TMB-8 as a pharmacologic tool in guinea pig myocardial tissues. II. Effects of TMB-8 on membrane currents in isolated ventricular cardiomyocytes.

In single isolated guinea pig ventricular cardiomyocytes, 8-(N,N-diethylamino)-octyl-3, 4, 5-trimethoxybenzoate hydrochloride (TMB-8) nonselectively inhibited membrane currents. TMB-8 concentration dependently and reversibly reduced calcium current (ICa) (pD2 5.0). The Ca++ channel blockade was only slightly use dependent. The steady-state inactivation curve of ICa was shifted to more negative membrane potentials by TMB-8; the curve for the normalized conductance of ICa was not significantly affected. The quasi steady-state K+ currents as a measure for K+ conductance were examined by means of slow depolarizing ramp pulses between -120 and +60 mV. In this potential range. TMB-8 (100 microM) reduced quasi steady-state potassium currents. The sodium current sodium current (INa) was investigated at low extracellular Na+ concentration (30 mM) after blocking ICa by Cd++ and reducing K+ currents (Cs+ substituted for K+). Under these conditions, TMB-8 concentration dependently and reversibly decreased INa (pD2 5.3), slightly shifted the steady-state inactivation curve of INa to more negative potentials and shifted the curve for the normalized conductance of INa to more positive potentials. We conclude that TMB-8 possesses both Ca++ channel- and Na+ channel-blocking properties and reduces the membrane K+ conductance. It is speculated that, because of its amphiphilic nature, TMB-8 accumulates at lipid-water interphase of biologic membranes and therefore interferes with the normal function of many membrane proteins.     

Cardiotonic action of [8]-gingerol, an activator of the Ca++-pumping adenosine triphosphatase of sarcoplasmic reticulum, in guinea pig atrial muscle

[8]-Gingerol (gingerol), a component of ginger, produced a concentration-dependent positive inotropic effect on guinea pig isolated left atria at concentrations of 1 X 10(-6) to 3 X 10(-5) M. Gingerol also exhibited positive inotropic and chronotropic effects on guinea pig right atria. The gingerol-induced inotropic effect was abolished by ryanodine, but was little affected by propranolol, chlorpheniramine, cimetidine, tetrodotoxin, diltiazem or reserpine. The time to peak tension and relaxation time within a single contraction were shortened by gingerol (1 X 10(-5) M) as well as isoproterenol, whereas they were prolonged by BAY K 8644. In guinea pig isolated atrial cells, gingerol (3 X 10(-6) M) caused an increase in the degree and the rate of longitudinal contractions. In guinea pig left atria, gingerol (1 X 10(-6) to 3 X 10(-5) M) gave little influence on the action potential, although it increased the contractile force of the atria. The whole-cell patch-clamp experiments showed that the slow inward current was little affected by gingerol (1 X 10(-6) to 3 X 10(-5) M) in voltage-clamped guinea pig cardiac myocytes. The measurement of extravesicular Ca++ concentration using a Ca++ electrode indicated that gingerol (3 X 10(-6) to 3 X 10(-5) M) accelerated the Ca++ uptake of fragmented sarcoplasmic reticulum (SR) prepared from canine cardiac muscle in a concentration-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beta adrenergic receptor-stimulated prostaglandin synthesis in the isolated rabbit heart: relationship to extra- and intracellular calcium

