Increase In Cardiac Contractility Research Articles

Diazepam potentiates the positive inotropic effect of isoprenaline in rat ventricle strips: role of cyclic AMP

The responses of the electrically driven right ventricle strip of the rat heart to isoprenaline and other cyclic AMP-related inotropic agents were recorded in the absence and in the presence of diazepam. Isoprenaline, in concentrations ranging from 10 nM to 1 μM, significantly increased, in a concentration-dependent manner, the contractile force in this preparation. Diazepam (10 μM) produced a leftward shift in the isoprenaline concentration-response curve and significantly reduced its EC 50. Higher concentrations of diazepam (100 μM) produced no further shift, but reduced the maximum of the concentration-response curve of isoprenaline. Forskolin (0.5–10 μM), which directly stimulates adenyl cyclase, also produced a concentration-dependent increase in cardiac contractility. Diazepam (10 μM) displaced to the left the concentration-response curve for forskolin and reduced its EC 50. The cyclic AMP analogous dibutyryl cyclic AMP (0.1–1 mM) produced concentration-dependent positive inotropic effects which were not significantly modified in the presence of diazepam (10 μM). Diazepam (10 μM) significantly enhanced the cyclic AMP production induced by isoprenaline (0.1 μM) and forskolin (10 μM) by about 136% and 35% respectively. These results indicate that diazepam potentiates the positive inotropic effect induced by β-adrenoceptor agonists, probably by increasing cyclic AMP production induced by these agents.

Cardiovascular response to acute and chronic endotoxemia in an awake, volume-resuscitated, canine model.

The cardiovascular response to endotoxemia was evaluated in an awake, intravascular volume-resuscitated canine model. The animals were chronically instrumented for ultrasonic crystal dimension analysis and pressure measurements of the left ventricle (LV), aorta, right atrium (RA), and pulmonary artery (PA) and for cardiac output (CO) measurement. Lipopolysaccharide (Escherichia coli 011:B4) (LPS) was administered intravenously either as an acute, high dose bolus (5 mg/kg; n = 5), a high dose bolus after complete beta-blockade with propranolol (n = 3), or a chronic, low dose infusion (5 micrograms/kg/h; n = 7). Relative to baseline values, cardiac contractility was increased after acute high dose LPS bolus, however this effect was negated by beta-blockade. Chronic, low dose LPS infusion produced an increase in cardiac contractility at 1 h, a return to baseline by 4 h, and maximal contractile depression by 24 h. No change was seen in LV compliance after the high dose LPS bolus. The LV end diastolic volume was decreased by the high dose LPS bolus. This change was blocked by propranolol administration. Chronic LPS administration was accompanied by a decrease in LV compliance and an increase in LV end diastolic volume. Other cardiovascular indices (heart rate, CO, systemic vascular resistance) changed in a fashion similar to human sepsis. These findings confirm that endotoxemia in conscious canine subjects causes changes in cardiovascular function similar to that seen in human and animal models of sepsis. This study also allows us to explain some of the discrepancies between earlier studies of human sepsis and animal models in which the appropriate clinical conditions and an intact neuro-endocrine axis were not maintained.

Angiotensin II induced alteration of cyclic adenosine 3',5'-monophosphate generation in the hypertrophic myocardium of Dahl salt-sensitive rat on a high-salt diet.

