Nonsteroidal cardiotonics. 2. The inotropic activity of linear, tricyclic 5-6-5 fused heterocycles

We previously reported the structure-activity relationships (SAR) of adibendan (1), a potent and long-acting cardiotonic. This paper describes the synthesis of a novel series of linear, tricyclic fused heterocycles of the 5-6-5 type. The compounds were evaluated for positive inotropic activity in anesthetized rats, cats, and dogs. Changes in left ventricular dP/dt were measured as an index of cardiac contractility. The increase in contractility was not mediated via stimulation of beta-adrenergic receptors. The data revealed the intrinsic positive inotropic activity of the parent compound of this series, 5,7-dihydro-7,7-dimethylpyrrolo[2,3-f]benzimidazol-6(1H)-one (2). The structural features that impart optimal inotropic activity are presented and compared with those of the 4,5-dihydro-3(2H)-pyridazinone series. The most potent compounds were evaluated orally in conscious dogs with implanted Konigsberg pressure transducers to measure ventricular pressures, and their effect on left ventricular dP/dt was compared with that of 1, pimobendan, and indolidan. After administration of 1 mg/kg, 1, 3, 7, 19, 22, 24, 31, 54, pimobendan, and indolidan were equipotent, but only with 1, 31, pimobendan, and indolidan, durations of action exceeded 6 h.     

Cardiotonic agents. Synthesis and cardiovascular properties of novel 2-arylbenzimidazoles and azabenzimidazoles.

Novel 2-arylbenzimidazoles and azabenzimidazoles were synthesized, and their inotropic action was evaluated. Changes in left ventricular pressure, dP/dt max, were measured as an index of cardiac contractility. The structural features that impart optimal inotropic activity are presented. The most potent compounds were evaluated orally in conscious dogs with implanted Konigsberg pressure transducers. To investigate the mechanism of action, the most potent compounds were tested for their calcium-sensitizing properties and their potential for the inhibition of phosphodiesterase. Two compounds, 1 and 41, showed interesting in vitro and oral activity without side effects. They have a more potent calcium-sensitizing effect than MCI-154 and are under further investigation.     

Relative inotropic and chronotropic activity of beta-adrenoceptor stimulants in anaesthetised, areflexic dogs

The use of different methods of measuring contractility and the effects of cardiovascular reflexes are among the factors which complicate the assessment of selective inotropic activity of beta-adrenoceptor agonists. The effects of dobutamine, prenalterol, noradrenaline and isoprenaline on heart rate, iliac blood flow, left ventricular pressure, max dP/dt and (dP/dt) divided by IIT (integrated isometric tension) were evaluated in anaesthetised dogs in which the hearts were denervated and blood pressure held constant. All the drugs caused dose-dependent increases in heart rate and contractility. The relative chronotropic and inotropic activity of each agonist was evaluated. At most doses studied the agonists exerted similar chronotropic and inotropic activity when compared to the non-selective agonist isoprenaline. It is likely that the inotropic selectivity observed with prenalterol and dobutamine in previous studies depends on factors other than direct drug action.     

The sinus node inhibitor UL-FS 49 lacks significant inotropic effect.

UL-FS 49 is a sinus node inhibitor that has been reported to reduce heart rate and may be useful in improving myocardial oxygen supply vs. demand. However, previous studies performed in a variety of preparations have produced mixed results regarding the independent inotropic effect of UL-FS 49. To determine whether UL-FS 49 has an inotropic effect, we measured both steady-state hemodynamic responses and transient hemodynamic responses to random preload and afterload changes, both with and without UL-FS 49. We found that under steady-state conditions, the effect of UL-FS 49 is so small that it would be of doubtful physiologic significance: a 3% increase in stroke volume (p = 0.007) and 7% increase in peak positive dP/dt (p = 0.051), in the presence of no statistically significant differences in end-diastolic pressure, end-diastolic volume, peak systolic pressure, end-systolic pressure, or heart rate. The more powerful multiple linear regression modeling of hemodynamic transient sequences resulting from random preload and afterload changes showed that UL-FS 49 is without a statistically significant direct effect on left ventricular function. We conclude that UL-FS 49 has no physiologically important direct effect on left ventricular pump function.     

