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.

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.

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.

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.

Electrophysiologic aspects of moricizine HCl.

Moricizine HCl, a phenothiazine derivative, is a new antiarrhythmic drug that has quinidine-like effects on the myocardium. Moricizine HCl, 1 X 10(-7) g/ml, significantly decreases Vmax of the transmembrane action potential of canine Purkinje fibers and reduces the duration of the action potential at 50% and 90% of repolarization, whereas resting potential is unchanged. The drug decreases the fast inward sodium current (lNa), as measured by the double sucrose gap technique in frog atrium trabeculae; the processes of activation, inactivation and reactivation of current did not change even when a high concentration of drug (5 X 10(-6) g/ml) was used. At the same dose, the slow inward calcium current (lCa) increased, but the total outward current did not change. Using the patch-clamp technique, it was shown that moricizine HCl, 1 X 10(-4) g/ml, did not alter single-channel conductance, but essentially decreased the mean open tonic and open-state probability of potassium channels either at positive or negative holding potentials. lNa measured in a single myocyte preparation decreased faster when drug was administered in the external bath compared with intracellular injection. Moricizine HCl action on lNa of the single cell and on Vmax of the action potential is frequency dependent. When lNa was recorded directly, there was a cumulative effect of drug. Similar to other quinidine-like agents, moricizine HCl enhanced arrhythmogenicity within the first few minutes after coronary artery occlusion, but prevented arrhythmias 24 hours after acute myocardial infarction.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal automatic rhythms in ischemic Purkinje fibers are modulated by a specific alpha 1-adrenergic receptor subtype.

BACKGROUND: Recent advances in adrenergic pharmacology have made possible the identification of alpha 1-adrenergic receptor subtypes using the specific blockers chloroethylclonidine and WB 4101. METHODS AND RESULTS: In the present study, we used these two blockers to determine the mechanisms responsible for automatic rhythms occurring during simulated ischemia and reperfusion of isolated canine Purkinje fibers. Experiments were done in the presence of propranolol to minimize beta-adrenergic contributions to the rhythms studied. In the control situation, all fibers showed membrane potentials greater than -90 mV and normal automatic rhythms. During simulated ischemia, membrane potential depolarized to the -60 mV range. Abnormal automaticity was seen in 20% of fibers not treated with phenylephrine and in 50% of those superfused with 1 x 10(-7) M phenylephrine. The incidence of abnormal automaticity was reduced to 0% by WB 4101 (which blocks phosphoinositide metabolic effects of alpha 1-adrenergic stimulation in the heart) and was increased to 90% by chloroethylclonidine (which blocks Na-K pump-stimulating effects of alpha-agonists). Moreover, the ischemic fibers were significantly more hyperpolarized during superfusion with WB 4101 than with chloroethylclonidine. Triggered activity induced by delayed or early after depolarizations was not seen in any experiment. CONCLUSIONS: Automatic arrhythmias induced by alpha 1-adrenergic stimulation during simulated ischemia may be attributed to a specific alpha 1-adrenergic receptor subtype that is blocked by WB 4101. These results have important implications with respect to the induction of arrhythmias in the setting of ischemia and the means for their prevention or treatment.

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.

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.

Influence of ionic modification on electrical activity of Purkinje fibers obtained from dogs with 1-day-old myocardial infarction.

A stable sustained rhythmic activity (SRA) occurred in 62 of 192 specimens isolated from the infarcted subendocardium of 48 dogs 24 h after a left descending coronary artery occlusion. Changes in [Na+]o and/or [Ca2+]o of the superfusate allowed us to distinguish two types of responses, suggestive of two different mechanisms for SRA. In type 1 responses at constant [Na+]o, the rate of SRA decreased when [Ca2+]o was increased and increased when [Ca2+]o was decreased. When [Ca2+]o was held constant, the rate of SRA was directly related to [Na+]o. In type 2 responses, at constant [Na+]o, the rate of SRA was directly proportional to [Ca2+]o. In contrast, at constant [Ca2+]o, SRA was inversely proportional to [Na+]o. When a constant [Ca2+]o/[Na+]o3 ratio was maintained, the type 2 response became indistinguishable from the type 1 response. The combination of lower temperature (36 degrees C) and high initial [Ca2+]o (2.7 mM) favored the type 2 response (28 of 32 preparations). In contrast, 25 of 30 preparations studied at 39 degrees C and 1.35 mM [Ca2+]o showed the type 1 response. These results suggest that SRA in the 24-h infarct model can be due to both abnormal automaticity (type 1) or triggered activity (type 2) and that changes in temperature and ionic milieu will largely determine which of the two mechanisms is responsible for SRA.

An alpha-1-adrenergic receptor subtype is responsible for delayed afterdepolarizations and triggered activity during simulated ischemia and reperfusion of isolated canine Purkinje fibers.

BACKGROUND: We used standard microelectrode techniques to study delayed afterdepolarizations and triggered activity induced by alpha 1-adrenergic stimulation in canine Purkinje fibers in the setting of simulated ischemia and reperfusion. METHODS AND RESULTS: The ischemic environment included 10.8 mM [Ca2+]o, 10 mM [K+]o, 40-50 mm Hg PO2, 20 mM lactate (pH 6.7), and 1 x 10(-7) M phenylephrine. During ischemia, there was the variable occurrence of abnormal automaticity, early afterdepolarizations, and delayed afterdepolarizations. During reperfusion, 100% of preparations manifested delayed afterdepolarizations and 40% manifested triggered activity. Decreasing PO2 to less than 20 mm Hg markedly reduced the incidence of delayed afterdepolarizations and triggered activity, as did increasing PO2 to values of more than 90 mm Hg. WB 4101, an alpha 1-subtype-selective competitive blocker that antagonizes the effects of alpha 1-agonists to induce phosphoinositide metabolism and increase [Ca2+]i, significantly reduced the incidence of delayed afterdepolarizations and triggered activity. In contrast, the alpha 1-subtype-selective blocker chloroethylclonidine, an alkylating agent, had no effect on afterdepolarizations or triggered activity. CONCLUSIONS: Our results indicate that a specific alpha 1-adrenergic pathway is involved in the induction of triggered activity in the setting of ischemia and reperfusion and suggest that interventions used to block this specific pathway have the potential to be antiarrhythmic. They also emphasize the importance of the magnitude of hypoxia in the expression of the arrhythmias.

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.

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.

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.

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 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.

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)

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.

The antiarrhythmic action of phosphocreatine in acute myocardial ischemia.

The mechanism of antiarrhythmic action of phosphocreatine on ischemic myocardium was studied by analyses of electrograms from normal and ischemic tissues. Ischemia induced significant changes in amplitude, duration, and conduction time of the electrograms, thereby showing depolarization of membranes and retarded conduction of excitation. Phosphocreatine administered in a single dose, 300 mg/kg iv, completely eliminated ventricular fibrillations in the ischemic hearts and significantly diminished the electrical instability occurring during reperfusion. The effects of phosphocreatine were completely reproduced by its structural analog phosphocreatine which is inactive in the creatine kinase reaction. It is concluded that the antiarrhythmic effect of both compounds is related to their specific chemical structure and that their specific effect is likely to be mediated via interaction with a sarcolemma site.