pressure-lowering effect because ET receptor antagonists and hydralazine reduced arterial pressure similarly. Arrhythmic severity during programmed stimulation in ET receptor antagonists-treated rats was significantly lower than that in vehicle-treated infarcted rats. Our data indicate that the ET system, especially via ETA receptors, plays an important role in attenuating sympathetic reinnervation after infarction. Independent of their hemodynamic effects, a chronic use of either ETA or ETA/ETB antagonists may modify the arrhythmogenic response to programmed electrical stimulation.

Physiological concentration of 17beta-estradiol on sympathetic reinnervation in ovariectomized infarcted rats

17beta-Estradiol (E2) has been shown to exert antiarrhythmic effect after myocardial infarction; however, the mechanisms remain unclear. This study was performed to determine whether E2 exerts beneficial effects through attenuated sympathetic hyperreinnervation after infarction. Two weeks after ovariectomy, female Wistar rats were assigned to coronary artery ligation or sham operation. Twenty-four hours after coronary ligation, rats underwent one of five treatments: 1) sc vehicle treatment (control), 2) sc E2 treatment, 3) sc E2 treatment + tamoxifen (a potent estrogen receptor antagonist), 4) bosentan (an endothelin receptor blocker), or 5) sc E2 treatment + bosentan and followed for 4 wk. Myocardial endothelin-1 and norepinephrine levels at the remote zone revealed a significant elevation in control infarcted rats, compared with sham-operated rats, which is consistent with sympathetic hyperinnervation after infarction. Sympathetic hyperinnervation was blunted after giving the rats either E2 or bosentan, assessed by immunohistochemical analysis of tyrosine hydroxylase, growth-associated protein 43 and neurofilament, and Western blotting and real-time quantitative RT-PCR of nerve growth factor. Arrhythmic scores during programmed stimulation in E2-treated infarcted rats were significantly lower than in control-infarcted rats. Addition of bosentan did not have additional beneficial effects, compared with rats treated with E2 alone. The beneficial effect of E2 on sympathetic hyperinnervation was abolished by tamoxifen. Our data indicated that E2 has a role for sympathetic hyperinnervation after infarction, probably through an endothelin-1-depedent pathway. Chronic administration of E2 after infarction may attenuate the arrhythmogenic response to programmed electrical stimulation.

Effect of pravastatin on sympathetic reinnervation in postinfarcted rats

We assessed whether pravastatin attenuates cardiac sympathetic reinnervation after myocardial infarction through the activation of ATP-sensitive K+(KATP) channels. Epidemiological studies have shown that men treated with statins appear to have a lower incidence of sudden death than men without statins. However, the specific factor for this has remained disappointingly elusive. Twenty-four hours after ligation of the anterior descending artery, male Wistar rats were randomized to groups treated with either vehicle, nicorandil (a specific mitochondrial KATPchannel agonist), pinacidil (a nonspecific KATPchannel agonist), pravastatin, glibenclamide (a KATPchannel blocker), or a combination of nicorandil and glibenclamide, pinacidil and glibenclamide, or pravastatin and glibenclamide for 4 wk. Myocardial norepinephrine levels revealed a significant elevation in vehicle-treated rats at the remote zone compared with sham-operated rats (2.54 ± 0.17 vs. 1.26 ± 0.36 μg/g protein, P < 0.0001), consistent with excessive sympathetic reinnervation after infarction. Immunohistochemical analysis for tyrosine hydroxylase, growth-associated factor 43, and neurofilament also confirmed the change of myocardial norepinephrine. This was paralleled by a significant upregulation of tyrosine hydroxylase protein expression and mRNA in vehicle-treated rats, which was reduced after the administration of either nicorandil, pinacidil, or pravastatin. Arrhythmic scores during programmed stimulation in vehicle-treated rats were significantly higher than those treated with pravastatin. In contrast, the beneficial effects of pravastatin were reversed by the addition of glibenclamide, implicating KATPchannels as the relevant target. The sympathetic reinnervation after infarction is modulated by the activation of KATPchannels. Chronic use of pravastatin after infarction, resulting in attenuated sympathetic reinnervation by the activation of KATPchannels, may modify the arrhythomogenic response to programmed electrical stimulation.

