Beta-adrenergic Blocking Activity Research Articles

Selective late INa inhibition by GS-458967 exerts parallel suppression of catecholamine-induced hemodynamically significant ventricular tachycardia and T-wave alternans in an intact porcine model

Catecholamines can elicit early and delayed afterdepolarizations (EADs and DADs), resulting in ventricular tachyarrhythmias. As inhibition of the cardiac late sodium current (I(Na)) suppresses EADs and DADs, we examined whether GS-458967 (GS-967), a potent inhibitor of this current that is devoid of beta-adrenergic blocking action, can prevent epinephrine-induced ventricular tachycardia (VT) induction in an intact porcine model. In 12 closed-chest anesthetized pigs, spontaneous VT was induced by epinephrine administration (2.0 µg/kg, intravenous, bolus over 1 minute). Effects of GS-967 (0.4 mg/kg, intravenous, infused over 30 minutes) on VT incidence, T-wave alternans (TWA) level, and hemodynamic and electrophysiologic parameters before and after epinephrine were analyzed (N = 6). Effects of vehicle control were investigated in 6 animals. TWA was measured using the Modified Moving Average method. Epinephrine elicited spontaneous hemodynamically significant nonsustained VT in all 6 pigs and increased TWA by 28-fold compared to baseline (P < .001). GS-967 reduced mean 3- to 7-beat VT incidence by 55% (from 9.5 ± 2.72 to 4.3 ± 0.76 beats/min, P = .020) and ≥8-beat VT incidence by 56% (from 1.6 ± 0.47 to 0.7 ± 0.42 beats/2 min, P = .033) and eliminated the VT-associated hypotension, with no changes in chronotropic and minimal attenuation of the inotropic responses to epinephrine. Concurrently, GS-967 at 30, 60, and 90 minutes reduced the magnitude of the epinephrine-induced surge in TWA by 56% (from 140 ± 13.2 to 62 ± 12.1 µV, P < .01), 62% (to 53 ± 8.3 µV, P < .01), and 51% (to 69 ± 14.0 µV, P < .01) (means ± SEM), respectively. Selective cardiac late INa inhibition with GS-967 confers significant protection against catecholamine-induced VT and TWA.

THE LATE SODIUM CURRENT INHIBITOR GS967 SUPPRESSES SPONTANEOUS ATRIAL FIBRILLATION IN AN INTACT PORCINE MODEL

The late sodium current (INa) has been increasingly implicated in atrial fibrillation (AF). This study evaluated anti-AF effects of GS967, a selective, potent inhibitor of late INa, devoid of beta-adrenergic blocking action, in a novel model of AF induction without electrical stimuli. In 6 closed-

ChemInform Abstract: Synthesis, Binding Affinity, and Selectivity of New β1- and β2-Adrenoceptor Blockers.

The synthesis of a new series of sesamol derivatives with beta-adrenergic blocking activity is described. The affinity and selectivity of these compounds for beta 1- and beta 2-adrenoceptors were studied in comparison with those of ICI-118551 and propranolol. Some of the synthesized compounds show very good affinity for the beta 2-receptors of rat lung membranes and two of them provide interesting selectivity.

Efficacy of Carvedilol for Ischemia/Reperfusion-Induced Oxidative Renal Injury in Rats

In renal transplantation, ischemia/reperfusion (I/R) injury is related to production of reactive oxygen species. In addition to its antihypertensive action due to nonselective beta-adrenergic blocking activity, carvedilol has potent antioxidant activity. This study was designed to investigate the effects of carvedilol on I/R injury in rats. On postoperative days 2 and 4, serum creatinine levels were higher among the control and the metoprolol treatment groups compared with the carvedilol treatment group ( P < .005). However, there were no significant differences on postoperative day 7. In conclusion, increased antioxidant modulation by carvedilol attenuated renal I/R injury.

The Role of β-Blockers as a Cornerstone of Cardiovascular Therapy

The beta-adrenergic blockade as a therapeutic approach first emerged in the 1950s. During the past five decades, the total number of indications that have been suggested, and the remarkable number approved by regulatory agencies, places beta-blockade far ahead of all competing treatments, not only in the cardiovascular area, but in all of therapeutics. Differentiation of beta-adrenergic blocking agents has been made on the basis of beta1 selectivity, duration of action, intrinsic sympathomimetic activity, lipophilicity, and whether or not the beta-adrenergic blocking action is accompanied by an alpha-adrenergic blocking action. With the development of nebivolol, a beta-adrenergic blocking agent that also activates nitric oxide synthase in blood vessels, comes a new therapeutic option. Endothelial dysfunction with loss of nitric oxide production is a common feature in many cardiovascular diseases. This fascinating class of drugs continues to provide us with new and important therapeutic opportunities.

