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.

Electrophysiological responses of canine atrial endocardium and epicardium to acetylcholine and 4-aminopyridine.

OBJECTIVES: Prior studies demonstrated marked electrophysiological and pharmacological differences between canine ventricular epicardium and endocardium. For atrium, however, it has been assumed that, because of the thin wall, electrical properties of epicardium and endocardium are similar. The aim of the present study was to compare the action potential (AP) characteristics in epicardial and endocardial atrial cells before and following addition of acetylcholine (ACh) and 4-aminopyridine (4-AP). METHODS AND RESULTS: Microelectrode techniques were used to study the effects of ACh (10(-7)-10(-5) M) and 4-AP (0.5 mM) on epicardial and endocardial AP of canine right atrial free wall at cycle lengths (CL) of 250 to 2000 ms. ACh hyperpolarized epicardial and endocardial cells (by 5-8 mV at 10(-5) M). In control, AP duration to 90% repolarization (APD90) was longer in endocardium at all CL. ACh shortened APD90 in either tissue with more prominent effect in endocardium (at 10(-5) M and CL = 2000 ms, from 179 +/- 10 to 90 +/- 11 ms in epicardium and from 209 +/- 10 to 65 +/- 6 ms in endocardium, P < 0.05). As a result, at 10(-5) M, APD90 in endocardium was shorter than in epicardium at all CL 4-AP effects on AP duration were similar in both tissue types. No effects of 4-AP was seen at CL = 250 ms and at long CL, the compound shortened APD90 and prolonged AP duration to 50% repolarization. CONCLUSIONS: (1) ACh exerts direct effects on atrial epicardial and endocardial AP; (2) 4-AP-sensitive transient outward current (Itol) is expressed both in canine atrial epicardial and endocardial cells; (3) differential response of epicardial and endocardial APD to ACh may alter the gradient of repolarization across the atrial wall and contribute to vagally induced atrial flutter and fibrillation.

Abnormal cardiac repolarization and impulse initiation in German shepherd dogs with inherited ventricular arrhythmias and sudden death.

OBJECTIVE: We tested the hypothesis that delayed afterdepolarization (DAD)-associated rhythms in German shepherd dogs with reduced anteroseptal left ventricular (LV) sympathetic innervation derive from abnormal beta-adrenergic receptor effector coupling. METHODS AND RESULTS: In anteroseptal LV midmyocardium of afflicted dogs, beta-receptor density was greater than that in normal dogs (P < .05), with affinity being equal in both groups. Basal and maximum isoproterenol (ISO) stimulated adenylyl cyclase activity of anteroseptal LV of afflicted dogs was greater than that in normal dogs (P < .05). Isolated anteroseptal M cell preparations of afflicted dogs studied with microelectrodes showed abnormal lengthening, rather than shortening of action potential duration in response to ISO, as well as a 61% incidence of 10(-7) mol/l ISO-induced triggered activity as compared to 12% in normals (P < .05). In contrast, there was no difference between afflicted and control dogs in triggered activity, beta-receptors or adenylyl cyclase activity in a normally innervated region of the ventricles. CONCLUSION: In this model there is an increase in beta-receptor density and beta-adrenergic stimulation of adenylyl cyclase and of triggered activity in anteroseptal myocardium but not in a normally innervated region of the heart. Hence, abnormal beta-adrenergic signal transduction appears associated with the neural abnormality identified in dogs with inherited VT.

Electrophysiological effects of LU111995 on canine hearts: in vivo and in vitro studies.

