Impulse conduction and gap junctional remodelling by endothelin-1 in cultured neonatal rat ventricular myocytes.

Endothelin-1 (ET-1) is an important contributor to ventricular hypertrophy and failure, which are associated with arrhythmogenesis and sudden death. To elucidate the mechanism(s) underlying the arrhythmogenic effects of ET-1 we tested the hypothesis that long-term (24 hrs) exposure to ET-1 impairs impulse conduction in cultures of neonatal rat ventricular myocytes (NRVM). NRVM were seeded on micro-electrode-arrays (MEAs, Multi Channel Systems, Reutlingen, Germany) and exposed to 50 nM ET-1 for 24 hrs. Hypertrophy was assessed by morphological and molecular methods. Consecutive recordings of paced activation times from the same cultures were conducted at baseline and after 3, 6 and 24 hrs, and activation maps for each time period constructed. Gap junctional Cx43 expression was assessed using Western blot and confocal microscopy of immunofluorescence staining using anti-Cx43 antibodies. ET-1 caused hypertrophy as indicated by a 70% increase in mRNA for atrial natriuretic peptide (P < 0.05), and increased cell areas (P < 0.05) compared to control. ET-1 also caused a time-dependent decrease in conduction velocity that was evident after 3 hrs of exposure to ET-1, and was augmented at 24 hrs, compared to controls (P < 0.01). ET-1 increased total Cx43 protein by approximately 40% (P < 0.05) without affecting non- phosphorylated Cx43 (NP-Cx43) protein expression. Quantitative confocal microscopy showed a approximately 30% decrease in the Cx43 immunofluorescence per field in the ET-1 group (P < 0.05) and a reduced field stain intensity (P < 0.05), compared to controls. ET-1-induced hypertrophy was accompanied by reduction in conduction velocity and gap junctional remodelling. The reduction in conduction velocity may play a role in ET-1 induced susceptibility to arrhythmogenesis.

Coupling an HCN2-expressing cell to a myocyte creates a two-cell pacing unit.

We examined whether coupling of a ventricular myocyte to a non-myocyte cell expressing HCN2 could create a two-cell syncytium capable of generating sustained pacing. Three non-myocyte cell types were transfected with the mHCN2 gene and used as sources of mHCN2-induced currents. They were human mesenchymal stem cells and HEK293 cells, both of which express connexin43 (Cx43), and HeLa cells transfected with Cx43. Cell-cell coupling between heterologous pairs increased with time in co-culture, and hyperpolarization of the myocyte induced HCN2 currents, indicating current transfer from the mHCN2-expressing cell to the myocyte via gap junctions. The magnitude of the HCN2 currents recorded in myocytes increased with increasing junctional conductance. Once a critical level of electrical cell-cell coupling between myocytes and mHCN2 transfected cells was exceeded spontaneous action potentials were generated at frequencies of approximately 0.6 to 1.7 Hz (1.09 +/- 0.05 Hz). Addition of carbenoxolone (200 microM), a gap junction channel blocker, to the media stopped spontaneous activity in heterologous cell pairs. Carbenoxolone washout restored activity. Blockade of HCN2 currents by 100 microM 9-amino-1,2,3,4-tetrahydroacridine (THA) stopped spontaneous activity and subsequent washout restored it. Neither THA nor carbenoxolone affected electrically stimulated action potentials in isolated single myocytes. In summary, the inward current evoked in the genetically engineered (HCN2-expressing) cell was delivered to the cardiac myocyte via gap junctions and generated action potentials such that the cell pair could function as a pacemaker unit. This finding lays the groundwork for understanding cell-based biological pacemakers in vivo once an understanding of delivery and target cell geometry is defined.

Effects of the renin-angiotensin system on the current I(to) in epicardial and endocardial ventricular myocytes from the canine heart.

