T5 spinal cord transection increases susceptibility to reperfusion-induced ventricular tachycardia by enhancing sympathetic activity in conscious rats

We recently documented that paraplegia (T(5) spinal cord transection) alters cardiac electrophysiology and increases the susceptibility to ventricular tachyarrhythmias induced by programmed electrical stimulation. However, coronary artery occlusion is the leading cause of death in industrially developed countries and will be the major cause of death in the world by the year 2020. The majority of these deaths result from tachyarrhythmias that culminate in ventricular fibrillation. beta-Adrenergic receptor antagonists have been shown to reduce the incidence of sudden cardiac death. Therefore, we tested the hypothesis that chronic T(5) spinal cord transection increases the susceptibility to clinically relevant ischemia-reperfusion-induced sustained ventricular tachycardia due to enhanced sympathetic activity. Intact and chronic (4 wk after transection) T(5) spinal cord-transected (T(5)X) male rats were instrumented to record arterial pressure, body temperature, and ECG. In addition, a snare was placed around the left main coronary artery. The susceptibility to sustained ventricular tachycardia produced by 2.5 min of occlusion and reperfusion of the left main coronary artery was determined in conscious rats by pulling on the snare. Reperfusion culminated in sustained ventricular tachycardia in 100% of T(5)X rats (susceptible T(5)X, 10 of 10) and 0% of intact rats [susceptible intact, 0 of 10 (P < 0.05, T(5)X vs. intact)]. Beta-adrenergic receptor blockade prevented reperfusion-induced sustained ventricular tachycardia in T(5)X rats [susceptible T(5)X 0 of 8, 0% (P < 0.05)]. Thus paraplegia increases the susceptibility to reperfusion-induced sustained ventricular tachycardia due to enhanced sympathetic activity.     

Sex differences to myocardial ischemia and beta-adrenergic receptor blockade in conscious rats

We recently documented sex differences in the susceptibility to reperfusion-induced sustained ventricular tachycardia and beta-adrenergic receptor blockade in conscious rats. However, the effect of sex on ischemia-induced ventricular arrhythmias and beta-adrenergic receptor blockade is under-investigated. Therefore, we tested the hypothesis that gonadal hormones influence the ventricular arrhythmia threshold (VAT) induced by coronary artery occlusion as well as the response to beta-adrenergic receptor blockade. The VAT was defined as the time from coronary occlusion to sustained ventricular tachycardia resulting in a reduction in arterial pressure. Male and female intact and gonadectomized (GnX) rats were instrumented with a radiotelemetry device for recording arterial pressure, temperature, and ECG, as well as a Doppler ultrasonic flow probe to measure cardiac output and a snare around the left main coronary artery. The VAT was determined in conscious rats by pulling on the snare. The VAT was significantly longer in intact females (5.56 +/- 0.19) vs. intact males (4.31 +/- 0.14 min). This sex difference was abolished by GnX. Specifically, GnX decreased the VAT in females (4.55 +/- 0.22) and increased the VAT in males (5.14 +/- 0.30 min). Thus male sex hormones increase and female sex hormones decrease the susceptibility to ischemia-induced sustained ventricular tachycardia. beta-Adrenergic receptor blockade increased the VAT in intact males and GnX females only. Thus gonadal hormones influence the response to beta-adrenergic receptor blockade. Uncovering major differences between males and females in the pathophysiology of the cardiovascular system may result in sex-specific optimization of patient treatments.     

Alpha 2-Adrenergic stimulation is protective against ischemia-reperfusion-induced ventricular arrhythmias in vivo

We previously reported that alpha(2)-adrenergic receptor (alpha(2)-AR) stimulation in Purkinje fibers in vitro prolongs action potential duration and suppresses beta-adrenergic-induced delayed afterdepolarizations and sustained triggered activities. We examined the effects of alpha(2)-AR stimulation on reperfusion-induced ventricular arrhythmias [ventricular tachycardia/ventricular fibrillation (VT/VF)] in vivo. Arterial blood pressure, heart rate, surface electrocardiogram, and renal sympathetic nerve activities were recorded simultaneously in Sprague-Dawley rats. The incidence of VT/VF was 87.5% for controls, 50% for the beta-blocker group, 72% for the alpha(1)-blocker group, and 12.5% for the alpha(1) + beta-blockers group (unopposed alpha(2)-adrenergic activation). Direct alpha(2)-AR stimulation with UK-14304 also prevented VT/VF. These effects were reversed by the alpha(2)-adrenergic antagonist yohimbine. Increases in renal sympathetic nerve activity were associated with left anterior descending coronary artery ligation and reperfusion (33 +/- 1.5 and 62 +/- 1.7% over baseline, respectively) in controls. Similar patterns were observed among all experimental groups irrespective of the incidence of VT/VF on reperfusion. We conclude that alpha(2)-AR stimulation has a potent antiarrhythmic effect on ischemia-reperfusion-induced VT/VF in vivo and that this effect is not centrally mediated.     