Attenuation of aortic banding-induced cardiac hypertrophy by propranolol is independent of beta-adrenoceptor blockade.

Racemic propranolol attenuates cardiac hypertrophy secondary to abdominal aortic banding-induced pressure overload by a mechanism independent of its effect on cardiac work load. This was only observed, however, using doses of propranolol that were much higher than those needed to induce beta-adrenoceptor blockade. Thus, the question remains as to whether the antihypertrophic effect of propranolol depends on its ability to antagonize cardiac beta-adrenoceptor-mediated action (positive chronotropic effect, trophic effect) or on beta-adrenoceptor-independent action. In a rat model of chronic pressure overload induced by abdominal aortic banding, we evaluated the effects on left ventricular hypertrophy (LVH) of the propranolol isomers, L-propranolol and D-propranolol, which compared to L-isomer is approximately 50-fold less potent as a beta-adrenoceptor antagonist, but is similarly potent as a membrane-stabilizer, as well as of timolol, a non-selective beta-adrenergic antagonist devoid of membrane stabilizing activity, and disopyramide, which is a membrane stabilizer, but not a beta-adrenoceptor blocker. Compared to sham-operated rats, banded rats had 30% greater left ventricular to body weight (LVW/BW) ratio (P < 0.01). The increase in LVW/BW ratio was significantly attenuated by treatment with 40 and 80 (but not 10) mg/kg per day of L-propranolol. Left ventricular hypertrophy was also prevented by D-propranolol, 40 and 80 mg/kg per day, and disopyramide, 50 mg/kg per day, whereas timolol, 30 and 60 mg/kg per day, showed no antihypertrophic effect. In separate groups of banded rats in which the reduction in heart rate induced by propranolol (80 mg/kg per day) was prevented by chronic cardiac pacing at 375 b.p.m., hypertrophy was again prevented, indicating that the effects of L-propranolol on LVH are not related to a reduction in cardiac work load. In the aortic banding-induced model of LVH: (i) the antihypertrophic effect of propranolol is independent of its beta-adrenergic blocking activity; and Iii) since disopyramide and D-propranolol also proved to be able to antagonize banding-induced LVH, the hypothesis is proposed that membrane-stabilizing activity, among the ancillary properties of propranolol, most likely accounts for the antihypertrophic effect of this drug.

A highly selective beta1-adrenergic blocker with partial beta2-agonist activity derived from ferulic acid, an active component of Ligusticum wallichii Franch.

Short-term injection of ferulinolol (0.1, 0.5, and 1.0 mg/kg, i.v.) produced dose-dependent bradycardia responses in pentobarbital-anesthetized Wistar rats, whereas it had no significant effects on the blood pressure. Ferulinolol markedly inhibited the tachycardia effects induced by (-)isoproterenol but did not show any blocking effect on the arterial pressor responses induced by (-)phenylephrine. These findings clearly suggested that ferulinolol had a beta-adrenergic blocking activity; nevertheless, it did not involve an alpha-adrenergic blocking action. In isolated guinea pig tissues, ferulinolol competitively antagonized (-)isoproterenol-induced positive inotropic and chronotropic effects of the atria and tracheal relaxation responses. The parallel shift to the right of the concentration-response curve of (-)isoproterenol suggested that ferulinolol was a beta-adrenoceptor-competitive antagonist. The apparent pA2 values for ferulinolol on right atria, left atria, and trachea were 7.62 +/- 0.05, 7.54 +/- 0.01, and 6.28 +/- 0.11, respectively. Ferulinolol was more potent on the atria than on tracheal tissues, demonstrating that it possessed beta1-adrenoceptor selectivity. The intrinsic sympathomimetic activity (ISA) of ferulinolol and propranolol were determined on isolated atria and trachea from reserpine-treated guinea pig. Propranolol caused significantly negative inotropic and chronotropic effects at > or =1 microM, whereas ferulinolol possessed fewer cardiodepressant activities than propranolol. In reserpine-treated tracheal strips, ferulinolol produced dose-dependent relaxant responses, but propranolol was without effectiveness. Preincubating the preparations with ICI 118,551 (0.1, 1.0, and 10 nM), a beta2-adrenoceptor antagonist, significantly shifted the concentration-relaxation curves of ferulinolol to a region of higher concentrations. These results implied that ferulinolol had a partial beta2-agonist activity. Further, binding characteristics of ferulinolol and various beta-adrenoceptor antagonists were evaluated in [3H]CGP-12177 binding to rat ventricular or lung membranes. The Ki values of ferulinolol, atenolol, metoprolol, and (-)propranolol were 103, 262, 123, and 0.23 nM, respectively, in ventricular membranes, and 2,412, 7,539, 2,186, and 0.72 nM, respectively, in lung membranes. In conclusion, ferulinolol was found to be a highly selective beta1-adrenoceptor antagonist with partial beta2-agonist activity but was devoid of alpha-adrenoceptor blocking action.