Cardiac electrophysiologic and inotropic actions of new and potent methanesulfonanilide class III antiarrhythmic agents in anesthetized dogs.

The effects of cumulative intravenous (i.v.) administration of potent and selective methanesulfonanilide class III antiarrhythmic agents on cardiac electrophysiologic and hemodynamic parameters were compared with those of D-sotalol in chloralose-anesthetized dogs. The new class III agents tested were E-4031 [1-(2-(6-methyl-2-pyridyl)ethyl)-(4-methanesulfonamidobenzoyl)pipe ridine]; UK-66,914 [N-(4-(1-hydroxy-2-(4-(4-pyridinyl)-1-piperazinyl)ethyl)phenyl) methanesulfonamide], and UK-68,798 [1-(4-methanesulfonamidophenoxy)-2-(N- (4-methanesulfonamidophenethyl)-N-methylamino)ethane]. The class III agents produced significant and dose-dependent increases in ventricular refractoriness, with effective doses required to increase ventricular relative refractory period 20 ms above baseline (ED20, micrograms/kg i.v., with 95% confidence limits) of 5.2 (4.2-6.6) for UK-68,798, 17 (13-23) for E-4031, 75 (58-99) for UK-66,914, and 3,700 (2,600-5,800) for D-sotalol. Significant increases in the electrocardiographic QT and QTc intervals paralleled the increases in ventricular refractoriness for the four class III agents. Significant increases in left ventricular (LV) + dP/dt also paralleled increases in ventricular refractoriness and QT intervals for E-4031 (10-1,000 micrograms/kg i.v.), UK-66,914 (100-1,000 micrograms/kg i.v.), and UK-68,798 (30-1,000 micrograms/kg i.v.), but not for D-sotalol. No concomitant alterations in LV-dP/dt were observed for the new and potent methanesulfonanilide class III agents, resulting in significant increases in the ratio of LV + dP/dt/-dP/dt for E-4031, UK-66,914, and UK-68,798. Potent and selective methanesulfonanilide class III agents therefore may augment cardiac contractility in addition to prolonging ventricular refractoriness.     

Comparative effects of inotropic agents on coronary and systemic hemodynamics of conscious dogs: actions of milrinone, dopamine, ouabain and MCI-154

The hemodynamic actions of a new inotropic agent, MCI-154, were compared to dopamine, ouabain and milrinone in conscious, chronically instrumented dogs. MCI-154 and milrinone produced similar hemodynamic changes: increases in heart rate, diastolic coronary blood flow velocity and peak positive dP/dt. Neither agent had significant effects on arterial pressure while both drugs reduced left ventricular end-diastolic pressure in a dose-related fashion and myocardial segment length, indicating a decrease in diastolic left-ventricular size. MCI-154 was found to be approximately twice as potent as milrinone. In contrast, dopamine and ouabain produced little change in left ventricular end-diastolic pressure or myocardial segment length during diastole, while both drugs produced increases in arterial and left ventricular systolic pressures. An increase in left ventricular afterload was not observed with either MCI-154 or milrinone, highlighting an important advantage of the latter compounds.     

Negative inotropic and lusitropic effects of intravenous amiodarone in conscious rats

1. The acute effect of amiodarone on haemodynamics (mean arterial pressure and heart rate) and ventricular function (+dP/dt(max) and -dP/dt(max)) was investigated in conscious rats. In addition, the effects of amiodarone on dobutamine stress were determined. 2. Catheters were inserted in rats into the left ventricle and femoral artery and vein. Three groups of rats received 25 or 50 mg/kg, i.v., amiodarone or vehicle (a 1:1:8 mixture of Tween 80:99.5% ethanol:distilled water), followed by dobutamine (10 microg/kg). 3. The hypotensive effect of 50 mg/kg amiodarone was combined with marked bradycardia and attenuation of +dP/dt(max) and -dP/dt(max). A slight, but significant, hypotension was caused by 25 mg/kg amiodarone, without affecting heart rate, +dP/dt(max) and -dP/dt(max). However, although both doses of amiodarone attenuated the tachycardia caused by dobutamine, neither 25 nor 50 mg/kg amiodarone affected the increase in mean arterial pressure or the enhanced response of +dP/dt(max) and -dP/dt(max). 4. In conclusion, amiodarone caused hypotension, bradycardia, negative inotropic (+dP/dt(max)) and lusitropic (-dP/dt(max)) effects in conscious rats. In addition, amiodarone attenuated the tachycardia without affecting the hypertensive, contractile (+dP/dt(max)) and lusitropic (-dP/dt(max)) responses to dobutamine stress.     