The objective of this study was to explore the action of angiotensin II (AII) on cardiac contractility and cyclic AMP (cAMP) generation by forskolin and isoproterenol in the hypertrophic myocardium of the salt-sensitive Dahl rat. Inbred Dahl S and Dahl R rats that had been on a diet supplemented with 6% NaCl were studied. The functional effects of the interaction of AII and forskolin on cardiac contractility were assessed in the isolated heart preparation. The effect on the cAMP signal transduction pathway was assessed in cardiomyocytes isolated from hearts of Dahl S and R rats. Dahl S rats developed cardiac hypertrophy on a high-salt diet, whereas Dahl R rats did not. Forskolin increased cardiac contractility, which was differently affected by AII, depending on whether the heart was from hypertrophied Dahl S rat or from the control Dahl R rat. AII accentuated forskolin-induced increases in cardiac contractility in hypertrophic hearts but diminished forskolin-induced increases in contractility in the nonhypertrophied hearts. This response was reflected in the cAMP response to forskolin, in that AII decreased forskolin-induced increases in cAMP in the nonhypertrophic heart. AII had the reverse effect in cardiomyocytes from hypertrophied hearts, as AII increased forskolin-induced cAMP production. This was shown to be due to an AII receptor mediated effect, as it was antagonized by the AII receptor antagonist saralasin. The same effects of AII were found on isoproterenol-induced increases in cAMP. Similar results occurred in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), suggesting that the effect was on cAMP production rather than cAMP degradation. This was attributed to an inhibitory G protein (Gi) mechanism, as the muscarinic agonist carbachol, which acts through Gi, produced the same effects as AII. Furthermore, the effects of AII were abolished by pertussis toxin, which inactivates Gi. These data indicate a reversal of control by AII in the hypertrophic Dahl S heart in response to forskolin, which activates adenylyl cyclase directly on the catalytic subunit, converting the substrate, ATP, to cAMP, independent of the guanine nucleotide activated protein, and in response to isoproterenol, which activates adenylyl cyclase through G protein mechanisms. All accentuated the forskolin-induced increase in cardiac contractility and cAMP generation in the hypertrophied ventricle but decreased both contractility and cAMP generation in nonhypertrophied hearts, suggesting that the process of cardiac hypertrophy in salt-sensitive Dahl rat may compensate for the reduction in intracellular cAMP by altering its regulatory control by AII.

Alpha-adrenoceptor modulation of the efferent function of capsaicin-sensitive sensory neurones in guinea-pig isolated atria.

1. Transmural nerve stimulation of guinea-pig atria, obtained from animals pretreated with reserpine (5 mg kg-1, i.p.), in the presence of atropine 1 microM and of the beta-adrenoceptor blocker CGP 20712A 1 microM, induced a positive inotropic effect which was reduced by the calcitonin gene-related peptide (CGRP) antagonist hCGRP-(8-37) and abolished by pretreatment with capsaicin 1 microM. 2. Noradrenaline concentration-dependently (0.01-10 microM) reduced the increase in cardiac contractility induced by transmural nerve stimulation. The inhibitory effect of noradrenaline was antagonized by yohimbine (0.5-1 microM), in a dose-dependent manner. Prazosin (0.5-1 microM) antagonized the effect of noradrenaline and this effect was independent of concentration. 3. In the presence of yohimbine, the lower part of the inhibitory-response curve for noradrenaline was slightly but significantly shifted by prazosin. A similar degree of antagonism was observed in the presence of 1 microM phenoxybenzamine. 4. The selective alpha 2 agonists BHT 920 and clonidine reduced, in the same concentration-range (0.01-1 microM), the cardiac response to transmural nerve stimulation in a yohimbine-sensitive fashion. 5. Phenylephrine (0.1-100 microM) and methoxamine (1-300 microM) also induced an inhibitory effect on transmural nerve stimulation. The effect of phenylephrine was antagonized by yohimbine (1 microM) more efficiently than by prazosin (0.5 microM). 6. These results are in keeping with the presence of inhibitory prejunctional alpha 2-adrenoceptors on cardiac sensory nerve endings which modulate the efferent function of capsaicin-sensitive neurones.

Neurogenic Pulmonary Edema: Treatment with Dobutamine

In the case of a patient with complicating subarachnoid hemorrhage, an infusion of dobutamine was followed by a massive diuresis and regression of severe neurogenic pulmonary edema. It is suggested that the reduction in total peripheral vascular resistance and the increase in cardiac contractility accounts for the observed beneficial effect and indicate that dobutamine is a suitable drug for the treatment of neurogenic pulmonary edema.

