Efficacy of the beta3-adrenergic receptor agonist CL-316243 on experimental bladder hyperreflexia and detrusor instability in the rat.

PURPOSE: Recent evidence indicates that in a number of species detrusor relaxation is mediated through activation of the beta3-adrenergic receptor. We determined whether activation of the beta3-adrenergic receptor would be a useful therapeutic approach for bladder instability. We profiled in vitro activity of the beta3-adrenergic receptor agonist CL-616243 and the efficacy of this compound in experimental models of detrusor instability and bladder hyperreflexia. MATERIALS AND METHODS: Isolated rat detrusor strips were contracted by depolarizing the preparations with 20 mM. KCl. CL-316423 was added to the tissue bath in increasing concentrations and contraction inhibition was assessed. Efficacy against bladder instability was evaluated using the obstructed hypertrophied bladder model in the rat. The acetic acid bladder cystometry model was used to assess the efficacy of CL-316423 in bladder hyperreflexia. Isovolumetric contractions were evoked by electrical stimulation using a silver bipolar electrode. Data are expressed as the mean plus or minus standard error of mean. RESULTS: CL-316243 inhibited spontaneously contracting, isolated rat detrusor strips in a concentration dependent manner with a mean concentration inhibiting 50% of maximal response of 2.65 +/- 0.36 nM. Intrinsic activity relative to forskolin was 1. In vivo CL-316243 administered intravenously or orally significantly increased the voiding interval and bladder compliance. In addition, there was a decrease in the number of spontaneous contractions during the filling phase in a model of neurogenic and obstruction induced bladder instability. The amplitude of electrically evoked isovolumetric contractions was significantly smaller after CL-316243 exposure. CONCLUSIONS: These data suggest that activating the beta3-adrenergic receptor in rat bladder using CL-316243 may directly inhibit smooth muscle contractility, experimental hyperreflexia and detrusor instability, and be useful for urge urinary incontinence.

Synthesis and potassium channel opening activity of substituted 10H-benzo[4,5]furo[3,2-b]indole-and 5,10-dihydro-indeno[1,2-b]indole-1-carboxylic acids.

Compounds in a structurally novel series of substituted 10H-benzo[4,5]furo[3,2-b]indole-1-carboxylic acids and related 5,10-dihydro-indeno[1,2-b]indole-1-carboxylic acids were prepared and shown to possess potent, bladder-selective smooth muscle relaxant properties and thus are potentially useful for the treatment of urge urinary incontinence. Electrophysiological studies using rat detrusor myocytes have demonstrated that prototype compound 7 produces a significant increase in hyperpolarizing current, which is iberiotoxin (IbTx)-reversed, thus consistent with activation of the large-conductance Ca(2+)-activated potassium channel (BK(Ca)).

Design and SAR of novel potassium channel openers targeted for urge urinary incontinence. 2. Selective and potent benzylamino cyclobutenediones.

A novel series of benzylamine, potassium channel openers (KCOs) is presented as part of our program toward designing new, bladder-selective compounds for the treatment of urge urinary incontinence (UUI). We have found that the in vitro potency of (R)-4-[3,4-dioxo-2-(1,2, 2-trimethyl-propylamino)-cyclobut-1-enylamino]-3-ethyl-benzo nitrile 1 in the relaxation of precontracted rat detrusor strips can also be obtained with cyanobenzylamine derivative 4 (IC(50) = 0.29 microM) (Figure 3). Addition of a 2-Cl substituted benzylamine moiety and changing the alkylamino substituent of 4 to a t-Bu amine gives 31 (IC(50) = 0.14 microM)-a compound with similar in vitro potency as 4 as well as relaxant activity on bladder smooth muscle in vivo when administered orally (31, ED(50) = 3 mg/kg) in a rodent model of bladder instability. Further modifications, particularly the replacement of the t-Bu amino substituent with a tert-amylamine, gave a similarly active compound 60 (IC(50) = 0.10 microM) which shows excellent in vivo efficacy (ED(50) = 0.6 mg/kg). Moreover, 60, 3-(2,4-dichloro-6-methyl-benzylamino)-4-(1, 1-dimethyl-propylamino)-cyclobut-3-ene-1,2-dione (WAY-151616), shows excellent tissue selectivity for bladder K channels over arterial tissue (60, MAP ED(20) = 100 mg/kg; selectivity: MAP ED(20)/bladder ED(50) = 166). Other manipulations of the benzylamino cyclobutenediones, acylation of the benzylamine, conversion of the benzylamine substituent to a benzamide, homologation of the benzylamine to a phenethylamine, and incorporation of a methyl group at the benzyl carbon, all led to substantial loss of in vitro activity, although some in vivo activity was maintained in the acylated analogues. Compound 60 represents an attractive candidate for development in the treatment of UUI.