Positive inotropic effects of histamine in anaesthetized dogs.

1 The cardiovascular effects of histamine were examined in dogs anaesthetized with pentobarbitone 2 The effect of histamine on heart rate, blood pressure, left ventricular pressure, dP/dtmax and dP/dt: IIT (integrated isometric tension) was compared in the presence and absence of autonomic reflexes and blood pressure control. 3 In innervated animals with no attempt to control blood pressure, histamine produced dose-dependent decreases in blood pressure and heart rate and either positive or negative inotropic actions. 4 When autonomic reflexes were abolished, this variability in inotropic response was reduced and histamine produced a slight positive inotropic response. There was a decrease in blood pressure and a positive chronotropic response to histamine. 5 When blood pressure was controlled and the cardiac nerves were intact, histamine produced a decrease in heart rate. However, in the denervated animals, there was a slight increase in heart rate. 6 Inotropic responses to histamine in the blood pressure controlled groups were less variable than when blood pressure was uncontrolled. In all of these animals there was an increase in contractility, the increase being more marked in the denervated group. 7 The H2-receptor agonist impromidine produced a positive inotropic action in intact animals with uncontrolled blood pressure.     

Activation of peripheral chemoreceptors causes positive inotropic effects in a working heart-brainstem preparation of the rat

1. The aim of the present study was to evaluate the effects of peripheral chemoreceptor activation on myocardial contractility in an anaesthetic-free decerebrated rat preparation. 2. In the decerebrated and retrogradely perfused working heart-brainstem preparation, we recorded phrenic nerve activity, left ventricular (LV) pressure (microtip Millar catheter), LV dP/dT, heart rate and aortic perfusion pressure before and after activating peripheral chemoreceptors with bolus intra-arterial injections of KCN. 3. Without cardiac pacing, chemoreflex activation caused falls in heart rate (-108 +/- 21 b.p.m.) and complex polyphasic changes in LV pressure and LV dP/dT. If the heart was paced, chemoreflex activation caused significant rises in LP pressure (+16 +/- 3 mmHg) and LV dP/dt (+778 +/- 93 mmHg/s). These positive inotropic effects were significantly and substantially attenuated by beta-adrenoceptor blockade with atenolol. In all instances, chemoreflex activation elicited potent tachypnoeic responses. 4. In conclusion, activation of peripheral chemoreceptors in non-anaesthetized rats evokes a positive inotropic response that is sympathetically mediated. This observation may be relevant for the evaluation of neurally induced effects of acute hypoxia on the ventricular myocardium.     

Hemodynamic effects of a new inotropic compound,PST-2744, in dogs with chronic ischemic heart failure

Inotropic agents for acute decompensated heart failure are associated with a lack of efficacy or increased mortality. New compounds are needed to support patients with acute exacerbations of heart failure. This study examined the hemodynamic effects of a new inotropic agent (PST-2744) in dogs with chronic ischemic heart failure. Eight mongrel dogs at low risk for postmyocardial infarction (MI) sudden death entered the protocol. Dogs were studied after ischemic left ventricular dysfunction was induced by repeated injections of latex microspheres into the circumflex artery until the ejection fraction reached 35%. Hemodynamic parameters were measured at baseline and peak drug effect (PST-2744 5 microg.kg-1.min-1). In 5 animals, PST-2744 effects were compared with dobutamine. Heart rates, PR intervals and QT intervals were unchanged following PST-2744 administration. PST-2744 increased contractility (+dP/dt) by 56% from 1881 +/- 282 mm Hg/s to 2939 +/- 734 mm Hg/s (P < 0.01). The inotropic effect of PST-2744 was equal to that produced by 5-microg.kg-1.min-1 dobutamine (56% increase in +dP/dt), but peak heart rates were significantly higher with dobutamine (129 +/- 24 bpm PST-2744 versus 160 +/- 6 bpm 5-microg.kg-1.min-1 dobutamine, P < 0.002). No arrhythmias or conduction delays were seen with either compound. PST-2744 is an effective inotropic agent without positive chronotropic effect in subjects with stable moderate left ventricular dysfunction.     

