Metabolic requirement for the inotropic effects of digitalis and BAY K 8644.

The positive inotropic effects of ouabain and of BAY K 8644 are not apparent in rabbit atria suspended in substrate free medium or in a medium containing 5 mM pyruvate. Addition of glucose in graded concentrations (1-11 mM) during continued exposure of the preparations to the inotropic agents yields graded inotropic effects. The possible involvement of the glycolytic pathway to the development of the inotropic effect of ouabain and BAY K 8644 was tested by using inhibitors of glycolysis that act at two different steps. Iodoacetic acid completely blocks the inotropic effect of ouabain in atria at 0.1 mM and papillary muscles at 0.05 mM. The inotropic effect of BAY K 8644 was blocked partially in atria and papillary muscles. Iodoacetic acid did not change the inotropic effects of isoproterenol and Ca2+. Addition of 1 mM fluoride did not affect significantly the inotropic effect of either ouabain or BAY K 8644 in atria but partially blocked the effect of BAY K 8644 in papillary muscles. The response of atria to ouabain was not changed significantly when glyceraldehyde (10 mM) was substituted for glucose. We suggest that glycolytic ATP may be important for the full inotropic effect of ouabain and BAY K 8644.

Blockade of the inotropic effect of Bay K 8644 by cytochalasin-B and phloretin.

1. The positive inotropic effect in rabbit atria and papillary muscles of Bay K 8644 is blocked by cytochalasin-B (Cyto-B) and phloretin, two compounds known to block the facilitated diffusion of glucose. These compounds do not change the concentration-response curve of calcium. 2. Cyto-B is more potent in atria than in papillary muscles, 10(-7) M having a maximal effect in atria whereas 2 x 10(-5) M was required for a maximal effect in papillary muscles. Phloretin was fully effective at 10(-4) M, the only concentration tested. 3. The inotropic effect of Bay K 8644 was virtually abolished in atria bathed in a glucose-free medium or one containing 5 mM pyruvate. The contractile response to Bay K 8644 of papillary muscles was not changed significantly in glucose-free or in pyruvate-containing medium. 4. Cyto-B (2 x 10(-5) M) caused a slight but significant increase in the KD for the binding of nitrendipine to a crude sarcolemnal preparation from rabbit ventricles. The Bmax was unchanged. 5. These results may best be explained by the hypothesis that there is a metabolic requirement for the inotropic effect of Bay K 8644.

Effects of cytochalasin-B and phloretin on digitalis inotropy.

Phloretin and cytochalasin-B are known to inhibit sugar transport across the cell membrane of many tissues. Both of these agents at concentrations of 100 and 20 microM, respectively, blocked the inotropic effects of ouabain and acetylstrophanthidin (AS) in isolated rabbit atria and papillary muscle preparations. Neither of these agents had any effect of its own on contractile force. Addition of phloretin or cytochalasin-B after the inotropic response to ouabain was fully established did not reverse the effect. The potency of cytochalasin-B was greater in atria than in papillary muscles, 1 microM of cytochalasin caused significant inhibition of the inotropic effect of ouabain in atria without significant effect in papillary muscles. Phloretin but not cytochalasin-B decreased the binding of [3H]ouabain to a semipurified sarcolemmal preparation isolated from canine left ventricular muscle. Neither ouabain nor AS had a substantial positive inotropic effect in atria suspended in substrate-free medium. Substitution of pyruvate (5 mM) for glucose did not fully support their inotropic effect in atria. Papillary muscles behaved differently, in that substrate-free as well as pyruvate media almost fully supported the inotropic effects of ouabain and of low concentrations of AS. Higher concentrations (greater than 250 ng/ml) of As produced a negative inotropic response in substrate-free medium. The possibility that an "active" sugar transport system is required for digitalis inotropy is ruled out by the observation that 2-deoxyglucose also prevents the inotropic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytochalasin-B and phloretin depress contraction and relaxation of aortic smooth muscle.

Cytochalasin-B (20 microM) and phloretin (100 microM) blocked by more than 80% the contractile responses to calcium ions in partially depolarized rabbit aortic strips. Both also blocked, but only by approximately 50%, the responses to noradrenaline and histamine in normal calcium medium. The responses to these agonists in calcium-free EGTA medium were also blocked partially. Cytochalasin-B partially blocked the acetylcholine-induced relaxation of aortic strips precontracted with phenylephrine but not the relaxation due to nitroglycerine or compound A 23187. The relaxation due to isoprenaline was potentiated by cytochalasin B. Since both compounds are known to block hexose transport but share no other known effects, it is suggested that a glycolytic intermediate could be required for the contraction in response to calcium. However, the concentrations of cytochalasin-B required for these effects were somewhat greater than those usually required to block hexose transport.