We have investigated the contribution of extra- and intracellular Ca++ and calmodulin to beta adrenergic receptor-stimulated prostaglandin synthesis in the isolated rabbit heart perfused with Krebs-Henseleit buffer. Administration of isoproterenol (100 ng) increased the output of immunoreactive 6-keto-prostaglandin F1 alpha and prostaglandin E2 as well as heart rate and developed tension; the coronary perfusion pressure was reduced. Isoproterenol-induced output of prostaglandins was positively correlated with the extracellular Ca++ concentration (0-5 mM). Infusion of the Ca++ channel blockers diltiazem (22 microM) or nifedipine (0.27 microM) inhibited isoproterenol-stimulated output of prostaglandins and the positive inotropic but not the positive chronotropic effect of the amine. Administration of the intracellular Ca++ antagonists 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride (23 microM) or ryanodine (1.6 microM) reduced the outflow of prostaglandins and the positive chronotropic and inotropic effect elicited by isoproterenol. The calmodulin inhibitors trifluoperazine (50 microM) or calmidazolium (1 microM) failed to alter isoproterenol-induced output of prostaglandins; trifluoperazine but not calmidazolium reduced the developed tension and coronary perfusion pressure without altering heart rate. The prostaglandin synthesis elicited by arachidonic acid (3 micrograms) was inhibited by indomethacin but not by alterations in extracellular Ca++, Ca++ channel blockers, intracellular Ca++ antagonists or calmodulin inhibitors. These data suggest that activation of beta adrenergic receptors promotes cardiac prostaglandin synthesis and myocardial contractility by increasing the trans-sarcolemmal flux of Ca++, which releases intracellular Ca++.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vasodilation Effects and Action Mechanisms of TMB-8 on Basilar Artery in Rabbits

BACKGROUND: 8-(N,N'-diethylamino)-n-octyl-3,4,5-trimethoxybenzoate (TMB-8) is a potent Ca(2+)-antagonist that can prevent/treat ischemic stroke and inhibit the contractility of smooth, skeletal, and cardiac muscles. Further studies are warranted to elucidate the efficacy of TMB-8 on rabbit basilar artery preparation and its action mechanisms on vascular smooth muscle cell cultures. METHODS AND RESULTS: Effects of TMB-8 on the contractility of rabbit's basilar artery in vitro and those on intracellular free Ca(2+) concentrations, [Ca(2+)](i), were studies with isolated organ bath and Fura-2 methods. Histamine-induced concentration-response curves were shifted by TMB-8 in a mixed manner whereas those of norepinephrine and KCl were shifted in a non-competitive manner. In the presence of nifedipine or in a Ca(2+)-free medium, 2,5-di(tert-butyl)-1,4-benzohydroquinone (BHQ) (10 μM) induced an immediate transient contraction in rabbit basilar artery, whereas ryanodine showed a slow, weak, sustained contraction, followed by a weak, sustained relaxation. TMB-8 (30 μM) significantly inhibited these contractions of BHQ and ryanodine. Further, aminophylline enhanced the inhibitory action of TMB-8 on vasocontractions, suggesting that TMB-8's inhibitory actions may be related to the increase of cAMP level. The muscle contraction induced by BHQ was enhanced by pretreatment of the artery ring with TMB-8 for 15 minutes and then TMB-8 was rinsed out. These results indicate that TMB-8 pretreatment can increase Ca(2+) sequestration into sarcoplasmic reticulum, which leads to a larger subsequent Ca(2+) release by BHQ. KCl-induced increase of [Ca(2+)](i) in vascular smooth muscle cells was reduced when the cells were bathed in the medium containing nifedipine. TMB-8 made further reduction on KCl-induced [Ca(2+)](i) increase in nifedipine-containing solution, which had already blocked the voltage-operated Ca(2+) entry. CONCLUSION: These results indicate that (a) TMB-8 can enhance Ca(2+) sequestration into sarcoplasmic reticulum, which leads to a larger amount of Ca(2+) that can be released by BHQ; (b) TMB-8 can inhibit KCl-induced muscle contraction caused by the reduction of [Ca(2+)](i) through saturation of Ca(2+) inside the sarcoplasmic reticulum rather than a direct blockade of Ca(2+)-influx at cell membrane site; and (c) TMB-8 increases cAMP, which enhances Ca(2+) uptake into the sarcoplasmic reticulum.     