Propafenone: an effective agent for the management of supraventricular arrhythmias.

Propafenone is a sodium channel blocking antiarrhythmic drug. It also has beta-adrenergic, potassium channel, and weak calcium channel blocking activity. The drug is metabolized in the liver with rates dependent on the debrisoquin phenotype. The saturable metabolism results in nonlinear pharmacokinetics. The metabolites retain sodium channel blocking activity but little beta-adrenergic blocking activity. Both controlled and noncontrolled studies have documented its efficacy in a variety of supraventricular arrhythmias. Intravenous propafenone is effective in converting atrial fibrillation to normal sinus rhythm. Chronic oral administration decreases the frequency of recurrence of atrial fibrillation and paroxysmal supraventricular tachycardia. The drug is particularly effective in the Wolff-Parkinson-White syndrome. The drug may produce SA block in patients with underlying sinus node dysfunction. Propafenone has comparatively few noncardiac side effects. It is a useful primary drug or an alternative to more commonly used drugs used for the treatment of supraventricular arrhythmias.

Synthesis and pharmacological activity of adaprolol enantiomers: a new soft drug for treating glaucoma.

Adaprolol maleate is a new beta-adrenergic antagonist that is being developed to treat glaucoma. The soft drug was designed to minimize systemic activity through facile inactivation to an inactive metabolite. Studies with other potent beta-adrenergic antagonists indicated that tissue specific receptor differences might be more stringent for selected beta-adrenergic blocking activities and suggested that R enantiomers of traditional beta-blockers should be developed for controlling glaucoma. The present studies demonstrate that the potent ocular hypotensive effects of adaprolol are not stereoselective. In contrast, cardiac effects could be detected after intravenous S(+) adaprolol, but not R(-) adaprolol. The studies confirm that adaprolol functions as a potent beta-adrenergic antagonist. The negligible systemic beta-blocking activity detected with opthalmic administration of adaprolol is consistent with soft drug design.

Improved delivery through biological membranes. LVI. Pharmacological evaluation of alprenoxime--a new potential antiglaucoma agent.

A new site-specific chemical delivery system (CDS) for alprenolol was designed and investigated as a potential novel antiglaucoma agent. The effect of this compound, alprenoxime (AO), on the intraocular pressure (IOP) of rabbits was evaluated after its uni- and bilateral administration. AO produced significant reduction of the IOP starting at 30 min and lasting for more than 6 hr after its topical administration. Both in rats and in rabbits the i.v. bolus injection of AO (6 mg/kg) led to insignificant transient bradycardia, while no activity was found after oral or topical administration. Alprenolol (ALP) in a similar dose produced a sustained and significant bradycardia for more than 30 min. When the beta-adrenergic blocking activity was assessed against isoprenaline-tachycardia, the same results were obtained, i.e., AO led to a transient brief activity, whereas ALP produced a significant long-lasting beta blockade. These results support the potent ocular hypotensive action and the weak systemic beta-adrenergic blocking and cardiovascular activity of AO: a significant improvement in the therapeutic index. This finding recommends alprenoxime as a potent site-specific antiglaucoma agent with minimal systemic side effects.

Is Class III antiarrhythmic activity important?