The Ca(2+)-independent portion of transient outward K(+) current (I(to)) exhibits a transmural gradient in ventricle. To investigate control mechanisms for this gradient, we studied canine epicardial and endocardial ventricular myocytes with use of the whole-cell patch-clamp technique. I(to) was larger in amplitude, had a more negative voltage threshold for activation, and had a more negative midpoint of inactivation in epicardium. Recovery from inactivation was >10-fold slower in endocardium. Incubation of epicardial myocytes with angiotensin II for 2 to 52 hours altered I(to) to resemble unincubated endocardium and reduced the amplitude of the phase 1 notch of the action potential. In contrast, incubation of endocardial myocytes with losartan for 2 to 52 hours altered I(to) to resemble unincubated epicardium and induced a phase 1 notch in the action potential. With RNase protection assays, we determined that incubations with angiotensin II or losartan did not alter mRNA levels for either Kv4.3 or Kv1.4; thus, a change in the alpha subunit for I(to) is unlikely to be responsible. To test whether posttranslational modification produced the effects of angiotensin II, we coexpressed Kv4.3 and the angiotensin II type 1a receptor in Xenopus oocytes. Incubation with angiotensin II increased the time constant for recovery from inactivation of the expressed current by 2-fold with an incubation time constant of 3.7 hours. No effect on activation or inactivation voltage dependence was observed. These results demonstrate that the properties of I(to) in endocardium and epicardium are plastic and likely under the tonic-differing influence of the renin-angiotensin system.

History of arrhythmias.

A historical overview is given on the techniques to record the electrical activity of the heart, some anatomical aspects relevant for the understanding of arrhythmias, general mechanisms of arrhythmias, mechanisms of some specific arrhythmias and nonpharmacological forms of therapy. The unravelling of arrhythmia mechanisms depends, of course, on the ability to record the electrical activity of the heart. It is therefore no surprise that following the construction of the string galvanometer by Einthoven in 1901, which allowed high-fidelity recording of the body surface electrocardiogram, the study of arrhythmias developed in an explosive way. Still, papers from McWilliam (1887), Garrey (1914) and Mines (1913, 1914) in which neither mechanical nor electrical activity was recorded provided crucial insights into re-entry as a mechanism for atrial and ventricular fibrillation, atrioventricular nodal re-entry and atrioventricular re-entrant tachycardia in hearts with an accessory atrioventricular connection. The components of the electrocardiogram, and of extracellular electrograms directly recorded from the heart, could only be well understood by comparing such registrations with recordings of transmembrane potentials. The first intracellular potentials were recorded with microelectrodes in 1949 by Coraboeuf and Weidmann. It is remarkable that the interpretation of extracellular electrograms was still controversial in the 1950s, and it was not until 1962 that Dower showed that the transmembrane action potential upstroke coincided with the steep negative deflection in the electrogram. For many decades, mapping of the spread of activation during an arrhythmia was performed with a "roving" electrode that was subsequently placed on different sites on the cardiac surface with a simultaneous recording of another signal as time reference. This method could only provide reliable information if the arrhythmia was strictly regular. When multiplexing systems became available in the late 1970s, and optical mapping in the 1980s, simultaneous registrations could be made from many sites. The analysis of atrial and ventricular fibrillation then became much more precise. The old question whether an arrhythmia is due to a focal or a re-entrant mechanism could be answered, and for atrial fibrillation, for instance, the answer is that both mechanisms may be operative. The road from understanding the mechanism of an arrhythmia to its successful therapy has been long: the studies of Mines in 1913 and 1914, microelectrode studies in animal preparations in the 1960s and 1970s, experimental and clinical demonstrations of initiation and termination of tachycardias by premature stimuli in the 1960s and 1970s, successful surgery in the 1980s, the development of external and implantable defibrillators in the 1960s and 1980s, and finally catheter ablation at the end of the previous century, with success rates that approach 99% for supraventricular tachycardias.

Cardiac memory in canine atrium : identification and implications.