A Novel, Orally Active Dopamine Prodrug TA-870. V. Natriuretic and Positive Inotropic Effects in Rats

We investigated the acute natriuretic and positive inotropic effects of the dopamine prodrug TA-870 in rats before and after repeated administration for 2 weeks. Single intraduodenal (i.d.) administration of TA-870 (10-250 mg/kg) to saline-loaded anesthetized rats produced a dose-dependent increase in urinary flow and sodium excretion. It also produced a decrease in renal vascular resistance and an increase in renal blood flow. In another series of normal anesthetized rats, TA-870 caused dose-dependent increases in cardiac contractility [left ventricular dP/dtmax (LV dP/dtmax)] at i.d. doses of 10-250 mg/kg. Although the heart rate was also increased, this effect was much smaller than the effect on LV dP/dtmax. SCH-23390 (0.3 mg/kg i.v.), a selective DA1 dopamine receptor antagonist, strongly inhibited the above diuretic, natriuretic, and renal vasodilatory effects of TA-870. The positive inotropic effect of TA-870 was not inhibited by SCH-23390, but the latter effect was inhibited by pretreatment with propranolol (0.5 mg/kg i.v.). After repeated oral administration of TA-870 to rats (250 mg/kg twice a day for greater than 2 weeks), there was no significant differences in the natriuretic and positive inotropic responses to TA-870 between the TA-870-pretreated and control groups indicating a lack of pharmacological tolerance. In conclusion, TA-870, when administered enterally to rats, produced the natriuretic effect via the DA1 dopamine receptor and positive inotropic effects via the beta 1 adrenergic receptor stimulation, and these effects were not attenuated by chronic treatment with TA-870.

Abnormal cardiovascular reactivity in borderline and mild essential hypertension.

Cardiovascular and hemodynamic reactivity was evaluated with M-mode echocardiography, phonocardiography, and carotidography in correlation with circulating catecholamine levels in 25 normotensive subjects, 15 borderline hypertensive patients, and 42 mildly hypertensive patients during isometric exercise at 30% of the maximum force for 3 minutes. At rest, norepinephrine and epinephrine levels were significantly higher, and the cardiac index was similarly increased in both groups of hypertensive patients, but the cardiac mass index was significantly increased only in the mildly hypertensive group. During isometric exercise, the sympathoadrenal reactivity as well as the pressor and chronotropic responses were similar in normotensive subjects and both groups of hypertensive patients. However, the variations in blood pressure were achieved through totally different hemodynamic mechanisms in normotensive subjects and hypertensive patients. In normotensive subjects, the increase in blood pressure could be linked mainly to an increase in cardiac contractility and performance, whereas in either group of hypertensive patients, the increase in blood pressure was mainly associated with an increase in peripheral resistance. These observations are consistent with the hypothesis of a blunted beta-adrenergic reactivity and a predominance of alpha-adrenergic vascular reactivity in borderline and mildly hypertensive patients. This phenomenon, which appears to be unrelated to age or severity of hypertension, could be an important mechanism underlying the development of hypertension in humans.

Nonopioid effects of beta-casomorphin-5 in guinea pig heart: alterations to the beta-adrenoceptor-G-protein complex and inhibition of myocardial responses to isoproterenol.

The influence of beta-casomorphin-5 on the beta-adrenoceptor complex in guinea pig heart membranes was studied by means of binding studies, G-protein investigations and isolated heart preparations. In nanomolar concentrations beta-CM-5 induced an increase in receptor affinity towards the agonist isoproterenol whereas the antagonist affinity was reduced. The isoproterenol-stimulated increase in cardiac contractility, moreover, is reduced by 10 nM beta-CM-5. Furthermore, beta-CM-5 was found to inhibit the isoproterenol-induced GDP/GTP exchange as well as the [35S]GTP[S] binding to guinea pig heart membranes, indicating an involvement of G-proteins. These findings suggest that low concentrations of beta-CM-5 modulate the functional properties of the myocardial beta-adrenoceptor-G-protein complex, presumably resulting in its desensitization. The observed effects of beta-CM-5 are not prevented by naloxone and, therefore, are nonopioid in nature.

Prejunctional prostanoid receptors on cardiac adrenergic terminals belong to the EP3 subtype.

1. The effects of prostaglandin E2 (PGE2) and of several synthetic prostanoids on the cardiac response to sympathetic nerve stimulation in guinea-pig atria have been evaluated. 2. PGE2 (0.01-100 nM), sulprostone (0.01-100 nM) and misoprostol (0.1-100 nM), but not butaprost (0.1-100 nM), dose-dependently reduced the increase in cardiac contractility induced by electrical field stimulation of sympathetic terminals. 3. The EP1 antagonist AH6809 (1 microM) did not modify the inhibition of cardiac response induced by PGE2, sulprostone and misoprostol. 4. In preparations preloaded with [3H]-noradrenaline, tritium overflow induced by electrical field stimulation was greatly and significantly reduced by 100 nM PGE2 and by 100 nM sulprostone. 5. These results indicate that PGE2 and other synthetic prostanoids reduce noradrenaline release from cardiac adrenergic nerve terminals acting on prejunctional inhibitory receptors belonging to the EP3 subtype.