We studied the electrophysiological effects of LU111995 (1-15 mg/kg p.o.) in conscious dogs with chronic atrioventricular block and ventricular pacing at 50 to 130 beats/min. LU111995 had no effects on idioventricular rhythm, QRS duration, and ventricular conduction time. It significantly prolonged Q-T interval (by 5-8%) and effective refractory period (ERP) (by 5-12%) with the maximal effect at 4 h after a 10 mg/kg dose. At 10 and 15 mg/kg, it increased the ERP/Q-T ratio. In vitro, the effects of LU111995 (1 x 10(-7) to 1 x 10(-5) M) on action potentials of Purkinje fibers (PFs) and M cells were studied at cycle lengths (CL) of 300 to 2000 ms. It had no effects on maximum diastolic potential and action potential amplitude in either tissue. High concentrations induced a moderate, rate-independent decrease of Vmax in M cells. In PFs and M cells, it produced reverse use-dependent lengthening of action potential duration (APD). In PFs at long CL, the drug exhibited a biphasic concentration-dependent effect on APD: maximum prolongation (by 26% at a CL of 2000 ms) was attained at 1 x 10(-6) M, and a decrease of APD occurred at higher concentrations. In M cells, the maximum effect on APD occurred at 3 x 10(-6) M. Early afterdepolarizations were seen in 50% of M cell preparations but only at CL of 2000 ms. Triggered activity did not occur. In summary, LU111995 prolongs the Q-T interval to a limited degree and is not arrhythmogenic over the physiological range of CLs.

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 controversial M cell.

A recent publication by us has been interpreted by some as arguing against the existence and importance of M cells. We suppose this is the reason we have been asked to write this "controversy." Regrettably for the controversy, neither our work nor we deny the existence of M cells. Rather, we have confirmed, conceivably ad nauseum, that M cells do exist and contribute importantly to the expression of electrical activity in the intact myocardium. What controversy there is relates to (1) whether there is an inhomogeneous transmural gradient for ventricular repolarization in normal hearts, and (2) why the electrophysiologic properties of different myocardial sites differ so markedly at the level of the isolated tissue and single cell and yet become so much more homogenous in the intact ventricle. These issues are addressed on the following pages.

Evolution and resolution of long-term cardiac memory.

BACKGROUND: Cardiac memory (CM) refers to T-wave changes induced by ventricular pacing or arrhythmia that accumulate in magnitude and duration with repeated episodes of abnormal activation. We report herein the kinetics of long-term CM and its association with the ventricular action potential. METHODS AND RESULTS: Dogs were paced from the ventricles at rates of 110 to 120 bpm for approximately 3 weeks. CM characterized by gradual sinus rhythm T vector rotation toward the paced QRS vector evolved in all dogs regardless of pacing site (left ventricular [LV] anterior apex or base, posterior LV, or right ventricular free wall). Cardiac hemodynamics and myocardial flow (microsphere studies) were unaltered by the pacing. Recovery time for the memory T wave to return to control increased with duration of the previous pacing. The protein synthesis inhibitor cycloheximide markedly (P<.05) and reproducibly attenuated evolution of CM. When pacing was performed from the atrium, CM did not occur. Standard microelectrode techniques were used to study action potential from the LV free wall of control and CM dogs. CM was associated with increased action potential duration in epicardial and endocardial but not midmyocardial cells, significantly altering the transmyocardial gradient for repolarization. CONCLUSIONS: CM is a dynamic process for which the final T vector is predicted by the paced QRS vector and which is associated with significant changes in epicardial and endocardial but not midmyocardial cell action potential duration, such that the transmural gradient of repolarization is altered. It is unaccompanied by evidence of altered hemodynamics or flow, requires a change in pathway of activation, and appears to require new protein synthesis.

Effects of quinidine on repolarization in canine epicardium, midmyocardium, and endocardium: I. In vitro study.