BACKGROUND: Memory is a diverse biological phenomenon whose importance in the ventricle has been demonstrated. We hypothesized its occurrence in the atrium, contributing to the modulation of cardiac rhythm. METHODS AND RESULTS: We analyzed P and Ta waves in conscious chronically instrumented dogs with complete heart block. Animals were atrioventricularly sequentially paced at 5% greater than the sinus rate from the lateral right atrium (RA) during control, followed by 2 periods of 1-hour test pacing at 50% greater than the sinus rate, or by equivalent test pacing from the left atrial appendage (LAA) at 5% or 50% greater than the sinus rate. Recovery RA pacing periods of 20- and 30-minute duration, respectively, succeeded each test pacing period. RA test pacing at either rate did not affect the variables measured, but changing the pacing site from RA to LAA altered the P and Ta waves. Displacement of the spatial atrial gradient vector occurred during recovery from LAA pacing, was more marked at rapid pacing rates, and manifested accumulation and resolution consistent with cardiac memory. Concurrently, the right effective refractory period decreased. CONCLUSIONS: Memory is demonstrable in canine atrium, showing rapid onset, accumulation during successive pacing periods, and resolution on cessation of pacing. Given its association with a reduced effective refractory period, it may contribute to the substrate for atrial arrhythmias.

Impact of sex and gonadal steroids on prolongation of ventricular repolarization and arrhythmias induced by I(K)-blocking drugs.

BACKGROUND: Mechanisms for longer rate-corrected QT intervals and higher incidences of drug-induced torsade de pointes in women than in men are incompletely defined, although gonadal steroids are assumed to be important determinants of these differences. METHODS AND RESULTS: We used microelectrode techniques to study isolated rabbit right ventricular endocardium from control male and female and castrated male (ORCH) and female (OVX) rabbits. Action potential duration to 30% repolarization (APD(30)) was significantly shorter in male than female and in ORCH than OVX at a cycle length of 500 ms. The I(Ks) blocker chromanol 293B had no effect on APD in males or females. The I(Kr) blocker dofetilide prolonged APD in female and ORCH more than in male and OVX. At 10(-)(6) mol/L dofetilide (cycle length=1 second), the incidence of early afterdepolarizations was: female, 67%; ORCH, 56%; male, 40%; and OVX, 28%. Serum 17beta-estradiol levels were unrelated to the effects of dofetilide, but as testosterone levels increased, the dofetilide effect to increase APD diminished, as did early afterdepolarization incidence. CONCLUSIONS: Sex-related differences in basal right ventricular endocardial AP configuration persist in castrated rabbits, suggesting that extragonadal factors contribute to the differences in ventricular repolarization. In this model, drugs that block I(Kr) but not I(Ks) prolong repolarization in a way that suggests that protection from excess prolongation in males is attributable to testosterone, whereas the risk of excess prolongation of repolarization in females is related to sex-determined factors in addition to estrogen.

Transient outward current, Ito1, is altered in cardiac memory.

BACKGROUND: Cardiac memory refers to an altered T-wave morphology induced by ventricular pacing or arrhythmias that persist for variable intervals after resumption of sinus rhythm. METHODS AND RESULTS: We induced long-term cardiac memory (LTM) in conscious dogs by pacing the ventricles at 120 bpm for 3 weeks. ECGs were recorded daily for 1 hour, during which time pacing was discontinued. At terminal study, the heart was removed and the electrophysiology of left ventricular epicardial myocytes was investigated. Control (C) and LTM ECG did not differ, except for T-wave amplitude, which decreased from 0.12+/-0.18 to -0.34+/-0.21 mV (+/-SEM, P<0.05), and T-wave vector, which shifted from -37+/-12 degrees to -143+/-4 degrees (P<0.05). Epicardial action potentials revealed loss of the notch and lengthening of duration at 20 days (both P<0.05). Calcium-insensitive transient outward current (Ito) was investigated by whole-cell patch clamp. No difference in capacitance was seen in C and LTM myocytes. Ito activated on membrane depolarization to -25+/-1 mV in C and -7+/-1 mV (P<0.05) in LTM myocytes, indicating a positive voltage shift of activation. Ito density was reduced in LTM myocytes, and a decreased mRNA level for Kv4.3 was observed. Recovery of Ito from inactivation was significantly prolonged: it was 531+/-80 ms (n=10) in LTM and 27+/-6 ms (n=9) in C (P<0.05) at -65 mV. CONCLUSIONS: Ito changes are associated with and can provide at least a partial explanation for action-potential and T-wave changes occurring with LTM.