The beginning of thought about a discrete Class III antiarrhythmic action stemmed from the observation that sotalol produced a very marked increase in the time course of repolarization with little or no change in conduction velocity in cardiac muscle. It is known that in patients with hypothyroidism or hypocalcemia, both of which are associated with a marked lengthening of the cardiac action potential, arrhythmias are exceedingly rare. The acetyl metabolite of procainamide also produces, as its sole action, a marked increase in the action-potential duration, and it has antiarrhythmic activity. It is clear that lengthening the action-potential duration is antiarrhythmic. Sotalol is a unique beta blocker in not only antagonizing sympathetic stimulation competitively, but also in exerting a potent antiarrhythmic effect by prolonging repolarization. Because of the augmented tension that is associated with the increase in action-potential duration, sotalol produces a less negative inotropic effect than that seen with other beta blockers. Sotalol is a potent broad-spectrum antiarrhythmic agent because of its Class III properties, resulting from an increase in the length of cardiac repolarization, a property that is modulated by its beta-adrenergic blocking actions.

Stereoselective disposition of carvedilol in man after intravenous and oral administration of the racemic compound

The racemic compound carvedilol is a multiple-action oral antihypertensive drug that exhibits both vasodilator and non-selective beta-adrenergic blocking activities. The effects of the levorotatory S-enantiomer [S(-)-CARV] are vasodilatation and beta-blockade. The R(+)-enantiomer [R(+)-CARV] is a pure vasodilating agent. Quantitative determination of the enantiomers in human plasma by HPLC was carried out after formation of diastereoisomers with the chiral reagent 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl isothiocyanate (GITC). The pharmacokinetics of the enantiomers were studied following i.v. (12.5 mg in 1 h) and p.o. (50 mg) administration of racemic carvedilol in ten healthy male subjects according to a randomized crossover design. The AUCs of S(-)-CARV were significantly lower than those of R(+)-CARV after both i.v. and p.o. administration. The systemic clearance of the two enantiomers was significantly different, whereas half-lives and apparent distribution volumes were comparable. Following p.o. administration, the absolute bioavailability (31.1% and 15.1%, respectively) and maximal plasma concentrations of R(+)-CARV were twice those of S(-)-CARV. A similar difference was found in the half-lives. A close correlation existed between enantiomeric ratios after i.v. and after p.o. administration, demonstrating slight intraindividual variability. The preferential systemic clearance of the S(-)-enantiomer suggests stereoselective hepatic metabolism of carvedilol, becoming especially apparent after p.o. administration. The small intrasubject variability in enantiomer ratios indicates a relatively constant relation of beta-blockade to vasodilation during chronic treatment.

Structure-Activity Relationships as a Response to the Pharmacological Differences in Beta- Receptor Ligands

With a few notable exceptions, beta-receptor ligands (agonists and antagonists) belong to the aryl- or heteroaryl-ethanolamine series and to the aryl- or heteroaryl-oxypropanolamine series. Structure-activity relationships for beta-adrenergic agonists show that a secondary amine in the phenylethanolamine side chain ending is essential for receptor stimulation. The 3,4-dihydroxyphenyl groups may be replaced by "phenol equivalents" (-CH2OH, -NHCONH2, -CHOH, -NHSO2CH3). In contrast, substitution at carbon alpha of the phenyl-ethanolamine side chain decreases or suppresses beta-adrenergic activity. The general requirements for beta-adrenergic blocking activity in the aryl- or heteroaryl-oxypropanolamine are as follows: (1) the potency of beta-blockade is conferred by a branched alkyl group (isopropyl or tert-butyl) grafted on the terminal amino N, and by the nature and position of a substituent on the aromatic ring: ortho-substituted compounds (especially when they have an hetero-atom in alpha) are the most potent ones. (2) The cardioselectivity is improved by the attachment of 3,4-dimethoxyphenylethyl,4-amide-substituted phenoxyethyl or acylamino-alkyl moieties to the terminal amino N of the side chain. Para substitution on the aromatic ring (particularly 4-acylamido substitution) has also yielded cardioselective drugs. Finally, the beta 1-selectivity is strongly and negatively correlated with lipophilicity. (3) Intrinsic sympathomimetic activity can be modulated by aromatic nucleus variations, particularly by hydroxyl-equivalents (electron withdrawing groups) on meta- and para-positions (3,4-substitutions).