Hemodynamic and myocardial energetic effects of CK-3197, a selective positive inotropic agent.

CK-3197 was developed as a selective positive inotropic agent for the treatment of congestive heart failure. We compared the hemodynamic and myocardial energetic effects of CK-3197 to ouabain in the pentobarbital-anesthetized dog. Fifteen minutes after intravenous (i.v.) administration of CK-3197 (0.1, 0.3, and 1.0 mg/kg) to five dogs, mean left ventricular (LV) dP/dt increased by 24, 68, and 109% and mean arterial pressure (MAP) decreased by 4, 9, and 18%, respectively, from basal values. CK-3197 was 11 times more potent as a positive inotropic agent than as a vasodilator. Heart rate (HR) increased by 5, 14, and 24% after these doses of CK-3197, whereas LV end diastolic pressure (LVEDP) decreased by 4 mm Hg after the highest dose of compound. LV oxygen consumption (MVO2) and stroke MVO2 increased by 9, 25, and 102% and 1, 8, and 58%, respectively, at the peak of the increases in LV dP/dt. Ouabain (0.02 and 0.03 mg/kg, i.v.) increased MAP (12 and 22%), HR (2 and 20%), and LV dP/dt (19 and 36%), with a 14 and 16% increase in LV MVO2 and a 12 and -6% change in stroke MVO2. Thus, CK-3197 is a selective, positive inotropic agent with preload reducing activity in the dog. CK-3197, similar to ouabain, produced energy-efficient positive inotropic responses with either no increase in MVO2 or increases in myocardial oxygen consumption that were less than the expected 1:1 ratio with LV dP/dt. Therefore, CK-3197 may have significant utility in the clinical treatment of congestive heart failure.     

Celiprolol: a positive inotropic beta-adrenoceptor blocking agent in conscious dogs.

1. beta-Adrenoceptor blocking agents are used to manage various cardiovascular disorders. A limiting factor in their use is the suppression of the cardiac contractile state. In our study, we examined the cardiac effects of celiprolol, a new beta-adrenoceptor blocking agent with reported positive inotropic effects. 2. Dogs were instrumented by use of sterile surgical techniques for the study of myocardial inotropic state, heart rate and internal left ventricular dimensions. Following complete recovery from surgery, experiments were conducted in the conscious state. 3. Intravenous injection of celiprolol (3 mg kg-1) in nine dogs, increased LV dP/dt by 13 +/- 2.6%, velocity of shortening (LV dD/dt) by 9.2 +/- 3.4%, and heart rate by 19 +/- 4.6% and decreased LV end-diastolic diameter by 1.8 +/- 0.8%, all significantly (P less than 0.05). Celiprolol blocked the inotropic actions of isoprenaline (0.5 micrograms kg-1) but only partially reduced its hypotensive effects. Propranolol, in contrast, reduced LV dP/dt by 17 +/- 3.3% and heart rate by 8.1 +/- 2.7% (P less than 0.05) while totally abolishing the hypotension, tachycardia and increase in LV dP/dt caused by isoprenaline. Following beta-adrenoceptor blockade with propranolol and with heart rate held constant by electrical pacing, celiprolol increased LV dP/dt by 16 +/- 4.0%, LV dD/dt by 12 +/- 3.0% and reduced LV end-diastolic diameter by 3.5 +/- 0.5% (P less than 0.05). 4. Thus, in conscious dogs, celiprolol increases inotropic state and reduces preload independently of beta 1-adrenoceptor mechanisms and the Bowditch phenomenon, while effectively blocking beta 1-receptors in the heart. These properties would make celiprolol useful in patients where a conventional beta-adrenoceptor blocking agent might lead to pump failure.     

The positive inotropic effect of glucagon in the chronically failed right ventricle as demonstrated in the isolated cat heart.

