Effects of caffeine and ryanodine on delayed afterdepolarizations and sustained rhythmic activity in 1-day-old myocardial infarction in the dog

Calcium-calmodulin dependent protein kinase II (CaMKII): a main signal responsible for early reperfusion arrhythmias

To explore whether CaMKII-dependent phosphorylation events mediate reperfusion arrhythmias, Langendorff perfused hearts were submitted to global ischemia/reperfusion. Epicardial monophasic or transmembrane action potentials and contractility were recorded. In rat hearts, reperfusion significantly increased the number of premature beats (PBs) relative to pre-ischemic values. This arrhythmic pattern was associated with a significant increase in CaMKII-dependent phosphorylation of Ser2814 on Ca(2+)-release channels (RyR2) and Thr17 on phospholamban (PLN) at the sarcoplasmic reticulum (SR). These phenomena could be prevented by the CaMKII-inhibitor KN-93. In transgenic mice with targeted inhibition of CaMKII at the SR membranes (SR-AIP), PBs were significantly decreased from 31±6 to 5±1 beats/3min with a virtually complete disappearance of early-afterdepolarizations (EADs). In mice with genetic mutation of the CaMKII phosphorylation site on RyR2 (RyR2-S2814A), PBs decreased by 51.0±14.7%. In contrast, the number of PBs upon reperfusion did not change in transgenic mice with ablation of both PLN phosphorylation sites (PLN-DM). The experiments in SR-AIP mice, in which the CaMKII inhibitor peptide is anchored in the SR membrane but also inhibits CaMKII regulation of L-type Ca(2+) channels, indicated a critical role of CaMKII-dependent phosphorylation of SR proteins and/or L-type Ca(2+) channels in reperfusion arrhythmias. The experiments in RyR2-S2814A further indicate that up to 60% of PBs related to CaMKII are dependent on the phosphorylation of RyR2-Ser2814 site and could be ascribed to delayed-afterdepolarizations (DADs). Moreover, phosphorylation of PLN-Thr17 and L-type Ca(2+) channels might contribute to reperfusion-induced PBs, by increasing SR Ca(2+) content and Ca(2+) influx.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Sodium calcium exchange as a target for antiarrhythmic therapy

In search of better antiarrhythmic therapy, targeting the Na/Ca exchanger is an option to be explored. The rationale is that increased activity of the Na/Ca exchanger has been implicated in arrhythmogenesis in a number of conditions. The evidence is strong for triggered arrhythmias related to Ca2+ overload, due to increased Na+ load or during adrenergic stimulation; the Na/Ca exchanger may be important in triggered arrhythmias in heart failure and in atrial fibrillation. There is also evidence for a less direct role of the Na/Ca exchanger in contributing to remodelling processes. In this chapter, we review this evidence and discuss the consequences of inhibition of Na/Ca exchange in the perspective of its physiological role in Ca2+ homeostasis. We summarize the current data on the use of available blockers of Na/Ca exchange and propose a framework for further study and development of such drugs. Very selective agents have great potential as tools for further study of the role the Na/Ca exchanger plays in arrhythmogenesis. For therapy, they may have their specific indications, but they carry the risk of increasing Ca2+ load of the cell. Agents with a broader action that includes Ca2+ channel block may have advantages in other conditions, e.g. with Ca2+ overload. Additional actions such as block of K+ channels, which may be unwanted in e.g. heart failure, may be used to advantage as well.

Cardiovascular challenges in toxicology

The diagnoses and subsequent treatment of poisoned patients manifesting cardiovascular compromise challenges the most experienced emergency physician. Numerous drugs and chemicals cause cardiac and vascular disorders. Despite widely varying indications for therapeutic use, many agents share a common cardiovascular pharmacologic effect if taken in overdose. Standard advanced cardiac life support protocol care of these patients may not apply and may even result in harm if followed. This chapter discusses com-mon cardiovascular toxins and groups them into their common mechanisms of toxicity. Multiple agents exist that result in human cardiovascular toxicity. The management of the toxicity of each agent should follow a rationale approach. The first step in the care of all poisoned patients focuses on good supportive care.

Would modulation of intracellular Ca2+ be antiarrhythmic?

