Translational neurocardiology: preclinical models and cardioneural integrative aspects

Neuronal elements distributed throughout the cardiac nervous system, from the level of the insular cortex to the intrinsic cardiac nervous system, are in constant communication with one another to ensure that cardiac output matches the dynamic process of regional blood flow demand. Neural elements in their various 'levels' become differentially recruited in the transduction of sensory inputs arising from the heart, major vessels, other visceral organs and somatic structures to optimize neuronal coordination of regional cardiac function. This White Paper will review the relevant aspects of the structural and functional organization for autonomic control of the heart in normal conditions, how these systems remodel/adapt during cardiac disease, and finally how such knowledge can be leveraged in the evolving realm of autonomic regulation therapy for cardiac therapeutics.

n-3 (omega-3) polyunsaturated fatty acids prevent acute atrial electrophysiological remodeling

BACKGROUND AND PURPOSE

Recent reports suggest that n-3 (omega-3) polyunsaturated fatty acids (PUFAs) may reduce atrial fibrillation (AF). Reduction of the atrial effective refractory period (ERP) is believed to be an important early remodeling event that favors the development and perpetuation of AF. We hypothesized that n-3 PUFAs would attenuate early atrial electrophysiolgical remodeling in a canine model of acute atrial tachypacing.

EXPERIMENTAL APPROACH

Adult dogs of either sex received n-3 PUFAs (n=6), n-6 PUFAs (n=6), or saline (n=6) infused over 1 h. After a stable ERP was established, treatment was initiated concurrently with 6 h of rapid atrial pacing (400 b.p.m.). Serial right atrial ERPs were measured during rapid atrial pacing, and induction of atrial tachyarrhythmias was attempted at the conclusion of each study.

KEY RESULTS

There was no change in P wave duration or in the PQ, QRS, QT or QTc intervals in any of the treatment groups. N-3 PUFA treatment significantly reduced the shortening of atrial ERP, compared to both control groups (P<0.05). In separate experiments, the same n-3 PUFA infusion was given to dogs remaining in normal sinus rhythm. During sinus rhythm, n-3 PUFA infusion did not alter any electrocardiogram (ECG) parameter or the atrial ERP.

CONCLUSIONS AND IMPLICATIONS

We conclude that acute n-3 PUFA treatment prevents acute atrial electrophysiological remodeling during high rate activity, which may minimize the self-perpetuation of AF.

Membrane Effects of the n-3 Fish Oil Fatty Acids, which Prevent Fatal Ventricular Arrhythmias

Fish oil fatty acids are known to exert beneficial effects on the heart and vascular systems. We have studied the membrane effects on ion channel conductance by the n-3 fish oil fatty acids that account for these beneficial effects. We have confirmed that these fatty acids prevent fatal cardiac arrhythmias in a reliable dog model of sudden cardiac death. This finding was followed by experiments indicating that the n-3 fatty acids electrically stabilize heart cells and do so largely through modulation of the fast voltage-dependent Na(+) currents and the L-type Ca(2+) channels in a manner, which makes the heart cells resistant to arrhythmias. Others and we have demonstrated that these membrane effects on the heart can prevent fatal cardiac arrhythmias in humans.

KB-R7943. Kanebo

Kanebo is investigating KB-R7943, a Na+/Ca2+ ion exchange inhibitor, for the potential treatment of ischemia and reperfusion injury. It inhibited the outward Na+/Ca2+ exchange current (iNCX) more potently than the inward current under unidirectional flow conditions; however, inward and outward current were inhibited equally under bidirectional conditions. The drug was a competitive inhibitor to external calcium, and the inhibition was reversible with a recovery t1/2 of about 30 s. The mammalian Na+/Ca2+ exchanger forms a multigene family of homologous proteins comprising three isoforms, NCX1, NCX2 and NCX3. By examining chimeric constructs between NCX1 and NCX3 expressed in CCL39 cells, it has been demonstrated that it is the conserved internal repeat regions (alpha-1 and alpha-2) of the exchanger that are critical for the drug's action.

Enhanced in vivo and in vitro contractile responses to beta(2)-adrenergic receptor stimulation in dogs susceptible to lethal arrhythmias

The response to beta-adrenergic receptor (beta-AR) stimulation was evaluated in both isolated cardiomyocytes (video edge detection) and the intact animal (echocardiography) in dogs either susceptible (S) or resistant (R) to ventricular fibrillation induced by a 2-min coronary occlusion during the last minute of exercise. In the intact animal, velocity of circumferential fiber shortening (Vcf) was evaluated both before (n = 27, S = 12 and R = 15) and after myocardial infarction. Before infarction, increasing doses of isoproterenol provoked similar contractile and heart rate responses in each group of dogs. Either beta(1)-AR (bisoprolol) or beta(2)-AR (ICI-118551) antagonists reduced the isoproterenol response, with a larger reduction noted after the beta(1)-AR blockade. In contrast, after infarction, isoproterenol induced a significantly larger Vcf and heart rate response in the susceptible animals that was eliminated by beta(2)-AR blockade. The single-cell isotonic shortening response to isoproterenol (100 nM) was also larger in cells obtained from susceptible compared with resistant dogs and was reduced to a greater extent by beta(2)-AR blockade in the susceptible dog myocytes (S, -48%, n = 6; R, -15%, n = 9). When considered together, these data suggest that myocardial infarction provoked an enhanced beta(2)-AR response in susceptible, but not resistant, animals.

beta(2)-Adrenoceptors and ventricular fibrillation

beta-Adrenoceptor antagonists significantly reduce the incidence of sudden cardiac death in patients with contractile dysfunction. Contractile dysfunction is associated with a decline in beta(1)-adrenoceptors, no change in the number of beta(2)-adrenoceptors, and an increased responsiveness to beta(2)-adrenoceptor stimulation. Selective beta(2)-adrenoceptor blockade prevents ventricular fibrillation in a canine model of sudden cardiac death. Cardiac beta(2)-adrenoceptor stimulation increases L-type Ca(2+) currents, but unlike beta(1)-adrenoceptor stimulation, it fails to elicit phospholamban phosphorylation. Restoration of resting diastolic [Ca(2+)] following beta(2)-adrenoceptor-mediated increases in Ca(2+) influx is more dependent on Na(+)/Ca(2+) exchange, which generates an arrhythmogenic transient inward current that can trigger ventricular fibrillation.

Antiarrhythmic efficacy of selective blockade of the cardiac slowly activating delayed rectifier current, I(Ks), in canine models of malignant ischemic ventricular arrhythmia

BACKGROUND

To date, the lack of potent and selective inhibitors has hampered the physiological assessment of modulation of the cardiac slowly activating delayed rectifier current, I(Ks). The present study, using the I(Ks) blocker L-768,673, represents the first in vivo assessment of the cardiac electrophysiological and antiarrhythmic effects of selective I(Ks) blockade.

METHODS AND RESULTS

In an anesthetized canine model of recent (8.5+/-0.4 days) anterior myocardial infarction, 0.003 to 0.03 mg/kg L-768,673 IV significantly suppressed electrically induced ventricular tachyarrhythmias and reduced the incidence of lethal arrhythmias precipitated by acute, thrombotically induced posterolateral myocardial ischemia. Antiarrhythmic protection afforded by L-768,673 was accompanied by modest 7% to 10% increases in noninfarct zone ventricular effective refractory period, 3% to 5% increases in infarct zone ventricular effective refractory period, and 4% to 6% increases in QTc interval. In a conscious canine model of healed (3 to 4 weeks) anterior myocardial infarction, ventricular fibrillation was provoked by transient occlusion of the left circumflex coronary artery during submaximal exercise. Pretreatment with 0.03 mg/kg L-768,673 IV elicited a modest 7% increase in QTc, prevented ventricular fibrillation in 5 of 6 animals, and suppressed arrhythmias in 2 additional animals.

CONCLUSIONS

The present findings suggest that selective blockade of I(Ks) may be a potentially useful intervention for the prevention of malignant ischemic ventricular arrhythmias.

The antiarrhythmic and anticonvulsant effects of dietary N-3 fatty acids

It has been shown in animals and probably in humans, that n-3 polyunsaturated fatty acids (PUFAs) are antiarrhythmic. We report recent studies on the antiarrhythmic actions of PUFAs. The PUFAs stabilize the electrical activity of isolated cardiac myocytes by modulating sarcolemmal ion channels, so that a stronger electrical stimulus is required to elicit an action potential and the refractory period is markedly prolonged. Inhibition of voltage-dependent sodium currents, which initiate action potentials in excitable tissues, and of the L-type calcium currents, which initiate release of sarcoplasmic calcium stores that increase cytosolic free calcium concentrations and activate the contractile proteins in myocytes, appear at present to be the probable major antiarrhythmic mechanism of the PUFAs.