The effect of extracellular potassium on the intracellular potassium ion activity and transmembrane potentials of beating canine cardiac Purkinje fibers.

We used open tip microelectrodes containing a K+-sensitive liquid ion exchanger to determine directly the intracellular K+ activity in beating canine cardiac Purkinje fibers. For preparations superfused with Tyrode's solution in which the K+ concentration was 4.0 mM, intracellular K+ activity (ak) was 130.0+/-2.3 mM (mean+/-SE) at 37 degrees C. The calculated K+ equilibrium potential (EK) was -100.6+/-0.5 mV. Maximum diastolic potential (ED) and resting transmembrane potential (EM) were measured with conventional microelectrodes filled with 3 M KCl and were -90.6+/-0.3 and -84.4+/-0.4 mV, respectively. When [K+]o was decreased to 2.0 mM or increased to 6.0, 10.0, and 16.0 mM, ak remained the same. At [K+]o=2.0, ED was -97.3+/-0.4 and Em -86.0+/-0.7 mV; at [K+]o=16.0, ED fell to -53.8+/-0.4 mV and Em to the same value. Over this range of values for [K+]o, EK changed from -119.0+/-0.3 to -63.6+/-0.2 mV. These values for EK are consistent with those previously estimated indirectly by other techniques.

Cellular electrophysiology of digitalis toxicity.

The toxic effects of digitalis are attributable in part to poisoning of the enzyme Na+-K+ ATPase and in part to the interactions of digitalis with the sympathetic and parasympathetic nervous systems. Additional modifiers of the toxic effects of digitalis include the concentrations of ions such as K+ and Ca2+, the age of the subject and the extent and type of cardiac disease. At the cellular electrophysiologic level, digitalis toxicity is seen as a depolarization of the membrane with the occurrence--individually or simultaneously--of abnormalities of impulse initiation (including delayed afterdepolarizations and abnormal automaticity) and abnormalities of conduction. The afterdepolarizations result in triggered arrhythmias that differ partially in their characteristics of onset and termination from automatic and reentrant arrhythmias. The cellular electrophysiologic basis for these arrhythmias induced by toxic concentrations of digitalis and their implications with respect to arrhythmogenesis in the in situ heart are explored in detail.

Triggered activity as a cause of bigeminy.

Standard microelectrode techniques were used to study bigeminal rhythms occurring during otherwise stable triggered activity in ouabain-toxic canine Purkinje fibers. The basis for the bigeminy appeared to be an alternans phenomenon in the delayed afterdepolarizations that induced the triggered activity, as well as in the maximal diastolic potential. The occurrence of bigeminy, previously thought to result from reentry, from single delayed afterdepolarizations coupled to a triggered action potential or from parasystole, can also be considered a manifestation of sustained triggered activity.

Cellular electrophysiologic effects of vertebrate digitalis-like substances.

Substances structurally and functionally similar to digitalis glycosides are produced by several vertebrate species. There also is evidence for a digitalis-like substance of human origin. Standard microelectrode techniques were used to study the direct effects on the cellular electrophysiology of canine Purkinje fibers of 1) bufalin, an unconjugated cardiotonic steroid molecule that is produced by the toad Bufo marinus, and 2) an extract of human bile that showed digitalis-like immunoreactivity on radioimmunoassay. The goal of this study was to determine whether these substances have arrhythmogenic effects comparable with those seen with toxic doses of digitalis glycosides. Bufalin, 2 x 10(-8) M, significantly (p less than 0.05) reduced maximal diastolic potential, action potential amplitude and duration and maximal rate of rise of phase 0 (Vmax) within 40 min of onset of exposure. All six fibers developed delayed afterdepolarizations and two developed triggered rhythms. Ouabain was less potent, in that a 2 x 10(-7) M concentration was required to comparably reduce maximal diastolic potential, action potential amplitude and duration and Vmax within 30 min. These Purkinje fibers also developed delayed afterdepolarizations and triggered rhythms. A sample of an extract of human bile that showed digitalis-like immunoreactivity with an antibufalin serum also reduced maximal diastolic potential, action potential amplitude and duration and Vmax, and produced delayed afterdepolarizations and triggered activity. In contrast, immunologically unreactive bile extracts had no appreciable effect on the action potential. In summary, the cardiac toxicity of digitalis substances produced by lower vertebrates is comparable with that induced by the glycosides. Moreover, it appears that humans may produce digitalis-like substances that may be cardiotoxic.