Beta blockers as anti-arrhythmic agents

Although beta-adrenergic blocking agents are not always considered anti-arrhythmic drugs, the results of several recent trials have suggested an anti-arrhythmic mechanism for at least part of their mortality benefit in the treatment of chronic congestive heart failure. We review background experimental and clinical evidence for the anti-arrhythmic actions of beta-blockers and then review the results of published beta-blocker heart failure trials. A majority of trials showed improvement in overall survival as well as reduction in sudden death and ventricular arrhythmias with beta-blocker treatment. Although different effects were seen with different specific agents, these trials overall support a clinically significant anti-arrhythmic effect of several beta-blockers.

Wavelet transform analysis of heart rate variability during myocardial ischaemia

Analysis of heart rate variability (HRV) is a valuable, non-invasive method for quantifying autonomic cardiac control in humans. Frequency-domain analysis of HRV involving myocardial ischaemic episodes should take into account its non-stationary behaviour. The wavelet transform is an alternative tool for the analysis of non-stationary signals. Fourteen patients have been analysed, ranging from 40 to 64 years old and selected from the European Electrocardiographic ST-T Database (ESDB). These records contain 33 ST episodes, according to the notation of the ESDB, with durations of between 40s and 12 min. A method for analysing HRV signals using the wavelet transform was applied to obtain a time-scale representation for very low-frequency (VLF), low-frequency (LF) and high-frequency (HF) bands using the orthogonal multiresolution pyramidal algorithm. The design and implementation using fast algorithms included a specially adapted decomposition quadrature mirror filter bank for the frequency bands of interest. Comparing a normality zone against the ischaemic episode in the same record, increases in LF (0.0112 +/- 0.0101 against 0.0175 +/- 0.0208 s2 Hz(-1); p<0.1) and HF (0.0011 +/- 0.0008 against 0.00 17 +/- 0.0020 s2 Hz(-1); p<0.05) were obtained. The possibility of using these indexes to develop an ischaemic-episode classifier was also tested. Results suggest that wavelet analysis provides useful information for the assessment of dynamic changes and patterns of HRV during myocardial ischaemia.

Cardiac adrenergic innervation within the first 3 months after acute myocardial infarction

It is widely accepted that myocardial infarction results in adrenergic denervation of the infarcted and peri-infarcted myocardium. On the contrary, the concept of re-innervation of adrenergic nerve fibres is less well established. Although there is evidence of partial re-innervation occuring several months after myocardial infarction, the extent and time scale of re-innervation are only poorly known. In this study we investigated changes in cardiac adrenergic innervation and myocardial perfusion during the early convalescence period (the first 3 months) after an acute myocardial infarction. Single-photon emission computed tomographic imaging was conducted in 15 men 1 week and 3 months after an acute myocardial infarction with I123-metaiodobentzylguanidine (MIBG) and Tc99m-sestamibi (MIBI) to determine the extent of adrenergic denervation and impaired perfusion, respectively. A MIBG and MIBI defect was determined as regional uptake </=30% of maximal myocardial activity. The size of the MIBG defect calculated as a percentage of left ventricular mass remained unchanged between 1 week and 3 months after myocardial infarction (31.1 +/- 17.3% vs. 30. 5 +/- 16.8%, respectively). Accordingly, MIBG activity of the infarct and peri-infarct zones (expressed as a percentage of MIBG activity of the myocardium with normal perfusion) showed no significant change (23.7 +/- 10.0% vs. 25.3 +/- 11.0% and 39.0 +/- 11.3% vs. 40.8 +/- 12.8%, respectively) during the follow-up. On the other hand, the size of MIBI defect decreased significantly during the follow-up (14.2 +/- 11.5% vs. 11.4 +/- 9.7%, P<0.05, respectively) indicating improved myocardial perfusion. The results demonstrate that cardiac adrenergic re-innervation is a slow process; despite a significant increase in myocardial perfusion we found no evidence of adrenergic re-innervation during the first 3 months after acute myocardial infarction.

Scintigraphic assessment of regional cardiac sympathetic nervous system in patients with single-vessel coronary artery disease