BACKGROUND: The antiarrhythmic action of quinidine is associated with a slowing of conduction and prolongation of repolarization. The latter effect has no consistent correlation with quinidine actions on action potential duration (APD) in isolated tissue experiments. To enhance our understanding of the mechanisms of quinidine action, we studied its effect on APD in canine epicardial, midmyocardial, and endocardial tissues. METHODS AND RESULTS: Standard microelectrode techniques were used to study the effects of quinidine 2.5 to 20 micromol/L on APD in ventricular epicardial, endocardial, and transmural (M-cell) slabs at cycle lengths (CLs) from 300 to 4000 ms. Qualitatively different time courses of actions and concentration- and rate-dependent effects were seen in M cells compared with the others. In endocardium and epicardium, quinidine induced monotonic and concentration-dependent APD prolongation at all CLs. In contrast, the effects of quinidine in M cells varied from prolongation to shortening, depending on duration of superfusion, concentration, and CL. Experiments with E4031 and TTX suggested that in M cells, quinidine-induced APD lengthening was attributable to block of delayed rectifier potassium current and APD shortening was due to inhibition of TTX-sensitive steady-state sodium current. CONCLUSIONS: In vitro, there is a significant difference of quinidine effects in M cells versus epicardial and endocardial cells that appears to reflect differences in the contributions of specific ion channels to the APD at the three sites. The differences may influence the actions of quinidine on repolarization of the heart in situ and determine both the proarrhythmic and antiarrhythmic actions of the drug.

Effects of quinidine on repolarization in canine epicardium, midmyocardium, and endocardium: II. In vivo study.

BACKGROUND: In the companion article, we report a significant difference in quinidine effects on the action potential duration between surface (epicardial and endocardial) cells and midmyocardial cells (M cells) of canine left ventricle in vitro. This article considers two questions raised by the previous study: (1) Are the complex quinidine effects in vitro reflected in its actions on the heart in situ? (2) What are the cellular determinants of quinidine effects on QT interval in ECG? METHODS AND RESULTS: We used plunge and surface electrodes to measure activation-recovery intervals (ARIs) of bipolar electrograms obtained from epicardium, endocardium, and midmyocardium (3, 5, and 9 mm from epicardium) of canine left ventricle in conditions of AV block and right ventricular pacing. Quinidine was infused continuously; its plasma level increased from 1.6+/-0.1 microg/mL at 30 minutes to 7.6+/-0.7 microg/mL at 180 minutes. At cycle lengths (CLs) from 300 to 1500 ms, there was no ARI gradient across the ventricular wall before and during quinidine infusion. At a CL of 300 ms, therapeutic concentrations of quinidine prolonged ARIs and QT intervals. At a CL of 1500 ms, ARIs were significantly prolonged at low quinidine concentrations. With an increase of quinidine concentration, this effect subsided and disappeared. CONCLUSIONS: In situ, quinidine-induced prolongation of repolarization is uniform in all myocardial layers and follows the pattern observed in M cells in vitro. The ability of quinidine in therapeutic concentrations to prolong repolarization at rapid heart rates can contribute to its antiarrhythmic efficacy.

Responses to norepinephrine of normal and "ischemic" canine Purkinje fibers are consistent with activation of different alpha 1-receptor subtypes.

INTRODUCTION: Previously we found that WB4101 (WB) 10(-7) M competitively blocks three alpha 1-adrenergic receptor-effector responses: the increase in normal automaticity occurring in Purkinje fibers (PF) at high membrane potentials; the increase in abnormal automaticity occurring in PF at depolarized membrane potentials; and the prolongation of PF action potential duration. These observations are consistent with two different hypotheses: (1) WB blocks a single alpha 1-receptor subtype, which subserves different effector pathways; and (2) WB blocks different receptor subtypes, each of which subserves an independent pathway. The aim of this study was to test both hypotheses. METHODS AND RESULTS: We used standard microelectrode techniques to study the concentration-dependent actions of three alpha 1-adrenoreceptor blockers (WB [alpha 1A > or = alpha 1D], 5-methylurapidil [5-MU] [alpha 1A > > alpha 1D], and UK52,046 [nonselective]) or norepinephrine (NE) effects in normal PF and in PF depolarized with a simulated ischemic solution ([K+]o = 10 mM; pO2 < 20 mmHg; pH 6.8; maximum diastolic potential -60 +/- 1 mV). In normally polarized PF, concentration-dependent actions of all blockers on both the positive chronotropic response and the prolongation of action potential duration completely coincide. In contrast, the response to NE of abnormal automaticity in "ischemic" PF differs from normals: there is a high sensitivity to WB and 5-MU and no response to UK52,046. CONCLUSIONS: (1) A single receptor subtype appears responsible for both the alpha 1-induced prolongation of repolarization and the positive chronotropic effect in normal PF. (2) Two different receptor subtypes may be responsible for the alpha 1-induced effects on automaticity in normal and ischemic fibers. It is likely that the latter one is alpha 1A, and that consideration of antiarrhythmic therapy with alpha 1-adrenergic blockers should focus on this subtype as a potential target.