Electrophysiological effects of ethmozine in perfused rabbit hearts.

His-bundle electrocardiography was used to evaluate the effects of ethmozine on cardiac conduction in isolated perfused rabbit hearts electrically driven at cycle lengths of 320 and 250 ms. There was no significant change in conduction until high concentrations of ethmozine were reached. His-Purkinje and atrioventricular (AV) nodal conduction were slowed significantly at 0.1 microgram/mL and atrial conduction at 1.0 microgram/mL. Conduction block occurred at 10.0 micrograms/mL in all the hearts treated. Effects of the drug (0.1 and 0.01 microgram/mL) on conduction of extrasystoles were also studied in hearts driven at a basic cycle length of 270 ms. No significant change was observed in atrial conduction of extrasystoles throughout the coupling intervals tested at both concentrations. Ethmozine (0.01 and 0.1 microgram/mL) caused slowing of His-Purkinje conduction of extrasystoles but the effect of the drug did not change as a function of the coupling interval. An interval-dependent increase in AV-nodal conduction time was observed, with the maximum slowing of conduction occurring at coupling intervals close to the effective refractory period of the AV node. AV-nodal functional refractory period was increased significantly by ethmozine (0.01 and 0.1 microgram/mL). The effective refractory period was significantly increased only at the higher concentration.

Ca2+ and the sarcoplasmic reticulum.

The role of the sarcoplasmic reticulum in the regulation of cytosolic calcium is briefly reviewed. A method is described for monitoring calcium efflux from relatively large amounts of SR in a newly designed filtration apparatus using membrane filtration. Two phases of Ca2+ efflux from SR preloaded in the presence of oxalate were observed: an early exponential phase, followed by a phase of constant efflux. Changes in pH from 6.8 to 7.4 resulted in increased efflux. A change in the opposite direction resulted in an initial rapid increase in efflux before the expected decrease occurred. Caffeine (10 and 20 mM) caused increased efflux. Addition of 100 microM EGTA to the buffer resulted in major changes in the exponential phase but not in the phase of constant release, and potentiated the effect of caffeine. The method appears suitable for a variety of pharmacological studies in SR as well as other tissue fractions.

Effect of encainide and its two major metabolites on cardiac conduction.

The effect of encainide and its two major metabolites, O-demethylated encainide (MJ 9444) and 3-O-methoxy encainide (MJ 14030), on cardiac conduction was studied by recording His bundle potentials in isolated perfused rabbit hearts and Purkinje and muscle conduction in vivo in dog hearts after destruction of the atrioventricular node. Both metabolites are 4 to 15 times more potent than encainide in slowing conduction through the atria, the AV-node and the His-Purkinje system of the rabbit heart. They did not differ from each other in potency but MJ 9444 increased the duration and decreased the height of the ventricular potential whereas MJ 14030 had no effect at doses which caused conduction block. In the dog, encainide (0.8-3.2 mg/kg i.v.) slowed conduction of extrasystoles in both Purkinje and muscle at all coupling intervals, increased the effective refractory period and the functional refractory period of the Purkinje pathway. MJ 9444 (0.05-0.4 mg/kg) speeded Purkinje conduction of early (less than 300 msec) without affecting or while slowing conduction of late (greater than 350 msec) extrasystoles. Higher doses (0.4-1.6 mg/kg) slowed conduction at all intervals. The effective refractory period and the functional refractory period were decreased but in some cases returned to control values at the higher doses. Muscle conduction was slowed at doses of 0.4 mg/kg or more. MJ 14030 (0.05-3.2 mg/kg) had variable effects, behaving like MJ 9444 in three experiments but like the parent compound in two others. Only slowing of conduction was seen with the three drugs when heart rate was changed.(ABSTRACT TRUNCATED AT 250 WORDS)

Increase in ventricular fibrillation threshold following blockade of atrioventricular conduction.

The threshold for electrically induced ventricular fibrillation was determined in anesthetized open-chest dogs, before and after blockade of atrioventricular (A-V) conduction. Without exception, the threshold was significantly elevated after creation of the block. The precise mechanism of this effect is not known at this time. However, it is apparent that when ventricular fibrillation thresholds are being determined, the functional status of A-V conduction should be established and maintained throughout the study.

Effect of lidocaine on ventricular conduction in acutely ischemic dog hearts.