Electrophysiologic effects of ketanserin on canine Purkinje fibers, ventricular myocardium and the intact heart

We studied the actions of ketanserin (KT) on transmembrane action potentials (AP) of canine Purkinje fibers (PF) and ventricular muscle (VM) and on rhythm in vivo. PF AP duration (APD) was increased by KT (10(-8) to 10(-6) M) and shortened at 10(-5) M. KT effect on APD was greater during stimulation at longer cycle lengths and KT induced early afterdepolarizations in two of six PF at [K+]0 = 2.7 mM. Maximum diastolic potential, AP amplitude and Vmax were not changed by KT. In VM, KT (10(-8) to 10(-6) M) prolonged APD; but 10(-5) M KT did not shorten APD, reducing the difference in APD between VM and PF. KT had no effect on slow response Vmax or amplitude but prolonged APD. To analyze whether changes in Na plateau current or transient outward current contributed to KT effects on APD, we used tetrodotoxin (TTX) and 4-aminopyridine. TTX shortened APD and in its presence, KT (10(-5) M) induced no further shortening. In contrast, the effect of KT persisted in the presence of 4-aminopyridine. In six anesthetized, open chest dogs, KT prolonged the QT interval, but did not modify QRS duration and epicardial conduction time or induce arrhythmias. KT facilitated the onset of torsades de pointes during epicardial aconitine application.(ABSTRACT TRUNCATED AT 250 WORDS)     

A characterization of the antinociception produced by intracerebroventricular injection of 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate in mice

There is evidence that 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) acts as an intracellular Ca++ antagonist producing decreases in free intracellular Ca++ and inhibiting many cellular processes dependent upon intracellular Ca++. Intracerebroventricularly administered TMB-8 was active in the mouse tail-flick test (ED50 = 50 micrograms), and this antinociceptive response was antagonized by naloxone (AD50 = 0.28 mg/kg s.c.), Ca++ (0.2-0.4 mumol i.c.v.) and, to a lesser degree, by ethylene glycol bis (beta-aminoethyl ether)N,N1-tetraacetic acid (0.02 and 0.06 mumol i.c.v.). TMB-8 (i.c.v.) was only marginally active in the p-phenylquinone test. The potency of TMB-8 (i.c.v.) was potentiated 10-fold in morphine-tolerant mice in the tail-flick test (ED50 = 2.5 micrograms). TMB-8 inhibited contraction of stimulated ilea (IC50 = 2.2 microM), an effect which was neither antagonized nor reversed by naloxone (1 microM). TMB-8 did not potentiate morphine, or was it potentiated by morphine, in the stimulated guinea pig ileum. Procaine, but not lidocaine showed dose-dependent activity in the tail-flick and p-phenylquinone tests (ED50 values, 136 and 83 micrograms, respectively, i.c.v.). The antinociception produced by procaine (i.c.v.) in the tail-flick test was antagonized by naloxone (AD50 = 0.4 mg/kg s.c.) Lidocaine (100 micrograms i.c.v.) produced only 30% maximum possible effect in the tail-flick tests, and was inactive in p-phenylquinone tests. Doses of lidocaine greater than 100 micrograms i.c.v. resulted in lethality of greater than 50% of the animals tested. Thus, the activity of TMB-8 resembles that of opiates in that both are antagonized by Ca++ or naloxone in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)     

The influence of ketamine on inotropic and chronotropic responsiveness of heart muscle.

The influence of ketamine on the inotropic and chronotropic responsiveness of heart muscle was examined in spontaneously beating right atrial preparations and in electrically driven left atrial preparations of guinea pigs. Ketamine (2.63 X 10(-5) to 4.2 X 10(-4) M) decreased heart rate of right atria and decreased contractile tension and its maximum rate of increase in both right and left atrial preparations (right atria greater than left atria). Ketamine did not prevent the heart rate increase produced by norepinephrine (NE; 1 X 10(-8) to 1 X 10(-4) M) in right atria; however, the maximum heart rate was consistently lower in ketamine-treated than in control muscles even after exposure to NE. Although contractile tension was decreased by ketamine, the maximum inotropic response to NE was consistently greater in ketamine-treated atria than in control atria. An inhibitor of the slow Ca++ current in heart muscle, D600, depressed the contractile effects of NE but did not prevent the positive inotropic interaction of ketamine and NE. Ketamine similarly enhanced the inotropic responses to norepinephrine (1 X 10(-6) M), epinephrine (1 X 10(-6) M), isoproterenol (1 X 10(-7) M) and dibutyryl cyclic adenosine 3':5'-monophosphate (AMP; 4 X 10(-3) M) in left atria electrically paced at a constant frequency of contraction of 1 Hz; however, ketamine inhibited the positive inotropic response to increased frequency of stimulation (0.1-3.0 Hz) and to ouabain (3 X 10(-7) M). These findings demonstrate that ketamine can exert a selective positive inotropic influence in heart muscle independent of heart rate or direct or reflexogenic autonomic nervous system changes, and suggest that this activity could in some way be associated with an alteration of the intracellular disposition of cyclic AMP.     