In coronary artery disease, the cardiac sympathetic nervous system is closely associated with myocardial ischemia. I-123 metaiodobenzylguanidine (MIBG) imaging allows us to assess the cardiac sympathetic nervous system regionally. One-hundred and eleven patients with single-vessel disease underwent regional quantitative analysis of MIBG imaging before successful percutaneous transluminal coronary angioplasty (PTCA), and repeat angiography 6 months after PTCA. Based on the results of the follow-up left ventriculogram, patients were divided into 3 groups: 39 angina pectoris (AP), 48 prior myocardial infarction without asynergy (MI without asynergy) and 24 prior myocardial infarction with asynergy (MI with asynergy). AP and MI without asynergy had significant correlations between uptake parameters and regional washout in the territory of diseased vessels, among which the severity score in AP was the most closely correlated with regional washout (r = 0.79, p < 0.0001). These correlations disappeared in MI with asynergy. To compare regional MIBG parameters in the territory of the diseased vessel as well as in the territories of the other major coronary arteries among the 3 groups, we examined MIBG parameters in 57 patients with left anterior descending artery (LAD) disease selected from among the study patients. Regional washout in the territory of the LAD was significantly higher in the MI without asynergy group than in the other two groups. The left circumflex artery (LCX) region showed significantly reduced MIBG uptake and an increased extent score in the MI with asynergy group compared with the AP group, although only a difference in the extent score existed between the MI with asynergy group and the AP group in the right coronary artery (RCA) region. In addition, the global ejection fraction before PTCA showed a significant negative correlation with each regional washout rate. In this way, regional quantitative analysis of MIBG imaging can detect the regional differences in the cardiac sympathetic nervous system in coronary artery disease, which may be associated with the degree of regional left ventricular dysfunction due to myocardial ischemia.

[Ischemic sudden death: critical analysis of risk markers. Part VIII]

Coronary artery disease is responsible for approximately 75-80% of sudden cardiac deaths in most industrialized countries. Risk factors can be divided in those which suggest structural heart disease and those reflecting abnormal physiological markers. Therapeutic strategies for primary prevention of sudden cardiac death require careful scrutiny. The systematic use of risk markers to identify and stratify high risk groups may be of help to establish primary prevention measures in daily practice. Different methods to stratify risk factors using ejection fraction, ventricular arrhythmias, heart rate variability, baroreflex sensitivity, and dispersion of repolarization are discussed in this article.

Mismatch between myocardial accumulation of 123I-MIBG and 99mTc-MIBI and late ventricular potentials in patients after myocardial infarction: association with the development of ventricular arrhythmias

BACKGROUND

Late ventricular potentials are widely used to predict life-threatening arrhythmias, although the predictive value is low. To improve prediction, we correlated the incidence of ventricular arrhythmias with mismatches in myocardial 99mTc-methoxyisobutylisonitrile (MIBI)/(123)I-metaiodobenzylguanidine (MIBG) accumulation and late ventricular potentials (LP).

METHODS AND RESULTS

Fifty patients with old myocardial infarctions were divided into an LP-positive group (n = 19) and an LP-negative group (n = 31). On bull's-eye single photon emission computed tomographic MIBI and MIBG images, the heart was divided into 9 segments to evaluate the accumulation of the 2 nuclides. There was no difference in total defect score (TDS) for MIBI between the LP-positive and LP-negative groups. However, TDS for MIBG and differences TDS between MIBI and MIBG (ATDS) were significantly greater in the LP-positive group.

CONCLUSIONS

The incidence of severe ventricular arrhythmias was greater among patients with an increased ATDS in the LP-positive group. Thus the combination of these two methods may improve the prediction of ventricular arrhythmias after myocardial infarction.

Differential uptake of myocardial perfusion radiotracers in normal, infarcted, and acutely ischemic peri-infarction myocardium

OBJECTIVES

We measured the uptake of technetium-99m tetrofosmin (99m Tc) and thallium-201 (201 TI) in areas of healed transmural myocardial infarction and in the regions of acute peri-infarction ischemia.

METHODS

Anesthetised pigs with a 1-month old transmural infarction elicited by permanent ligature of the left anterior descending (LAD) coronary artery below the first branch underwent one hour of proximal LAD occlusion followed by injection of 99m Tc-tetrofosmin and 201TI either in the left atrium (GI, n= 19) or in the jugular vein (GII, n = 6). Twelve other pigs (GIII) with similar acute peri-infarction ischemia received 99m Tc-tetrofosmin and 201Tl into the left ventricle during cardiocirculatory arrest to rule out the effect of coronary collaterals. Radiotracer counting was determined in samples from normal, acute ischemic and necrotic regions.

RESULTS

Uptake of 99m Tc-tetrofosmin and 201 Tl was greater in the infarct scar (median % of normal tissue: 20 for 99m Tc and 8.6 for 201 Tl in GI; 22 and 15 in GII) than in acute ischemic myocardium (3.2 and 2.5 in GI; 6.4 and 3.3 in GII). Radiotracer injection in arrested hearts (GIII) depicted a similar pattern (median % of injected dose: 6.2 for 99m Tc and 10 for 201Tl in the scar; 2.3 and 4.0 in acute ischemia; 2.9 and 3.5 in normal tissue). The infarcted region showed connective tissue and lack of viable myocardium.