Design and SAR of novel potassium channel openers targeted for urge urinary incontinence. 1. N-Cyanoguanidine bioisosteres possessing in vivo bladder selectivity.

A structurally novel series of adenosine 5'-triphosphate-sensitive potassium (K(ATP)) channel openers is described. As part of our efforts directed toward identifying novel, bladder-selective potassium channel openers (KCOs) targeted for urge urinary incontinence (UUI), we found that bioisosteric replacement of the N-cyanoguanidine moiety of pinacidil (1, Figure 1) with a diaminocyclobutenedione template afforded squaric acid analogue 2, the prototype of a novel series of K(ATP) channel openers with unique selectivity for bladder smooth muscle in vivo. Further modification of the heterocyclic ring to give substituted aryl derivatives (3) afforded potent KCOs that possessed the desired detrusor selectivity when administered orally. The effects of these potassium channel agonists on bladder contractile function was studied in vitro using isolated rat detrusor strips. Potent relaxants were evaluated in vivo in a rat model of bladder instability. Lead compounds were evaluated concomitantly in normotensive rats for their effects on mean arterial blood pressure (MAP) and heart rate as a measure of in vivo bladder selectivity. (R)-4-[3,4-Dioxo-2-(1,2, 2-trimethyl-propylamino)-cyclobut-1-enylamino]-3-ethyl-benzo nitrile (79) met our potency and selectivity criteria and represents an attractive development candidate for the treatment of UUI. Electrophysiological studies using isolated rat bladder detrusor myocytes have demonstrated that compound 79 produces significant hyperpolarization which is glyburide-reversed, thus consistent with the activation of K(ATP). The design, synthesis, structure-activity relationships (SAR), and pharmacological activity associated with this series of novel KCOs will be discussed.

Comparison of the potassium channel openers, WAY-133537, ZD6169, and celikalim on isolated bladder tissue and In vivo bladder instability in rat.

The effects of the ATP-dependent potassium channel agonists ZD6169, celikalim, and WAY-133537 on bladder contractile function were examined in vitro on isolated bladder strips and in vivo on spontaneous bladder contractions. All three compounds produced a concentration-dependent relaxation of isolated rat detrusor strips (IC50 values = 0.93, 0.03, and 0.09 microM, respectively for ZD6169, celikalim, and WAY-133537. Contractile inhibition by all three compounds was fully reversed by 6 microM glyburide. These compounds also effectively inhibited spontaneous bladder contractions in the rat hypertrophied bladder model of detrusor instability. We also examined the electrophysiological properties of WAY-133537 on isolated rat bladder detrusor myocytes. Myocytes had an average resting membrane potential of -40 mV. Under patch current-clamp conditions, WAY-133537 (0.3 and 1.0 microM, n = 4-5) produced a significant hyperpolarization of 21 and 26 mV, respectively. Hyperpolarization was reversed by the addition of 5 microM glyburide. In patch voltage-clamp studies, WAY-133537 (0.3 microM, n = 3) significantly increased outward current in response to both voltage step and ramp protocols consistent with activation of the ATP-dependent potassium channel. In the detrusor instability model, WAY-133537 and celikalim had similar oral potencies (ED50 = 0.13 and 0.3 mg/kg, respectively), whereas ZD6169 was less potent (ED50 = 2.4 mg/kg). The antihypertensive agent celikalim exerted effects on the bladder at doses that significantly reduced systemic blood pressure. In contrast, both WAY-133537 and ZD6169 inhibited bladder hyperactivity at doses that produced minimal changes in both mean arterial blood pressure and heart rate. These data suggest that both WAY-133537 and ZD6169 may be useful in the treatment of bladder instability at doses associated with minimal hemodynamic side effects.