Negative chronotropic and positive inotropic actions of phencyclidine on isolated atrial muscle in guinea pigs and rats

Chronotropic and inotropic actions of phencyclidine were studied in spontaneously beating right atrial muscle and electrically paced left atrial muscle preparations isolated from guinea-pig or rat hearts. In right atrial muscle preparations, phencyclidine (10-100 microM) decreased the frequency of spontaneous beating. Guinea-pig and rat heart preparations had similar sensitivities to this action of phencyclidine. The negative chronotropic effect was not altered by atropine. A high concentration of naloxone failed to affect the chronotropic effect of phencyclidine in guinea-pig muscle, but significantly reduced the effect in rat heart muscle preparations. Phencyclidine (1-100 microM) caused positive inotropic effects in both guinea-pig and rat heart left atrial muscle electrically stimulated at 1.5 Hz; rat heart preparations had a higher sensitivity to the positive inotropic action of phencyclidine. The positive inotropic effect was reduced by verapamil, nifedipine and relatively high concentrations of diltiazem, but was not affected by propranolol, phentolamine, tripelennamine, atropine or ryanodine, indicating that the effect is not mediated by adrenergic, histaminergic or cholinergic systems or does not involve ryanodine-sensitive calcium pools. Inactivation of the fast sodium channels by partial membrane depolarization, and subsequent restoration of the contraction by raising the extracellular Ca++ concentration, did not abolish the positive inotropic action of phencyclidine. These results suggest that the negative chronotropic effect of phencyclidine is not mediated by a stimulation of the muscarinic receptor. The positive inotropic effects of phencyclidine seem to result from an increase in Ca++ influx through the slow channels of the cardiac cell membrane.     

Effects of mitoxantrone on excitation-contraction coupling in guinea pig ventricular myocytes

The mechanisms of the inotropic effect of mitoxantrone (MTO), a synthetic dihydroxyanthracenedione derivative with antineoplastic activity, was investigated in guinea pig ventricular myocytes using whole-cell patch-clamp methods combined with fura-2 fluorescence and cell-edge tracking techniques. In right ventricular papillary muscles, 30 microM MTO increased isometric force of contraction as well as action potential duration (APD) in a time-dependent manner. The force of contraction was increased approximately 3-fold within 4 h. This positive inotropic effect was accompanied by a prolongation of time to peak force and relaxation time. In current-clamped single myocytes treated with 30 microM MTO for 30 min, an increase of cell shortening by 77% and a prolongation of APD by 19% was observed. Peak amplitude of the intracellular Ca(2+) transients was also increased by 10%. The contribution of APD prolongation to the enhancement of cell shortening induced by MTO was assessed by clamping control myocytes with action potentials of various duration. Prolongation of APD(90) (ADP measured at 90% of repolarization) by 24% led to an increase of cell shortening by 13%. When the cells were clamped by an action potential with constant APD, MTO still caused an increase of cell shortening by 59% within 30 min. No increase of the peak intracellular Ca(2+) transients, however, was observed under this condition. We conclude that both the APD prolongation and a direct interaction with the contractile proteins contributed to the positive inotropic effect of MTO.     

Mechanism of inotropic action of xestoquinone, a novel cardiotonic agent isolated from a sea sponge.