Dextrorotatory isomer of sotalol: Electrophysiologic effects and interaction with verapamil

The dextrorotatory isomer of sotalol ( d-sotalol) has class III antiarrhythmic properties with known action potential duration (APD) prolonging effects, and is largely devoid of beta-adrenergic blocking activity. We studied its electrophysiologic effects and the mechanism of its APD prolonging effects in sheep Purkinje fibers by means of standard microelectrode techniques. At all concentrations (10 −6 to 10 −4 mol/L), d-sotalol had no effect on V̇ max. At 10 −6 mol/L, d-sotalol had no effect on APD. A concentration-dependent effect of 10 −5 to 10 −4 mol/L d-sotalol was found on APD. At peak effect, APD was prolonged by 56% and 49% at 50% (APD 50) and 90% (APD 90) of repolarization ( p < 0.01). Early afterdepolarizations (EADs) were noted at high concentration of d-sotalol (10 −4 mol/L). These EADs were increased in number by epinephrine. To assess the mechanism of APD prolongation, verapamil (2.0 × 10 −6 mol/L) was added before and after d-sotalol treatment. In the presence of verapamil, no APD prolongation was observed even at the highest d-sotalol concentration (10 −4 mol/L). Next, during APD prolongation at the highest d-sotalol concentration, we added verapamil, noticing a remarkable shortening of APD. In addition, d-sotalol-induced EADs disappeared with addition of verapamil. Thus (1) verapamil blocks d-sotalol-induced APD prolongation and EADs and (2) this may be consistent with a mechanism via slow inward current.

Lack of Concordance Between the Antiarrhythmic and Antifibrillatory Actions of UM-424, a Quaternary Ammonium Analogue of Propranolol

UM-424, 1-dimethyl isopropylamino-3-(2-phenylphenoxy)-propan-2-ol chloride, is a quaternary ammonium derivative of propranolol. Previous studies have demonstrated UM-424 to suppress the development of ventricular arrhythmias in a variety of experimental canine models, while lacking significant beta-adrenergic blocking and cardiodepressant actions. In the present studies, slight and transient reductions in heart rate, coronary flow, and indices of cardiac force and pressure developed only after the intravenous administration of 10.0 mg/kg UM-424. The positive inotropic response to intravenous isoproterenol was not altered significantly by 1.0-10.0 mg/kg UM-424. In conscious dogs 4-7 days after anterior myocardial infarction, UM-424 administered intravenously in a single (5.0 mg/kg) or multiple (5.0 mg/kg q 6 h for 24 h) dose schedule increased the ventricular refractory period from 146 +/- 4 to 180 +/- 3 ms (p less than 0.01), and suppressed the initiation of ventricular tachycardia by programmed ventricular stimulation in six of nine postinfarction dogs tested. However, the incidence of subsequent ventricular fibrillation developing in response to acute ischemia at a site remote from previous myocardial infarction was 100% in both UM-424 (n = 8)- and vehicle (n = 8)-treated postinfarction dogs. These findings suggest UM-424 is ineffective in preventing the development of ischemic ventricular fibrillation in the presence of previous myocardial injury, despite the efficacy of this agent in suppressing other experimentally induced ventricular arrhythmias.

Tolerance and beta-adrenergic blocking activity of flestolol, a short-acting beta blocker.

The tolerance and beta-adrenergic blocking activity of flestolol, a short-acting beta-blocker, was investigated in 30 subjects. Flestolol infused intravenously at doses up to 100 micrograms/kg/min was found to be well tolerated. A dose-dependent attenuation of isoproterenol-induced tachycardia and increase in systolic blood pressure occurred with flestolol at doses ranging from 0.5 to 15.0 micrograms/kg/min. The average percent reduction in isoproterenol-induced tachycardia (beta-blockade) at each dose of flestolol, 0.5, 2.5, 5.0, 15.0, and 50.0 micrograms/kg/min, was 15.1%, 45.9%, 67.0%, 85.9%, and 90.3%, respectively. The onset of beta-blockade occurred within 30 minutes. After the end of flestolol infusion there was a marked reduction in beta-blockade within 6 minutes, with complete recovery from beta-blockade within 30 to 45 minutes. There was a statistically significant (P less than 0.01) positive correlation between flestolol dosage and its blood levels (r = 0.91) as well as between the flestolol-induced beta-blockade and its dosage (r = 0.62).

Rising multiple-dose pharmacokinetics of labetalol in hypertensive patients.