Effect of cicletanine on reperfusion-induced arrhythmias and its relation to 6-keto-PGF1 alpha and thromboxane B2 release

Isolated hearts, excised from spontaneously hypertensive male rats treated orally with cicletanine, a new furopyridine anti-hypertensive drug, were subjected to 30 min of global ischemia followed by 10 min of reperfusion. The effect of cicletanine on reperfusion-induced arrhythmias in relation to 6-keto-PGF1 alpha and thromboxane (TXB2) release was studied. After 30 min of global ischemia, the incidence (total) of ventricular fibrillation (VF) and ventricular tachycardia (VT) was reduced by 2-week pretreatment of the rats with 30 and 100 mg/kg of cicletanine (VF, 33% at 30 mg/kg and 25% at 100 mg/kg vs. 91% in untreated rats; VT, 42% at 30 mg/kg and 42% at 100 mg/kg vs. 100% in untreated rats), while lower doses of cicletanine (3 and 10 mg/kg) failed to reduce the incidence of reperfusion-induced rhythm disturbances. Reperfusion of the ischemic myocardium resulted in a fivefold increase of 6-keto-PGF1 alpha and TXB2 release in the perfusion effluent of fibrillated hearts but not in the perfusion effluent of nonfibrillated hearts. Cicletanine failed to influence the reperfusion-stimulated release of 6-keto-PGF1 alpha and TXB2. These results indicate that the anti-arrhythmic effect of cicletanine in the reperfused myocardium is not related to PGI2 and thromboxane A2 release.     

Long-term treatment with glibenclamide increases susceptibility of streptozotocin-induced diabetic rat heart to reperfusion-induced ventricular tachycardia

This study investigated the effects of long-term treatment with glibenclamide (GLIB) on the susceptibility of streptozotocin (STZ)-induced diabetic heart to ischemia/reperfusion insults. Starting 4 weeks after the injection of STZ, rats were treated with GLIB (0.1 mg/kg, ip, three times a week, STZ-GLIB group) or vehicle (STZ-VEH group) for 8 weeks. The recovery of cardiac performance, released creatine kinase (CK), and incidence of ventricular arrhythmias were studied during the reperfusion period in isolated hearts from rats in STZ-GLIB (n = 14) and in STZ-VEH groups (n = 13) and from age-matched control rats (CNT group, n = 14). Each heart was subjected to 5 min of global low-flow ischemia followed by 25 min of no-flow ischemia, with a subsequent 30 min of reperfusion. Plasma glucose level was not significantly different between the STZ-GLIB and STZ-VEH groups. The recovery of cardiac performance and the released CK during reperfusion period were significantly lower in both STZ-VEH and STZ-GLIB groups than in the CNT group (P < 0.01 and P < 0.05, respectively). Reperfusion resulted in an incidence of ventricular fibrillation in 23% and 21% in STZ-VEH and STZ-GLIB groups, respectively (P = ns). These values were significantly lower than that of the CNT group (100%, P < 0.001 for both). More importantly, the incidence of ventricular tachycardia in the STZ-GLIB group was significantly higher than that in the STZ-VEH group (93% vs 54%, P < 0.05) and was not significantly different from that in the CNT group (93% vs 100%, P = ns). The results suggest that STZ-induced protection against reperfusion-induced ventricular arrhythmias in diabetic heart may be partially abrogated by long-term treatment with GLIB.     