Influence of adenosine on the stimulatory effect of isoprenaline and insulin on myocardial contractility in vivo

In vitro investigations have indicated that adenosine can inhibit beta adrenergic stimulated increases in cardiac contractility. The present study was designed to determine the ability of adenosine to inhibit isoprenaline induced increases in contractility in vivo. Adenosine has been reported to exert its inhibitory effects on contractility by inhibiting adenylate cyclase. Thus, adenosine should have no effect on positive inotropic agents that act independently of adenylate cyclase. We therefore assessed the ability of this nucleoside to inhibit the positive inotropic effect of insulin, a hormone that exerts a positive inotropic effect independently of alterations in cyclic AMP. Saline or adenosine (10 mumol.ml-1) was infused into the circumflex artery at 1 ml.min-1 as a background. Isoprenaline (20 or 200 pmol.min-1) was infused into the artery during saline or adenosine infusion. The response to insulin was determined during hyperinsulinaemic euglycaemic clamp. 16 adult mongrel dogs were anaesthetised with pentobarbitone. Five dogs were used in isoprenaline studies, and 11 dogs in insulin studies. Dogs were instrumented to obtain measurements of mean arterial blood pressure, heart rate, circumflex artery blood flow (Q), instantaneous left ventricular pressure, and posterior left ventricular wall thickness. We used the slope of the end systolic pressure-dimension relationship (Ees) as an index of myocardial contractility, previously shown to reflect changes in myocardial inotropic state independent of influence from afterload and preload. Left ventricular dP/dtmax was derived from left ventricular pressure with respect to time, and Ees was determined from left ventricular pressure and wall thickness. Neither adenosine, isoprenaline, nor insulin alone caused any significant changes in mean arterial pressure or heart rate. Adenosine caused a significant increase in Q. Both left ventricular dP/dtmax and Ees were significantly increased by either insulin or both doses of isoprenaline. Adenosine inhibited the increases in these indices caused by isoprenaline, but not those caused by insulin. Adenosine is capable of inhibiting the positive inotropic effect of isoprenaline in vivo. The results suggest that adenosine does not inhibit positive inotropic responses that act independently of the stimulation of adenylate cyclase.

Comparison of cardiorenal and emetic effects of dopamine prodrugs docarpamine (TA-870) and levodopa in dogs.

Positive inotropic and renal vasodilatory effects of a novel dopamine prodrug, docarpamine, [N-(N-acetyl-L-methionyl)-3,4-bis(ethoxycarbonyl)dopamine; TA-870] and those of levodopa were compared in anesthetized dogs, and emetic effects of the two drugs were compared in conscious dogs. Intraduodenal administrations of docarpamine and levodopa at 20 mg/kg produced similar increases in cardiac contractility. The maximal increases in the left ventricular dP/dtmax after docarpamine and levodopa were 36.6 +/- 14.2 and 39.2 +/- 12.1%, respectively. The two drugs also produced similar decreases in renal vascular resistance (21.2 +/- 2.2 and 13.6 +/- 3.4%, respectively) and increases in renal blood flow (27.9 +/- 3.2 and 17.9 +/- 5.2%, respectively), however, the duration of renal vasodilation after docarpamine was longer than that of levodopa. Oral administration of the two drugs in conscious dogs produced vomiting. The ED50 value of the emetic effect for docarpamine was greater than 160 mg/kg, and that for levodopa was 11.0 mg/kg. The emesis was inhibited by pretreatment with a DA2 antagonist, domperidone. In conclusion, docarpamine and levodopa produced similar cardiorenal effects, but the emetic effect of docarpamine was much weaker than that of levodopa. Docarpamine can be used as a selective dopamine prodrug for the peripheral circulation.