Previous in vitro studies had provided evidence to show that papillary muscles obtained from cats with chronic right ventricular failure had lost their ability to develop a positive inotropic response to glucagon. Since it is difficult to extrapolate from the isolated papillary muscle to the intact heart, studies were done to assess the effects of glucagon in the perfused isovolumically beating heart obtained from cats four months after surgical banding of the pulmonary artery for the experimental production of chronic right ventricular failure (CRVF). At the peak of the dose-response curve, glucagon increased right ventricular isovolumic pressure 25% (39.00 +/- 4.37 to 49.67 +/- 5.15 mm Hg; p less than 0.001) and right ventricular dP/dt 63% (522.2 +/- 93.9 to 852.6 +/- 159.9 mm Hg/sec; p less than 0.001) in 6 normal hearts. Similar dose related increases in right ventricular isovolumic pressure and dP/dt were obtained in 6 hearts taken from cats with chronic right ventricular failure. The respective increases in right ventricular isovolumic pressure and dP/dt were 43% (30.33 +/- 4.01 to 43.67 +/- 6.25 mm Hg; p less than 0.025) and 73% (317.50 +/- 30.29 to 550.83 +/- 89.04 mm Hg/sec; p less than 0.025). These results provide evidence that glucagon possesses the capacity to augment myocardial contractility in the heart with experimentally induced chronic right ventricular failure.     

Inotropic selectivity of salbutamol and terbutaline in anaesthetised, areflexic dogs

The cardiovascular effects of the selective beta 2-adrenoceptor agonists salbutamol and terbutaline have been evaluated in anaesthetised, areflexic dogs. The preparation was designed to reduce the effects of changes in cardiac function mediated via reflex responses to changes in blood pressure. The effects of the selective beta 2-adrenoceptor agonists on heart rate, hindlimb blood flow, left ventricular pressure, max dP/dt and (dP/dt)/IIT (integrated isometric tension) were compared to those of isoprenaline, while blood pressure was held constant. All three drugs produced dose-dependent increases in heart rate, myocardial contractility and iliac blood flow. When equiactive inotropic doses of isoprenaline and salbutamol were compared, salbutamol produced a significantly lower chronotropic effect. A similar inotropic selectivity was found when terbutaline was compared to isoprenaline. beta 2-Adrenoceptor blockade abolished this selectivity. It is concluded that, in the absence of autonomic reflex activity, the beta 2-selective adrenoceptor agonists are relatively selective inotropic stimulants.     

Paradoxical inotropic effects of clonidine and labetalol in the conscious rabbit

Both clonidine and labetalol when given by bolus intravenous injection into conscious rabbits produce an initial rise in left ventricular pressure associated with a decrease in myocardial contractility as assessed by left ventricular dP/dt. While clonidine also produces a marked bradycardiac effect, labetalol causes no change in heart rate. Acute beta-adrenoceptor blockade with propranolol, 1 mg/kg, i.v., does not significantly modify the response produced by either clonidine or labetalol. Following cardiac cholinergic blockade with scopolamine methylbromide, 50 microgram/kg, the negative inotropic effect produced by clonidine is abolished and is replaced by a positive inotropic effect, while the negative inotropic response produced by labetalol is attenuated. Following cardiac autonomic blockade with propranolol and scopolamine methyl-bromide, both clonidine and labetalol produce a positive inotropic response. These positive inotropic responses may be due to the partial alpha-adrenoceptor-agonist properties which both drugs possess and which could be unmasked by cardiac autonomic blockade.     

A comparative study of the cardiovascular and biochemical actions of the imidazo [4,5b] pyridine sulmazole and an imidazo [4,5c] pyridine analogue, BW A746C.