Reduced susceptibility to ventricular tachyarrhythmias in rats selectively bred for high aerobic capacity

Reperfusion after a brief period of cardiac ischemia can lead to potentially lethal arrhythmias. Human epidemiological studies and experimental work with animals indicate that regular physical activity is associated with reductions in cardiovascular disease (CVD) risk factors and sudden cardiac death. Similarly, artificial selection of rats for high aerobic treadmill-running capacity (high-capacity runners; HCR) has been shown to reduce CVD risk factors relative to rats selected as low-capacity runners (LCR). Therefore, we tested the hypothesis that HCR, relative to LCR rats, would be less susceptible to ischemia-reperfusion-mediated ventricular tachyarrhythmias. To test this hypothesis, we measured the susceptibility to ventricular tachyarrhythmias produced by 3 min of occlusion and reperfusion of the left main coronary artery in conscious LCR and HCR rats. Results document a significantly lower incidence of ventricular tachyarrhythmias in HCR (3 of 11, 27.3%) relative to LCR (6 of 7, 85.6%) rats. The decreased susceptibility to tachyarrhythmias in HCR rats was associated with a reduced cardiac metabolic demand during ischemia (lower rate-pressure product and ST segment elevation) as well as a wider range for the autonomic control of heart rate. The HCR and LCR represent a unique substrate for evaluation of the mechanisms underlying ischemia-mediated cardiac arrhythmogenesis.     

Electroacupuncture decreases the susceptibility to ventricular tachycardia in conscious rats by reducing cardiac metabolic demand

Reperfusion after a brief period of cardiac ischemia can lead to potentially lethal arrhythmias. Clinical observations and experimental work with animals suggest that acupuncture may have therapeutic effects for individuals with coronary heart disease, certain arrhythmias, and myocardial ischemia. Therefore, we tested the hypothesis that electroacupuncture reduces the susceptibility to ischemia-reperfusion-mediated ventricular tachyarrhythmias. To test this hypothesis, we measured the susceptibility to ventricular tachyarrhythmias produced by 3 min of occlusion and reperfusion of the left main coronary artery in conscious rats under two experimental conditions: 1) control and 2) with electroacupuncture. Acupuncture was simulated by electrically stimulating the median nerves, corresponding to the Jianshi-Neiguan [pericardial meridian (P) 5-6] acupoints. Results document a significantly lower incidence of ventricular tachyarrhythmias with electroacupuncture (2 of 8, 25%) relative to control (14 of 14, 100%) rats. The decreased susceptibility to tachyarrhythmias with electroacupuncture was associated with a reduced cardiac metabolic demand (lower rate-pressure product and ST-segment elevation) during ischemia.     

Susceptibility to ischemia-induced arrhythmias and the effect of preconditioning in the diabetic rat heart

Diabetic heart is suggested to exhibit either increased or decreased resistance to ischemic injury. Ischemic preconditioning suppresses arrhythmias in the normal heart, whereas relatively little is known about its effects in the diseased myocardium. Our objective was to investigate whether development of diabetes mellitus modifies the susceptibility to ischemia-induced arrhythmias and affects preconditioning in the rat heart. Following 1 and 9 weeks of streptozotocin-induced (45 mg/kg, i.v.) diabetes, the hearts were Langendorff-perfused at constant pressure of 70 mm Hg and subjected to test ischemia induced by 30 min occlusion of the left anterior descending (LAD) coronary artery. Preconditioning consisted of one cycle of 5 min ischemia and 10 min reperfusion, prior to test ischemia. Susceptibility to ischemia-induced arrhythmias was lower in 1-week diabetics: only 42 % of diabetic hearts exhibited ventricular tachycardia (VT) and 16 % had short episodes of ventricular fibrillation (VF) as compared to VT 100 % and VF 70 % (including sustained VF 36 %) in the non-diabetics (P<0.05). Development of the disease was associated with an increased incidence of VT (VT 92 %, not significantly different from non-diabetics) and longer total duration of VT and VF at 9-weeks, as compared to 1-week diabetics. Preconditioning effectively suppressed arrhythmias in the normal hearts (VT 33 %, VF 0 %). However, it did not provide any additional antiarrhythmic protection in the acute diabetes. On the other hand, in the preconditioned 9-weeks diabetic hearts, the incidence of arrhythmias tended to decrease (VT 50 %, transient VF 10 %) and their severity was reduced. Diabetic rat hearts are thus less susceptible to ischemia-induced arrhythmias in the acute phase of the disease. Development of diabetes attenuates increased ischemic tolerance, however, diabetic hearts in the chronic phase can benefit more from ischemic preconditioning, due to its persisting influence.     