Inhibition of cardiac delayed rectifier K+ current by overexpression of the long-QT syndrome HERG G628S mutation in transgenic mice.

Mutations in the HERG gene are linked to the LQT2 form of the inherited long-QT syndrome. Transgenic mice were generated expressing high myocardial levels of a particularly severe form of LQT2-associated HERG mutation (G628S). Hearts from G628S mice appeared normal except for a modest enlargement seen only in females. Ventricular myocytes isolated from adult wild-type hearts consistently exhibited an inwardly rectifying E-4031-sensitive K+ current resembling the rapidly activating cardiac delayed rectifier K+ current (Ikr) in its time and voltage dependence; this current was not found in cells isolated from G628S mice. Action potential duration was significantly prolonged in single myocytes from G628S ventricle (cycle length=1 second, 26 degrees C) but not in recordings from intact ventricular strips studied at more physiological rates and temperature (200 to 400 bpm, 37 degrees C). ECG intervals, including QT duration, were unchanged, although minor aberrancies were noted in 20% (16/80) of the G628S mice studied, primarily involving the QRS complex and, more rarely, T-wave morphology. The aberrations were more commonly observed in females than males but could not be correlated with sex-based differences in action potential duration. These results establish the presence of IKr in the adult mouse ventricle and demonstrate the ability of the G628S mutation to exert a dominant negative effect on endogenous IKr in vivo, leading to the expected LQT2 phenotype of prolonged repolarization at the single cell level but not QT prolongation in the intact animal. The model may be useful in dissecting repolarization currents in the mouse heart and as a means of examining the mechanism(s) by which the G628S mutation exerts its dominant negative effect on native cardiac cells in vivo.

Inhibition of guinea pig detrusor contraction by NS-1619 is associated with activation of BKCa and inhibition of calcium currents.

The effects of NS-1619 on bladder contractile function and on transmembrane currents were evaluated in vitro on isolated guinea pig detrusor strips and isolated detrusor myocytes, respectively. In the isolated bladder strip, NS-1619 inhibited KCl-induced contractions in a concentration-dependent manner (IC50 = 12.2 +/- 3. 2 microM). Isolated detrusor myocytes were quiescent and had resting membrane potentials that averaged -45.3 +/- 2.7 mV. With patch-clamp techniques we demonstrated that exposure to 10 to 100 microM NS-1619 increased an iberiotoxin-sensitive current consistent with the activation of the large conductance calcium-dependent potassium channel (BKCa). Single-channel analysis confirmed that NS-1619 increased the open probability of BKCa channels. NS-1619 also appeared to decrease inward calcium current (ICa). After exposure to 30 microM NS-1619, peak current amplitude significantly decreased by approximately 50%. Analysis of the current voltage relationship revealed a significant decrease in maximal conductance from 10.5 +/- 4 to 6.2 +/- 3 nS. The voltage dependence of calcium current activation and inactivation was well fit by a Boltzmann relationship. Besides the decrease in conductance, there was a small, but significant shift in the half-inactivation voltage, which suggests that NS-1619 preferentially blocks the open state of the channel. Steady-state (window) calcium current was also decreased. Analysis of the theoretical window current revealed a 71% decrease in this noninactivating current. These data indicate that NS-1619 inhibits detrusor smooth muscle contraction in a concentration-dependent manner and that the underlying mechanism of action for this effect involves inhibition of calcium current, and may also include activation of the BKCa channel. Compounds with this profile may be useful in the treatment of bladder instability.