1. BW A746C is a chemical analogue of the imidazo [4,5b] pyridine, sulmazole (AR-L115 BS). Like sulmazole, BW A746C possesses positive inotropic and vasodilator activity in vivo. 2. In anaesthetized guinea-pigs, dogs and primates, a bolus i.v. injection of BW A746C, (0.001-1.0 mg kg-1) caused a significant, dose-related increase in ventricular dP/dt, and reduction in diastolic blood pressure, with small increases in heart rate. In these species, a significantly higher dose of BW A746C was required to lower blood pressure by 30% from basal, than was needed to raise ventricular dP/dt by 50% over basal. 3. In anaesthetized guinea-pigs and dogs, bolus i.v. injections of sulmazole (0.1-10.0 mg kg-1) caused similar effects to those observed with BW A746C. In these species, however, there was no significant difference between the dose of sulmazole required to lower blood pressure by 30% from basal and that required to raise ventricular dP/dt by 50%. 4. In conscious dogs, i.v. infusion of BW A746C (to a total dose of 0.3 mg kg-1) caused a significant increase in ventricular dP/dt, but no significant change in either diastolic blood pressure or heart rate. 5. In cell-free biochemical assays, there were no clear differences between the observed activities of BW A746C and sulmazole. Both compounds are cyclic nucleotide phosphodiesterase inhibitors with similar potencies and selectivities for the Type III enzyme (IC50 BW A746C = 3.0 +/- 0.5 X 10(-5) M, sulmazole 5.0 +/- 1.9 X 10(-5) M). The compounds had little or no effects on sarcolemmal Na+/K+-ATPase, Ca2+ ATPase or Na+/Ca2+ exchange, and sulmazole, but not BW A746C, had a small, stimulatory effect on myofibrillar ATPase. 6. In anaesthetized guinea-pigs and dogs, BW A746C was significantly more potent as a positive inotrope than sulmazole. In contrast with sulmazole, BW A746C produced its inotropic effects at significantly lower doses than those required to reduce diastolic blood pressure. This was also apparent from the results obtained in the anaesthetised primates and the conscious dogs. It was therefore concluded that the inotropic/vasodilator profile of BW A746C favours its positive inotrope activity. This profile cannot be explained on the basis of any biochemical differences from sulmazole.     

Cardiovascular actions of prostaglandin C in the cat and dog

1 Prostaglandin C(2) causes a prolonged fall in arterial blood pressure in the cat.2 At constant heart rate this fall in arterial pressure is accompanied by falls in stroke volume, left ventricular end diastolic pressure, and left ventricular dP/dt max.3 If mean aortic pressure and left ventricular end diastolic pressure are held constant as well as heart rate, prostaglandins C(2) and E(2) do not affect dP/dt max in the cat.4 In the dog, under similarly controlled conditions, prostaglandins C(2) and E(2) raise dP/dt max.5 We conclude that prostaglandins E(2) and C(2) have no direct inotropic action in the cat, but both have a direct positive inotropic action in the dog.     

The negative inotropic effect of beta3-adrenoceptor stimulation in the beating guinea pig heart

Although beta3-adrenoceptors (ARs) have been extensively characterized in brown and white adipocytes, their actions in the beating heart are unclear. We examined the effects of a beta3-AR agonist, BRL37344, on cardiac function and calcium transients in Langendorff-perfused guinea pig hearts by simultaneously measuring left ventricular (LV) pressure and Ca2+-dependent indo-1 fluorescence. BRL37344 induced a dose-dependent negative inotropic effect at concentrations from 10(-11) to 10(-8) M. Maximally, LV developed pressure decreased to 80+/-2%, +dP/dt to 81+/-2%. and -dP/dt to 81+/-3% of their respective control values (p < 0.01). The amplitude of the Ca2+ transient also decreased (to 92+/-3% of the control level; p < 0.01). The BRL37344 dose-response curve was not altered by nadolol (10(-5) M), a potent beta1- and beta2-AR antagonist, but completely suppressed by bupranolol (10(-6) M), a potent beta1-, beta2- and beta3-AR antagonist. To assess the potential role of a nitric oxide synthase (NOS) pathway, we determined whether the NOS inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), modified the contractile response to BRL37344. L-NAME (10(-7) and 10(-4) M) attenuated the negative inotropic effects on LV developed pressure by 35 and 50%, suggesting that beta3-AR stimulation induces a negative inotropic effect on guinea pig hearts partly through a decrease in the Ca2+ transient and partly by the NOS pathway.     

Interaction between negative inotropic effects of halothane and nifedipine in the isolated rat heart

Isobolographic analysis was used to assess quantitatively the combined direct negative inotropic effect of halothane and nifedipine as well as to define the type of interaction between these two agents. Experiments were performed on an isolated rat heart preparation. Developed left ventricular pressure and dP/dtmax were used as indices of inotropic action. It was found that the combination of halothane and nifedipine gives mostly additive direct negative inotropic effect. However, with a small level of myocardial depression (ED25), there was some deviation from the additive interaction toward an infra-additive effect (p less than 0.01), and with a high level of depression (ED75), some deviation toward a supra-additive effect (p less than 0.05).