Experimental studies on antiarrhythmic and antiseizure effects of polyunsaturated fatty acids in excitable tissues

It has been shown that in animals, and probably in humans, n-3 polyunsaturated fatty acids (PUFAs) are antiarrhythmic. We discuss our recent studies on the antiarrhythmic actions of PUFAs. PUFAs stabilize the electrical activity of isolated cardiac myocytes by requiring a stronger electrical stimulus to elicit an action potential and by markedly prolonging the refractory period. These electrophysiologic effects are the result of specific modulation of ion currents, particularly of the voltage-dependent sodium current and of the L-type calcium currents across sarcolemmal phospholipid membranes. This appears to be the probable major antiarrhythmic mechanism of PUFAs. However, they also similarly affect neuronal ion channels with potentially important functional effects on the nervous system.

n-3 fatty acids in the prevention of cardiac arrhythmias

In animals and probably in humans n-3 polyunsaturated fatty acids (PUFA) are antiarrhythmic. A report follows on the recent studies of the antiarrhythmic actions of PUFA. The PUFA stabilize the electrical activity of isolated cardiac myocytes by inhibiting sarcolemmal ion channels, so that a stronger electrical stimulus is required to elicit an action potential and the relative refractory period is markedly prolonged. This appears at present to be the probable major antiarrhythmic mechanism of PUFA.

Prevention of sudden cardiac death by dietary pure omega-3 polyunsaturated fatty acids in dogs

BACKGROUND

Rat diets high in fish oil have been shown to be protective against ischemia-induced fatal ventricular arrhythmias. Increasing evidence suggests that this may also apply to humans. To confirm the evidence in animals, we tested a concentrate of the free fish-oil fatty acids and found them to be antiarrhythmic. In this study, we tested the pure free fatty acids of the 2 major dietary omega-3 polyunsaturated fatty acids in fish oil: cis-5,8,11,14, 17-eicosapentaenoic acid (C20:5omega-3) and cis-4,7,10,13,16, 19-docosahexaenoic acid (C22:6omega-3), and the parent omega-3 fatty acid in some vegetable oils, cis-9,12,15-alpha-linolenic acid (C18:3omega-3), administered intravenously on albumin or a phospholipid emulsion.

METHODS AND RESULTS

The tests were performed in a dog model of cardiac sudden death. Dogs were prepared with a large anterior wall myocardial infarction produced surgically and an inflatable cuff placed around the left circumflex coronary artery. With the dogs running on a treadmill 1 month after the surgery, occlusion of the left circumflex artery regularly produced ventricular fibrillation in the control tests done 1 week before and after the test, with the omega-3 fatty acids administered intravenously as their pure free fatty acid. With infusion of the eicosapentaenoic acid, 5 of 7 dogs were protected from fatal ventricular arrhythmias (P<0.02). With docosahexaenoic acid, 6 of 8 dogs were protected, and with alpha-linolenic acid, 6 of 8 dogs were also protected (P<0.004 for each). The before and after control studies performed on the same animal all resulted in fatal ventricular arrhythmias, from which they were defibrillated.

CONCLUSIONS

These results indicate that purified omega-3 fatty acids can prevent ischemia-induced ventricular fibrillation in this dog model of sudden cardiac death.

Functional and electrophysiologic effects of polyunsaturated fatty acids on exictable tissues: heart and brain

It has been shown in animals and probably in humans, that n-3 polyunsaturated fatty acids (PUFAs) are antiarrhythmic. The free PUFAs stabilize the electrical activity of isolated cardiac myocytes by inhibiting sarcolemmal ion channels, so that a stronger electrical stimulus is required to elicit an action potential and the relative refractory period is markedly prolonged. This appears at present to be the probable major antiarrhythmic mechanism of the PUFAs. They similarly inhibit the Na+ and Ca2+ currents in rat hippocampal neurons which results in an increase in the electrical threshold for generalized seizures using the cortical stimulation model in rats.

Low-frequency component of the heart rate variability spectrum: a poor marker of sympathetic activity

The low-frequency component of the heart rate variability spectrum (0.06-0.10 Hz) is often used as an accurate reflection of sympathetic activity. Therefore, interventions that enhance cardiac sympathetic drive, e.g., exercise and myocardial ischemia, should elicit increases in the low-frequency power. Furthermore, because an enhanced sympathetic activation has been linked to an increased propensity for malignant arrhythmias, one might also predict a greater low-frequency power in animals that are susceptible to ventricular fibrillation than in resistant animals. To test these hypotheses, a 2-min coronary occlusion was made during the last minute of exercise in 71 dogs with healed myocardial infarctions: 43 had ventricular fibrillation (susceptible) and 28 did not experience arrhythmias (resistant). Exercise or ischemia alone provoked significant heart rate increases in both groups of animals, with the largest increase in the susceptible animals. These heart rate increases were attenuated by beta-adrenergic receptor blockade. Despite the sympathetically mediated increases in heart rate, the low-frequency power decreased, rather than increased, in both groups, with the largest decrease again in the susceptible animals: 4.0 +/- 0.2 (susceptible) vs. 4.1 +/- 0.2 ln ms2 (resistant) in preexercise control and 2.2 +/- 0.2 (susceptible) vs. 2.9 +/- 0.2 ln ms2 (resistant) at highest exercise level. In a similar manner the parasympathetic antagonist atropine sulfate elicited significant reductions in the low-frequency power. Although sympathetic nerve activity was not directly recorded, these data suggest that the low-frequency component of the heart rate power spectrum probably results from an interaction of the sympathetic and parasympathetic nervous systems and, as such, does not accurately reflect changes in the sympathetic activity.

HMR 1883, a novel cardioselective inhibitor of the ATP-sensitive potassium channel. Part II: effects on susceptibility to ventricular fibrillation induced by myocardial ischemia in conscious dogs

The activation of the ATP-sensitive potassium channel (KATP) during myocardial ischemia leads to potassium efflux, reductions in action potential duration and the formation of ventricular fibrillation (VF). Drugs that inactivate KATP should prevent these changes and thereby prevent VF. However, most KATP antagonists also alter pancreatic channels, which promote insulin release and hypoglycemia. Recently, a cardioselective KATP antagonist, HMR 1883, has been developed that may offer cardioprotection without the untoward side effects of existing compounds. Therefore, VF was induced in 13 mongrel dogs with healed myocardial infarctions by a 2-min coronary artery occlusion during the last minute of a submaximal exercise test. On subsequent days, the exercise-plus-ischemia test was repeated after pretreatment with HMR 1883 (3.0 mg/kg i.v., n = 13) or glibenclamide (1.0 mg/kg i.v., n = 7). HMR 1883 (P < .001) and glibenclamide (P < .01) prevented VF in 11 of 13 and 6 of 7 animals, respectively. Glibenclamide, but not HMR 1883, elicited increases in plasma insulin and reductions in blood glucose. Glibenclamide also reduced (P < .01) both mean coronary blood flow and left ventricular dP/dt maximum as well as the reactive hyperemia induced by 15-sec coronary occlusions (-30.3 +/- 11%), whereas HMR 1883 did not alter this increase in coronary flow (-3.0 +/- 4.7%). Finally, myocardial ischemia (n = 10) significantly (P < .01) reduced refractory period (control, 121 +/- 2 msec; occlusion, 115 +/- 2 msec), which was prevented by either glibenclamide or HMR 1883. Thus, the cardioselective KATP antagonist HMR 1883 can prevent ischemically induced reductions in refractory period and VF without major hemodynamic effects or alterations in blood glucose levels. These data further suggest that the activation of KATPs may play a particularly important role in both the reductions in refractory period and lethal arrhythmia formation associated with myocardial ischemia.

Effect of a 'vagomimetic' atropine dose on canine cardiac vagal tone and susceptibility to sudden cardiac death

We manipulated the level of cardiac vagal tone in dogs with healed myocardial infarctions during exercise plus acute ischemia, to explore vagal involvement in the pathophysiology of sudden cardiac death. We occluded the circumflex coronary artery during the last minute of treadmill exercise in 32 dogs with healed anterior myocardial infarctions. Twenty-one dogs experienced ventricular fibrillation (susceptible) and 11 did not (resistant). On a subsequent day, we gave intravenous low-dose atropine to susceptible dogs to increase their levels of cardiac vagal tone, as estimated by moving polynomial time-series analysis of R-R interval variability (0.24-1.04 Hz). We also measured vagal responses to coronary occlusion at rest, before and after low-dose atropine. In susceptible dogs, atropine increased the average vagal tone index at rest (atropine: 7.3 +/- 0.4 versus control: 6.6 +/- 0.5 ln ms2, P < 0.01) and during maximum exercise (atropine: 2.5 +/- 0.4 versus control: 1.6 +/- 0.3 ln ms2, P < 0.01), but failed to prevent ventricular fibrillation actually decreased from 63 +/- 3 to 42 +/- 2s (P < 0.01), and R-R interval shortening elicited by coronary occlusion increased (atropine: delta -144 +/- 64 versus control: delta -55 +/- 32 ms, P < 0.01). In resting susceptible dogs, atropine significantly increased preocclusion indexes of vagal tone (atropine: 7.8 +/- 0.3 versus control: 6.9 +/- 0.4 ln ms2, P < 0.01), but did not prevent large reductions of vagal tone during ischemia (atropine: delta -4.4 +/- 0.6 versus control: delta -3.8 +/- 0.4 ln ms2, P > 0.05). We conclude that increases of resting vagal tone after low-dose atropine in dogs with healed anterior myocardial infarctions do not protect against sudden cardiac death. Quite the contrary, vagal tone is withdrawn more completely during ischemia, and the time to ventricular fibrillation during exercise plus ischemia is shortened.