Under several types of conditions, reversal of steps of excitation-contraction coupling (RECC) can give rise to nondriven electrical activity. In this review we explore those conditions for several cardiac cell types (SA, atrial, Purkinje, ventricular cells). We find that abnormal spontaneous Ca2+ release from intracellular Ca2+ stores, aberrant Ca2+ influx from sarcolemmal channels or abnormal Ca2+ surges in nonuniform muscle can be the initiators of the RECC. Often, with such increases in Ca2+, spontaneous Ca2+ waves occur and lead to membrane depolarizations. Because the change in membrane voltage is produced by Ca2+-dependent changes in ion channel function, we also review here what is known about the molecular interaction of Ca2+ and several Ca2+-dependent processes, including the intracellular Ca2+ release channels implicated in the genetic basis of some forms of human arrhythmias. Finally, we review what is known about the effectiveness of several agents in modifying such Ca2+-dependent arrhythmias.

2APB- and JTV519(K201)-sensitive micro Ca2+ waves in arrhythmogenic Purkinje cells that survive in infarcted canine heart

OBJECTIVES/BACKGROUND

Studies from several laboratories have implicated intracellular Ca(2+) dynamics in the modulation of electrical activity. We have reported that abnormal Ca(2+) wave activity is the underlying cause of afterdepolarization-induced electrical activity in subendocardial Purkinje cells that survive in the 48-hour infarcted canine heart. These cells form the focus of arrhythmias at this time postcoronary artery occlusion.

METHODS

We studied the effects of agonists and antagonists on the abnormal Ca(2+) release activity of Purkinje cell aggregates dispersed from the subendocardium 48 hours postcoronary artery occlusion (IZPCs). Studies were completed using epifluorescent microscopy of Fluo-3 loaded Purkinje cells.

RESULTS

Similar to our previous report, highly frequent traveling micro Ca(2+) transients (muCaiTs) and cell-wide Ca(2+) waves were seen in IZPCs in the absence of any drug. Isoproterenol (ISO) increased muCaiTs and cell-wide Ca(2+) waves in Purkinje cells dispersed from the normal heart (NZPCs). In IZPCs, ISO increased cell-wide wave frequency but had no effect on the already highly frequent micro Ca(2+) wave transient activity, suggesting that ISO lowers the threshold of cell-wide generators responding to micro Ca(2+) transients. Drugs that block inward sodium or calcium currents (verapamil, tetrodotoxin) had no effect on Ca(2+) activity in Purkinje cells. Antagonists of intracellular Ca(2+) release channels [ryanodine, JTV519(K201)] greatly suppressed spontaneous Ca(2+) release events in IZPCs. 2APB, an agent that blocks IP(3) receptors, greatly reduced the frequency of Ca(2+) events in IZPCs.

CONCLUSIONS

In arrhythmogenic Purkinje cells that survive in the infarcted heart, agents that block or inhibit intracellular Ca(2+) release channel activity reduced Ca(2+) waves and could be antiarrhythmic.

Massive Caffeine Overdose Requiring Vasopressin Infusion and Hemodialysis

INTRODUCTION

Massive caffeine overdose is associated with life-threatening hemodynamic complications that present challenges for clinicians. We describe the highest-reported serum concentration of caffeine in a patient who survived and discuss the first-reported use of vasopressin and hemodialysis in a caffeine-poisoned patient.

CASE REPORT

A 41-yr-old woman presented 3 h after ingesting approximately 50 g of caffeine. She subsequently underwent cardiopulmonary resuscitation and received multiple medications in an attempt to raise her blood pressure and control her heart rate without success. Vasopressin infusion increased her blood pressure to the point where hemodialysis could be performed. Despite ensuing multisystem organ failure, she survived and has made a complete recovery.

CONCLUSION

Hemodialysis and vasopressin infusions may be of benefit in the management of caffeine-intoxicated patients who fail to respond to standard therapies.

Nonuniform Ca2+ transients in arrhythmogenic Purkinje cells that survive in the infarcted canine heart

OBJECTIVE AND METHODS

In this study, we investigated whether Ca(2+) transients are altered in Purkinje cell aggregates dispersed from the subendocardium overlying the infarcted zone of the left ventricle (IZPCs) 48 h after coronary artery occlusion. To do so, we combined epifluorescent imaging with microelectrode recordings of IZPCs and normal canine Purkinje cell aggregates (NZPCs).