Xestoquinone (XQN) isolated from the sea sponge Xestospongia sapra produced dose-dependent cardiotonic effects on guinea pig left and right atria. A direct action of XQN (1-30 microM) on the contractile machinery of cardiac myofilaments was demonstrated in chemically skinned fiber preparations from guinea pig papillary muscles. In atrial preparations, the XQN-induced inotropic effect was markedly inhibited by verapamil or nifedipine, but was not affected by practolol, chlorpheniramine, cimetidine, tetrodotoxin or reserpine. The Ca++ dependence curve for the contractile response of the atria was substantially shifted to the left by XQN (10 microM), and this XQN-induced shift was reversed by verapamil. The time-to-peak tension and relaxation times of the atrial contractions were shortened by XQN, and the action potential duration was markedly prolonged. Whole-cell patch clamp recordings in left atrial strips confirmed that XQN (30 microM) increased the slow inward current. However, there was a temporal dissociation between altered tension development and prolongation of the action potential duration. Cyclic AMP phosphodiesterase activity was inhibited and tissue cyclic AMP content of guinea pig left atria was increased by XQN (0.3-10 microM) in a concentration-dependent manner, but increases in cyclic AMP content did not occur in parallel with increases in contractile response. These observations suggest that an enhancement of intracellular cyclic AMP content and Ca++ influx across the cell membrane contribute to the late phase of XQN-caused cardiotonic responses, whereas the early phase may largely be elicited through direct activation of contractile elements. XQN may provide a novel leading compound for valuable cardiotonic agents.     

Effects of the calcium antagonists perhexiline and cinnarizine on vascular and cardiac contractile protein function

The weakly basic, lipophilic Ca++ antagonists perhexiline and cinnarizine have been compared with the calmodulin inhibitor W-7 and the cardiotonics Vardax and APP-201-533 for the ability to modulate Ca++-dependent contractile protein interactions directly, as well as Ca++-calmodulin-mediated myosin light chain phosphorylation, in arterial actomyosin or cardiac myofibrils. Both perhexiline and cinnarizine inhibited arterial myosin P-light chain phosphorylation and superprecipitation of arterial actomyosin over the concentration range of 10 to 200 microM. Concomitant inhibition of arterial superprecipitation and phosphorylation by perhexiline (IC50 = 33 microM) and cinnarizine (IC50 = 60 microM) was similar to W-7 (IC50 = 35 microM), and was characterized by a rightward shift in the pCa superprecipitation and pCa-light chain phosphorylation relationships, depressed maximum activity and attenuation by 2 microM exogenous calmodulin. However, whereas inhibition of superprecipitation and P-light chain phosphorylation by W-7 was equal at different Mg++ concentrations, relatively greater inhibition with perhexiline and less inhibition with cinnarizine was apparent as the free Mg++ concentration was lowered. In cardiac myofibrils prepared from both bovine and canine ventricles, perhexiline stimulated Mg-adenosine triphosphatase (ATPase) activity and cinnarizine was without effect, whereas W-7 significantly depressed ATPase activity. Perhexiline was 10-fold more potent and 3-fold more efficacious than either Vardax or APP-201-533 in canine cardiac myofibrils. Whereas APP-201-533 increased Ca++ sensitivity and maximum ATPase activity (Vmax), perhexiline increased Ca++ sensitivity, but not Vmax, and W-7 depressed both parameters.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiac adenylate cyclase activity, positive chronotropic and inotropic effects of forskolin analogs with either low, medium or high binding site affinity