Age related differences in the response to acidosis, hypoxia, and hyperkalaemia in canine cardiac Purkinje fibers.

OBJECTIVE: In the clinical setting, the response of adult and neonatal cardiac rhythms to hypoxia and to acidosis differs, the former leading to tachyarrhythmias and the latter to bradyarrhythmias. In this study, the aim was to determine whether a cellular electrophysiological substrate could be identified to explain the clinical observation. METHODS: Conventional microelectrode techniques were used to study the electrophysiological responses of automatic Purkinje fibres to acidosis, hypoxia, and hyperkalaemia individually. RESULTS: Adult Purkinje fibres showed decreases in maximum diastolic potential, activation voltage, and automaticity as pH was decreased from 7.3 to 6.1. Triggered activity due to early afterdepolarisations developed in 70% of adult Purkinje fibres at pH 6.1. Neonatal Purkinje fibres showed decreased automaticity as pH decreased and, in contrast to adults, quiescence occurred at pH 6.1 At PO2 < 2.9 kPa automaticity decreased significantly in adult Purkinje fibres, whereas neonatal fibres were unaffected. The effect of raising [K+]o was comparable at both ages. CONCLUSIONS: The response to acidosis and to hypoxia differs significantly between neonatal and adult Purkinje fibres and may explain some developmental differences in the expression of cardiac arrhythmias.

Modulation of delayed afterdepolarisations by alpha 1 adrenergic receptor subtypes.

OBJECTIVE: The aim was to evaluate the role of alpha 1 adrenoceptor stimulation and selective receptor subtype blockade on delayed afterdepolarisations induced by high extracellular calcium ([Ca2+]o = 8.1 mM) and by ouabain (2 x 10(-7) M) in canine Purkinje fibres. METHODS: Standard microelectrode techniques were used to record transmembrane action potentials from Purkinje fibres placed at cycle length = 500 ms. The drive was interrupted for 5 s every minute to allow observation of delayed afterdepolarisations or any subsequent spontaneous rhythms. RESULTS: The alpha 1 adrenoceptor agonist phenylephrine, 1 x 10(-7) M, increased the amplitude of delayed afterdepolarisations induced by high [Ca] from 4.2(SEM 0.4) to 5.4(0.2) mV (p < 0.05), and by ouabain from 4.3(0.6) to 6.9(1.0) mV (p < 0.05). The selective alpha 1 adrenoceptor subtype agonist WB 4101, 1 x 10(-7) M, blocked the effect of phenylephrine on delayed afterdepolarisations induced by ouabain + phenylephrine [DAD = 4.6(0.6), p < 0.05] but decreased the amplitude of those induced by Ca + phenylephrine, to 2.0(0.3) mV (p < 0.05). In contrast, the selective alpha 1b adrenoceptor antagonist chloroethylclonidine, 1 x 10(-7) M, increased the amplitude of delayed afterdepolarisations induced by Ca and phenylephrine to 6.6(0.6) mV (p < 0.05) and by ouabain and phenylephrine to 12.6(2.0) mV (p < 0.05). CONCLUSIONS: alpha 1 Adrenoceptor subtypes modulate the amplitude of delayed afterdepolarisations induced by ouabain and by high [Ca]. The chloroethylclonidine-blocked alpha 1 adrenoceptor subtype decreases, and the WB 4101-blocked subtype potentiates, this arrhythmogenic mechanism.

Sympathetic innervation modulates ventricular impulse propagation and repolarisation in the immature rat heart.