Interactions of peripheral mu-opioid receptors and K(ATP)-channels in regulation of cardiac electrical stability in ischemia,reperfusion, and postinfarction cardiosclerosis

It has been shown that mu-opioid receptor stimulation by intravenous administration of the selective mu receptor agonist DALDA in a dose of 0.1 mg/kg prevented ischemic and reperfusion arrhythmias in rats subjected to coronary artery occlusion (10 min) and reperfusion (10 min), and also increased the ventricular fibrillation threshold in rats with postinfarction cardiac fibrosis. These effects were abolished by pre-treatment with the selective mu receptor antagonist CTAP in a dose of 0.5 mg/kg or by prior injection of the opioid receptor antagonist naloxone methiodide (2 mg/kg) which does not penetrate the blood-braib barrier. Both antagonists by themselves had no effect on the incidence of occlusion or reperfusion-induced arrhythmias or on the ventricular fibrillation threshold. Pre-treatment with ATP-sensitive K+ channel (KATP channel) blocker glibenclamide in a dose of 0.3 mg/kg completely abolished the antiarrhythmic effect of DALDA. We believe that DALDA prevents occurrence of electrical instability during ischemia and reperfusion and increases the ventricular fibrillation threshold in rats with postinfarction cardiac fibrosis via stimulation of peripheral mu-opioid receptor which appear to be coupled to the KATP channel.     

The role of NO in ischemia/reperfusion injury in isolated rat heart

Nitric oxide (NO) is an important regulator of myocardial function and vascular tone under physiological conditions. However, its role in the pathological situations, such as myocardial ischemia is not unequivocal, and both positive and negative effects have been demonstrated in different experimental settings including human pathology. The aim of the study was to investigate the role of NO in the rat hearts adapted and non-adapted to ischemia. Isolated Langendorff-perfused hearts were subjected to test ischemic (TI) challenge induced by 25 min global ischemia followed by 35 min reperfusion. Short-term adaptation to ischemia (ischemic preconditioning, IP) was evoked by 2 cycles of 5 min ischemia and 5 min reperfusion, before TI. Recovery of function at the end of reperfusion and reperfusion-induced arrhythmias served as the end-points of injury. Coronary flow (CF), left ventricular developed pressure (LVDP), and dP/dt(max) (index of contraction) were measured at the end of stabilization and throughout the remainder of the protocol until the end of reperfusion. The role of NO was investigated by subjecting the hearts to 15 min perfusion with NO synthase (NOS) inhibitor L-NAME (100 mmol/l), prior to sustained ischemia. At the end of reperfusion, LVDP in the controls recovered to 29.0 +/- 3.9 % of baseline value, whereas preconditioned hearts showed a significantly increased recovery (LVDP 66.4 +/- 5.7 %, p < 0.05). Recovery of both CF and dP/dt(max) after TI was also significantly higher in the adapted hearts (101.5 +/- 5.8 % and 83.64 +/- 3.92 % ) as compared with the controls (71.9 +/- 6.3 % and 35.7 +/- 4.87 %, respectively, p < 0.05). NOS inhibition improved contractile recovery in the non-adapted group (LVDP 53.8 +/- 3.1 %; dP/dt(max) 67.5 +/- 5.92 %) and increased CF to 82.4 +/- 5.2 %. In contrast, in the adapted group, it abolished the protective effect of IP (LVDP 31.8 +/- 3.1 %; CF 70.3 +/- 3.4 % and dP/dt(max) 43.25 +/- 2.19 %). Control group exhibited 100 % occurrence of ventricular tachycardia (VT), 57 % incidence of ventricular fibrillation (VF) - 21 % of them was sustained VF (SVF); application of L-NAME attenuated reperfusion arrhythmias (VT 70 %, VF 20 %, SVF 0 %). Adaptation by IP also reduced arrhythmias, however, L-NAME in the preconditioned hearts increased the incidence of arrhythmias (VT 100 %, VF 58 %, SVF 17 %). In conclusion: our results indicate that administration of L-NAME might be cardioprotective in the normal hearts exposed to ischemia/reperfusion (I/R) alone, suggesting that NO contributes to low ischemic tolerance in the non-adapted hearts. On the other hand, blockade of cardioprotective effect of IP by L-NAME points out to a dual role of NO in the heart: a negative role in the non-adapted myocardium subjected to I/R, and a positive one, due to its involvement in the mechanisms of protection triggered by short-term cardiac adaptation by preconditioning.     