A series of in vitro studies were conducted examining the adenylate cyclase stimulation, positive chronotropic and inotropic effects of forskolin and nine analogs which exhibited a range of [3H]forskolin binding site affinities (K1) from 0.020 to 3.174 microM. A significant (P less than .001) linear correlation (r = 0.94) was found between binding site affinity and adenylate cyclase stimulation (EC50) for forskolin and the nine structural analogs. Adenylate cyclase activity was also significantly correlated with the positive chronotropic and inotropic effects of these substances on isolated guinea pig atria. Compounds with K1 values between 0.020 and 1.136 microM produced concentration-dependent increases in heart rate and contractile force in isolated spontaneous and electrically paced guinea pig atria, respectively. In contrast, an analog with a K1 of 3.174 microM caused significant (P less than .05) negative chronotropic and inotropic effects at concentrations above 10 microM. The optimal separation between positive inotropic and chronotropic activity was found with compounds displaying potent [3H]forskolin binding site affinity but moderate adenylate cyclase stimulation, i.e., K1 and EC50 values of approximately 0.05 to 0.10 and 3 microM, respectively. The results of this study show that the forskolin analog, P87-7692 [7-desacetyl-7-(O-propionyl)-hydroxyl amino-carbonyl-forskolin], has marked activity with a wide separation between positive inotropic (248 +/- 41%) and chronotropic effects (43 +/- 13%) at 6.2 microM and may serve as a prototype for a forskolin-based cardiotonic.     

Stereoselective modulation of ryanodine-sensitive calcium channels by the delta isomer of hexachlorocyclohexane (delta-HCH).

delta-Hexachlorocyclohexane (delta-HCH) is shown to be 30-fold more potent as a positive inotropic agent with rat atrial strips compared with lindane (gamma-HCH). Threshold and ED50 values for enhanced contractile force at a pacing frequency of 0.5 Hz are less than 1 microM and 2.2 microM for delta-HCH and 40 microM and 63 microM for gamma-HCH, respectively. Contracture developed in atria exposed to greater than 4 microM delta-HCH (ED50 = 11 microM) but not in atria exposed to gamma-HCH. Uptake and release of Ca++ measured from actively loaded cardiac sarcoplasmic reticulum (SR) vesicles is measured with antipyrylazo III. Although delta-HCH (30 microM) decreases Ca(++)-dependent ATPase by 20%, it does not significantly alter Ca++ loading in the presence of ruthenium red. Addition of delta-HCH (5-50 microM) after loading is complete causes rapid, dose-dependent release of Ca++ from SR. Ca++ release induced by delta-HCH is markedly stereoselective. Compared with gamma-HCH (50 microM), delta-HCH (50 microM) induces a nearly 20-fold higher initial rate of Ca++ release (4.3 nmol of Ca++/mg/sec). Studies with [3H]ryanodine demonstrate that delta-HCH sharply inhibits Ca(++)- or daunorubicin-activated radioligand binding (IC50 = 37 and 25 microM, respectively, logit slope = 2). Inhibition of [3H]ryanodine-binding by delta-HCH is stereoselective inasmuch as IC50 values for alpha, beta and gamma isomers are greater than 100 microM. The delta-HCH modified Ca++ channel appears to proceed by a noncompetitive mechanism (reducing Bmax in equilibrium experiments) with respect to the conformationally sensitive binding site for [3H]ryanodine.(ABSTRACT TRUNCATED AT 250 WORDS)     

A sodium channel blocker, pilsicainide, produces atrial post-repolarization refractoriness through the reduction of sodium channel availability

Atrial fibrillation (AF) is the most common tachyarrhythmia. Shortening of atrial action potential duration (APD) and effective refractory period (ERP) is one of the crucial factors in the occurrence and maintenance of AF. ERP is usually shorter than APD, but ERP can be prolonged beyond action potential repolarization in some situations. It is termed as post-repolarization refractoriness (PRR) that is thought to be one of main anti-arrhythmic mechanisms of class I sodium channel blockers (SCBs). Most of anti-arrhythmic agents, including SCBs, have multi-channel blocking effects. It is unknown whether atrial PRR with SCBs is associated with the reduction of sodium channel availability. We therefore explored the relationship between the reduction of sodium channel availability with a pure SCB (pilsicainide or tetrodotoxin) and atrial PRR using the left atrial appendage from male guinea pigs (each group, n = 3~10). Employing a standard microelectrode technique, we evaluated APD measured at 90% repolarization (APD(90)) and the sodium channel availability, judged from the maximal rate of rise of action potential (Vmax). At a 500-msec basic cycle length (BCL), pilsicainide prolonged atrial ERP assessed by a single extra-stimulus in response to the decrement of the Vmax in a dose-dependent manner without affecting APD(90). Furthermore, pilsicainide increased the ERP and decreased the Vmax in a rate-dependent manner without APD(90) prolongation at a shorter BCL (200 msec). Importantly, tetrodotoxin reproduced the effects of pilsicainide on atrial ERP, APD(90), and Vmax. These results indicate that SCBs produce atrial PRR through the reduction of sodium channel availability.     