OBJECTIVE: When cardiac sympathetic innervation in neonatal rats is retarded by antiserum to nerve growth factor, there is a corresponding increase in the QT interval on ECG. Since the propagation of the cardiac impulse and the repolarisation of cardiac cells both contribute to the QT interval, the aim of this study was to determine the role of sympathetic innervation in modulating ventricular impulse propagation and repolarisation. METHODS: Neonatal rats were treated for the first 10 days of life with nerve growth factor (NGF), its antiserum (As), or placebo. Standard microelectrode techniques were used to study the transmembrane action potential characteristics of subendocardial (ventricular septal) and subepicardial ventricular myocardium. Bipolar surface electrograms were used to record the velocity of impulse propagation and electron microscopy to examine the intercalated discs. RESULTS: In the subendocardium, the phase 0 upstroke velocity of the action potential (dV/dtmax) was lowest in the As treated rats. The latter group also showed the slowest conduction velocity. There were no differences in control action potential durations in the endocardium among the three groups, but in the epicardial tissues, action potential duration was longest in the As treated group. Thus the dispersion in action potential duration was smallest in the As treated animals. Electron microscopic studies of the intercalated discs of ventricular myocytes showed significant enhancement of nexal junction formation in NGF treated rats, whereas As treated animals showed a retarded pattern of both nexal and desmosomal junction formation. CONCLUSIONS: The differences in ultrastructure, conduction, and repolarisation seen in As and NGF treated animals may explain the prolonged QT interval seen in the As treated group.

Deaminovasopressin has direct and modulatory effects on ventricular automaticity in the rat heart.

OBJECTIVE: Some neuropeptides have direct cardiac effects and also modulate the cardiac effects of catecholamines. Vasopressin is an abundantly available neuropeptide having well known interactions with catecholamines in vascular smooth muscle. The aim of this study was to determine the direct and modulatory effects of vasopressin on ventricular automaticity. METHODS: The cardiac effects of deaminovasopressin (dAVP), a long acting synthetic analogue of vasopressin, were tested on basal and alpha 1 agonist induced changes in automaticity in isolated ventricular septal preparations from adult and neonatal rats after chronic exposure (10 micrograms.kg-1 x d-1 subcutaneously for 10 d) and acute exposure (in vitro bath superfusion with 10(-8) M dAVP for 1 h). RESULTS: Chronic exposure to dAVP decreased basal ventricular automaticity in the adult and in 10-11 d old rats. Although alpha 1 agonists tended to decrease automaticity in adult rat heart, prior chronic dAVP exposure altered the chronotropic response to alpha 1 agonist so that only an increase in automaticity was observed. A similar result was seen in adult ventricular septal preparations upon acute superfusion with dAVP. Acute dAVP exposure reduced basal ventricular automaticity, and modified the alpha 1 adrenergic chronotropic response, such that only an increase in automaticity occurred. Acute dAVP exposure in adult ventricular septal preparations did not significantly change total alpha 1 adrenergic receptor density or antagonist affinity, alpha 1 adrenergic receptor subtype expression, or the amount of pertussis toxin sensitive G protein measured in an ADP ribosylation assay. CONCLUSION: dAVP not only exerted direct effects of chronotropy, but also influenced the expression of alpha 1 adrenergic chronotropic responsiveness. If vasopressin has a similar action, this may have important implications in instances where levels of this peptide are raised. For example, surgical stress and cardiopulmonary bypass are clinical situations associated with increases in both vasopressin and catecholamine levels. An interaction between the two may contribute to the development of tachyarrhythmias in these settings.

Vagal release of vasoactive intestinal peptide can promote vagotonic tachycardia in the isolated innervated rat heart.