CONCLUSION

A 1-month old infarct scar with no viable myocardial tissue can take up significant fractions of 99mTc-tetrofosmin and 201Tl even in the absence of coronary collateral perfusion. Data suggest that the infarct scar can extract these radiotracers from the intraventricular blood.

Simultaneous cardiac and renal sympathetic neural responses to activation of left ventricular sympathetic afferents

In separate sets of experiments, we observed that activation of left ventricular sympathetic afferents by transmural myocardial ischemia (TMI) appeared to elicit greater reflex increases in efferent sympathetic nerve activity (SNA) to the heart than to the kidney. To assess this observation more rigorously, we simultaneously measured changes in cardiac and renal SNA elicited by TMI and by epicardial and intracoronary bradykinin (BK). Experiments were performed in 19 chloralose-anesthetized dogs with sinoaortic denervation and vagotomy. TMI was created by a 2 min complete occlusion of the anterior descending coronary artery while a collateral flow limiting stenosis was present on the circumflex coronary artery. Epicardial BK was applied to small sponges (1 cm2) which were placed on the anterior wall of the left ventricle. Intracoronary BK was injected into a branch of the anterior descending artery. We observed that mean maximal reflex increases in SNA during TMI and intracoronary BK were significantly greater in cardiac than in renal nerves (TMI; 58 +/- 11% versus 36 +/- 9%, p = 0.01; intracoronary BK; 144 +/- 48% versus 77 +/- 26%, p = 0.05). Epicardial BK elicited reflex increases in cardiac and renal SNA which were not significantly different (167 +/- 44% versus 127 +/- 36%; p = 0.72). Our results indicate that activation of left ventricular sympathetic afferents by TMI and intracoronary BK elicits greater reflex increases in sympathetic outflow to the heart than to other end-organs such as the kidney. We speculate that these augmented excitatory responses are most likely related to engagement of cardio-cardiac spinal sympathetic reflexes during intense stimulation of sympathetic afferent endings.

Scintigraphic evaluation of cardiac autonomic innervation

Alterations in cardiac autonomic neuronal function have become the focus of intense research in various cardiovascular diseases. Both single-photon emission tomography (SPECT) and the positron emission tomography (PET) imaging techniques in combination with radio-labeled neurotransmitters and receptor ligands have become available for the scintigraphic visualization of presynaptic and postsynaptic neuronal function. Several clinical studies have shown changes in tracer distribution in different clinical conditions, such as ischemic heart disease, congestive heart failure, malignant arrhythmias, heart transplantation, and in patients with diabetes. In patients with congestive heart failure, previous in vitro investigations have concentrated on the postsynaptic level of the sympathetic innervation. However, alterations in presynaptic nerve function have been demonstrated with scintigraphic investigations by decreased presynaptic tracer retention. Moreover, correlation between scintigraphic findings and clinical outcome was shown in patients with heart failure, providing important prognostic information superior to conventional risk assessment. In conclusion, scintigraphic evaluation by SPECT and PET allows functional characterization of cardiac presynaptic and postsynaptic neurons. Regional tracer uptake can be used as an index for the integrity of innervation in various diseases. Newer tracer approaches may allow the noninvasive quantification of neuronal function by PET.

Altered pharmacologic responsiveness of reduced L-type calcium currents in myocytes surviving in the infarcted heart

The pharmacologic responses of macroscopic L-type calcium channel currents to the dihydropyridine agonist, Bay K 8644, and beta-adrenergic receptor stimulation by isoproterenol were studied in myocytes enzymatically dissociated from the epicardial border zone of the arrhythmic 5-day infarcted canine heart (IZs). Calcium currents were recorded at 36 degrees to 37 degrees C using the whole cell, patch clamp method and elicited by applying step depolarizations from a holding potential of -40 mV to various test potentials for 250-msec duration at 8-second intervals. A Cs+ -rich and 10 mM EGTA-containing pipette solution and a Na+ -and K+ -free external solutions were used to isolate calcium currents from other contaminating currents. During control, peak ICa,L density was found to be significantly less in IZs (4.0 +/- 1.1 pA/pF) than in myocytes dispersed from the epicardium of the normal noninfarcted heart (NZs; 6.5 +/- 1.8 pA/pF). Bay K 8644 (1 micro M) significantly increased peak ICa,L density 3.5-fold above control levels in both NZs (to 22.5 +/- 6.2 pA/pF; n = 7) and IZs (to 12.8 +/- 3.0 pA/pF; n = 5), yet peak ICa,L density in the presence of drug was significantly less in IZs than NZs. The effects of Bay K 8644 on kinetics of current decay and steady-state inactivation relations of peak ICa,L were similar in the two cell types. In contrast, the response of peak L-type current density to isoproterenol (1 micro M) was significantly diminished in IZs compared to NZs regardless of whether Ba2+ or Ca2+ ions carried the current. Thus, these results indicate an altered responsiveness to beta-adrenergic stimulation in cells that survive in the infarcted heart. Furthermore, application of forskolin (1 micro M and 10 micro M) or intracellular cAMP (200 micro M), agents known to act downstream of the beta-receptor, also produced a smaller increase in peak IBa density in IZs versus NZs, suggesting that multiple defects exist in the beta-adrenergic signaling pathway of IZs. In conclusion, these studies illustrate that reduced macroscopic calcium currents of cells in the infarcted heart exhibit an altered pharmacologic profile that has important implications in the development of drugs for the diseased heart.

