Sympathetic Influences in Myocardial Ischemia: Possible Mechanisms of Arrhythmogenesis

Antiarrhythmic effect of acupuncture pretreatment in rats subjected to simulative global ischemia and reperfusion--involvement of adenylate cyclase, protein kinase A, and L-type Ca2+ channel

Our previous study showed that electro-acupuncture (EA) pretreatment protects the heart from injury of ischemia. The present study explored further whether adenylate cyclase (AC), protein kinase A (PKA), and L-type Ca(2+) channel, the beta(1)-AR signaling components modulating intracellular Ca(2+) ([Ca(2+)](i)), are involved in the mediation of the antiarrhythmic effect of EA pretreatment in the rats from which the hearts were subsequently isolated and subjected to simulative global ischemia and reperfusion (SGIR). SGIR was performed by perfusing the isolated heart at a low flow followed by normal perfusion. Adult rats were randomized into four groups, namely, normal control (NC), SGIR, EA, and NC plus EA (NCEA) groups. The rats in the EA and NCEA groups were given EA pretreatment at bilateral Neiguan points (PC6) for 30 min once a day in 3 consecutive days before the hearts were isolated and perfused. The arrhythmia score and the response of [Ca(2+)](i) to the activators of AC, PKA, and L-type Ca(2+) channel in single ventricular myocyte isolated from the hearts subjected to SGIR were compared among the groups. The results showed that the arrhythmia score was significantly higher in the SGIR group as compared with the NC and NCEA groups. The SGIR-enhanced arrhythmia score was significantly attenuated in the EA group. More interesting, EA pretreatment also attenuated the SGIR-enhanced response of [Ca(2+)](i) to the activators of AC, PKA, and the L-type Ca(2+) channel in the myocytes isolated from the hearts subjected to SGIR. In conclusion, EA pretreatment can produce an antiarrhythmic effect in the rat of SGIR. AC, PKA and the L-type Ca(2+) channel are involved in the mediation of the antiarrhythmic effect of EA pretreatment.

Sympathetic neural responses to coronary occlusion during balloon angioplasty

OBJECTIVE

The sympathetic nervous system is a key modulator of prognosis and outcome in cardiac ischaemia and infarction. The effects of acute cardiac ischaemia on sympathetic neural traffic in humans are unknown. We tested the hypothesis that angioplasty, and associated transient myocardial ischaemia, elicits changes in neural circulatory control, including direct intraneural measures of sympathetic traffic.

METHODS

We measured muscle sympathetic nerve activity (MSNA), blood pressure and heart rate in 12 patients (11 men, one woman) undergoing clinically indicated elective coronary angioplasty of the left anterior descending (n = 7) or circumflex (n = 5) coronary artery. Baseline data were obtained for 2 min before occlusion. Each balloon inflation was designed to last up to 120 s.

RESULTS

Coronary occlusion had no significant effect on blood pressure or heart rate. In contrast, occlusion resulted in a significant increase in MSNA, in 10 of the 12 patients, and in all seven of those patients undergoing angioplasty of the left anterior descending artery. The group mean increase in MSNA during occlusion was 36 +/- 11% (P = 0.008 versus preocclusion). MSNA increased within 60 s of occlusion (129 +/- 12% of baseline; n = 12; P = 0.04), and increased further during the next 60 s to 141 +/- 12% of baseline levels (n = 10; P < 0.001). Increases in MSNA were similar in patients with and without significant chest pain (39 +/- 9% versus 34 +/- 13%, respectively; P = 0.84).

CONCLUSION

Acute coronary occlusion during angioplasty increases central sympathetic outflow, but there is no systematic change in heart rate. The increase in sympathetic nerve traffic cannot be explained by blood pressure changes or occlusion-related chest pain.