RESULTS

NZPCs respond to an action potential (AP) by a small Ca(2+) transient at the cell surface immediately after the AP upstroke followed by a large [Ca(2+)] transient, which propagates to the cell core. In addition, focal Ca(2+) waves can originate spontaneously later during the AP or during the diastolic interval (Circ Res 2000;86:448-55) and then propagate throughout the aggregate as 'cell-wide Ca(2+) waves'. Electrically-evoked Ca(2+) transients in IZPCs arose significantly faster than those in NZPCs, and showed substantial spatiotemporal nonuniformity within an IZPC aggregate as well as between IZPC aggregates. IZPCs showed, hitherto undetected, low amplitude, micro Ca(2+) transients (extent <or=5 microm) at a fivefold higher incidence than in NZPCs. Micro Ca(2+) transients appeared to meander over distances <or=100 microm and reduced the local Ca(2+) transient of the next paced beat. Micro Ca(2+) transients nearly always preceded the cell-wide Ca(2+)waves, which occurred more frequently in IZPCs than in NZPCs and caused non-driven electrical activity of the Purkinje aggregate.

CONCLUSIONS

Micro Ca(2+) transients preceded cell-wide Ca(2+) waves so often that it is probable that micro Ca(2+) transients induced cell-wide Ca(2+) waves. Cell-wide Ca(2+) waves, in turn, clearly elicited spontaneous APs. We propose that the high incidence of micro Ca(2+) transients in IZPCs is a fundamental element of the abnormal Ca(2+) handling of diseased Purkinje cells, underlying arrhythmias originating in the subendocardial Purkinje network post myocardial infarction.

Effects of mibefradil, a T-type calcium current antagonist, on electrophysiology of Purkinje fibers that survived in the infarcted canine heart

INTRODUCTION

We studied the effects of mibefradil (MIB), a nondihydropyridine T-type Ca2+ channel antagonist, on T- and L-type Ca2+ (I(CaT), I(CaL)) currents in Purkinje myocytes dispersed from the subendocardium of the left ventricle of normal (NZPC) and 48-hour infarcted (IZPC) hearts.

METHODS AND RESULTS

Currents were recorded with Cs+- and EGTA-rich pipettes and in Na+-K+-free external solutions to eliminate overlapping currents. In all cells, I(Ca) was reduced by MIB (0.1 to 10 microM). No change in the time course of decay of peak I(Ca) was noted. Average peak T/L ratio decreased in NZPCs but not IZPCs with 1 microM MIB. Steady-state availability of I(CaL) was altered with 1 microM MIB in both cell types (mean +/- SEM) (V0.5 = -22 +/- 4 mV for NZPC and -25 +/- 5 mV for IZPC before drug; -63 +/- 9 mV for NZPC and -67 +/- 6 mV for IZPC after drug; P < 0.05). For I(CaT), V0.5 (-50 +/- 3 mV for NZPC and -52 +/- 1 mV for IZPC before drug) shifted to -60 +/- 2 mV (NZPC) and -62 +/- 3 mV (IZPC) (P < 0.05) after drug. We also determined the effects of MIB on spontaneously beating Purkinje normal fibers and on depolarized abnormally automatic fibers from the infarcted heart using standard microelectrode techniques. When NZPC and IZPC fibers were superfused with [K+]o = 2.7 mM, MIB 3 microM and 10 microM had no effect on rate or the maximum diastolic potential, but action potential plateau shifted to more negative values, the slope of repolarization phase 3 decreased, and action potential duration increased.

CONCLUSION

MIB blocks L- and T-type Ca2+ currents in Purkinje myocytes but lacks an effect on either normal or abnormal automaticity in Purkinje fibers.

Prevention of reoxygenation-induced arrhythmias in guinea pig papillary muscles

Effects of various agents on reoxygenation-induced arrhythmias, action potentials, and tension of guinea pig papillary muscles were recorded to investigate the site of action. Triggered activities due to delayed afterdepolarizations (DADs) and aftercontractions were elicited on reoxygenation after 60-min substrate-free hypoxia. Low extracellular Ca2+ (0.1 mM) abolished arrhythmias, and high Ca2+ (4.9 mM) increased the amplitudes of DADs and aftercontractions. D-600 at the high concentration (20 microM) decreased the incidence of arrhythmias (p < 0.05 vs. no drug) and decreased the recovery of developed tension after reoxygenation (p < 0.001). Ryanodine (1 microM) abolished aftercontractions and arrhythmias but did not affect the recovery of developed tension. Tetrodotoxin (TTX 3 microM) and nicorandil (100 microM) decreased the incidence of arrhythmias (p < 0.05), but did not affect the recovery of developed tension or the amplitudes of aftercontractions. TTX caused only a slight decrease in Ca2+ transients in a fluo-3-loaded guinea pig ventricular myocyte. The Ca2+ entry through the Ca2+ channels apparently synchronized Ca2+ release from the sarcoplasmic reticulum, and D-600 at the high concentration apparently decreased the incidence of arrhythmias. TTX and nicorandil decreased arrhythmias, probably by decreasing the Na+ current or by increasing the ATP-sensitive K+ current, respectively.