Dietary n-3 fatty acids in the prevention of cardiac arrhythmias

It has been shown that in animals, and probably in humans, n-3 polyunsaturated fatty acids (PUFAs) are antiarrhythmic. We discuss recent studies on the antiarrhythmic actions of polyunsaturated fatty acids. Polyunsaturated fatty acids stabilize the electrical activity of isolated cardiac myocytes by inhibiting sarcolemmal ion channels so that a stronger electrical stimulus is required to elicit an action potential and the relative refractory period is markedly prolonged. This appears at present to be the probable major antiarrhythmic mechanism of polyunsaturated fatty acids.

Prevention of ischemia-induced cardiac sudden death by n-3 polyunsaturated fatty acids in dogs

The objective of this study was to obtain functional information associated with the prevention by n-3 polyunsaturated fatty acids (PUFA) of ischemia-induced fatal cardiac ventricular arrhythmias in the intact, conscious, exercising dog. Thirteen dogs susceptible to ischemia-induced ventricular fibrillation were prepared surgically by ligation of their anterior descending left coronary artery and placement of an inflatable cuff around their left circumflex artery. After 4 wk of recovery, exercise-plus-ischemia tests were performed without and then with an intravenous infusion of an emulsion of free n-3 PUFA just prior to occluding the left circumflex artery while the animals were running on a treadmill. One week later the exercise-plus-ischemia test was repeated but with a control infusion replacing the emulsion of n-3 PUFA. The infusion of the free n-3 PUFA in quantities of 1.0 to 10 g prevented ventricular fibrillation in 10 of the 13 dogs tested (P < 0.005), apparently without esterification of the PUFA into membrane phospholipids. The antiarrhythmic effect of the n-3 PUFA was associated with slowing of the heart rate, shortening of the QT-interval (electrical action potential duration), reduction of left ventricular systolic pressure, and prolongation of the electrocardiographic atrial-ventricular conduction time (P-R interval). These effects are comparable with those we have reported in studies with cultured neonatal rat cardiac myocytes.

Beta2-adrenergic receptor antagonists protect against ventricular fibrillation: in vivo and in vitro evidence for enhanced sensitivity to beta2-adrenergic stimulation in animals susceptible to sudden death

BACKGROUND

The ventricular myocardium contains functional beta2-adrenergic receptors that when activated increase intracellular Ca2+ transients. Because elevated Ca2+ has been implicated in the induction of ventricular fibrillation (VF), it is possible that the activation of these receptors may also provoke malignant arrhythmias.

METHODS AND RESULTS

To test this hypothesis, a 2-minute occlusion of the left circumflex coronary artery was made during the last minute of exercise in 28 dogs with healed anterior myocardial infarctions: 17 had VF (susceptible) and 11 did not (resistant). On a subsequent day, this test was repeated after administration of the beta2-adrenergic receptor antagonist ICI 118,551 (0.2 mg/kg). This drug did not alter the hemodynamic response to the coronary occlusion, yet it prevented VF in 10 of 11 animals tested (P<.001). However, heart rate was reduced in 6 animals. Therefore, the ICI 118,551 exercise-plus-ischemia test was repeated with heart rate held constant by ventricular pacing (n=3). ICI 118,551 still prevented VF when heart rate was maintained. Next, the effects of increasing doses of the beta2-adrenergic receptor agonist zinterol on Ca2+ transient amplitudes were examined in ventricular myocytes. Zinterol elicited significantly greater increases in Ca2+ transient amplitudes at all doses tested (10(-9) to 10(-6) mol/L) in myocytes prepared from susceptible versus resistant animals. The cardiomyocyte response to isoproterenol (10(-7) mol/L) in the presence or absence of the selective beta1- (CGP-20712A, 300 nmol/L) or beta2- (ICI 118,551, 100 nmol/L) adrenergic receptor antagonist was also examined. Isoproterenol elicited larger Ca2+ transient increases in the susceptible myocytes, which were eliminated by ICI but not by CGP.

CONCLUSIONS

When considered together, these data demonstrate that canine myocytes contain functional beta2-adrenergic receptors that are activated to a greater extent in the susceptible animals. The resulting cytosolic Ca2+ transient increases may lead to afterpotentials that ultimately trigger VF in these animals.

Effects of calcium channel antagonists on Ca2+ transients in rat and canine cardiomyocytes

First-generation Ca2+ channel antagonists depress myocardial contractility, but many of the newer Ca2+ channel blockers have a high degree of "vascular selectivity". This study compares the effects of the Ca2+ antagonists felodipine, amlodipine, mibefradil, verapamil and nifedipine, and the Ca2+ channel agonist. (S)(-)-Bay K-8644 on Ca2+ transient amplitudes in fura-2/AM-loaded rat and canine ventricular cardiomyocytes. At 10(-11) and 10(-10) M, felodipine increased [Ca2+]i transient amplitudes by 10-25% in field-stimulated fura-2-loaded cells from both species while at 10(-6) M it depressed [Ca2+]i transients by 80%. Mibefradil increased [Ca2+]i transient amplitudes by 16% at 10(-11) and 10(-10) M and decreased the transients by 25% at 10(-6) M. The calcium channel agonist, (S)(-)-Bay K-8644 increased [Ca2+]i transient amplitudes at 10(-10)-10(-6) M (maximally 37% at 10(-7) M) but depressed [Ca2+]i transients by 10% at 10(-5) M. Nifedipine was inhibitory at all concentrations tested (10(-11)-10(-6) M) in canine myocytes, but in rat cells. 10(-10) M nifedipine increased [Ca2+]i transient amplitudes by 37%. All concentrations of verapamil and amlodipine (10(-11)-10(-6) M) depressed [Ca2+]i transients in both rat and canine myocytes. We conclude that: (1) felodipine and mibefradil may be positive rather than negative inotropes at low concentrations, which are therapeutically relevant: and (2) low concentrations of nifedipine may have a positive inotropic effect in the rat but not the dog heart.

The effects of mibefradil, a novel calcium channel antagonist on ventricular arrhythmias induced by myocardial ischemia and programmed electrical stimulation

Calcium channel antagonists can reduce calcium overload induced by myocardial ischemia and thereby protect against malignant arrhythmias. However, these drugs may also adversely affect cardiac contractile function. Mibefradil is a new calcium antagonist that can inhibit cardiac calcium current without reducing myocardial force development. The effects of mibefradil on the inducibility of arrhythmias both before and during ischemia were therefore evaluated in animals with healed infarctions. First, a 2-min coronary occlusion was made during the last minute of exercise (n = 48): 25 animals had ventricular fibrillation (susceptible), whereas 23 did not (resistant). On a subsequent day, programmed electrical stimulation (PES, 8 paced beats followed by two extrastimuli) induced ventricular tachycardia in 19 of 25 susceptible animals but in none of the resistant animals (chi square = 24.6, P < .001). Verapamil (n = 14), diltiazem (n = 13) and mibefradil (n = 14) elicited significant dose-dependent decreases in refractory period and in the Q-Tc interval (except mibefradil) yet failed to prevent PES-induced arrhythmias. Diltiazem and verapamil also increased P-R interval and reduced the maximum rate of change of left ventricular pressure, whereas mibefradil did not. However, all three drugs abolished arrhythmias induced by PES during ischemia. In contrast, lidocaine suppressed PES-induced arrhythmias but failed to prevent ischemically induced arrhythmias. Thus mibefradil can prevent ischemically induced ventricular fibrillation without adverse actions on either A-V nodal conduction or contractile function. These data further suggest that calcium entry may play a critical role in the initiation of ventricular fibrillation during ischemia, whereas other factors must be responsible for the extrasystoles induced by PES.

Response of failing canine and human heart cells to beta 2-adrenergic stimulation

BACKGROUND

Failing human hearts lose beta 1- but not beta 2-adrenergic receptors. In canine hearts with tachypacing failure, the ratio of beta 2- to beta 1-adrenergic receptors is increased. The present study was designed to determine whether heart failure increases sensitivity to beta 2-adrenergic stimulation in isolated canine ventricular cardiomyocytes and to verify that myocytes from failing human ventricles contain functional beta 2-adrenergic receptors.