Protective role of protein kinase C epsilon activation in ischemia-reperfusion arrhythmia

PURPOSE: Ischemic heart disease carries an increased risk of malignant ventricular tachycardia (VT), fibrillation (VF), and sudden cardiac death. Protein kinase C (PKC) epsilon activation has been shown to improve the hemodynamics in hearts subjected to ischemia/reperfusion. However, very little is known about the role of epsilon PKC in reperfusion arrhythmias. Here we show that epsilon PKC activation is anti-arrhythmic and its inhibition is pro-arrhythmic. METHOD: Langendorff-perfused isolated hearts from epsilonPKC agonist (epsilonPKC activation), antagonist (epsilonPKC inhibition) transgenic (TG), and wild-type control mice were subjected to 30 min stabilization period, 10 min global ischemia, and 30 min reperfusion. Action potentials (APs) and calcium transients (CaiT) were recorded simultaneously at 37 degrees C using optical mapping techniques. The incidence of VT and VF was assessed during reperfusion. RESULTS: No VT/VF was seen in any group during the stabilization period in which hearts were perfused with Tyrode's solution. Upon reperfusion, 3 out of the 16 (19%) wild-type mice developed VT but no VF. In epsilonPKC antagonist group, in which epsilonPKC activity was downregulated, 10 out of 13 (76.9%) TG mice developed VT, of which six (46.2%) degenerated into sustained VF upon reperfusion. Interestingly, in epsilonPKC agonist mice, in which the activity of epsilonPKC was upregulated, no VF was observed and only 1 out of 12 mice showed only transient VT during reperfusion. During ischemia and reperfusion, CaiT decay was exceedingly slower in the antagonist mice compared to the other two groups. CONCLUSION: Moderate in vivo activation of epsilonPKC exerts beneficial antiarrhythmic effect vis-a-vis the lethal reperfusion arrhythmias. Abnormal CaiT decay may, in part, contribute to the high incidence of reperfusion arrhythmias in the antagonist mice. These findings have important implications for the development of PKC isozyme targeted therapeutics and subsequently for the treatment of ischemic heart diseases.     

Mechanisms responsible for the cardiotoxic effects of cocaine

Cocaine can induce lethal cardiovascular events, including myocardial infarction and ventricular fibrillation. The mechanisms responsible for these cardiotoxic effects of cocaine remain largely to be determined. Cocaine has both sympathomimetic (inhibition of neuronal uptake of norepinephrine) and local anesthetic (Na+ channel blockade) properties. Neurotransmitters released from cardiac sympathetic nerves bind to both alpha- and beta-adrenergic receptors eliciting a cascade of intracellular responses. Stimulation of beta-adrenergic receptors activates adenylate cyclase, increasing cyclic AMP levels, whereas alpha-adrenergic receptor stimulation activates phospholipase C, increasing inositol trisphosphate. These second messengers, in turn, elicit increases in cystolic calcium. Elevations in cystolic calcium can provoke oscillatory depolarizations of the cardiac membrane, triggering sustained action potential generation and extrasystoles. Cocaine also acts as a local anesthetic by inhibiting sodium influx into cardiac cells, which impairs impulse conduction and creates an ideal substrate for reentrant circuits. Thus, the adrenergic and anesthetic properties of cocaine could act synergistically to elicit and maintain ventricular fibrillation. Adrenergic receptor activation would trigger the event whereas sodium channel blockade would create the reentrant substrate to perpetuate the malignant arrhythmias.     

Ischaemic preconditioning in the rat heart: The role of G-proteins and adrenergic stimulation