Caffeine induces ventricular tachyarrhythmias possibly due to triggered activity in rabbits in vivo

Caffeine induces delayed afterdepolarizations (DADs) and triggered activity in isolated cardiac tissue. We investigated the ability of caffeine to induce triggered ventricular arrhythmias in rabbits in vivo. During continuous infusion of caffeine at doses of 0.3 or 1.0 mg/kg per min, ventricular pacing was performed with 50 stimuli with a cycle length of 220 msec (basic pacing train) every 5 min until ventricular tachycardia (VT) was induced. The effects of programmed stimulation and pharmacologic agents on the induction of ventricular ectopic beats (VEBs) were examined. Pacing protocols were carried out in the presence of vagal-induced slowing of sinus rhythm. VT was induced by a basic pacing train during the infusion of caffeine at 1.0 mg/kg per min, but not at 0.3 mg/kg per min. An increase in the pacing rate or the number of stimuli resulted in 1) a decrease in the first postpacing interval, and 2) an increase in the number of postpacing VEBs. Induction of VT was suppressed by intravenous bolus injections of verapamil, propranolol and adenosine. At the time of the initial induction of VT, the plasma concentration of caffeine was 87 +/- 2 micrograms/ml and the plasma level of norepinephrine increased from 666 +/- 166 pg/ml at baseline to 1121 +/- 245 pg/ml. These results suggest that catecholamine-associated triggered activity may be responsible for caffeine-induced VT.

Electrophysiologic effects of cocaine on subendocardial Purkinje fibers surviving 1 day of myocardial infarction

INTRODUCTION

Cocaine has been shown to have broad cardiovascular effects that could be life threatening. Most of the reported electrophysiologic effects of cocaine have been studied in normal but not infarcted myocardium.

METHODS AND RESULTS

Using microelectrode techniques, we investigated the electrophysiologic effects of cocaine on endocardial canine Purkinje fibers that survived 1 day of myocardial infarction. In quiescent infarcted preparations, stimulated trains were followed by subthreshold delayed afterdepolarizations (DADs), in the presence of propranolol (1 microM). Cocaine (10 microM) decreased the amplitude of DADs from 6.1 +/- 1.8 mV to 3.0 +/- 1.3 mV (P < 0.05, n = 6). When stimulated preparations (n = 23) showing no triggered activity during control (+propranolol) were superfused with a low concentration of caffeine (1 mM) or high extracellular Ca2+ (8.1 mM), triggered activity was induced. Subsequent cocaine (10 microM) superfusion prevented the induction of caffeine- and high Ca(2+)-induced triggered activity. Cocaine's effects were reversible upon washout. In preparations that showed triggered activity during control conditions (+propranolol), the mean cycle length of triggered activity was 755 +/- 45 msec. Cocaine (10 microM) superfusion lengthened the cycle length to 1030 +/- 141 msec and terminated triggered activity with a subthreshold DAD (n = 12). In addition, cocaine and ryanodine (10 microM) suppressed triggered activity in a similar manner when tested in the same preparations (n = 4). During control conditions, cocaine did not cause any significant change on the rate of rise of action potential upstroke (from 55.6 +/- 24.3 to 54.5 +/- 28.6 V/sec, n = 8) and maximum diastolic potential (from -58.4 +/- 4.3 to -56.6 +/- 6.5 mV, n = 8). In the absence of propranolol, 50 microM but not 10 microM cocaine induced early afterdepolarizations in 62% of the preparations exhibiting triggered activity during control conditions.

CONCLUSION

The results suggest that cocaine modulates DADs and triggered activity in infarcted endocardial fibers via direct inhibition of cyclic release of Ca2+ from sarcoplasmic reticulum (SR) independently from a local anesthetic or sympathomimetic effect. This SR inhibition could account for the myocardial depressant effect of cocaine. However, while cocaine suppressed DADs, its induction of EADs can precipitate malignant ventricular arrhythmias in the setting of cocaine overdose and infarction.