OBJECTIVE: The aim was to determine the extent to which endogenous release of vasoactive intestinal polypeptide (VIP) might be implicated in the modulation of sinoatrial rate in the presence and absence of muscarinic blockade or beta blockade. METHODS: Langendorff perfused rat hearts were studied with the right vagus intact. The hearts were maintained in sinus rhythm and subjected to right vagal stimuli of 5, 10, 20, and 30 Hz. RESULTS: Administration of exogenous VIP, 10(-8) M, increased sinus rate by 20% (p < 0.05). This increase in heart rate was reduced significantly to 8% by the VIP antagonist [D-p-Cl-Phe6, Leu17]VIP, 10(-7) M, which alone had no effect on sinus rate. Vagal stimulation reduced sinus rate from a control of 254(SEM 2) to 164(17) beats.min-1 (p < 0.05) at 20 Hz. VIP, 10(-8) M, increased these rates to 284(6) and 220(21) beats.min-1 (p < 0.05). In another eight vagally stimulated hearts, frequencies of 5-20 Hz reduced sinus rate. At 30 Hz heart rate increased in five, and the resultant rate was significantly faster in these [154(10) beats.min-1] than in the remainder [98(12) beats.min-1, p < 0.05]. Vagal stimulation also increased sinus rate (p < 0.05) in four of seven additional hearts perfused with atropine, 2 x 10(-6) M. This increase was completely abolished by [D-p-Cl-Phe6, Leu17]VIP. That the effect was not beta adrenergic was demonstrated in eight experiments using atropine plus propranolol, 1 x 10(-7) M. A vagally induced increment in rate still occurred (p < 0.05) and was abolished by [D-p-CL-Phe6, Leu17]VIP. The ability to ascribe a rate change to VIP release was maximal in the presence of propranolol and atropine, intermediate in the presence of atropine alone, and minimal in the absence of muscarinic or beta blockade. CONCLUSIONS: Vagally released VIP is capable of limiting the decrement in sinus rate that occurs at high frequencies of vagal stimulation, and in some circumstances can actually increment sinus rate. Its role as an endogenous modulator of vagal effects on heart rate and as a possible cause of vagal and postvagal tachycardias should be further explored.

Steady-state and nonsteady-state action potentials in fibrillating canine atrium: abnormal rate adaptation and its possible mechanisms.

OBJECTIVE: Our goal was to study rate adaptation of atrial action potentials in non-steady and steady states to further our understanding of mechanisms determining inducibility and stability of atrial fibrillation. METHODS: We used standard microelectrode techniques to examine the characteristics of steady-state action potentials paced at regular cycle lengths (CL) and of nonsteady-state action potentials observed after an abrupt change of CL in atria from canine hearts that had been rapidly paced. RESULTS: We compared action potential characteristics among normal atria, atria in which chronic atrial fibrillation (cAF, lasting more than 3 days) had been induced and atria in which only nonsustained atrial fibrillation (nAF, lasting less than 12 h) had been induced. In steady-state, the rate adaptation of maximum diastolic potential (MDP) and action potential duration (APD) and markedly reduced in both cAF and nAF. Action potential characteristics did not differ between cAF and nAF atria, suggesting that factors other than electrophysiological properties determine the chronicity of AF. The time course of change in APD after an abrupt change of CL was altered in nAF/cAF atria; i.e., when CL was prolonged, APD also prolonged at the first beat, and then shortened during several subsequent beats (initial phase). Thereafter, APD slowly prolonged to a new steady-state (slow phase). In nAF/cAF atria, the initial phase was enhanced (greater shortening of APD) and the slow phase was reduced (less prolongation of APD). This latter phase was modified by ryanodine. CONCLUSIONS: Thus the reduced rate adaptation of steady-state APD is explained mainly by the loss of a slow phase of APD adaptation in nAF/cAF which is reversed in the presence of ryanodine. Therefore, in both nAF and cAF atria, rate adaptation of MDP as well as APD are reduced, nonsteady state as well as steady state, AP characteristics are markedly altered and these changes are partially explicable by Ca, -dependent processes.

Long-term electrophysiological effects of regional cardiac sympathetic denervation of the neonatal dog.