Electrophysiologic effects of nibentan (HE-11) on canine cardiac tissue.

We studied the effects of nibentan on transmembrane action potentials of canine Purkinje fibers (PF), ventricular epicardial and endocardial tissues and atrial tissue. Nibentan (1 x 10(-8) to 5 x 10(-6) M) had no effects on maximum diastolic potential of all tissues and produced a modest concentration- and use-dependent decrease in V(max). However, a remarkable tissue specificity was observed in its effects on action potential duration (APD). In PF, the concentration-dependent effect was biphasic: maximum APD prolongation was attained at 10(-7) M, and a decrease in APD was seen at higher concentrations. In contrast, in ventricular tissue, nibentan prolonged APD monotonically to a steady state at 10(-6) M. In atrial tissue, a monotonic, concentration-dependent increase in APD was observed through the highest concentration. The ability of nibentan to prolong PF APD significantly diminished as the cycle length shortened (from 2000 to 300 ms), whereas in ventricular and atrial tissues, it showed no reverse use-dependence. In the physiological range of cycle length, nibentan did not enhance the spatial inhomogeneity of repolarization. In PF, it prolonged APD, slightly inhibited V(max) of Ca++-induced action potentials and completely eliminated the effects of isoproterenol on normal automaticity. We conclude that 1) nibentan is an antiarrhythmic with a profound ability to prolong repolarization while decreasing heterogeneity of repolarization and 2) the extent of nibentan's APD prolongation effect is significantly different in different cardiac tissues.

Regional differences in electrophysiological properties of epicardium, midmyocardium, and endocardium. In vitro and in vivo correlations.

BACKGROUND: Microelectrode studies have described a population of cells within the midmyocardium (M cells) displaying a steep rate dependence of action potential duration (APD) and high Vmax compared with endocardial (Endo) and epicardial (Epi) cells. METHODS AND RESULTS: We studied repolarization in different myocardial layers in vitro and in situ. In addition to confirming the results of earlier studies, we found that after abrupt lengthening of the cycle length (CL), APDs in M cells reached a new steady state faster than in Epi or Endo cells: the time to achieve 90% of the difference in APD (t90) was 13.3 +/- 0.7 minutes in Endo cells, 12.8 +/- 1.1 minutes in Epi cells, and 2.6 +/- 0.4 minutes in M cells (P < .05 compared with Epi or Endo) when CL changed from 400 to 1000 ms. In situ, we registered activation-recovery intervals (ARIs) in bipolar electrograms obtained from different myocardial layers in conditions of AV block and His-bundle pacing. At all CLs from 300 to 2000 ms, ARIs were equal in all myocardial layers from Epi to Endo cells. Steady-state ARIs coincided with APD of M cells registered in vitro in the physiological range of CL from 300 to 700 ms. When CL was changed from 300 to 1000 ms, the ARI followed the rapid time course typical of M cells (t90 = 2.6 +/- 0.5, 2.2 +/- 0.4, 2.5 +/- 0.4, 2.6 +/- 0.5, and 2.3 +/- 0.4 minutes for Epi; 3-, 5-, and 7-mm sub-Epi; and Endo cells, respectively). CONCLUSIONS: In contrast to in vitro results, there is no significant difference in repolarization among myocardial layers in the intact normal canine heart.