Imaging of myocardial sympathetic innervation with metaiodobenzylguanidine

The sympathetic nervous system has profound influences on myocardial function and pathophysiology. The heart is densely innervated with sympathetic nerves, which are distributed on a regional basis. Heterogeneity of myocardial sympathetic innervation, or autonomic imbalance, has long been hypothesized as a major mechanism underlying sudden cardiac death. Only in the past few years has it been possible to evaluate abnormalities in heart innervation in the intact animal. Recent developments in cardiac imaging have lead to the ability to map the distribution of the sympathetic nerves in vivo, with radiolabeled metaiodobenzylguanidine. As a result, pathophysiologic mechanisms that relate alterations in sympathetic nerve activity to disease processes are now being explored.

Effects of canine myocardial infarction on sympathetic efferent neuronal function: scintigraphic and electrophysiologic correlates

We studied the effects of nondecentralized left stellate ganglion stimulation on regional epicardial monophasic action potential duration at 50% (APD50) and 90% (APD90) repolarization from 104 sites in 10 surviving dogs with a chronic myocardial infarction model. These effects were correlated with thallium-201 and iodine-123 metiodobenzylguanidine (MIBG) imaging to identify areas of viable but denervated myocardium. Mean infarct size was 5.2% +/- 0.8% total heart weight, and the planimetered areas of denervation were always larger (18% +/- 4% total heart area). During constant ventricular pacing, stellate stimulation tended to shorten the APD90 only in normally innervated areas (364 +/- 5 to 358 +/- 5 msec) and to increase in denervated areas (358 +/- 5 to 362 +/- 5 msec), (p value not significant (NS) for prestellate and poststellate stimulation; p < 0.05 for difference between denervated vs innervated). The APD50 significantly shortened in innervated areas from 287 +/- 5 to 270 +/- 3 msec (p < 0.05) compared with denervated areas (283 +/- 4 to 274 +/- 5 msec, p = NS). We conclude that MIBG imaging demonstration of denervation identifies areas with impaired shortening of the epicardial APD50 in response to stellate stimulation and that nontransmural myocardial infarction produces areas of denervation larger than areas of necrosis.

Significance of cardiac innervation on spontaneous ventricular arrhythmias elicited by left stellate ganglion stimulation in dogs 4 days after myocardial infarction: comparison of two experimental models

The effects of cardiac sympathetic overactivity on spontaneous arrhythmias and transmural left ventricular effective refractory period (LVERP) were assessed by left stellate stimulation (LSS) in 16 anesthetized dogs. The experiments were performed 4 days after proximal occlusion of the left anterior descending (LAD) coronary artery produced by either ligation (9 dogs) or embolization with histoacryl (7 dogs). The innervation of left ventricular myocardium was studied by light and electron microscopies. Synaptophysin (SYN)- and neuropeptide Y (NPY)-immunoreactive nerve fibers and terminals were thereby detected. In dogs subjected to ligation, LSS elicited negligible arrhythmias in spite of a decrease in LVERP by 6.9 +/- 2.2% (mean +/- SD, p < 0.001). However, dogs with intravascular occlusion were more susceptible to LSS, as indicated by development of sustained ventricular rhythms. In these animals, the LVERP decreased with LSS by 14.6 +/- 3.4% (p < 0.001). The innervation of the anterior left ventricular wall distal to the place of occlusion revealed a higher reduction of SYN- and NPY-immunoreactive nerves in infarcted myocardium and a more heterogeneous distribution of nerves in undamaged regions after ligation, compared to intravascular occlusion. Ultrastructurally, nerve terminals containing small agranular and large dense-core vesicles were found innervating ischemically damaged myocardiocytes. Our findings indicate a higher preservation of nerves in infarcted and noninfarcted myocardium of animals subjected to embolic occlusion of the LAD. Because LSS apparently elicited more arrhythmias in these animals, we suggested a proarrhythmic effect of intact myocardial innervation after infarction.