Acute administration of 17 beta-estradiol inhibits calcium currents in isolated guinea pig detrusor myocytes.

The acute effects of 17 beta-estradiol on bladder contractile function and on calcium currents were evaluated in vitro on isolated guinea pig bladder strips and detrusor myocytes, respectively. In the isolated bladder strip, 17 beta-estradiol inhibited KCl-induced contractions with an IC50 of 1.7 +/- 0.3 microM. In isolated detrusor myocytes, single cell capacitance was 52.9 +/- 2.2 pF. This corresponded to a mean cell surface area of 5297.5 +/- 214.8 micron2. The specific membrane resistance was 97.8 +/- 31.6 K omega cm2. The effects of 17 beta-estradiol (0.1 to 3 microM) on peak transmembrane calcium currents were evaluated using the whole cell voltage clamp technique. Peak calcium currents were decreased by approximately 50% (427.8 +/- 57.6 to 226.1 +/- 70.2 pA) at 1.0 microM. Additional analyses were performed at 1.0 microM. Evaluation of the current voltage relationship indicated a decrease in the maximum conductance from 12.31 +/- 1.85 to 7.29 +/- 1.91 nS. Current activations were reasonably fit to a Boltzmann logistic. After exposure to 17 beta-estradiol, the activation curve was shifted to the right by approximately 13 mV. The voltage-dependence of calcium current inactivation was U-shaped, but well described by a Boltzmann relation at membrane potentials between -80 and 0 mV. 17 beta-Estradiol had no effect on the voltage-dependence of calcium current inactivation. The combination of effects on peak current amplitude and voltage dependence of current activation produced a significant decrease in the calcium "window current." Quasi steady-state ramp currents were characteristically "N-shaped," and after exposure to 17 beta-estradiol became flattened.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiologic activity and antiarrhythmic efficacy of CK-3579, a new class III antiarrhythmic agent with beta-adrenergic blocking properties.

CK-3579 (N-[4-[2-hydroxy-3-[[2-[4-(1H-inidazol-1- yl)phenoxy]ethyl]amino]propoxy]phenyl]methanesulfonamide, HCL-[RS]), is a new class III antiarrhythmic agent with beta-adrenoceptor blocking properties shown to be effective in preventing ventricular arrhythmias in experimental animals. We examined the beta 1- and beta 2-adrenoceptor binding properties, cellular electrophysiology, and antiarrhythmic efficacy of CK-3579 and the two enantiomers CK-4000 (S) and CK-4001 (R). Both CK-3579 and CK-4000 were equipotent in displacing [3H]dihydroalprenolol from cardiac membranes (IC50 = 2.4 +/- 0.3 and 1.7 +/- 0.4 microM, respectively) and were approximately 23-59 times more selective for the heart receptor than the lung receptor. The IC50 for CK-4001 at the beta 1-adrenoceptor was 38.1 +/- 8 microM and > 100 microM at the beta 2-adrenoceptor. In isolated canine Purkinje fibers and ventricular muscle preparations, all three compounds increased action potential duration at 95% repolarization (APD95) with equal potency, having an average EC20 of approximately 1 microM in Purkinje fiber and 2 microM in ventricular muscle. No significant effects were observed on any other action potential (AP) parameters. In Purkinje fibers with APs shortened by isoproterenol, the class III activity of CK-4000 was significantly greater than that of CK-4001. CK-4000 inhibits the potassium delayed rectifier current Ik in a concentration-dependent manner in isolated feline ventricular myocytes. The IC50 for inhibition of fully activated current was 0.4 +/- 0.2 microM. Steady-state currents negative to -20 mV were unchanged by CK-4000, but the "hump" in the outward current between -20 and +30 mV was flattened.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of synaptic transmission in reinnervating rat skeletal muscle.