Chronic in vivo and in vitro effects of amiodarone on guinea pig hearts.

To study the electrophysiologic effects of chronically administered amiodarone and its interaction with an I(Kr) blocker, amiodarone was injected i.p. daily for 7 days into male guinea pigs. Control animals received vehicle only. At 80 mg/kg, RR and rate corrected QT (QT(C)) intervals increased after 4 days from 209 +/- 5 ms and 162 +/- 3 respectively to 285 +/- 13 ms and 176 +/- 3 (P < .05, n = 10), respectively, and remained significantly high on the 8th day (256 +/- 14 ms and 173 +/- 4). Neither RR nor QT(C) intervals changed significantly in control animals. Twenty-four hours after the last injection, papillary muscles were isolated from both ventricles and superfused with Tyrode's solution not containing amiodarone. The preparations from amiodarone-treated animals manifested a statistically significant prolongation of action potential duration (APD) at all pacing cycle lengths (CL) (from 300 to 1500 ms). The amiodarone-induced increase of APD diminished with elevation of potassium concentration ([K+]O). Amiodarone did not modify the dependence of Vmax on membrane potential at different [K+]O. There was minimal to no summation of effects of chronic amiodarone and acute super-fusion of the I(Kr) blocker, E-4031 (3 x 10(-6) M) on APD at CL = 1500 ms. The data demonstrate that in chronically treated guinea pigs, amiodarone prolongs repolarization, manifests minimum reverse use-dependent in APD prolongation, and, at low pacing rate, shows no additive actions with an acutely superfused blocker of I(Kr).

Effects of exogenous neuropeptide Y on automaticity of isolated Purkinje fibers and atrium.

Neuropeptide Y exerts prejunctional effects on automaticity in cardiac pacemaker tissue and postjunctional effects on contractile activity of cardiomyocytes. It is uncertain whether neuropeptide Y has postjunctional effects on cardiac automaticity. This paper reports a study of the actions of exogenous neuropeptide Y (10(-10)-10(-6) M) on automaticity of isolated preparations of canine Purkinje fibers and guinea-pig right atrium. Neuropeptide Y had no effect on the rate of normal and abnormal (barium-induced) automaticity and did not modify the effect of norepinephrine on canine Purkinje fibers. Neuropeptide Y did not affect normal sinus rhythm in guinea-pig right atrium. The influence of neuropeptide Y (5 x 10(-7) M) on the response to field stimulation in guinea-pig right atrium was also studied: neuropeptide Y reduced the vagal component of response three-fold (P < 0.05) and insignificantly diminished the sympathetic component. Neuropeptide Y fragment 18-36 suppressed the vagal effect of neuropeptide Y by approximately 50% (P < 0.05). These results suggest that neuropeptide Y does not influence automaticity directly in canine Purkinje fibers and guinea-pig right atria. A prejunctional action to inhibit release of acetylcholine from parasympathetic nerve endings is implied by experiments on field-stimulated right atrium, but based on results with fragment 18-36, postjunctional actions may also occur here.

Mechanisms for vagal modulation of ventricular repolarization and of coronary occlusion-induced lethal arrhythmias in cats.