Since recent findings indicate the involvement of G-proteins in the mechanisms of ischaemic preconditioning (PC), the present study was aimed to investigate the role of adrenergic mechanisms, such as G-proteins and stimulation of adrenergic receptors, in this phenomenon. For this purpose, isolated Langendorff-perfused rat hearts were subjected to regional ischaemia (30 min occlusion of LAD) followed by reperfusion. The effect of PC (a single 5 min occlusion/reperfusion before a long occlusion) on ischaemia- and reperfusion-induced arrhythmias was studied in conjunction with an assessment of G-proteins in the myocardial tissue by means of Western blotting and ADP-ribosylation with bacterial toxins. To follow the link between G-proteins and adrenergic receptors, their stimulation by exogenous norepinephrine (NE) was applied to test whether it can mimic the effect of PC on arrhythmias. Thirty min ischaemia and subsequent reperfusion induced high incidence of ventricular tachycardia (VT) and fibrillation (VF). PC significantly reduced a total number of extrasystoles, incidence of VT and abolished VF. It was, however, insufficient to suppress reperfusion-induced sustained VF. Measurement of G-proteins revealed that PC led to a reduction of stimulatory Gs proteins, whereas inhibitory Gi proteins were increased. NE (50 nmol) introduced in a manner similar to PC (5 min infusion, 10 min normal perfusion) reduced ischaemic arrhythmias in the same way, as PC. In addition, in NE-pretreated hearts reperfusion induced mostly transient VF, which was spontaneously reverted to a normal sinus rhythm. A transient increase in heart rate and perfusion pressure during NE infusion completely waned before the onset of ischaemia, indicating that antiarrhythmic effect was not related to haemodynamic changes and to conditions of myocardial perfusion. Conclusion: Antiarrhythmic effect of PC may be mediated by a stimulation of adrenergic receptors coupled to appropriate G-proteins. Consequently, the inhibition of adenylate cyclase activity and reduction in cAMP level, as well as the activation of protein kinase C may be considered as two possible pathways leading to a final response.     

Effects of HNS-32, a novel antiarrhythmic drug,on ventricular arrhythmias induced by coronary artery occlusion and reperfusion in anesthetized rats

HNS-32 (N¹,N¹-dimethyl-N²-(2-pyridylmethyl)-5-isopropyl-3, 8-dimethylazulene-1-carboxamidine: CAS 186086-10-2) is a newly synthesized compound, and possesses antiarrhythmic properties with vasodilator action in dog hearts. The aim of this study was to investigate the dose-dependent effects of HNS-32 on ischemia- and/or reperfusion-induced ventricular arrhythmias in anesthetized rats in vivo and compared with those of mexiletine. Saline or drugs were administered intravenously 5 min prior to coronary artery occlusion. On the ischemia-induced ventricular arrhythmias, HNS-32 showed dose-dependent reduction of total number of premature ventricular complexes (PVC) from 2091 ± 225 to 656 ± 116 and 286 ± 69 beats/30 min (p < 0.05), the ventricular tachycardia (VT) duration from 183 ± 33 to 28 ± 9 and 4 ± 2 sec (p < 0.05), the incidence of VT from 100 to 90 (n.s.) and 40% (p < 0.05), and the incidence of ventricular fibrillation (VF) from 50 to 0 and 0% (p < 0.05) with 3 and 5 mg/kg, respectively. Mexiletine also reduced these parameters to 936 ± 159 beats/30 min (p < 0.05), 39 ± 22 sec (p < 0.05), 90% (n.s.) and 10% (n.s.), respectively. HNS-32 completely suppressed the late reperfusion-induced arrhythmias, however mexiletine did not affect them. On the early reperfusion-induced ventricular arrhythmias, HNS-32 showed dose-dependent reduction of VT duration from 126 ± 34 to 37 ± 12 and 3 ± 2 sec (p < 0.05), incidence of VT from 100 to 90 (n.s.) and 40% (p < 0.05), incidence of VF from 100 to 10 and 0% (p < 0.05), and mortality rate from 90 to 0 and 0% (p < 0.05), with 3 and 5 mg/kg, respectively. Mexiletine also reduced these parameters to 16 ± 9 sec (p < 0.05), 80 (n.s.), 50 (p < 0.05), and 10% (p < 0.05), respectively. HNS-32 significantly reduced the heart rate in a dose-dependent manner, from 399 ± 14 to 350 ± 8 and 299 ± 10 beats/min (p < 0.05) with 3 and 5 mg/kg, respectively. The antiarrhythmic effects of HNS-32 were more potent than that of the similar dose of mexiletine against occlusion-induced and reperfusion-induced arrhythmias in in vivo rats.     