Synaptic transmission, and its sensitivity to the effects of 3,4-diaminopyridine (3,4-DAP) and the phosphatase 2,3-butanedione monoxime (BDM), was examined for "crushed fiber" preparations of rat extensor digitorum longus muscle undergoing reinnervation after nerve crush. While mean quantal content (m) of endplate potentials (EPPs) was low early during reinnervation (10-24 days after nerve crush), elevation of temperature or extracellular calcium concentration restored m toward normal. However, m achieved control values for reinnervating preparations exposed to 3,4-DAP. 3,4-DAP also activated quiescent motor nerve terminals: after exposure to this drug, synaptic transmission was detected as early as 8 days after nerve crush. BDM too activated quiescent regenerating motor nerve terminals and increased m to normal. It also prolonged EPP and endplate current decay, suggesting a pre-synaptic effect on the synchrony of transmitter release and/or a post-synaptic effect on the open time of acetylcholine-gated endplate channels. While the effects of temperature, extracellular calcium, 3,4-DAP, and BDM suggest that regenerating nerve terminals can mobilise a reserve of quanta, this reserve is abnormally low, since hemicholinium-3 caused rapid rundown of EPP amplitude at repetitively stimulated regenerating endplates.

Synthesis, cardiac electrophysiology, and beta-blocking activity of novel arylpiperazines with potential as class II/III antiarrhythmic agents.

A series of novel arylpiperazines have been prepared in an attempt to incorporate both class II (beta-receptor blocking) and class III antiarrhythmic properties in a single molecule. The key step in the preparation of the new compounds involves a regioselective heterocyclic ring formation. All but four compounds significantly prolonged action potential duration in canine cardiac Purkinje fibers (class III activity). All but one of the compounds demonstrated beta-receptor affinity in a competitive binding assay and three had beta 1-receptor selectivity. Compared to sotalol, a reference class II/III agent, arylpiperazine 7a (4-[(methylsulfonyl)amino]-N-[(4- phenylpiperazin-2-yl)methyl]benzamide) demonstrated beta 1-selectivity and was 1 order of magnitude more potent in the in vitro class III and the beta 1-receptor screens. Compound 7a was evaluated further and found to be effective in preventing programmed electrical stimulation-induced arrhythmias in conscious dogs (class III activity) and against epinephrine-induced arrhythmias in halothane anesthetized dogs (class II activity).

Cellular electrophysiological effects of the class III antiarrhythmic agents sematilide and clofilium on rabbit atrial tissues.

Sematilide (N-[2-(diethylamino)ethyl]-4- [(methylsulfonyl)amino]benzamide HCl) is a new class III antiarrhythmic agent that has been shown to be effective in preventing reentrant ventricular arrhythmias in experimental animals and humans. In this study, we examined the in vitro effects of sematilide (1-100 microM) on isolated sinoatrial (SA) node, atrioventricular (AV) node, and atrial muscle. These results were then compared to another class III agent, clofilium (1-30 microM). In SA nodal tissue, sematilide increased the action potential duration (APD) and spontaneous cycle length (SCL) in a concentration-dependent manner (EC20% = 15 +/- 3 and 54 +/- 13 microM, respectively). In addition, there was a slight reduction in maximum diastolic potential at 100 microM. Clofilium had similar class III effects, but was approximately 3 to 18 times more potent (EC20% = 6 +/- 2 and 3 +/- 1 microM for the APD and SCL, respectively). Neither agent had a significant effect on the slope of phase 4 nor on other action potential parameters. Results in AV nodal preparations were similar. Both sematilide and clofilium increased the APD and SCL in a concentration-dependent manner, with clofilium being approximately four to six times more potent than sematilide (EC20% for the APD and SCL for sematilide = 12 +/- 4 and 12 +/- 8 microM, respectively, and for clofilium = 2 +/- 1 and 3 +/- 2 microM, respectively). No significant effects were observed on other action potential parameters. Sematilide and clofilium increased the APD and effective refractory period (ERP) in atrial trabeculae in a concentration-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and cardiac electrophysiological activity of aryl-substituted derivatives of the class III antiarrhythmic agent sematilide. Potential class I/III agents.