ChemInform Abstract: 7-Heteroaryl-1,2,3,5-tetrahydroimidazo(2,1-b)quinazolin-2(1H)-one Derivatives with Cardiac Stimulant Activity.

A series of 7-heteroaryl-1,2,3,5-tetrahydroimidazo[2,1-b]quinazolin++ +-2 (1H)-ones was synthesized and evaluated in dogs for cardiac stimulant activity. Compounds were obtained by a palladium-catalyzed cross-coupling reaction between a heteroarylzinc chloride and a 7-iodo-1,2,3,5-tetrahydroimidazo [2,1-b]quinazolin-2(1H)-one or by cyclization of an N-[(2-aminophenyl)methyl]glycinate with cyanogen bromide. Compared to the parent ring system (3), introduction of a 2,6-dimethylpyridin-3-yl (6), 2,4-dimethylimidazol-1-yl (7), or 1,2,4-triazol-1-yl (8) moiety at the 7-position led to a 13-17-fold increase in positive inotropic activity (percentage increase in dP/dtmax) in anesthetized dogs. Potency could be further enhanced with a 9-methyl substituent (10-12). The most potent member of the series, 7-(2,4-dimethylimidazol-1-yl)-9-methyl-1,2,3,5-tetrahydroimidaz o [2,1-b]quinazolin-2(1H)-one (11) (23% increase in dP/dtmax, 2 micrograms/kg), was 80 times more active than 3 and displayed a 5-fold advantage over milrinone. In conscious dogs, 6 elicited marked and sustained positive inotropic activity (decrease in QA interval) after oral administration (1 mg/kg), whereas 10-12 were 10 times more potent. 11 produced an obvious increase in cardiac contractility (20% increase in dP/dtmax) at low dose levels (25 micrograms/kg) while, after 100 micrograms/kg, the marked response (50% increase in dP/dtmax) was maintained for the whole 7-h test period. In these experiments, 11 had no effect on heart rate, and the compound also displayed exceptional selectivity for increasing the force rather than the rate of cardiac contraction (greater than 150% increase in dP/dtmax) in the Starling heart-lung preparation. These studies demonstrate that the tetrahydroimidazoquinazolinone nucleus is an effective bioisostere for the 2(1H)-quinolinone system and that 11 displays improved cardiac stimulant activity and duration of action when compared to milrinone.

Ｃｅｌｉｐｒｏｌｏｌの薬理学的研究 Ｉ β‐遮断作用，内因性交感神経刺激様作用，血管拡張作用並びに降圧作用

The beta-blocking effect, intrinsic sympathomimetic activity (ISA), and vasodilating and hypotensive effects of celiprolol were tested. 1. Celiprolol competitively antagonized the isoproterenol-induced positive chronotropic and inotropic effects in guinea pig right atrial and papillary muscles and isoproterenol-induced guinea pig tracheal relaxation with pA2 values of 8.03, 7.98 and 6.43, respectively. 2. In dogs, isoproterenol-induced increases in cardiac contractility and heart rate were markedly inhibited by celiprolol (83.1 and 84.8%, respectively), while isoproterenol-induced hypotension was suppressed only by 16.8%. 3. Celiprolol increased the beating rate of the right atrium and relaxed the trachea isolated from normal and reserpinized guinea pigs. Celiprolol also potentiated the contractility of the left atrium isolated from reserpinized animals. These effects of celiprolol were antagonized by propranolol. 4. Celiprolol concentration-dependently relaxed rat femoral arteries contracted by methoxamine. The concentration-relaxation curve was shifted to the right by pretreatment with propranolol. 5. Orally administered celiprolol produced long lasting hypotension in conscious SHR without any heart rate change. 6. These results indicate that celiprolol is a highly cardioselective beta-blocker with a significant ISA, which contributes to its vasodilatory and hypotensive effects.