The effect of rapid lipid lowering with atorvastatin on autonomic parameters in patients with coronary artery disease

Many cardiovascular disease states are associated with autonomic dysfunction, specifically sympathetic activation and parasympathetic withdrawal. Both these autonomic derangements are independently associated with adverse prognostic outcomes. HMG CoA reductase inhibitors (statins) reduce cardiovascular mortality and morbidity when compared to placebo in subjects with proven coronary artery disease (CAD), including sudden presumed arrhythmic death. As autonomic dysfunction is associated with arrhythmogenesis, statins may be having a beneficial effect on autonomic function in these subjects. We conducted a randomised, double-blind, placebo-controlled, cross-over study examining the effect of rapid short-term lipid lowering with a statin on autonomic function in CAD patients. Ten subjects with proven CAD (8 male, 2 female; mean age 63.4 years) were randomised to receive either 80 mg atorvastatin or placebo over a 4 week period followed by a 4 week washout, then the alternative treatment for a further 4 weeks. Autonomic parameters assessed were plasma noradrenaline levels on recumbency and 80 degrees head-up tilt, cold pressor testing, and heart rate variability (HRV) analysis. Plasma noradrenaline levels were significantly reduced (p=0.050) after 20 min rest in the recumbent position, with atorvastatin compared to placebo. A nonsignificant reduction in plasma noradrenaline with atorvastatin compared to placebo was observed in the prolonged 80 degrees head-up position (p=0.207). In addition, sympathovagal balance was shifted to greater vagal predominance with atorvastatin (low-frequency/high-frequency ratio in the HRV frequency domain) when compared to placebo, p=0.06. We found that rapid lipid lowering with atorvastatin reduces sympathetic nervous system in this pilot study of CAD patients. Larger trials are required to definitively address the effects of statins on autonomic activity in these patients.

Autonomic control of ventricular tachycardia. III. Effects of adenosine and N6-R-1-phenyl-2-propyladenosine

The purpose of this study was to determine whether adenosine or the adenosine deaminase-resistant analogue, N6-R-1-phenyl-2-propyladenosine (RPIA), could slow the rate of spontaneous ventricular tachycardia occurring 24 hours after left anterior descending coronary artery occlusion. Chloralose-anesthetized, open chest dogs (n = 25) with ventricular tachycardia were studied. The left anterior descending artery was cannulated distally. Intracoronary infusions of adenosine, 10(-7) to 10(-5) M, did not alter the rate of ventricular tachycardia. Ventricular tachycardia slowed by 4.6% with adenosine, 10(-4) M. RPIA, 10(-6) to 10(-4) M, produced a concentration-dependent decrease in the rate of ventricular tachycardia when injected into the left anterior descending coronary artery. This effect of RPIA was reversed by the adenosine antagonist aminophylline, 10(-5) M. After bilateral stellate ganglionectomy, RPIA, 10(-5) M, did not, but metoprolol, 0.5 mg, did slow ventricular tachycardia after intracoronary injection. However, RPIA, 10(-5) M, produced a 43% decrease in the increment in ventricular tachycardia occurring during sympathetic neural stimulation. Therefore, when injected into the left anterior descending artery, adenosine, 10(-4) M, and RPIA, 10(-6) to 10(-4) M, decrease the rate of ventricular tachycardia in 24 hour old myocardial infarction. Furthermore, this decrease in the rate of ventricular tachycardia is the result of prejunctional sympathetic antagonism.

Autonomic control of ventricular tachycardia: direct effects of beta-adrenergic blockade in 24 hour old canine myocardial infarction

The purpose of this study was to determine whether alpha- or beta-adrenergic influences directly modulate the rate of spontaneous ventricular tachycardia occurring 24 hours after left anterior descending coronary artery occlusion. Chloralose-anesthetized, open chest dogs (n = 41) with ventricular tachycardia were studied. The left anterior descending artery was cannulated distally. Neither intracoronary saline solution nor phenylephrine (0.3 to 12 micrograms) changed the rate of ventricular tachycardia; however, isoproterenol (0.01 to 10 micrograms) produced dose-dependent increases in the rate. In six dogs, metoprolol, 5 mg given intravenously, slowed ventricular tachycardia from 174 +/- 10 (mean +/- SE) to 140 +/- 17 beats/min (p less than 0.05). This was accompanied by decreases in mean arterial pressure from 106 +/- 7 to 95 +/- 8 mm Hg, cardiac output from 2.6 +/- 0.3 to 1.6 +/- 0.3 liters/min and prolongation of atrioventricular conduction from 134 +/- 10 to 189 +/- 29 ms (all p less than 0.05) during atrial pacing at a cycle length of 300 ms. In 10 dogs, metoprolol (0.5 mg) given intracoronary, a dose that shifted the isoproterenol dose-response curve to the right, slowed ventricular tachycardia from 174 +/- 7.2 to 140 +/- 9.7 beats/min (p less than 0.05) without hemodynamic changes. Additional metoprolol (4.5 mg) given intravenously produced hemodynamic alterations, but ventricular tachycardia did not slow further. Therefore, beta- but not alpha-adrenergic influences control the rate of ventricular tachycardia occurring 24 hours after left anterior descending coronary artery occlusion. Furthermore, beta-adrenergic blockade slows ventricular tachycardia solely by a direct electrophysiologic effect on the tachycardia foci and not indirectly as a result of hemodynamic effects.