METHODS AND RESULTS

Myocytes from healthy dogs, dogs with tachypacing failure, and human transplant recipients were loaded with fura 2-AM and subjected to electric field stimulation in the presence of zinterol, a highly selective beta 2-adrenergic agonist. Zinterol significantly increased [Ca2+]i transient amplitudes in all three groups. The failing canine myocytes were significantly more responsive than normal to beta 2-adrenergic stimulation. We also measured isotonic twitches, indo-1 fluorescence transients, and L-type Ca2+ currents in healthy canine myocytes. Zinterol (10(-5) mol/L) elicited large increases in the amplitudes of simultaneously recorded twitches and [Ca2+]i transients. Zinterol also increased L-type Ca2+ currents in the normal canine myocytes; this augmentation was abolished by 10(-7) mol/L ICI 118,551. cAMP production by suspensions of healthy and failing canine myocytes was not increased by zinterol (10(-9) to 10(-5) mol/L), nor did 10(-5) mol/L zinterol elicit phospholamban phosphorylation.

CONCLUSIONS

Failing human ventricular cardiomyocytes contain functional beta 2-adrenergic receptors. Canine myocytes also contain functional beta 2-adrenergic receptors. The canine ventricular response to beta 2-agonists is increased in tachypacing failure. Positive inotropic responses to beta 2-stimulation are not mediated by increases in cAMP or cAMP-dependent phosphorylation of phospholamban.

Cocaine: a review of its toxic actions on cardiac function

It has become increasingly apparent that cocaine abuse can provoke lethal cardiac events, including myocardial infarction and ventricular fibrillation. The mechanisms responsible for these cardiotoxic actions of cocaine largely remain to be determined. Cocaine has two primary pharmacological properties that can adversely affect the heart and vasculature. Cocaine acts both as a local anesthetic (sodium and potassium channel blockade) and as a powerful cardiac stimulant that accentuates the actions of the sympathetic nervous system (inhibition of central and peripheral neuronal catecholamine uptake). The local anesthetic properties could impair impulse conduction, as well as elicit inhomogeneities in repolarization (refractory period), which creates an ideal substrate for reentrant arrhythmias. In addition, high doses of cocaine can depress contractile function due to inhibition of sodium/calcium exchange that results from decreased sodium influx (local anesthetic action). These actions are particularly obvious when sympathomimetic effects of cocaine are blunted. In a similar manner, the cocaine-induced accumulation of catecholamines potentiates the activation of alpha- and beta-adrenergic receptors, which can provoke coronary vasospasm (myocardial ischemia and infarction), increased contractile force (increased metabolic demand), and cardiac arrhythmias. The activation of adrenergic receptors will elicit a cascade of second messengers, ultimately provoking an increase in cytosolic calcium. These elevations in cytosolic calcium can provoke oscillations in membrane potential, triggering sustained action potential generation and extrasystoles. In particular, activation of the alpha IA-adrenergic receptor subtype and corresponding increase in calcium influx via voltage sensitive (L type) channels may play a critical role in the genesis of malignant arrhythmias. Thus, the adrenergic and local anesthetic properties of cocaine could act synergistically to elicit toxic actions on the heart.

Effect of alpha 1-adrenergic receptor antagonists on susceptibility to malignant arrhythmias: protection from ventricular fibrillation

alpha-Adrenergic receptor responsiveness has been reported to increase during myocardial ischemia, correlating with onset of malignant arrhythmias. If alpha-adrenoceptor mechanisms play a significant role in induction of life-threatening arrhythmias, inhibition of these receptors with specific alpha-adrenoceptor antagonists should protect against disturbances in cardiac rhythm. To test this hypothesis, we induced ventricular fibrillation (VF) in 21 mongrel dogs with healed myocardial infarctions (MI) by 2-min coronary artery occlusion during exercise. On a subsequent day, the exercise plus ischemia test was repeated after the alpha 1-adrenoceptor antagonist prazosin HCl (0.5 mg/kg intravenously, i.v.; n = 14) or the alpha 1A-adrenergic receptor subtype antagonist WB4101 (2.0 mg/kg i.v., n = 9). Prazosin elicited a significant reduction in left ventricular systolic pressure (LVSP, control 157.0 +/- 6.5 vs. prazosin 118.5 +/- 2.7 mm Hg) and prevented arrhythmias in 13 of 14 animals (chi square p < 0.001). No other hemodynamic parameters, either before or during the coronary occlusion, were altered by prazosin. WB4101 did not alter any hemodynamic parameters either before or during coronary artery occlusion, yet prevented VF in 7 of 9 animals (chi square p < 0.025), delaying onset of malignant arrhythmias in the remaining animals. A second control exercise plus ischemia test reproducibly induced VF in all animals. Together these data demonstrate that alpha-adrenoceptor antagonists can prevent VF independent of hemodynamic changes. In particular, the data suggest that activation of the alpha 1A-adrenergic receptor subtype may contribute importantly to development of malignant arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of ATP sensitive potassium channel in extracellular potassium accumulation and cardiac arrhythmias during myocardial ischaemia

Extracellular potassium rises rapidly during myocardial ischaemia, correlating with the onset of ventricular arrhythmias. The extracellular accumulation of potassium can induce abnormalities in both impulse conduction and impulse generation. Inhomogeneities of potassium conductance will elicit regional differences in action potential duration and repolarisation. The resulting spatial dispersion of refractory period will allow for fragmentation of impulse conduction on ensuing beats, the formation of irregular reentrant pathways and ventricular fibrillation. In a similar manner, the spread of injury current from the ischaemic tissue to surrounding normal tissue can trigger extrasystoles (depolarisation induced automaticity). It has been hypothesised that the activation of the ATP sensitive potassium channel contributes significantly to reductions in action potential duration and increases in extracellular potassium accumulation during myocardial ischaemia. ATP sensitive potassium channel antagonists prevent ischaemically induced reductions in action potential duration and the dispersion of refractory period but may induce oscillatory afterpotentials under some conditions (for example, calcium overload). In contrast, potassium channel agonists enhance the dispersion of refractory period ischaemia, which promotes the formation of re-entrant arrhythmias. The pharmacological modulation of the ATP sensitive potassium channels could therefore offer a novel approach for the management of cardiac arrhythmias in patients with ischaemic heart disease. In general, channel antagonists prevent ventricular fibrillation, while high (hypotensive) doses of channel agonists can induce malignant arrhythmias during ischaemia in animal models. However, recent evidence also suggests that potassium channel agonists may promote a better preservation of myocardial mechanical performance during reperfusion while ATP sensitive potassium channel antagonists exacerbate mechanical depression during ischaemia in experimental models.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevention of ischemia-induced ventricular fibrillation by omega 3 fatty acids

A specially prepared dog model of myocardial infarction was used to test the efficacy of the long-chain polyunsaturated fish oil omega 3 fatty acids eicosapentaenoic (20:5 n-3) and docosahexaenoic (22:6 n-3) acids to prevent ischemia-induced malignant cardiac arrhythmias. The dogs had sustained a prior experimental myocardial infarction from ligation of the left anterior descending coronary artery, and a hydraulic cuff was implanted around the left circumflex artery at that operation. After recovery from that procedure the animals were tested during a treadmill exercise test. With compression of the left circumflex artery sensitive animals will predictably develop ventricular fibrillation (VF). In such prepared dogs an emulsion of fish oil fatty acids was infused i.v. over a 50- to 60-min period just before the exercise-plus-ischemia test, and the effect on development of VF was recorded. The infusion was 100 ml of a 10% (vol/vol) emulsion of a fish oil concentrate containing 70% omega 3 fatty acids with free eicosapentaenoic acid and docosahexaenoic acid composing 33.9% and 25.0% of that total, respectively. Alternatively, some animals similarly received an emulsion containing 5 ml of the free fatty acid concentrate plus 5 ml of a triacylglyerol concentrate containing 65% omega 3 fatty acids with eicosapentaenoic acid and docosahexaenoic acid composing 34.0% and 23.6% of that total, respectively. In seven of eight animals the infusion of the fish oil emulsion completely prevented the acute occurrence of VF in the susceptible animals (P < 0.005). In five of five of these animals the subsequent exercise-plus-ischemia test after a similar infusion of an emulsion in which soy bean oil replaced the fish oil fatty acid concentrates resulted in prompt development of VF. Possible mechanisms for this protective effect of omega 3 fatty acids against exercise and ischemia-induced malignant arrhythmias are considered.