OBJECTIVE: In many cardiac arrhythmias, both a triggering factor and a favorable myocardial substrate are required. Whereas the sympathetic nervous system may trigger tachyarrhythmias, its function as a long-term modulator of the myocardial substrate is less well understood. Therefore, we tested the hypothesis that regional sympathetic denervation at birth would produce an abnormal myocardial substrate. The comparator was the substrate associated with inherited, lethal tachyarrhythmias at 5 months of age in German shepherd dogs with incomplete sympathetic innervation. METHODS: Mongrel dogs underwent right cardiac stellectomy (RSX) within the first day of life and were terminally studied with control littermates at 5 months of age. RESULTS: On days 1-21 of life, RSX animals manifested significant QT prolongation on ECG and sudden, asystolic death. Beyond this age, QT intervals normalized and deaths did not occur. At 5 months, action potentials (AP) were recorded from Purkinje fibers (PF) and midmyocardial preparations in anteroseptal (AS) and posterobasal (PB) left ventricle. Early afterdepolarizations occurred only in left ventricular PF from RSX dogs. Isoproterenol prolonged AP duration in AS and shortened it in PB of RSX but not control dogs. The incidence of isoproterenol-initiated triggered activity and the amplitude of delayed afterdepolarizations were greater in RSX than control dogs. CONCLUSION: Five months after RSX heterogeneous alterations of LV electrophysiological properties were similar to those previously observed in animals having inherited deficits in sympathetic innervation and sudden death. This implicates the sympathetic nerves as long-term modulators of an arrhythmogenic substrate. That 5-month-old RSX dogs did not experience tachyarrhythmias or sudden death indicates that further anomalies--beyond those explicable by the substrate change--must exist to induce sudden death.

beta(1) and beta(2)-adrenergic receptor subtype effects in German shepherd dogs with inherited lethal ventricular arrhythmias.

OBJECTIVE: Delayed afterdepolarization-induced triggered activity originating in ventricular myocardium is a mechanism for some age-dependent, inherited ventricular tachycardias in a colony of German shepherd dogs. METHODS: We used standard microelectrode techniques to study beta-adrenergic receptor subtype modulation of the triggered activity in anteroseptal left ventricular myocardium from eleven of these dogs and seven unafflicted, age-matched German shepherd controls. RESULTS: During sustained stimulation at cycle lengths of 300-4000 ms, 10(-9)-10(-7) M isoproterenol concentration-dependently shortened action potential duration (APD) to 90% repolarization more in myocardium from afflicted than from unafflicted dogs. This shortening was prevented by a beta(1)-blocker CGP20712A (10(-7) M) while a beta(2)-blocker ICI118551 (10(-7) M) did not modify the effect of isoproterenol in either group. The beta(2)-agonist zinterol 10(-8)-10(-6) M had no effect on APD. Stimulation at a cycle length of 250 ms in the presence of 10(-7) M isoproterenol induced more triggered AP in myocardium from afflicted than unafflicted dogs. beta(1)-Blockade completely eliminated, while beta(2)-blockade facilitated, and the beta(2)-agonist zinterol did not induce triggered activity in the two groups. CONCLUSION: Isoproterenol effects on APD and triggered activity in the myocardium of dogs with inherited arrhythmias are due primarily to an abnormality of beta(1)-adrenoceptor mediated signaling that is subject to beta(2)-adrenergic modulation.

Interactions between antiarrhythmic drugs and cardiac memory.

OBJECTIVE: Ventricular pacing or arrhythmias can induce cardiac memory (CM). We hypothesized that clinically administered antiarrhythmic drugs alter the expression of CM, and that the repolarization changes characteristic of CM can modulate the effects of antiarrhythmic drugs. METHODS: We studied conscious, chronically-instrumented dogs paced for two 1-h periods to study the effects of drugs on the evolution of memory (protocol 1) or for 21 days (protocol 2) to observe the effects of steady-state memory on drug actions. Dogs were treated in both settings with quinidine, lidocaine or E4031, in random order, and within therapeutic serum concentration ranges. RESULTS: Pacing, alone, for 2 h significantly prolonged ERP only near the left ventricular pacing site, whereas pacing alone for 21 days prolonged ERP at all sites (P<0.05). Quinidine and E4031, but not lidocaine, prolonged repolarization and ERP and suppressed evolution of CM in protocol 1. However, quinidine's effect in prolonging repolarization was diminished in both protocols, while its effect in prolonging ERP was diminished in the 21-day protocol only. In contrast, the effects of E4031 were additive to those of CM, prolonging repolarization and ERP in both protocols, while lidocaine showed no changes in effect at all. CONCLUSIONS: Pacing to induce CM significantly affects ventricular repolarization and refractoriness, and there are interactions between CM, quinidine and E4031. Depending on the specific drug, these interactions have the potential to be anti- or proarrhythmic, and may impact importantly on the clinical efficacy of drugs as well as on electrophysiologic testing of drug actions.