Autonomic control of ventricular tachycardia: sympathetic neural influence on spontaneous tachycardia 24 hours after coronary occlusion

This study was performed to determine whether sympathetic nerves influence the rate of ventricular tachycardia occurring spontaneously in dogs 24 hr after occlusion of the anterior descending coronary artery. Seventeen chloralose-anesthetized dogs underwent activation mapping during spontaneous ventricular tachycardia with QRS morphologies similar to those recorded in the conscious state. Bilateral stellate ganglionectomy (n = 8) decreased mean arterial pressure from 71 +/- 4 (mean +/- SE) to 52 +/- 5 mm Hg (p less than .001) and heart rate from 121 +/- 9 to 79 +/- 15 beats/min (p less than .025) by decreasing the number of complexes of ventricular tachycardia from 120 +/- 9 to 49 +/- 15 per minute (p less than .001). Subsequent unilateral sympathetic nerve stimulation (n = 4) was shown to accelerate ventricular tachycardia foci originating from the ipsilateral aspect of the infarction. Regional sympathetic denervation (n = 7) was performed by application of phenol to the epicardium surrounding an electrode at the site of origin of at least one morphology of ventricular tachycardia. Mean arterial pressure did not change, but total heart rate decreased from 122 +/- 9 to 106 +/- 9 beats/min (p less than .01) and the number of complexes of ventricular tachycardia with a morphology arising from the phenol-treated area fell from 68 +/- 12 to 28 +/- 9 (p less than .001). Evidence for regional denervation was documented by prolongation of duration of electrograms and local repolarization times limited to the phenol-treated area. We conclude that sympathetic nerves directly control rate of spontaneous ventricular tachycardia 24 hr after myocardial infarction in the dog.

Carotid sinus baroreflex influence on electrophysiologic properties of the canine atrioventricular node and ventricle

This study examines the efferent mechanisms of carotid sinus baroreflex influence on ventricular repolarization and refractory period compared with effects on atrioventricular (AV) nodal conduction. Pressure was controlled in both carotid sinuses by the Moisejeff technique in 16 chloralose-anesthetized dogs. Increases in carotid sinus pressure during pacing produced graded prolongation of AV nodal conduction, ventricular repolarization and refractory period with a threshold at a carotid sinus pressure of 120 mm Hg and a peak response at 200 mm Hg. Atropine, 0.4 mg/kg, attenuated the peak percent change in ventricular repolarization interval by only 12 +/- 14% (+/- standard error of the mean) despite a significantly greater attenuation (48 +/- 11%, p less than 0.05) in peak percent change in AV nodal conduction. However, stellate ganglionectomy attenuated the peak percent change in ventricular repolarization (42 +/- 19%), similar to effects on AV nodal conduction (59 +/- 21%, p greater than 0.25). Changes in mean arterial pressure, ventricular end-diastolic segment length or segment length shortening with systole (sonomicrometer technique) did not account for the electrophysiologic responses. Latency to peak effect on ventricular repolarization (43 +/- 7 seconds) was slower than that on AV nodal conduction (23 +/- 6 seconds, p less than 0.05). This difference in time course was not abolished by atropine. Thus, the carotid sinus baroreflex prolongs ventricular repolarization and refractoriness mainly by withdrawal of sympathetic influence; AV nodal conduction is prolonged by both vagal activation and sympathetic withdrawal. In addition, differences in time course between ventricular and AV nodal electrophysiologic responses are not explained by different efferent autonomic mechanisms.