The effect of adrenergic receptor antagonists on cocaine-induced ventricular fibrillation: alpha but not beta adrenergic receptor antagonists prevent malignant arrhythmias independent of heart rate

Cocaine-induced increases in catecholamines and the resulting enhanced activation of myocardial adrenergic receptors could contribute significantly to the formation of ventricular fibrillation (VF). In order to test this hypothesis, a 2-min coronary occlusion was initiated during the last minute of exercise in instrumented mongrel dogs. Forty-one animals were selected in which this test failed to provoke ventricular arrhythmias. The test was repeated after cocaine HCl (1.0 mg/kg). Cocaine significantly (P < .01) increased heart rate, left ventricular systolic pressure and positive left ventricular dP/dt, as well as elicited VF in 34 animals. The cocaine exercise plus ischemia test was repeated in animals after pretreatment with either the beta adrenergic receptor antagonist propranolol HCl (1.0 mg/kg, n = 14) or the alpha adrenergic receptor antagonist prazosin HCl (0.5 mg/kg, n = 15). Both propranolol and prazosin reduced the hemodynamic effects of cocaine and prevented VF in 12 of 14 and 12 of 15 animals, respectively. The studies were then repeated with heart rate matched to the cocaine heart rate by ventricular pacing. Prazosin (n = 5) but not propranolol (n = 4) still prevented VF even with heart rate held constant. Finally, the alpha-1A adrenergic receptor subtype antagonist WB4101 (2.0 mg/kg, n = 10) also prevented cocaine VF in 7 of 10 animals without changing heart rate. In contrast, the alpha-1B adrenergic receptor antagonist chloroethylclonidine (2.0 mg/kg, n = 3) failed to prevent VF. Thus, alpha but not beta adrenergic receptor antagonists can prevent cocaine-induced malignant arrhythmias independently of their action on heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of interventions that increase cyclic AMP levels on susceptibility to ventricular fibrillation in unanesthetized dogs

Although the autonomic nervous system has been implicated in the formation of ventricular fibrillation, the precise mechanism by which this is mediated remains undetermined. In particular, the role of second messengers, generated by beta-adrenoceptor activation, has been postulated to mediate the pro-arrhythmic effects of the sympathetic nervous system. Thus, a 2 min occlusion of the left circumflex coronary artery was initiated during the last minute of exercise in canines with healed myocardial infarctions (produced by ligation of left anterior descending artery). Fifteen dogs were found to be susceptible to the formation of ventricular fibrillation while 17 animals were resistant. Nine resistant dogs were treated with the phosphodiesterase inhibitor isobutylmethyl xanthine (IBMX, 1 mg/kg) in combination with an infusion of 8-bromo-cAMP (100-150 micrograms/kg/min beginning 45 min prior to exercise). Heart rate and left ventricular dP/dtmax significantly increased, but failed to elicit, arrhythmias during the exercise and ischemia test. Nine resistant animals were also treated with the adenylate cyclase activator forskolin, (100 micrograms/kg), which provoked the same hemodynamic changes as the cyclic AMP infusion but also failed to induce ventricular fibrillation. Both forskolin (n = 3) and IBMX (n = 3) induced large increases in myocardial cAMP levels (control 5.2 +/- 0.5, forskolin 8.1 +/- 0.8 pmol/mg non-collagen protein; control 5.0 +/- 0.8, IBMX 6.8 +/- 0.3 pmol/mg non-collagen protein). Ten resistant animals were treated with the beta-adrenoceptor agonist isoproterenol (1-10 micrograms/kg/min), which failed to cause ventricular fibrillation despite significant increases in the hemodynamic parameters described above. Finally, experiments were repeated after 8-bromo-cAMP infusion and IBMX pretreatment in 8 susceptible animals with pharmacologic denervation (atropine+propranolol+prazosin). In spite of hemodynamic increases indicative of an increase in myocardial cyclic AMP levels, arrhythmias were not re-introduced. These data suggest that changes in cAMP may not be responsible for ventricular fibrillation in this model of sudden cardiac death.

Effects of cocaine on cardiac vagal tone before and during coronary artery occlusion: cocaine exacerbates the autonomic response to myocardial ischemia

Cocaine is a potent sympathomimetic drug that can provoke lethal cardiac events. Cocaine-induced alterations in autonomic balance, particularly during myocardial ischemia, could contribute significantly to these adverse reactions. To test this hypothesis, we produced a 2-min left circumflex coronary artery (LCX) occlusion in unanesthetized mongrel dogs (n = 7) instrumented to measure left ventricular pressure (LVP), ventricular electrogram, and coronary blood flow (CBF) with and without various doses of cocaine (0.0, 0.5, 1, 2, and 4 mg/kg). At least 24 h elapsed between cocaine doses, which were given in random order. Time series analysis of heart rate (HR) variability was used as an index of cardiac vagal tone (0.24-1.04 Hz). Cocaine elicited dose-dependent increases in HR that were accompanied by corresponding decreases in cardiac vagal tone. The peak response was achieved approximately 1 min after cocaine was given and returned to precocaine values 15 (0.5 and 1 mg/kg), 30 (2 mg/kg), or 60 (4 mg/kg) min later. Myocardial ischemia elicited significant increases in HR and reductions in cardiac vagal tone that were accentuated by cocaine (1, 2, and 4 mg/kg); e.g., cocaine (2 mg/kg) elicited a greater HR (control 119.3 +/- 5.9, occlusion 149.7 +/- 9.6; cocaine 144 +/- 11.9, occlusion 178.3 +/- 10.4 beats/min) and vagal tone (control 5.6 +/- 0.7, occlusion 2.6 +/- 0.3; cocaine 5.2 +/- 0.7, occlusion 1.3 +/- 0.5 ln s2) response to 2-min coronary occlusion. beta-Adrenoceptor blockade (propranolol HCl 1 mg/kg) attenuated the HR response but elicited greater reduction (lower values were achieved) in vagal tone during coronary artery occlusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of ryanodine on ventricular fibrillation induced by myocardial ischaemia

OBJECTIVE

Myocardial ischaemia can provoke a rise in cytosolic calcium which may in turn trigger malignant ventricular arrhythmias. Recently, inhibition of calcium entry has been shown to prevent these lethal arrhythmias. However, the contributions of calcium release from cytosolic stores to these disruptions in cardiac rhythm have not been investigated. This study examines the role of calcium release from the sarcoplasmic reticulum in the initiation of lethal ventricular arrhythmias.

METHODS

Mongrel dogs were chronically instrumented to measure left ventricular pressure, coronary blood flow, and cardiac electrical activity (ventricular electrocardiogram). The left anterior descending coronary artery was ligated during the surgery to produce a myocardial infarction. In addition, a hydraulic occluder was placed around the left circumflex artery. The susceptibility to ventricular fibrillation was then evaluated by the combination of acute myocardial ischaemia and exercise.

RESULTS

Ventricular fibrillation was induced in 10 animals during the exercise plus ischaemia test. On a subsequent day the exercise plus ischaemia test was repeated after pretreatment with ryanodine (10 micrograms.kg-1, n = 10), a drug which impairs calcium efflux from the sarcoplasmic reticulum. Ryanodine failed to prevent ventricular fibrillation induced by ischaemia. Ryanodine significantly (p < 0.01) increased heart rate [control 115.3(SEM 6.3) v ryanodine 156.4(14.7) beats.min-1] but reduced left ventricular systolic pressure [control 141.8(4.9) v ryanodine 111.1(12.7) mm Hg] and positive left ventricular dP/dt [3312.9(217.4) v ryanodine 1462.9(226.3) mm Hg.s-1] both at rest and during exercise. In contrast, this drug abolished ventricular tachycardia induced by ouabain toxicity (n = 10, 40 micrograms.kg-1 bolus followed by 0.076 microgram.kg-1.min-1 for 1 h, then 20 micrograms.kg-1 bolus, intravenously).

CONCLUSIONS

These data suggest that calcium release from ryanodine sensitive channels in the sarcoplasmic reticulum may contribute significantly to the arrhythmias induced by ouabain toxicity but not to ventricular fibrillation provoked by ischaemia.

Intracellular calcium chelator, BAPTA-AM, prevents cocaine-induced ventricular fibrillation

Cocaine is a potent cardiac stimulant that can provoke lethal cardiac events, including ventricular fibrillation (VF). The cocaine-induced accumulation of intracellular calcium could contribute significantly to the development of these lethal arrhythmias. To test this hypothesis, VF was induced in 12 mongrel dogs by the combination of cocaine (1.0 mg/kg) and a 2-min coronary occlusion during exercise. This test without cocaine failed to induce arrhythmias. Pretreatment with the intracellular calcium-specific chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA-AM; 1.0 mg/kg iv) prevented VF in 8 of 12 animals (P < 0.001) and delayed the onset of lethal arrhythmias in 3 of the remaining animals. Cocaine induced significant increases in left ventricular (LV) systolic pressure (control 154.7 +/- 8.7, cocaine 167.4 +/- 8.4 mmHg), heart rate (control 195.9 +/- 6.1, cocaine 222.3 +/- 10.6 beats/min), and LV maximum rate of pressure development (dP/dtmax; control 5,251 +/- 317.6, cocaine 6,016 +/- 435.1 mmHg/s). BAPTA-AM attenuated the increase in LV dP/dtmax (BAPTA-AM 4,591 +/- 479.3 mmHg/s) and LV systolic pressure (BAPTA-AM 154.5 +/- 6.8 mmHg). Because vascular muscle relaxation could contribute to the cardioprotection, the cocaine and exercise plus ischemia test was repeated after nitroprusside. The nitroprusside prevented cocaine-induced increases in LV systolic pressure but failed to prevent VF. These data suggest that BAPTA-AM may prevent cocaine-induced VF independently of its vascular actions.