Neuromodulation of calcium current by extracellular ATP in isolated ventricular myocytes

The effects of extracellular ATP on the L-tpe Ca2+ current (ICa), action potential, and resting and intracellular Ca2+ levels were examined in enzymatically isolated myocytes from the right ventricles of ferrets. Extracellular ATP decreased the peak amplitude of ICa in a time- and concentration-dependent manner. The concentration-response relationship for ATP inhibition of ICa was well described by a conventional Michaelis-Menten relationship with a half maximal inhibitory concentration of 1 microM and a maximal effect of 50%. Extracellular ATP did not change the resting myoplasmic Ca2+ levels; however, it did decrease the Ca2+ transient. The effects of extracellular ATP were mediated independently of adenosine A1 receptors and a pertussis-toxin sensitive G protein. Pharmacological characterization of receptor subtype using ATP analogs was consistent with ATP binding to a P2Y type receptor.

Enhanced induction of ventricular arrhythmias during sympathetic stimulation before and during coronary artery occlusion

We used programmed electrical stimulation to examine the arrhythmogenic influence of the sympathetic nervous system before and during coronary artery occlusion. In 29 anesthetized dogs the left and/or right stellate ganglia were stimulated at 2-8 hertz. Program-induced ventricular arrhythmias included single premature ventricular depolarizations, doublets, triplets, ventricular tachycardia and ventricular fibrillation. Both the number of extrastimuli and the duration of coronary occlusion significantly influenced ventricular arrhythmia induction. After pooling the number of extrastimuli, type of artery occluded, and the duration of occlusion, the influences of unilateral and bilateral stellate stimulations were evaluated. The incidence of induced ventricular arrhythmias was 54% during control conditions (prior to sympathetic stimulation). Right stellate stimulation had no influence on arrhythmogenesis, causing ventricular arrhythmia induction in 52% (NS) of the trials. Left stellate stimulation resulted in increased ventricular arrhythmias (68%; P less than 0.05) in response to programmed electrical stimulation. Bilateral stellate stimulation elevated program-induced ventricular arrhythmias (63%; P less than 0.05). The effects of the stellate stimulations on arrhythmia induction were similar during and up to 180 minutes of coronary occlusion. Thus, the arrhythmogenic influence of sympathetic stimulation was present before and during coronary artery occlusion.

The role of thrombolytic therapy in patients with acute myocardial infarction presenting later than six hours after the onset of symptoms

Thrombolytic therapy clearly reduces mortality in patients with acute myocardial infarction, especially when initiated within 6 hours of onset of symptoms. Some studies have also suggested that thrombolytic therapy may improve survival even when initiated 6-24 hours after the onset of symptoms by mechanisms other than infarct size limitation, such as reduced expansion, reduced electrical instability, and improved healing of the infarct. However, in view of the possibility that late thrombolytic therapy may also be associated with an increased risk of cardiac rupture, the risk-benefit ratio needs to be more clearly defined. Ongoing randomized trials are expected to clarify the situation in the near future. In the meantime, efforts to initiate reperfusion as soon after the onset of myocardial infarction as possible should continue, since early treatment is still the best treatment.

Nerve fibers in human myocardial scars

The relationships between ischemic heart disease, myocardial scars, ventricular nerve fibers, and ventricular arrhythmias have not been established despite considerable evidence suggesting important correlations. We recently described the reactions of nerve fibers in necrotic, healing, and healed rat myocardium. Prompted by these studies and by the lack of similar information for humans, we studied the structural relationships between nerve fibers and human myocardial scars. Hearts were obtained from transplant surgery and autopsy. Nerve fibers were labeled with antibody to S-100 protein. Light and electron microscopy of left ventricular scars revealed (1) fiber densities greater than those in adjacent intact myocardium, (2) fiber aggregates concentrated irregularly along the periphery of lesions, (3) fibers few in number or absent in the deeper aspects of scars, and (4) axonal enlargements containing clear and dense storage granules within the fiber aggregates. Like all other elements of the scars, the nerve fibers appeared to be oriented predominantly in the long axis of myocytes located at the edges of the lesions. Based on our experimental findings in rat hearts, these studies suggest that human myocardial nerve fibers regenerate after necrotizing injuries and that at least some of the resulting scar-associated fibers have structural features differing from those in uninjured myocardium. We suspect that these structural differences might be associated with functional alterations that could affect the triggering of ventricular arrhythmias.