Twelve novel derivatives of the selective class III antiarrhythmic agent sematilide were prepared in an attempt to incorporate both class I and class III electrophysiological properties into a single molecule. Electrophysiological activity was determined by standard microelectrode techniques in canine cardiac Purkinje fibers. Initial assessment of class I efficacy was carried out in a ouabain-induced arrhythmia model in guinea pigs. All of the compounds prolonged action potential duration in Purkinje fibers (class III activity), and three were active against ouabain-induced arrhythmias (class I activity). Selected compounds were evaluated further in dogs for efficacy against arrhythmias occurring 24 h following coronary ligation (automatic arrhythmias) and induced by using programmed electrical stimulation techniques (reentrant arrhythmias). The most effective compounds from the series are 3g and -j, which were effective in both canine models. Molecular modeling and structure-activity relationships are discussed.

Electrical properties of canine subendocardial Purkinje fibers surviving in 1-day-old experimental myocardial infarction.

The passive electrical properties of subendocardial Purkinje fibers surviving in infarcted regions of canine ventricle 24 hours after coronary ligation were studied by using microelectrode techniques and cable theory. In normal hearts, cells within the subendocardial Purkinje fiber strands were found to be well coupled to each other but electrically isolated from neighboring myocardium. Voltage response to intracellular current injection was consistent with one-dimensional cable behavior and yielded estimates of passive electrical properties in general agreement with previous work on free-running Purkinje strands (membrane length constant, 1.2 +/- 0.1 mm; membrane time constant, 7.3 +/- 0.8 msec; input resistance, 67.4 +/- 7.4 K omega; membrane resistance, 8.2 +/- 0.7 K omega.cm; axial resistance, 0.52 +/- 0.06 M omega/cm; membrane capacitance, 960 +/- 102 nF/cm) (n = 21). On the day after coronary ligation, subendocardial Purkinje fiber action potentials were prolonged and slightly depolarized. Significant increases were measured in input resistance (+40.5%), membrane resistance (+43.9%), and axial resistance (+47.5%), whereas membrane capacitance was found to be significantly decreased (-24.3%) (n = 19). Conduction velocity, membrane length constant, membrane time constant, and the time constant and capacitance for the foot of the action potential remained unchanged. These results are consistent with electrical uncoupling between adjacent cells, which will increase internal resistivity, accompanied by changes in cellular phospholipid content, which can increase membrane resistance and alter membrane capacitance. Alternatively, the results can be explained by a simple model in which the apparent electrical structure is altered by changes in electrical coupling alone, with specific electrical properties remaining constant. Although the mechanisms underlying the observed changes remain uncertain, the present study indicates that myocardial infarction is associated with alterations in the passive electrical structure of surviving subendocardial Purkinje fibers, which, together with changes in action potential configuration, may provide a substrate for the generation of ventricular arrhythmias 24 hours after coronary ligation.

Antiarrhythmic peptide has no direct cardiac actions.

The electrophysiologic, inotropic, and muscarinic effects of antiarrhythmic peptide (AAP) were examined in canine cardiac Purkinje fibers, ferret papillary muscle, and canine cardiac membranes, respectively. Aside from a prolongation of time to peak force in papillary muscle, no biologically significant effects of AAP could be determined in any preparation, suggesting that its antiarrhythmic effects are not mediated by direct membrane actions.

Synthesis and cardiac electrophysiological activity of 2- and 3-[(substituted phenyl)alkyl]quinuclidines. Structure-activity relationships.