Transnitrosation as a predominant mechanism in the hypotensive effect of S-nitrosoglutathione

S-Nitrosoglutathione, a possible intermediate in the pharmacological mechanisms of many vasodilators, undergoes hemolysis at pH 7.4 and 37 degrees C to give oxidized glutathione and nitric oxide with a second-order rate constant of approximately 3 x 10(-4) M-1sec-1. At the dosages normally employed, this reaction is, therefore, too slow to be an obligatory step in the pharmacological mechanisms of those, usually, fast-acting drugs. Transfers of the nitroso moiety to another thiol or to certain hemoproteins are, however, both much faster and could be involved in those pharmacological mechanisms. Intravenously administered S-nitrosoglutathione reduced the blood pressure of anesthesized dogs and monkeys to the same extent and with essentially the same rapid onset and dissipation as sodium nitroprusside, which is the fastest-acting of those vasodilators.

Effect of calcium channel antagonists on cocaine-induced malignant arrhythmias: protection against ventricular fibrillation

It is increasingly apparent that cocaine can provoke lethal cardiac events, including ventricular fibrillation (VF). Cocaine-induced accumulation of intracellular calcium could contribute significantly to the development of these lethal arrhythmias. In order to test this hypothesis, a 2-min coronary occlusion was initiated during the last minute of exercise in instrumented mongrel dogs. Twenty-eight animals were selected in which this test failed to induce ventricular arrhythmias. The test was repeated after cocaine HCl (1.0 mg/kg). Cocaine significantly (P < .01) increased heart rate, left ventricular pressure and d(left ventricular pressure)/dt maximum, and elicited VF in 21 animals. The cocaine exercise+ischemia test was repeated in the animals that developed VF after the pretreatment with the following calcium channel antagonists: diltiazem (n = 8, 1.0 mg/kg), flunarizine (n = 7, 2.5 mg/kg), magnesium sulfate (n = 7, 100 mg/kg), nifedipine (n = 7 100 micrograms/kg), Ro 40-5967 (n = 7, 1.0 mg/kg) and verapamil (n = 6, 250 micrograms/kg). Diltiazem, flunarizine, nifedipine, Ro 40-5967 and verapamil completely suppressed cocaine-induced VF, whereas magnesium prevented VF in five of seven animals. Many of the calcium channel antagonists attenuated the heart rate and systolic pressure increases provoked by cocaine, as well as the heart rate increase induced by the ischemia. Heart rate was therefore matched to the cocaine values by ventricular pacing (verapamil+pace, n = 5), whereas the pressure increases were prevented by nitroprusside (n = 4). Even with heart rate held constant, verapamil prevented VF, whereas nitroprusside failed to protect any animal. Thus, myocardial calcium entry may play a critical role in cocaine-induced VF, whereas calcium antagonists can prevent these malignant arrhythmias independently of their vascular action.

The effects of the ATP-dependent potassium channel antagonist, glyburide, on coronary blood flow and susceptibility to ventricular fibrillation in unanesthetized dogs

Extracellular potassium rapidly increases during myocardial ischemia and has been implicated in the development of ventricular fibrillation (VF). Recent evidence suggests that ischemically induced potassium efflux results from the opening of ATP-dependent potassium channels. If extracellular potassium accumulation contributes to VF, one would predict that drugs that block the ATP-dependent potassium channels should protect against these arrhythmias. Therefore, VF was induced in 15 mongrel dogs with healed myocardial infarctions by a 2-min coronary occlusion during exercise. This exercise (Ex) plus ischemia test consistently induced VF during each control (vehicle) presentation. However, glyburide, a sulfonylurea drug that selectively blocks the ATP-dependent potassium channel, prevented VF in 13 of 15 animals tested (p < 0.001, chi 2). Glyburide (G) elicited significant reductions in left ventricular dP/dtmax (Ex: control, 5,031 +/- 386.4; G, 3,286 +/- 116.6 mm Hg/s) and mean coronary blood flow (Ex: control, 49.6 +/- 8.7; G, 31.3 +/- 5.5 ml/min) both before and during exercise. The heart rate responses to exercise and coronary occlusion were also reduced. Since heart rate reductions could contribute to the cardioprotection, the exercise plus ischemia test was repeated with the heart rate held constant by ventricular pacing. Glyburide still protected four of five animals under these conditions. These data indicate that glyburide, a drug that blocks the ATP-dependent potassium channels, can prevent VF independently of changes in the heart rate. The data further suggest that these channels may contribute to the coronary vasodilation elicited by an increase in metabolic demand during exercise.

Ro 40-5967, a novel calcium channel antagonist, protects against ventricular fibrillation

Ro 40-5967 is a new calcium channel antagonist that binds at the same membrane sites as verapamil, yet has minimal negative inotropic effects. The effects of Ro 40-5967 on the susceptibility to ventricular fibrillation were investigated and compared to diltiazem. Ventricular fibrillation (VF) was induced in 40 mongrel dogs with healed myocardial infarctions by a 2-min coronary occlusion during exercise. Twenty-four animals were found to be susceptible to VF and were given the treatments described below. Pretreatment with Ro 40-5967 (n = 17, 1000 micrograms/kg i.v.) significantly (P < 0.001) reduced the incidence of VF (13 of 17 protected) during the exercise plus ischemia test. Diltiazem (n = 8, 1000 micrograms/kg) completely suppressed VF. Lower doses of diltiazem and Ro 40-5967 did not prevent VF. The hemodynamic effects of Ro 40-5967 were also compared to diltiazem and verapamil. Diltiazem and verapamil, but not Ro 40-5967, increased P-R interval in a dose-dependent manner. Even when reflex tachycardia was controlled by beta-adrenoceptor blockade, Ro 40-5967 still exerted only minimal effects on P-R interval. Verapamil, but neither Ro 40-5967 nor diltiazem, provoked a dose-dependent negative inotropic response. All three drugs elicited large increases in coronary blood flow. These data support the hypothesis that calcium entry may play a critical role in the development of malignant arrhythmias during ischemia. Further, Ro 40-5967 can protect against ventricular fibrillation without significant negative inotropic or dromotropic effects.

Effect of general anesthesia on cardiac vagal tone

Although it is accepted that general anesthetics alter cardiac vagal tone, the magnitude of this effect was difficult to quantify by noninvasive methods until recently. Twenty-eight mongrel dogs were anesthetized using one of four representative anesthetics: pentobarbital sodium (25 mg/kg iv), morphine (1 mg/kg sc) with alpha-chloralose (50 mg/kg iv), urethan (500 mg/kg iv), thiopental sodium (25 mg/kg iv), and halothane (2% inhalation). Heart period (R-R interval) was recorded, from which the amplitude of the respiratory sinus arrhythmias (RSAs; frequency 0.24-1.04 Hz) was determined by time-series analysis. Morphine with alpha-chloralose-urethan significantly (P less than 0.01) increased RSA (control 8.3 +/- 0.6 ln ms2, anesthesia 9.4 +/- 0.3 ln ms2), whereas thiopental and halothane both significantly (P less than 0.01) decreased RSA (control 8.7 +/- 0.4 ln ms2, thiopental 1.3 +/- 0.4 ln ms2; and control 8.5 +/- 0.6 ln ms2, halothane 3.6 +/- 0.8 ln ms2). Pentobarbital failed to elicit a consistent change in RSA. These data suggest that vagal tone was maintained during morphine with alpha-chloralose-urethan anesthesia but was reduced during thiopental and halothane anesthesia.

The calcium channel antagonist, flunarizine, protects against ventricular fibrillation

Elevations in intracellular calcium during myocardial ischemia have been implicated in the development of lethal cardiac arrhythmias. The calcium antagonist, flunarizine, has been shown to suppress the accumulation of intracellular calcium and has been proposed to protect against triggered activity due to calcium overload. Using 13 mongrel dogs with healed myocardial infarctions, ventricular fibrillation (VF) was induced by a 2 min coronary occlusion during exercise. This exercise plus ischemia test consistently induced VF during control (C, vehicle) presentations. Pretreatment with flunarizine (2.5 mg/kg i.v.) completely suppressed VF in all the animals (P less than 0.001 Chi-squared). Flunarizine (F) elicited significant (P less than 0.01 ANOVA) reductions in left ventricular (LV) systolic pressure (C 143.2 +/- 12.0 F 92.3 +/- 10.5 mm Hg), LVdP/dt max (C 4256 +/- 251.9, F 1784 +/- 297.2 mm Hg/s) and heart rate (C 118.8 +/- 7.4, F 104.7 +/- 9.0 beats/min). Since heart rate can contribute significantly to the development of VF, the exercise plus ischemia test was repeated with heart rate held constant with ventricular pacing (n = 3, 230.0 +/- 10 beats/min). Flunarizine pretreatment still prevented VF under these conditions.