Late potentials on signal-averaged electrocardiograms and patency of the infarct-related artery in survivors of acute myocardial infarction

This study evaluated the relation between patency of the infarct-related artery and the presence of late potentials on the signal-averaged electrocardiogram (ECG) in 124 consecutive patients (98 men, 26 women; mean age 59 years) with acute myocardial infarction receiving thrombolytic therapy, acute percutaneous transluminal coronary angioplasty or standard care. All patients were studied by coronary angiography, measurement of ejection fraction and signal-averaged ECG. The infarct-related artery was closed in 51 patients and open in 73. Among patients with no prior myocardial infarction undergoing early attempted reperfusion therapy, a patent artery was associated with a decreased incidence of late potentials (20% versus 71%; no significant difference in ejection fraction). In the 48 patients receiving thrombolytic agents within 4 h of symptom onset, the incidence of late potentials was 24% and 83% among patients with an open or closed artery, respectively (p less than 0.04). The most powerful predictors of late potentials were the presence of a closed infarct-related artery, followed by prior infarction and patient age. Among patients receiving thrombolytic agents within 4 h of symptom onset, the only variable that was predictive of the presence of late potentials was a closed infarct-related artery. These data imply that reperfusion of an infarct-related artery has a beneficial effect on the electrophysiologic substrate for serious ventricular arrhythmias that is independent of change in left ventricular ejection fraction as an index of infarct size. These findings might explain, in part, the low late mortality rate in survivors of myocardial infarction with documented reperfusion of the infarct-related artery.

Effect of isoproterenol on induction of ventricular tachyarrhythmias in the normal and infarcted canine heart

The influence of isoproterenol on induction of ventricular arrhythmias was evaluated in 10 normal dogs and 17 dogs with experimentally induced myocardial infarction. Programmed stimulation (using up to 6 extrastimuli) was performed before and then during infusion of isoproterenol (2 micrograms/minute followed by 4 micrograms/minute). Isoproterenol facilitated induction of sustained monomorphic ventricular tachycardia (cycle length 163 +/- 26 msec) in 5 of the 10 animals with no inducible baseline arrhythmia (P less than 0.05). Isoproterenol did not affect cycle length or the number of extrastimuli required in animals with baseline ventricular tachycardia (cycle length 158 +/- 15 msec before versus 163 +/- 17 msec during isoproterenol, P = 0.3; extrastimuli 3.8 +/- 0.6 before versus 3.8 +/- 0.4 during isoproterenol infusion, P = 0.3). Isoproterenol did not significantly facilitate induction of ventricular fibrillation in either normal dogs or those studied after production of myocardial infarction. We conclude that infusion of isoproterenol increases the incidence of inducible ventricular tachycardia in the infarcted heart, but does not facilitate the induction of ventricular fibrillation in infarcted or normal hearts, despite the use of an aggressive protocol for programmed stimulation. Isoproterenol is, therefore, a safe and useful adjunct to programmed stimulation in this setting.

Beta-adrenergic-mediated influences on microscopic conduction in epicardial regions overlying infarcted myocardium