The effect of graded doses of lidocaine (1.25-10 mg/kg) on endocardial Purkinje and transmural conduction at different heart rates as well as during extrasystolic stimulation was studied in dogs, 30 min after ligation of the anterior descending coronary artery. The drug had no effect on endocardial conduction within the ischemic zone except at the highest dose. Transmural conduction time was increased by ischemia in only 50% of dogs. Transmural conduction time was increased further at high heart rates but not by short coupled extrasystoles. Lidocaine slowed conduction further in the ischemic myocardium by a process that was both rate and interval dependent. "Apparent" supernormal transmural conduction was observed during short coupled extrasystoles but not at fast drive rates. This phenomenon was blocked only by administration of high doses of lidocaine.

Absence of supernormal conduction in canine myocardium.

Transmural conduction time, measured as the difference in arrival time of impulses from a distant stimulating site at endocardial and epicardial electrodes near the left ventricular apex, has been reported to decrease when closely coupled extrasystoles are interpolated, indicating that muscle conduction could be supernormal. We have now determined that reduction in transmural conduction time is accounted for completely by the relatively late arrival time of the extrasystolic wave front at the endocardial recording site. The endocardial recording site was activated later than an immediately adjacent site within the wall in two out of eight animals, which could be interpreted as retrograde conduction. No evidence for supernormal conduction within the ventricular wall could be obtained by multiple electrode recordings. Supernormal conduction throughout the myocardial wall could not be demonstrated on stimulation of the endocardial site. We conclude that supernormal conduction in myocardial muscle cannot be demonstrated and that changes in transmural conduction time do not always measure myocardial conduction velocity.

Site of origin of halothane--epinephrine arrhythmia determined by direct and echocardiographic recordings.

A constantly coupled bigeminal arrhythmia was induced in dogs anesthetized with thiopental-halothane by infusion of epinephrine (1 microgram.kg-1.min-1). The site of origin of the abnormal beat was localized to the interventricular septum by bipolar recordings from the left and right ventricular epicardium and two sites in the septum. Simultaneous echocardiograms showed early movement of the septum with a pattern similar to that seen in the left bundle branch block. Simulation at the recording sites resulted in movement patterns which indicate that assessment of septal and posterior left ventricular motion can be helpful in localization of the origin of ventricular arrhythmias, although only with left ventricular arrhythmias is there potential for anatomic localization to a small area of muscle.

Effect of disopyramide and disobutamide on conduction in perfused rabbit hearts.

The effects of disopyramide (DP) and a new antiarrhythmic agent, disobutamide (DB) on cardiac conduction were studied using His bundle recording from modified rabbit Langendorff preparations electrically driven at 3 and 4 Hz. Both disopyramide (4-16 microgram/mL) and disobutamide (1-30 microgram/ml) showed conduction throughout the atrioventricular conduction system, i.e., SA, AH, and HV intervals were increased in a dose-related manner. Conversion of the conduction time changes to percent changes indicates that disobutamide had a relatively equal effect on each part of the system whereas disopyramide exhibited significantly less effect on AV nodal conduction. Slowing of conduction in the AV node by DP was clearly related to rate. Changes in SA and HV intervals were rate related to a lesser degree. No such rate-related effect was evident with disobutamide. Block of arterial conduction occurred in two out of six hearts when the rate was increased 8 microgram/mL of DP and in three additional hearts at 16 microgram/mL. This was interpreted to indicate a change in atrial excitability such that 2 X threshold currents no longer excited the tissues. This was not observed at any concentration of DB.

Effects of lidocaine on conduction of extrasystoles in the normal canine heart.

The effect of lidocaine on the conduction of extrasystoles was studied in 8 open-chest dogs after atrioventricular nodal block. Simultaneous recording of endocardial and epicardial activation provided separate measures of endocardial (Purkinje) conduction as well as myocardial (muscle) conduction. Lidocaine (1.25--10.0 mg/kg) caused a dose-dependent slowing of conduction of midrange extrasystoles (250--400 ms) in both the Purkinje system and the myocardium, which became statistically significant at doses larger than 1.25 mg/kg. On the other hand, low doses of lidocaine caused speeding of early extrasystoles, i.e., coupling intervals (less than 250 ms) in the Purkinje system but not in the myocardium. Measurement of transmural conduction time as a function of coupling interval revealed a period of "apparent" supernormal conduction through ventricular muscle that was eliminated at high doses of lidocaine.

Interaction of lidocaine and calcium on the electrical characteristics of rabbit atria.