Sevoflurane postconditioning attenuates reperfusion-induced ventricular arrhythmias in isolated rat hearts exposed to ischemia/reperfusion injury

Sevoflurane postconditioning has been proven to protect the hearts against ischemia/reperfusion injury, manifested mainly by improved cardiac function, reduced myocardial specific biomarker release, and decreased infarct size. This study is to observe the effects of sevoflurane postconditioning on reperfusion-induced ventricular arrhythmias and reactive oxygen species generation in Langendorff perfused rat hearts. Compared with the unprotected hearts subjected to 25 min of global ischemia followed by 30 min of reperfusion, exposure of 3% sevoflurane during the first 15 min of reperfusion significantly improved cardiac function, reduced cardiac troponin I release, decreased infarct size and attenuated reperfusion-induced ventricular arrhythmia. Further analysis on arrhythmia during the 30 min of reperfusion showed that, sevoflurane postconditioning decreased both the duration and incidence of ventricular tachycardia and ventricular fibrillation. In the meantime, intracellular malondialdehyde and reactive oxygen species levels were also reduced. These above results demonstrate that sevoflurane postconditioning protects the hearts against ischemia/reperfusion injury and attenuates reperfusion-induced arrhythmia, which may be associated with the regulation of lipid peroxidation and reactive oxygen species generation.     

Is Performance Time a Prerequisite for the Onset and Intensity of the Occlusion-Reperfusion Arrhythmias in Anaesthetised Rats?

The aim of the present study is to investigate the onset and the intensity of arrhythmias in anaesthetized rats as a function of time under a standardized experimental condition, which is composed of 30 min occlusion and 60 min reperfusion. Local bred rats (250-350 g) housed in a 12-h light-dark cycle (lights on at 09.00 h, lights off at 21.00 h) were anaesthetized by sodium thiopentone (60 mg kg^-1 i.p.) and left anterior descending coronary artery ligation method using 6/0 braided silk ligature was used to induce 30 min occlusion and 60-min reperfusion. Animals were randomly allocated into three groups to exposure to 30-min occlusion at 9.00 h and 60 min reperfusion at 9:30 h (Group I, n = 6); to 30 min occlusion at 15.00 h and 60 min reperfusion at 15:30 h (Group II, n = 6); and to 30 min occlusion at 21.00 h and 60 min reperfusion at 21.30 h (Group III, n = 6). ECG and haemodynamic parameters were recorded throughout the experiments. The onset of ventricular ectopic beats (VEBs), number of VEBs, incidences of ventricular tachycardia (VT) and ventricular fibrillation (VF) during the periods of occlusion-reperfusion were analysed. Total VF incidence during occlusion were lower than the VT incidence in all groups. Either VT or VF incidences during reperfusion showed same profiles in all groups but VT incidence was 2-fold higher than VF. Time-dependent application of occlusion-reperfusion induced by coronary artery ligation method in the anaesthetized rats did not result in a variation in the onset and the intensity of arrhythmias. The duration of the experimental ischaemia was the principal factor, which determines the time of onset and intensity of the occlusion-reperfusion arrhythmias.     

Cardioprotective actions of wild garlic (Allium ursinum) in ischemia and reperfusion

The susceptibility to ventricular arrhythmias under the conditions of cardiac ischemia and reperfusion was investigated in the Langendorff heart preparation of rats fed for eight weeks a standard chow enriched with 2% of pulverized wild garlic leaves. The isolated hearts were perfused with a modified Krebs-Henseleit solution. The incidence of ventricular fibrillation (VF) during 20 min occlusion of thedescending branch of the left coronary artery (LAD) was significantly reduced in the wild garlic group as compared to untreated controls (20% vs 88%). The same holds for the size of the ischemic zone (33.6% vs 40.9% of heart weight). In the reperfusion experiments (5 min after 10 min ischemia), ventricular tachycardia (VT) occurred in 70% of the wild garlic group vs 100% in untreated controls and VF in 50% vs 90%. The time until occurrence of extrasytoles, VT or VR was prolonged. No significant alterations in cardiac fatty acid composition could be observed. Although the prostacyclin production was slightly increased in hearts of the wild garlic group, inhibition of cyclooxygenase by acetylsalicylic acid (ASA; aspirin) could not completely prevent the cardioprotective effects suggesting that the prostaglandin system, does not play, a decisive role in the cardioprotective action of wild garlic. Furthermore, a moderate angiotensin converting enzyme (ACE) inhibiting action of wild garlic was foundin vitro as well asin vivo that could contribute to the cardioprotective and blood pressure lowering action of wild garlic. Whether a free radiacal scavenging activity of wild garlic is involved in its cardioprotective effects remains to be established.     

Gender difference in baroreflex-mediated bradycardia in young rats: role of cardiac sympathetic and parasympathetic components.