Regional expression of sodium pump subunits isoforms and Na+-Ca++ exchanger in the human heart.

Cardiac glycosides exert a positive inotropic effect by inhibiting sodium pump (Na,K-ATPase) activity, decreasing the driving force for Na+-Ca++ exchange, and increasing cellular content and release of Ca++ during depolarization. Since the inotropic response will be a function of the level of expression of sodium pumps, which are alpha(beta) heterodimers, and of Na+-Ca++ exchangers, this study aimed to determine the regional pattern of expression of these transporters in the heart. Immunoblot assays of homogenate from atria, ventricles, and septa of 14 nonfailing human hearts established expression of Na,K-ATPase alpha1, alpha2, alpha3, beta1, and Na+-Ca++ exchangers in all regions. Na,K-ATPase beta2 expression is negligible, indicating that the human cardiac glycoside receptors are alpha1beta1, alpha2beta1, and alpha3beta1. alpha3, beta1, sodium pump activity, and Na+-Ca++ exchanger levels were 30-50% lower in atria compared to ventricles and/or septum; differences between ventricles and septum were insignificant. Functionally, the EC50 of the sodium channel activator BDF 9148 to increase force of contraction was lower in atria than ventricle muscle strips (0.36 vs. 1.54 microM). These results define the distribution of the cardiac glycoside receptor isoforms in the human heart and they demonstrate that atria have fewer sodium pumps, fewer Na+-Ca++ exchangers, and enhanced sensitivity to inotropic stimulation compared to ventricles.     

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.     

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.     

SR 33557, a novel calcium entry blocker. I. In vitro isolated tissue studies.

The effects of SR 33557 on isolated cardiovascular preparations were compared to those of nifedipine, verapamil and diltiazem. In rat aortic strips, SR 33557, like nifedipine, verapamil and diltiazem, caused a significant and simultaneous inhibition of potassium-induced 45Ca++ influx and contractile responses (nifedipine greater than SR 33557 greater than verapamil greater than diltiazem). SR 33557 also antagonized Ca(++)-induced contractions in K(+)-depolarized aorta preparations (pA2:9.08 +/- 0.03) and is the first calcium channel antagonist, structurally not related to 1,4-dihydropyridines, to inhibit competitively contractions induced by BAY K8644. In spike-generating vascular smooth muscle (rat portal vein), contractures evoked by noradrenaline (4 microM) or KCl (100 mM) were reduced by all four antagonists, the pharmacological potency being nifedipine greater than SR 33557 greater than verapamil greater than diltiazem. Unlike SR 33557, nifedipine, verapamil and diltiazem showed a parallel enhancement of frequency of spontaneous contractions in rat portal vein in spite of a concentration-related reduction in amplitude. By using rabbit atrial preparations, spontaneous right atrial rate and electrically stimulated (120/min) basal contractions of left atria were used as indices of chronotropy and inotropy. The potency series for negative chronotropic effects was nifedipine greater than SR 33557 greater than verapamil greater than diltiazem. For negative inotropic effects the potency order was verapamil greater than nifedipine greater than SR 33557 greater than diltiazem, respectively. Thus, SR 33557 should depress heart rate to a greater extent than ventricular contractility. These results suggest that SR 33557 is a potent calcium entry blocker that (unlike verapamil and diltiazem) is particularly selective for vascular smooth muscle and devoid of any potent negative inotropic actions.     