The syntheses and cardiac electrophysiological effects of 21 2- and 3-substituted quinuclidines and some quaternary ammonium derivatives are described. The 2-substituted quinuclidines 2-8 were prepared by alkylation of 2-methylene-3-quinuclidinone. The Wittig reaction with 3-quinuclidinone afforded the 3-substituted derivative 9, which was subsequently converted to 10 and 11. The electrophysiological profiles of the compounds were determined in canine cardiac Purkinje fibers and ventricular muscle strips. The 3-[(substituted phenyl)alkyl]quinuclidines selectively increased action potential duration (Vaughan Williams class III activity). In the 2-substituted series some of the compounds both increased action potential duration and decreased conduction velocity (class I activity). For some of the 2-substituted quinuclidines, appropriate substitution of the phenyl ring was shown to be a requirement for significant class III electrophysiological activity. Selected compounds were efficacious in a programmed electrical stimulation model in the anesthetized dog.

Synthesis and antiarrhythmic activity of novel 3-alkyl-1-[omega-[4-[(alkylsulfonyl)amino]phenyl]-omega- hydroxyalkyl]-1H-imidazolium salts and related compounds. 2.

Novel analogues of the class III antiarrhythmic agent 1-[2-hydroxy-2-[4-[(methylsulfonyl)amino]phenyl]ethyl]-3-methyl-1H- imidazolium chloride, 1 (CK-1649), were prepared and investigated for their class III electrophysiological activity on isolated canine cardiac Purkinje fibers and ventricular muscle tissue. Structure-activity relationships are discussed for a series of 11 compounds. One compound, N-[4-[1-hydroxy-2-(4,5-dihydro-2-methyl-1H-imidazol-1- yl)ethyl]phenyl]methanesulfonamide hydrochloride, 9, was comparable in activity to 1 in vitro and prolonged the functional refractory period in anesthetized dogs when given intraduodenally. Unlike 1, compound 9 was ineffective at preventing ventricular tachycardia induced by programmed electrical stimulation in anesthetized dogs 24 h after an acute myocardial infarction.

Metabolism and electrophysiology in subendocardial Purkinje fibers after infarction.

Subendocardial Purkinje fibers (SEPF) have been implicated in the genesis of fatal arrhythmias that occur 24-48 h after infarction but little is known about the metabolic processes involved. Quantitative microchemical and electrophysiological studies were performed on normal and infarcted hearts removed 24 h after coronary artery occlusion. ATP, ADP, AMP, total adenine nucleotide content, phosphocreatine (PCr), and inorganic phosphate in superficial subendocardial Purkinje fibers from infarct preparations decreased approximately 30% compared with normal preparations. The phosphate potential decreased 45% in the infarct group. Similar changes were observed in adjacent contractile muscle between normals and infarcts. Action potentials of SEPF from infarct hearts had increased automaticity, markedly prolonged action potential durations at 50 and 90% repolarization (APD50 or APD90), but unchanged resting membrane potentials. The decrease in ATP, total adenine nucleotides, and the phosphate potential correlated linearly with APD50 and APD90. No correlation was found between PCr and APD90. This combined biochemical and electrophysiological approach provides a promising new way to further probe the biochemical basis of the abnormal electrical properties of subendocardial Purkinje fibers after myocardial infarction.

Antiarrhythmic-like actions of the smooth muscle spasmolytic agent, cinnamedrine, on action potentials of mammalian ventricular tissue.

Cinnamedrine is an active ingredient in preparations used to relieve dysmenorrhea. It has been reported to have local anesthetic properties in nerve. This property prompted us to evaluate the effects of cinnamedrine on the cardiac action potential. Cinnamedrine (10-35 microM) significantly reduced the overshoot and maximum rate of rise, and also prolonged the duration of action potentials recorded from rat and guinea pig ventricular tissues. The action of cinnamedrine to depress upstroke velocity in guinea pig papillary muscle was dependent on the rate of stimulation. In addition, the refractory period of rat ventricular muscle was prolonged markedly. Finally, cinnamedrine (10 microM) significantly reduced, or totally abolished, epinephrine-induced automatic activity in both dog and rat myocardium in vitro. Prolongation of action potential duration and refractory period, and depression of upstroke velocity are characteristics which cinnamedrine shares with antiarrhythmic drugs.