Our goal was to better understand the mechanisms underlying muscarinic receptor actions on the ventricle in vivo. Therefore, we studied the effects of vagal stimulation on ventricular repolarization and of vagal tone on lethal arrhythmias induced by 30 minutes of left anterior descending coronary artery ligation in anesthetized cats. Experimental groups included normal control cats subjected only to coronary ligation and cats pretreated with atropine, pertussis toxin (PTX), or propranolol. All cats received bilateral cervical vagal stimulation (Vstim) at 1, 3, and 5 Hz for 1 minute at 10-minute intervals. Before coronary ligation, Vstim slowed sinus rate, prolonged the PR interval, and lowered blood pressure. Most important from the point of view of electrophysiological function was a vagally induced acceleration of ventricular repolarization in paced and unpaced hearts, which could be explained by the effects of acetylcholine (ie, shortening the subepicardial muscle action potentials). The effect on repolarization was blocked by atropine or PTX but not by propranolol. The extent of sinus slowing and acceleration of repolarization was directly related to the level of functional PTX-sensitive G protein (P < .05). Coronary occlusion was performed during atrial pacing such that the heart rate in all groups was equal. The incidence of ventricular fibrillation (VF) was 10% in the control group and 50% and 54% in atropine and PTX groups, respectively (P < .05). During atrial pacing before coronary occlusion, a vagal index was calculated as percent QTc shortening during Vstim. When the vagal index was 13% to 26%, the incidence of VF during occlusion was zero. When the vagal index was 0% to 12%, VF was 52% (P < .01). Conclusions are as follows: (1) Vstim accelerates ventricular repolarization in cats via a pathway that incorporates a PTX-sensitive G protein and involves an altered gradient between epicardium and endocardium. (2) Removal of vagal tone during ischemia favors VF, as predicted by a vagal index.

Electrophysiologic effects of alprafenone on canine cardiac tissue.

We studied the actions of the propafenone derivative alprafenone on transmembrane action potentials (APs) of canine Purkinje fibers (PF) and ventricular and atrial muscle. In PF with normal maximum diastolic potentials (MDPs), alprafenone (5 x 10(-8)-1 x 10(-6) M) produced concentration-dependent decreases in AP amplitude (APA), Vmax, AP duration (APD), and conduction velocity. The effects on Vmax were use dependent. The decrease in PF APD was the result of the drug's action on the slope of phase 2. In contrast to its effect on PF, alprafenone induced a significant increase in APD in ventricular epicardial and endocardial muscle and had no effect on atrial APD. In PF, alprafenone prolonged the effective refractory period (ERP) with respect to APD, and at high [K+]0, prolonged refractoriness beyond repolarization. Alprafenone decreased Vmax and slow-response APA. It had no effect on normal automaticity or isoproterenol (ISO)-induced automaticity, but significantly suppressed BaCl2-induced abnormal automaticity at low levels of membrane potential. Alprafenone inhibited both early and delayed afterdepolarizations and eliminated triggered activity. We conclude that alprafenone's range of actions is consistent with that of other effective antiarrhythmic drugs and is very similar to that of propafenone.

Receptor-effector coupling pathway for alpha 1-adrenergic modulation of abnormal automaticity in 'ischemic' canine Purkinje fibers.

We studied the receptor-effector coupling mechanism responsible for alpha 1-adrenergic receptor-induced increases in abnormal automaticity (AA) occurring at low membrane potentials in "ischemic" Purkinje fibers, superfused with Tyrode's solution containing [K+]o 10 mmol/L, pH 6.8, PO2 < 25 mm Hg. To exclude beta-adrenergic actions, propranolol was added to all solutions. We derived membrane slope resistance (Rsl) from the current-voltage relation obtained with two microelectrodes for intracellular current injection and transmembrane voltage recording. We also measured the membrane time constant, Tm, to assess changes in membrane resistance (Rm). Phenylephrine effects on Rsl in simulated ischemia were studied in the absence or presence of the alpha 1-subtype blockers WB 4101 (WB) or chloroethylclonidine (CEC), both 0.1 mumol/L, and in Purkinje fibers from dogs injected with pertussis toxin (PTX) (30 micrograms/kg i.v., 60 to 72 hours before study). There were no significant differences in mean values of Rsl before phenylephrine superfusion among all groups of Purkinje fibers. Tm increased by 23% during phenylephrine 0.1 mumol/L superfusion, and Rsl increased by 11%. These two results suggest a 23% increase in Rm with no concordant change in longitudinal resistance. In the presence of CEC, phenylephrine increased Rsl by 12%. In contrast, WB blocked phenylephrine effects on Rsl (0.3%). In PTX-treated Purkinje fibers, the levels of PTX-sensitive G protein as well as phenylephrine effects on Rsl (3%) were significantly reduced. In the absence of WB and of CEC, the phenylephrine effects both on Rsl and on the incidence of AA were directly related to the level of PTX-sensitive substrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Combination ethacizin and ethmozin treatment of resistant ventricular ectopy: theoretical, experimental, and clinical study.