The effect of unilateral stellectomy on the regulation of heart rate during behavioral stress

Changes in heart rate were examined during classical aversive conditioning before and after either left (n = 10) or right stellectomy (n = 8). Heart rate (HR) significantly (p less than 0.01) increased in response to the conditional stimulus with a further increase noted during the unconditional stimulus. After right stellectomy (RSGx) the aversive stress elicited a significantly (p less than 0.01) smaller increase in heart rate (peak HR change: control 66.8 +/- 6.0; RSGx 36.1 +/- 6.8 beats/min). In contrast left stellectomy did not significantly alter the heart rate conditional response. Cardioselective beta-adrenergic receptor blockade, metoprolol HC1 (BB, 1.0 mg/kg, n = 8), significantly (p less than 0.01) reduced the heart-rate conditional response (peak HR change: control 66.8 +/- 6; BB 25.1 +/- 3.8 beats/min) to the same extent as noted during RSGx. These data suggest that the heart-rate increases elicited by aversive stress are mediated in part by sympathetic nerves that originate or pass through the right stellate ganglion. The residual HR increase noted after RSGx or BB probably results from the withdrawal of cardiac parasympathetic activity.

Elevated myocardial calcium and its role in sudden cardiac death

The contribution of intracellular calcium to ventricular fibrillation (VF) was investigated using chronically instrumented dogs with healed myocardial infarctions. A 2-minute coronary occlusion was initiated during the last minute of exercise. Fourteen animals developed ventricular fibrillation (susceptible) whereas the remaining 12 did not (resistant) during this exercise plus ischemia test. The test was then repeated for the susceptible animals after pretreatment with the intracellular calcium chelator BAPTA-AM (1.0 mg/kg). BAPTA-AM significantly reduced left ventricular dp/dt max and prevented VF in 8 of 12 susceptible animals. Conversely, myocardial cytosolic calcium levels were increased in resistant animals using the calcium channel agonist Bay K 8644 (30 micrograms/kg) or phenylephrine (10 micrograms.kg-1.min-1 3-5 min before occlusion). Bay K 8644 induced VF in all 5 resistant animals tested whereas phenylephrine induced VF in 8 of 12 resistant animals. BAPTA-AM pretreatment attenuated the hemodynamic effects of Bay K 8644 or phenylephrine and prevented VF in five of five Bay K 8644- and four of seven phenylephrine-treated animals. Finally, the endogenous level of calcium/calmodulin (Ca-CaM)-dependent phosphorylation of 170- and 55-kDa substrate proteins was measured (as an index of intracellular free calcium concentration). In the susceptible dog heart, the endogenous level of Ca-CaM-dependent phosphorylation was estimated to be two- to threefold higher than that observed in resistant dog heart. Treatment of resistant dog tissue with the calcium ionophore A23187 increased the level of Ca-CaM-dependent phosphorylation of these two proteins to the level observed in susceptible dog heart. These data suggest that elevated cytosolic calcium facilitates development of malignant arrhythmias and that elevated cytosolic calcium levels may be present in animals particularly susceptible to ventricular fibrillation.

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.

Effect of carbachol and cyclic GMP on susceptibility to ventricular fibrillation

Reductions in cardiac vagal tone have been shown to correlate with increased susceptibility to ventricular fibrillation (VF). If these reductions in vagal tone contribute to VF, one would predict that interventions that increase vagal tone should protect against these lethal arrhythmias. Therefore, VF was induced in 17 mongrel dogs with healed myocardial infarctions by a 2-min coronary occlusion during exercise. On a subsequent day, the cholinergic agonist carbachol (20 micrograms/kg, i.v.) was given before the exercise plus ischemia test (n = 14). Carbachol elicited significant reductions in heart rate (control 204.5 +/- 27.7 vs. carbachol 147.0 +/- 49.6 beats/min) and prevented VF in 11 of 14 animals. When the decline in heart rate was prevented by ventricular pacing, carbachol prevented VF in five of six animals. Cyclic GMP may act as an intracellular messenger of cholinergic activation; therefore, 8-bromo cyclic GMP (n = 9) was infused (100-150 micrograms.kg-1.min-1, i.v.) throughout the exercise beginning 45 min before onset of exercise. Heart rate increased but VF was prevented in eight of nine animals. Similar results were noted for dibutyryl cyclic GMP (n = 5). These data suggest that cholinergic agonists and cyclic GMP can prevent VF in susceptible animals independently of heart rate changes.

Dynamic changes in cardiac vagal tone as measured by time-series analysis

A time-series analysis of heart rate variability was evaluated as a marker of cardiac vagal tone using well-characterized autonomic interventions. Heart period (R-R interval) was recorded in 14 mongrel dogs from which the amplitude of the respiratory sinus arrhythmia (0.24-1.04 Hz) was determined. Exercise elicited significant (P less than 0.01) reductions in the index of vagal tone (control 6.3 +/- 0.3 ln ms2 vs. exercise 2.4 +/- 0.4 ln ms2) that were accompanied by significant (P less than 0.01) increases in heart rate (control 123.1 +/- 5 vs. exercise 201.0 +/- 7.7 beats/min). The vagal tone index remained greater than 0 throughout exercise. After propranolol HCl pretreatment, the vagal tone index rapidly decreased toward zero (control 6.2 +/- 0.5; exercise 0.7 +/- 0.3 ln ms2), despite significantly lower increases in heart rate (control 109.3 +/- 4.2; exercise 178.0 +/- 7.6 beats/min). Atropine given during exercise evoked significantly greater increases in heart rate in the control (+48.7 +/- 7.9 beats/min) vs. propranolol (+14.2 +/- 6.7 beats/min) conditions. These data suggest that 1) high levels of cardiac vagal tone remain during exercise; 2) vagal withdrawal is largely responsible for the heart rate increase after beta-adrenergic receptor blockade; and 3) time-series analysis of the R-R interval can provide a dynamic and noninvasive index of cardiac vagal tone.

Hemodynamic and arrhythmogenic effects of aversive stress during myocardial ischemia

Pavlovian conditioning was accomplished in two groups of dogs by following a 30-s tone (the CS+) with a short (less than or equal to 1 s) electric shock). An inflatable occluder was implanted around the left circumflex coronary artery in all animals. The CS+ was presented to Group I dogs (n = 6) during control (i.e., no coronary occlusion) and at 30 s and 3 min after the onset of a 4-min left circumflex coronary occlusion. The CS+ evoked a robust increase in heart rate and blood pressure in the control state. A conditional cardiovascular response was still evoked during the acute coronary occlusion despite the evolving myocardial ischemia. No increase in ventricular ectopic beats occurred during the CS+. An anterior myocardial infarction (16.8 +/- 1.7% of left ventricular mass, mean +/- SEM) was created in Group II dogs (n = 8) at the time of surgery. A CS+ was presented, as above, both alone and 1 min after the beginning of a 2-min left circumflex coronary occlusion. There were significant differences in the conditional response in heart rate, left ventricular (systolic) pressure (LVP) and d(LVP)/dt for trials given during the coronary occlusion compared with no occlusion. These dogs were then classified as 'resistant' or 'susceptible' to ventricular arrhythmias approx. 4 weeks later using an exercise and coronary occlusion test. Three of five susceptible dogs, but none of three resistant dogs, evidenced increased severity of arrhythmias during the CS+. We conclude that the nature of the cardiovascular response to behavioral stress during myocardial ischemia depends in part upon the timing of the CS+, the presence or absence of a resident myocardial infarction and the status of the autonomic reflexes controlling the heart.