Slowing in conduction and functional block in the epicardial tissue overlying a region of transmural necrosis are pivotal in the genesis of many lethal arrhythmias during the healing phase of myocardial infarction. The mechanisms responsible for these alterations in conduction in the epicardial region have not been completely elucidated. In the present study, the microscopic abnormalities in conduction were investigated in vitro using high-density mapping with 224 bipolar electrodes (interelectrode distance, 350 microns) in isolated epicardial tissue excised from the hearts of dogs 2 weeks after anterior myocardial infarction. Seven epicardial tissue slices (group 1) exhibited uninterrupted sequential activation after stimulation at the basic cycle length as well as after premature stimuli. Four tissues (group 2) demonstrated microscopic alterations in conduction both during basic drive and premature stimuli, with the extent of conduction delay dependent on the stimulation site. In three tissues (group 3), the activation sequence was normal during the basic drive but became abnormal after premature stimulation, with the appearance of functional block in the direction transverse to fiber orientation. Superfusion with isoproterenol (10(-6) M) did not significantly modify resting membrane potential, amplitude, or maximum rate change of voltage (Vmax) of phase 0 depolarization but decreased action potential duration. Isoproterenol did not alter the activation sequence during basic drive, but it reduced the slowing in conduction elicited by premature stimulation in group 1 (p less than 0.01 in the transverse direction), alleviated microcircuitous conduction in group 2, and prevented the occurrence of functional block in tissues in group 3. Despite this salutary effect on conduction velocity, arrhythmias occurred after isoproterenol because of focal activity (three of 14, 21%) or reentry (one of 14, 7%) at very short coupling intervals of premature stimulation. These findings indicate that microscopic abnormalities in conduction occur in response to premature stimulation in epicardial regions overlying an infarct, eliciting conduction block that is more prone to occur in the direction transverse to fiber orientation and is dependent on the direction of activation. beta-Adrenergic stimulation prevents or attenuates the alterations in the pattern of activation elicited by premature stimulation. However, this potential beneficial effect of beta-adrenergic stimulation is offset by the occurrence of focal activity and reentry at short coupling intervals of premature stimulation.

Electrophysiological effects of adenosine in the transplanted human heart. Evidence of supersensitivity

After cardiac transplantation, the denervated donor atria and ventricles demonstrate increased sensitivity to infusions of sympathomimetic amines. Recently, supersensitivity of the canine sinus and atrioventricular (AV) nodes to acetylcholine has also been demonstrated after parasympathetic denervation. Acetylcholine and the endogenous nucleoside adenosine exert similar electrophysiological effects in both the sinus and AV nodes, and share a common transduction process. We, therefore, hypothesized that after orthotopic cardiac transplantation, the donor (denervated) sinus node would demonstrate greater sensitivity to exogenous adenosine than the recipient (innervated) sinus node. The effects of incremental doses of intravenous adenosine (37-112 micrograms/kg) on changes in sinus cycle length (SCL) (delta SCLmax%), changes in PR interval (delta PRmax%), time to peak effect (sec), and duration of electrophysiological effects (sec) were prospectively measured in 28 orthotopic cardiac transplant patients and nine control subjects. The baseline SCL was 795 +/- 71 msec for the control subjects, 891 +/- 43 msec for the recipient atria, and 700 +/- 18 msec for the donor atria (p less than 0.05, donor vs. recipient). The delta SCLmax% for each dose of adenosine was similar in the innervated control and recipient atria. In contrast, the donor sinus node demonstrated a threefold to fourfold increased response to adenosine as compared with the recipient sinus node and a threefold to sixfold increased response as compared with control subjects. Similarly, the donor AV node demonstrated a threefold to fivefold increase in PR interval as compared with control subjects. The duration of sinus node slowing in the denervated atria was threefold to fivefold longer than in the recipient and control atria (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

New insights into pacemaker syndrome gained from hemodynamic, humoral and vascular responses during ventriculo-atrial pacing

Ventricular pacing is performed during programmed electrical stimulation and during normal functioning of single chamber (VVI or VVIR) pacemakers. In many patients, retrograde ventriculoatrial (V-A) conduction may occur and evoke hemodynamic and reflex neurohumoral responses, which are unique to this pacing mode. Accordingly, forearm blood flow, forearm vascular resistance, mean and phasic arterial pressure, cardiac output and plasma norepinephrine, epinephrine and dopamine were measured during atrial, ventricular and V-A pacing at a cycle length of 600 ms (100 beats/min) before and after regional alpha blockade with intraarterial phentolamine in 16 patients with a left ventricular ejection fraction greater than 35% and little or no symptoms of congestive heart failure. During V-A pacing, cardiac output decreased by 10%, whereas forearm vascular resistance increased from 52 +/- 7 to 70 +/- 9 U (p less than 0.001) and plasma norepinephrine increased from 183 +/- 27 to 232 +/- 27 pg/ml (p less than 0.01). Phentolamine nearly abolished the increase in forearm vascular resistance in response to V-A pacing (18 +/- 4.1 U before vs 5.8 +/- 1.5 U after, p less than 0.05). The change in forearm vascular resistance with V-A pacing correlated with systolic arterial pressure, but not with changes in mean arterial pressure, pulse pressure, cardiac output, mean or peak right atrial pressure, pulmonary artery or pulmonary capillary wedge pressure. These results suggest that forearm vascular resistance responses to V-A pacing are mediated mainly by alpha-adrenergic receptors, through the arterial baroreflexes.(ABSTRACT TRUNCATED AT 250 WORDS)