In a previous clinical study we have demonstrated a significantly lower baroreflex-mediated bradycardic response in young women compared with men. The present study determined whether sexual dimorphism in baroreflex sensitivity in young rats also covers the reflex tachycardic response. The study was then extended to test the hypothesis that an attenuated cardiac cholinergic component of the baroreflex heart rate response in females may account for the gender difference. Baroreflex sensitivity (BRS) was expressed as the regression coefficient of the reciprocal relationship between evoked changes in blood pressure and heart rate. BRS measured in conscious rats with phenylephrine (BRS(PE)) and nitroprusside (BRS(NP)) represented the reflex bradycardic and tachycardic responses, respectively. Female rats exhibited significantly lower BRS(PE) compared with male rats (-1.53+/-0.1 vs. -2.36+/-0.13 beats x min(-1) x mmHg(-1); p < 0.05) but similar BRS(NP) (-2.60+/-0.20 vs. -2.29+/-0.17 beats x min(-1) x mmHg(-1)). Blockade of cardiac muscarinic receptors with atropine methyl bromide elicited greater attenuation of BRS(PE) in male than in female rats (72+/-4.6 vs. 53+/-6.7% inhibition; p < 0.01) and abolished the gender difference. In male rats cardiac muscarinic blockade attenuated BRS(PE) significantly more than did cardiac beta-adrenergic receptor blockade with propranolol (72+/-4.6 vs. 43+/-2.7; p < 0.01), which suggests greater dependence of BRS(PE) on the parasympathetic component. In females, muscarinic and beta-adrenergic blockade elicited similar attenuation of BRS(PE). The findings suggest that (i) BRS is differentially influenced by gender; female rats exhibit substantially lower BRS(PE) but similar BRS(NP) compared with age-matched male rats and (ii) the sexual dimorphism in BRS(PE) results, at least partly, from a smaller increase in vagal outflow to the heart in response to baroreceptor activation.     

Selective inactivation of redox-sensitive mitochondrial enzymes during cardiac reperfusion

Reperfusion of ischemic myocardial tissue results in an increase in mitochondrial free radical production and declines in respiratory activity. The effects of ischemia and reperfusion on the activities of Krebs cycle enzymes, as well as enzymes involved in electron transport, were evaluated to provide insight into whether free radical events are likely to affect enzymatic and mitochondrial function(s). An in vivo rat model was utilized in which ischemia is induced by ligating the left anterior descending coronary artery. Reperfusion, initiated by release of the ligature, resulted in a significant decline in NADH-linked ADP-dependent mitochondrial respiration as assessed in isolated cardiac mitochondria. Assays of respiratory chain complexes revealed reduction in the activities of complex I and, to a lesser extent, complex IV exclusively during reperfusion, with no alterations in the activities of complexes II and III. Moreover, Krebs cycle enzymes alpha-ketoglutarate dehydrogenase and aconitase were susceptible to reperfusion-induced inactivation with no decline in the activities of other Krebs cycle enzymes. The decline in alpha-ketoglutarate dehydrogenase activity during reperfusion was associated with a loss in native lipoic acid on the E2 subunit, suggesting oxidative inactivation. Inhibition of complex I in vitro promotes free radical generation. alpha-Ketoglutarate dehydrogenase and aconitase are uniquely susceptible to in vitro oxidative inactivation. Thus, our results suggest a scenario in which inhibition of complex I promotes free radical production leading to oxidative inactivation of alpha-ketoglutarate dehydrogenase and aconitase.     

Chronic propranolol treatment decreases pulmonary artery pressure in conscious dogs

Daily administration of propranolol to 9 chronically instrumented, trained dogs for 2 weeks caused significant (p less than 0.05) decreases in heart rate (70 +/- 8 to 57 +/- 6 beats/min), cardiac output (3.6 +/- 0.3 to 2.9 +/- 0.2 liters/min), pulmonary arterial pressure (15.7 +/- 0.5 to 10.0 +/- 0.5 mm Hg) and total pulmonary vascular resistance (4.6 +/- 0.6 to 3.3 +/- 0.4 units). Nadolol, a structurally dissimilar beta-adrenergic receptor antagonist, caused a similar decrease in total pulmonary resistance. Acute meclofenamate administration did not return to normal pulmonary arterial pressure and resistance in the dogs chronically treated with beta-adrenergic receptor blockers. We therefore conclude that chronic beta-adrenergic receptor blockade lowered pulmonary arterial pressure and resistance by a mechanism independent of cyclooxygenase. In addition, chronic beta-adrenergic receptor blockade did not affect the potential for hypoxic vasoconstriction.