Autonomic response to coronary occlusion in animals susceptible to ventricular fibrillation

Disturbances in autonomic control during myocardial ischemia may contribute significantly to the development of malignant cardiac arrhythmias. Therefore acute ischemia was induced in 29 mongrel dogs with healed myocardial infarctions during an exercise test. Seventeen animals developed ventricular fibrillation (susceptible, S), whereas 12 dogs did not (resistant, R). Before the exercise plus ischemia test a coronary occlusion was made at rest. The amplitude of respiratory sinus arrhythmia (0.24- to 1.04-Hz component of R-R interval fluctuation) was used as an index of cardiac vagal tone. Acute ischemia elicited a significantly larger heart rate increase in susceptible animals (S: control 115.6 +/- 0.8, occlusion 176.4 +/- 8.2 beats/min vs. R: control 114.6 +/- 8.9, occlusion 145.7 +/- 7.5 beats/min). Accompanying the heart rate increase were significantly greater reductions in the cardiac vagal tone index in the susceptible animals. (S: control 6.4 +/- 0.3, occlusion 2.2 +/- 0.6 ln ms2 vs. R: control 6.6 +/- 0.4, occlusion 5.1 +/- 0.5 ln ms2). beta-Adrenergic receptor blockade reduced the heart rate increases but exacerbated the reductions in the cardiac vagal tone index. These data suggest that coronary artery occlusion elicits a significantly greater increase in sympathetic activity coupled with a greater reduction in parasympathetic activity in animals subsequently shown to be susceptible to ventricular fibrillation.

Time-series analysis of heart rate variability during submaximal exercise. Evidence for reduced cardiac vagal tone in animals susceptible to ventricular fibrillation

Periodic fluctuations in the R-R interval have been used as noninvasive measures of cardiac autonomic tone. For example, a reduced heart rate variability has been shown to correlate with an increased mortality in patients recovering from myocardial infarction. The effects that physiologic perturbations such as exercise have on this heart rate variability have not been investigated. Therefore, heart rate variability was measured throughout a submaximal exercise test in 36 mongrel dogs with healed anterior myocardial infarctions. The amplitude of the respiratory component (0.24-1.04 Hz) was determined by time-series analysis techniques and was used as an index of cardiac vagal tone. On a subsequent day, a 2-minute coronary occlusion was initiated during the last minute of exercise. Twenty-two animals developed ventricular fibrillation (susceptible), whereas 14 animals did not (resistant). Exercise elicited a significantly greater increase in heart rate (resistant, 205.4 +/- 7.1; susceptible, 227.0 +/- 5.4 beats/min) in susceptible animals, which was accompanied by a greater reduction in the cardiac vagal tone index (resistant, 2.7 +/- 0.3; susceptible, 1.1 +/- 0.2 ln msec2) as compared with resistant animals. Conversely, atropine sulfate (50 micrograms/kg) given during exercise elicited a greater heart rate increase in the resistant dogs (heart rate change: resistant, 54.2 +/- 7.0; susceptible, 18.7 +/- 4.4 beats/min). Taken together, these data suggest that exercise elicited a greater reduction in cardiac vagal tone in animals known to be susceptible to ventricular fibrillation.

Effect of myocardial ischemia on hemodynamic response to carotid occlusion

Fourteen mongrel dogs were anesthetized and instrumented to measure arterial pressure (AP), left ventricular pressure (LVP), aortic blood flow, and heart rate (HR). Hydraulic occluders were placed around the left anterior descending (LAD, n = 9) and left circumflex (LCC, n = 14) coronary arteries. A bilateral carotid occlusion (BCO) was made before and during either anterior (LAD occlusion) or posterior (LCC occlusion) ischemia. Posterior ischemia significantly (P less than 0.01) reduced the BCO-induced increases in mean AP (by 44.3 +/- 7.3%), systolic LVP (by 65.5 +/- 6.9%), first derivative of LVP (dLVP/dt, by 95.7 +/- 44.3%), and aortic resistance (by 117.7 +/- 26.9%). In contrast, anterior ischemia failed to alter significantly the hemodynamic response to BCO. Bilateral vagotomy attenuated or eliminated many of the effects of posterior ischemia on the BCO response. In fact, the change in aortic resistance was no longer affected by the ischemia and increased to the same extent, as noted during the control BCO. However, mean AP (38.7 +/- 6.8%), systolic LVP (40.3 +/- 8.7%), and dLVP/dt (62.4 +/- 11.0%) remained significantly reduced when compared with the control (no coronary occlusion) response. These data suggest that 1) posterior ischemia elicits a greater reduction in the BCO response than anterior ischemia, and 2) vagal afferents as well as depression of contractile function may both contribute to the BCO response inhibition noted during posterior ischemia.

Effect of calcium channel antagonists on susceptibility to sudden cardiac death: protection from ventricular fibrillation

The effects of calcium channel antagonists and an agonist on susceptibility to ventricular fibrillation were investigated using chronically instrumented dogs with a healed anterior wall myocardial infarction. Three to 4 weeks after the infarction, a 2-min coronary occlusion was initiated during the last minute of exercise and continued for 1 min after cessation of exercise. Twenty-two dogs developed ventricular fibrillation (susceptible) whereas the remaining 14 animals did not (resistant) during this exercise plus ischemia test. The exercise plus ischemia test was repeated in the susceptible dogs after the following treatments: verapamil (n = 17, 250 micrograms/kg), nifedipine (n = 9, 10 and 100 micrograms/kg) and magnesium sulfate (n = 9, 100 mg/kg). Verapamil prevented ventricular fibrillation in all dogs whereas the high dose of nifedipine protected eight of nine animals; the low dose of nifedipine protected one of nine animals and magnesium sulfate protected seven of nine dogs. Finally, the calcium channel agonist Bay K8644 (n = 9, 30 micrograms/kg) was given to resistant animals. All resistant dogs developed ventricular fibrillation when the exercise plus ischemia test was repeated after Bay K8644. These data suggest that calcium entry may play a critical role in the development of arrhythmias during ischemia, with increased calcium entry provoking arrhythmias and calcium entry blockade preventing the lethal events.

Selective vagal postganglionic innervation of the sinoatrial and atrioventricular nodes in the non-human primate

The distribution of parasympathetic postganglionic nerves to the atrioventricular (AVN) and sinoatrial nodal (SAN) regions was investigated in the non-human primate heart. Eight male monkeys (Macaca fascicularis) weighing 5.5-7.0 kg. were anesthetized (alpha-chloralose, 50 mg/kg and urethane, 500 mg/kg) and instrumented to measure arterial pressure, electrocardiogram, atrial and ventricular electrograms. The cervical vagi were electrically stimulated (20 Hz, 4 V, 2 ms) before and after selective denervation (D) of the AVN and/or SAN. Vagal stimulation was repeated during atrial pacing to assess parasympathetic modulation of AVN conduction. Ablation of parasympathetic pathways to the AVN, accomplished by the disruption of the epicardial fat and surface muscle layer at the junction of the inferior vena cava and inferior left atrium eliminated (P less than 0.01) the dromotropic effects of vagal stimulation without affecting the heart rate response (right vagus, before D, paced: atrial rate 218.0 +/- 6.3, ventricular rate 67.1 +/- 23.7; after D: atrial rate 210.3 +/- 6.4, ventricular rate 210.3 +/- 6.4 beats/min, means +/- S.D.). In sharp contrast, surgical dissection of the fat pad overlying the right pulmonary vein-superior vena cava junction significantly (P greater than 0.01) attenuated negative chronotropic effects of vagal stimulation (left vagus, before D the R-R interval increased by 832.7 +/- 146.4 ms, 209.5% increase; after D 37.4 +/- 18.0 ms, 8.8% increase). These data demonstrate discrete vagal efferent pathways innervate both the SAN and AVN regions of the non-human primate heart.

Prevention of ventricular fibrillation with magnesium sulfate

Mongrel dogs with healed myocardial infarctions were given a 2 min coronary occlusion during an exercise test. The exercise plus ischemia test induced ventricular fibrillation in nine animals. One week later, the test was repeated after pretreatment with magnesium sulfate (100 mg/kg i.v.). Magnesium prevented ventricular fibrillation in seven of the nine animals without adverse side effects. Thus, magnesium may be useful in the management of ventricular fibrillation during ischemia.

Cocaine-induced ventricular fibrillation: protection afforded by the calcium antagonist verapamil

There is increasing evidence that the use of cocaine can trigger lethal cardiac events, including ventricular fibrillation. The mechanism responsible for these lethal cardiac arrhythmias remains to be determined. Therefore, 13 mongrel dogs were instrumented so that heart rate, left ventricular pressure (LVP), and d(LVP)/dt could be measured. After a 3- to 4-wk recovery period, the left circumflex coronary artery was occluded for 2 min, beginning with the last minute of an exercise stress test and continuing for 1 min after the cessation of exercise. None of the dogs developed cardiac arrhythmias during the control exercise plus ischemia test. On a subsequent day, the test was repeated after the injection of cocaine HCl (1.0 mg/kg). Cocaine significantly (P less than 0.01) elevated heart rate, systolic LVP, and d(LVP)/dt, and it elicited cardiac arrhythmias in 12 of the 13 animals during the exercise plus test. In fact, 11 animals developed ventricular fibrillation. Verapamil, a calcium channel antagonist (250 micrograms/kg), attenuated the hemodynamic effects of cocaine and prevented the development of ventricular arrhythmias. These data suggest that cocaine can induce ventricular fibrillation during myocardial ischemia and that these lethal arrhythmias may be prevented by a calcium channel antagonist.