The interactions of lidocaine (1-5 X 10(-5) M) and calcium ions (1.25-5.0 mM) on electrical characteristics of atrial potentials were determined with standard microelectrode techniques with major reference to the maximum rate of rise of the action potential (Vmax of AP), the time constant of recovery of the rapid sodium carrier (gamma) and repetitive firing due to early extra stimuli (arrhythmia). Lowering Ca caused depolarization and decreased Vmax and gamma; high Ca caused changes in the opposite direction. The relation of gamma to membrane potential was downward concave when membrane potential was changed by Ca but upward concave when equivalent changes in membrane potential were induced by changing the external potassium concentration. Lidocaine (1 X 10(-5) M) had no significant effect at 2.5 mM Ca but significantly decreased the overshoot and Vmax of AP, increased gamma and effective refractory period and was antiarrhythmic at 1.25 mM Ca. These changes were closely similar to the effects of lidocaine (5 X 10(-5) M) at 2.5 mM Ca. The effects of this high concentration were decreased when Ca was changed to 5.0 mM. The effect of lidocaine most clearly predictive of efficacy for the type of arrhythmia was that on Vmax, with changes in gamma in particular not being related to antiarrhythmic activity.

The effect of propranolol and dimethylpropranolol on cardiac conduction.

Isolated dog hearts perfused with blood from a donor dogand driven at two heart rates were used to compare the effects of propranolol with those of its quaternary ammonium derivative on atrial, atrioventricular (AV) nodal, and His-Purkinje conduction. Propranolol slowed only AV-nodal conduction, increasing the minimal conduction time and the effect of prematurity, without affecting fatigue. Practolol did not have this effect. Dimethylpropranolol had similar but not identical effects on the AV node, but also slowed atrial and ventricular conduction. In contrast with the quaternary derivative of lidocaine, dimethylpropranolol's effect on atrial and ventricular conduction was not dependent on the heart rate. The effect of dimethylpropranolol on ventricular conduction was observed at doses lower than those reported by others to be antiarrhythmic.

A specific effect of lidocaine and tocainide on ventricular conduction of mid-range extrasystoles.

Lidocaine and tocainide had no effect on ventricular conduction of extrasystoles with coupling intervals longer than 500 msec in isolated blood-perfused dog hearts, but caused interval-related increases in conduction time of extrasystoles in the range of 250--400 msec, here called mid-range extrasystoles (MRE). Quinidine, procainamide, disopyramide, and methyl lidocaine increased conduction times of extrasystoles at all coupling intervals, and no additional slowing of MRE was observed. The slowing of MRE specific to lidocaine and tocainide was confirmed in the intact dog heart. During acute myocardial ischemia in the intact dog heart, conduction was slowed and additional slowing of MRE was found. Lidocaine and tocainide caused further slowing of conduction of MRE. This unique effect of lidocaine and tocainide on the conduction of MRE may be important in the suppression of reentrant arrhythmias. However, lidocaine and tocainide were also found to be arrhythmogenic when extrasystoles were introduced, after acute coronary occlusion, in those animals in which such occlusion alone did not allow demonstration of arrhythmias due to extrasystoles.

Effects of lidocaine in rabbit atria.

Standard microelectrode recordings were obtained from rabbit right and left atria. Lidocaine (1 X 10(-5) M) had no effect on these, but 5 X 10(-5) M lidocaine significantly slowed rate and Vmax. This concentration had no effect on the duration of the action potential, a result clearly different from the effect of this drug in Purkinje tissue. Lidocaine had much less effect on the 'steady-state' relation of membrane potential to Vmax of phase 0 of the action potential than on the 'membrane responsiveness curve' obtained by the extra stimulus technique. We have demonstrated time-related recovery from sodium inactivation in rabbit left atria and have shown that lidocaine slows recovery in this tissue as it does in Purkinje fibres.

Prediction of the functional refractory period of the atrioventricular node.

Based on the new description of AV-nodal conductivity from our laboratory, we developed an equation which describes AV-nodal conduction time in the dog heart at all coupling intervals. This allows us to predict the functional refractory period (FRP) of the AV-node based on the AV-nodal conduction times of only five extra stimuli having relatively long coupling intervals. The equation predicted the FRP within acceptable limits when compared to values obtained by the classical technique in which the full range of extrasystolic intervals must be tested. Changes in FRP after the administration of antiarrhythmic drugs (lidocaine and quinidine) or due to changes in heart rate were predicted accurately. The ease and accuracy of the present method could allow determination of FRP in a short time.