The contribution of nonreentrant mechanisms to malignant ventricular arrhythmias

Evidence obtained from experimental animals and man indicates that reentry is a major mechanism underlying arrhythmogenesis. However, focal or nonreentrant mechanisms also appear to be operative under a wide variety of pathophysiologic conditions. For example, results obtained using three-dimensional (3D) mapping from 232 simultaneous sites in the feline heart in vivo revealed that nonreentrant or focal mechanisms were prominent during both ischemia and reperfusion. During early ischemia, nonreentrant mechanisms were responsible for initiation of ventricular tachycardia (VT) in 25% of cases and, in cases where VT was initiated by reentry, it often could be maintained by a nonreentrant mechanism. During reperfusion of ischemic myocardium, nonreentrant mechanisms were responsible for initiation of VT in 75% of cases. Most importantly, the transition from VT to ventricular fibrillation in response to reperfusion was secondary to acceleration of a nonreentrant mechanism in either the subendocardium or subepicardium. Potential cellular mechanisms include: 1) sarcolemmal accumulation of amphiphiles such as long-chain acylcarnitines and lysophosphatidylcholine; 2) alpha- and beta-adrenergic mediated effects of catecholamines on the transient inward current (ITI) secondary to an increase in intracellular Ca2+; and 3) alpha-adrenergic receptor-induced decrease in IK mediated by activation of protein kinase C. Recent findings obtained using 3D intraoperative mapping in patients with refractory VT and a previous myocardial infarction also indicate that both reentrant and nonreentrant or focal mechanisms contribute. For example, in 13 selected patients, mapping was of a sufficient resolution to define the mechanisms of 10 runs of VT. Intraoperative mapping indicated that five runs of VT were initiated by intramural reentry, whereas five runs of VT were initiated by a focal or nonreentrant mechanism. The mechanisms underlying ventricular arrhythmias associated with ischemic cardiomyopathy have recently been delineated in dogs after multiple sequential intracoronary embolizations with microspheres (with a decrease in mean ejection fraction from 64% to 25%). Spontaneous VT initiated by focal mechanisms from the subendocardium in 82% and epicardium in 18%, with no evidence of macroreentry. Thus, in divergent pathophysiologic settings, nonreentrant mechanisms appear to contribute importantly to the genesis of lethal ventricular arrhythmias, suggesting that development of novel therapeutic approaches should be directed at inhibition of not only reentrant circuits, but also nonreentrant mechanisms, including triggered activity.

Electrophysiologic mechanisms of ventricular arrhythmias

In this work the electrophysiologic mechanisms of ventricular arrhythmias have been briefly summarized. Ventricular arrhythmias can be caused either by pacemaker activity or by reentrant excitation. Enhancement of normal automaticity can generate a parasystolic rhythm in normal fibers. Abnormal automaticity may arise from fibers in which maximum diastolic potential has been reduced by a variety of interventions. Triggered activity is caused by either an early (EAD) or delayed (DAD) afterdepolarization and requires a prior normal action potential for initiation. While there is growing evidence that EAD-induced triggered activity plays a significant role in the Long QTU syndrome and Torsade de Pointes, no clinical arrhythmias has definitely been ascribed to DADs, although DADs have been recorded in man after acute digoxin intoxication. Ventricular arrhythmias can be also caused by reentrant excitation, which can be subdivided into reflection or circus movement reentry (CMR). In the reflection model impulses in both directions are transmitted over the same pathway. In the CMR three models can be differentiated: the ring model, which requires a fixed anatomical obstacle; the figure-eight model and the leading circle model, where functional rather than fixed anatomical obstacles are involved.

Effects of pretreatment with caffeine or ryanodine on the myocardial response to simulated ischaemia

1. The cytoplasmic calcium concentration ([Ca]c) of rat isolated atrial myocardium was assessed with the dye indo-1. Dye-loaded atria were superfused with physiological salt solution and excited with radiation at 360 nm, while epifluorescence emissions were collected simultaneously at 400 nm and 500 nm. The ratio of these emissions was used as a measure of [Ca]c. 2. Dye-loaded atria showed a phasic rise and fall in [Ca]c with each applied electrical pacing stimulus. The amplitude of these oscillations was reduced by the presence of caffeine (10(-3)-10(-2) M) or of ryanodine (10(-8)-10(-6) M) in a concentration-dependent manner. 3. Atria superfused with a solution the composition of which resembled that found extracellularly in regions of myocardial ischaemia rapidly lost systolic increments in [Ca]c, while end-diastolic [Ca]c values gradually rose. 4. Pretreatment with caffeine (10(-2) M) or ryanodine (10(-7) M) protected atria against the rise in end-diastolic [Ca]c that occurred when the tissue was exposed to conditions of simulated ischaemia.