Ethmozin (Moricizine HCl) and ethacizin are two class I antiarrhythmic drugs with different rate constants of interaction with the sodium channel. Computer simulation using the "guarded-receptor" model predicted that the combination of ethacizin and ethmozin should exert a greater decrease in excitability and conduction at short coupling intervals, but little effect at normal heart rates (HR). To test this prediction, we measured intraventricular conduction delay in canine hearts in vivo. In agreement with the model, the combination more potently prolonged the delay only at intervals < 600 ms as compared with ethacizin alone. Combination therapy was tested in 6 patients with idiopathic ventricular ectopic depolarizations (VEDs). Three patients were resistant to either ethmozin or ethacizin monotherapy, and three could not tolerate effective doses because of side effects. Quantitative continuous ECG monitoring showed that total VEDs in the resistant group decreased 0 and 17 +/- 13% for 400 and 800 mg/day ethmozin and 18 +/- 12 and 55 +/- 12% for 100 and 200 mg/day ethacizin, respectively. Combined therapy with ethmozin (400 mg/day) and ethacizin (100 mg/day) reduced the number of VEDs by 78 +/- 2% in these patients without side effects. In the "nonresistant" but intolerant group of patients, use of the combination allowed relief of symptomatic ectopy without side effects. A theoretical model correctly predicted an effective combination of class I antiarrhythmic drugs, one with "fast-off" and one with "slow-off" kinetics, which may provide a general rationale for choosing drug combinations.

Positive chronotropic responses induced by alpha 1-adrenergic stimulation of normal and "ischemic" Purkinje fibers have different receptor-effector coupling mechanisms.

We studied the mechanisms underlying the increase in automaticity induced by alpha 1-adrenergic stimulation of normal and "ischemic" canine Purkinje fibers. Fibers were superfused with a control Tyrode's solution, followed by an ischemic superfusate that included 10 mM KCl, 5 mM NaHCO3, Po2 of 10-25 mm Hg, and pH 6.7. To exclude beta-adrenergic actions, propranolol was added to all solutions. In the presence of phenylephrine, normal automaticity at high membrane potentials usually decreased, whereas the incidence of abnormal automaticity during ischemia was increased from a control value of 10% to 30%. Block of an alpha 1-receptor subtype with chloroethylclonidine in the presence of phenylephrine caused normal automaticity to increase in all fibers studied and significantly increased abnormal automaticity to 70%. The alpha-adrenergic-induced increase in automaticity did not occur in ischemic fibers from animals pretreated with pertussis toxin (PTX), which ADP-ribosylated and functionally inactivated the 41-kd family of GTP regulatory proteins. In contrast, the use of PTX enhanced the increase in automaticity induced by phenylephrine in normally polarized Purkinje fibers. Ryanodine, which blocks sarcoplasmic reticulum Ca2+ release, attenuated the increase in normal automaticity in nonischemic fibers but had no effect on abnormal automaticity in ischemic fibers. The increase in abnormal automaticity was, however, blocked by the alpha 1 subtype blocker WB 4101, which also blocks the increase in automaticity in normal fibers. In conclusion, the increase in abnormal automaticity in ischemic Purkinje fibers depends on a WB 4101-sensitive alpha 1-adrenergic receptor subtype whose actions are transduced by a PTX-sensitive 41-kd G protein and are not blocked by ryanodine.(ABSTRACT TRUNCATED AT 250 WORDS)