Labetalol, a drug possessing both alpha- and beta-adrenergic blocking activities, is used in the treatment of hypertension. The current study was undertaken to elucidate the steady-state pharmacokinetics of labetalol following a rising oral multiple-dosage regimen. Twelve patients received oral labetalol every 12 hours for 18 days. An initial dose of 100 mg was increased at three-day intervals to 200, 300, 400, and 600 mg q12h. Selected blood samples were taken at various times following drug administration at each dose level and analyzed for labetalol levels by a specific high-performance liquid chromography assay. The pharmacokinetics of labetalol are best described by a two-compartment open model with first-order absorption. The half-lives of the absorption, distribution, and elimination phases are 0.6, 1.3, and 8.3 hours, respectively. The steady-state plasma drug concentrations are predictable from the pharmacokinetic data and are in good agreement with the observed values. Steady-state levels are reached by the third day at each dose level studied and increase proportionally with dose.

P-[(Thienylcarbonyl)amino]phenoxy]propanolamines derivatives as diuretic and beta-adrenergic receptor blocking agents.

The synthesis of [[(thienylcarbonyl)amino]phenoxy]propanolamines and their beta-adrenergic blocking and diuretic activity are described. Structure-activity relationships demonstrated that ortho substitution of the phenoxy ring with an hydrogen or an ester function leads to compounds possessing both activities. Ethyl 2-[3-[(1,1-dimethylethyl) amino]-2-hydroxypropoxy]-5-[(2-thienylcarbonyl)amino]benzoate (3d) was selected as the most active compound for further investigation.

An endocrine experimental model for myofibrillar disarray as found in hypertrophic cardiomyopathy

The various etiologic suggestions for hypertrophic cardiomyopathy in man have been reviewed and experimental studies for endogenous pathways have been investigated experimentally. Intramuscular administration of triac to adult rats resulted in severe myocardial hypertrophy but no disarray. When studying the effect of triac on the myocardium of developing rats profound changes of disarray as well as hypertrophy were produced, mimicking the ultrastructural changes of hypertrophic cardiomyopathy in man. By using a variety of compounds with beta-adrenergic blocking action or predominantly membrane stabilizing properties or agonist action together with triac, the site where triac exerts its effect has been shown to be the cell membranes. A mechanisms for production of cellular disarray has been delineated. Extrapolating to man, these experiments lend support to the suggestion that an endogenous hormonal mechanism may be operative in some patients with hypertrophic cardiomyopathy.

Effects of S-596, a new beta-adrenoceptor blocking agent on the left ventricular performance of normal subjects during exercise.

The effects of d1-2-(3'-t-butylamino-2'-hydroxypropylthio)-4-(5'-carbamoyl-2'-thi enyl) thiazole hydrochloride (S-596) were evaluated in 9 normal volunteers. Exercise echocardiography was performed in the semi-supine position before and 2, 4 and 24 hours after the oral administration of 15 mg of S-596. Two hours after administration, resting heart rate was unchanged, compared with control, but systolic blood pressure at rest was slightly decreased (114 +/- 6 vs 106 +/- 10 mmHg, p less than 0.05). Left ventricular dimensions were unchanged, but shortening fraction was increased in the resting state (30.3 +/- 4.6 vs 33.1 +/- 4.8%, p less than 0.05). During exercise on oral S-596, heart rate and systolic blood pressure responses were reduced (127 +/- 11 in control vs 108 +/- 6 beats/min on S-596, 198 +/- 25 vs 168 +/- 23 mmHg, p less than 0.001 and 0.05, respectively). Left ventricular end-diastolic dimension was not significantly altered by S-596 compared with the preceding control exercise test; however, end-systolic dimension was significantly larger after beta blockade with S-596 (2.6 +/- 0.4 vs 3.0 +/- 0.4 cm, p less than 0.05). Shortening fraction and cardiac output decreased significantly (43.7 +/- 3.7 vs 40.0 +/- 5.7%, 9.5 +/- 1.4 vs 8.4 +/- 1.3 L/min, p less than 0.05 and 0.01, respectively). These effects of S-596 were maximal at 2 hours after oral administration with reduced response of heart rate to exercise lasting for 24 hours. Compared with the effects of propranolol (30 mg, given orally) in the same subjects, beta-adrenergic blockade during exercise with S-596 was equivalent to or greater than that of propranolol. Thus, S-596 has little, if any, effect on resting left ventricular performance, but demonstrates potent negative chronotropic and inotropic effects during exercise in normal human subjects. Its beta-adrenergic blocking action also has long acting properties, especially its chronotropic effect.