临床抗心律失常药物对牵张引起的豚鼠心房心肌细胞动作电位变化的影响

目的：研究机械牵张对离体豚鼠左心耳心肌细胞动作电位（AP）的影响，观察几类抗心律失常药物对机械牵张引起的心房心肌细胞AP改变的作用。方法：采用常规做电极技术记录心肌细胞AP,观察机械牵张对其的影响，并分析抗心律失常药物对机械牵张引起的心房心肌细胞AP和有效不应期（ERP）变化的作用。结果：①机械牵张可加快心房心肌细胞整个复极过程，牵张效果呈心肌牵张长度依赖性，但牵张刺激对静息膜电位（RMP）和动作电位幅度（APA）无影响；②进一步分析抗心律失常药物对机械牵张后的心房心肌细胞RMP、APA、动作电位时间（APD）和ERP的影响后发现，Ⅰa类抗心律失常药物奎尼丁可使该AP复极至90％所需的时间（APD50）和ERP均显著延长（P均〈0．01），APA显著降低（P均〈0．01）；Ⅰb类抗心律失常药物利多母因可使该AP复极至50％所需的时间（APD50）、APD50和ERP显著缩短（P均〈0．01）；Ⅰc类抗心律失常药物氟卡尼可使该APA、AP复极至20％，所需时间（APD20）和APD50进一步缩短，但未发现Ⅱ类抗心律失常药物普萘洛尔有改变机械牵张后的RMP、APD和ERP作用（P〉0．05）；Ⅲ类抗心律失常药物胺碘酮可使机械牵张后的APD20、APD50、APD90和ERP显著延长（P均〈0．01）；Ⅳ类抗心律失常药物维拉帕米可使该APD50和APD90显著延长（P均〈0．01）。结论：①Ⅰa、Ⅲ和Ⅳ类抗心律失常药物可防止机械牵张引起的心房心肌细胞APD和ERP缩短；②Ⅰb和Ⅰc类抗心律失常药物可促进机械牵张引起的心房心肌细胞APD和ERP的缩短；③Ⅱ类抗心律失常药物对牵张引起的心房心肌细胞APD和ERP改变无影响。     

The electrophysiologic effects of low and high digoxin concentrations on isolated mammalian cardiac tissue: reversal by digoxin-specific antibody

The effects of digoxin on electrophysiologic properties were evaluated in isolated perfused cardiac tissue. In canine Purkinje fiber (PF)-ventricular muscle (VM) preparations, control measurements, using microelectrode technique, were made of: resting potential (RP), action potential (AP) amplitude, rate of rise, overshoot, duration (APD), membrane responsiveness, conduction velocity (CV), and refractory period. The preparation was then exposed to 1 x 10(-7) M digoxin and repeat measurements were carried out every 15 min. At slow (30/min) rates of stimulation APD initially prolonged then markedly shortened. With more rapid stimulation (75 and 120/min) no initial APD prolongation was observed. When stimulated at 75/min, RP and AP rate of rise, amplitude, and CV remained near control values for 60-75 min then rapidly decreased until electrical inexcitability (110+/-15 min). At that time fibers were perfused with serum containing digoxin-specific antibody (DSA) or one of a group of test solutions. In the preparations exposed to DSA, membrane characteristics improved by 15 min, and by 60 min approximated control values. No beneficial effect was seen with the various test solutions. DSA also reversed digoxin-induced enhanced phase 4 depolarization in PF.Effective (ERP) and Functional (FRP) refractory periods of rabbit atrioventricular (AV) node preparations were measured in the control state. The tissue was then exposed to 1 x 10(-7) M digoxin and refractory period measurements repeated. At a time when AV conduction prolonged by 20%, associated with marked prolongation of ERP and FRP, DSA or various test solutions were perfused. The prolongation in ERP, FRP, and AV conduction time rapidly returned to normal only in the DSA perfused tissue. It is concluded that DSA has the ability to reverse pronounced toxic electrophysiological effects of digoxin in in vitro cardiac tissue.