Effects of moderate hypoxia, hypercapnia and acidosis on haemodynamic changes induced by endothelin‐1 in the pithed rat

1. Pithed rats were respired at a fixed rate of 54 cycles min-1 and with a ventilation volume of either 20 (control) or 10 ml kg-1. In these two preparations, the dose-response relationships for the systemic blood pressure responses to endothelin-1, administered i.v., were examined. Also, cardiac output, its distribution, tissue blood flows and vascular resistances were determined at both respiratory volumes in pithed rats given saline or during pressor responses to endothelin-1 (750 ng, i.v.). Finally, a comparison was made of the pressor responses to endothelin-1 in the blood perfused superior mesenteric arterial bed of pithed rats respired at 10 or 20 ml kg-1. 2. In control rats the systemic blood pressure responses to i.v. endothelin-1 were biphasic with an initial, transient (30 s) decrease in blood pressure followed by a well sustained pressor response. These responses were dose-dependent (the ED50 for the pressor response being 0.27 +/- 0.04 micrograms). The pressor effect of endothelin-1 was due to an increase in total peripheral resistance with no change in heart rate or cardiac output. This increased total peripheral resistance was due to vasoconstriction of the spleen, stomach, large intestine, small intestine and the pancreas/mesentery (in which it was most severe). Endothelin-1 also increased blood flow through the heart, lungs, liver, epididimides, fat and skin through redistribution of cardiac output to these vascular beds. 3. At the lower ventilation volume there was moderate acidosis, hypoxia and hypercapnia relative to those rats respired at 20 ml kg-1. With respiration at 10 ml kg-1, the pressor response to endothelin-1 was not sustained and, after oscillations in both blood pressure and heart rate, death occurred 15-20 min after administration. The pressor effect resulted from increases in cardiac output (due to increased stroke volume) and total peripheral resistance: the latter was caused by vasoconstriction in the stomach, small intestine, large intestine and pancreas/mesentery. Endothelin-1 increased blood flow through the heart, lungs, liver, kidneys, testes, fat and skin due to either an increase in cardiac output, redistribution of cardiac output or both. 4. Endothelin-1 induced dose-dependent pressor responses in the mesenteric bed in situ. At the lower ventilation volume the potency of endothelin-1 in this vascular bed was increased approximately two fold with the ED50 being 68 +/- 7 pmol compared to 113 +/- 15 pmol in the rats respired at 20 ml kg-1. 5. This study indicates that, in normoxic control pithed rats, the pressor response to endothelin-1 was due largely to vasoconstriction of the splanchnic vascular bed. In rats with moderate hypoxia, hypercapnia and acidosis, the pressor response was due to vasoconstriction of the gastrointestinal tract as well as an increase in cardiac output. Endothelin-1 induced profound vasoconstriction in the mesenteric bed of the pithed rat both in vivo and in situ. The potency of endothelin-1 on this bed in situ was doubled by lowering the ventilation volume. An increase in cardiac contractility and severe gastrointestinal vasoconstriction may be the initial events leading to the eventual toxic effect of endothelin-1 in the hypoxic pithed rat.

Effects of opioid drugs on capsaicin-sensitive neurones in guinea-pig atria

Transmural nerve stimulation of isolated guinea-pig atria in the presence of atropine induced a biphasic positive inotropic effect but only a slow increase in contractility (NANC response) in atria obtained from 6-hydroxydopamine-pretreated animals. The latter effect disappeared after exposure of the preparations to capsaicin. The effects of some opioid peptides were investigated on NANC responses. [D-Ala 2,D-Leu 5]enkephalin (DADLE) and [D-Ala 2,N-Me-Phe 4 Gly 5-ol]enkephalin (DAGO, 0.1–10 μM) inhibited the cardiac response to transmural nerve stimulation in a dose-dependent and naloxone-sensitive manner. Dynorphin-(1–13) and morphine, at 10-fold higher concentrations (1–10 μM), reduced the response in a naloxone-sensitive manner. Naloxone alone however did not affect the response. Opioid peptides were not able to reduce the positive inotropic effect induced by calcitonin gene-related peptide (CGRP), or the increase in cardiac contractility produced by capsaicin. These results suggest that opioid receptors exert a modulatory role on peripheral terminals of capsaicin-sensitive sensory nerves.

7-Heteroaryl-1,2,3,5-tetrahydroimidazo[2,1-b]quinazolin -2(1H)-one derivatives with cardiac stimulant activity.

A series of 7-heteroaryl-1,2,3,5-tetrahydroimidazo[2,1-b]quinazolin++ +-2 (1H)-ones was synthesized and evaluated in dogs for cardiac stimulant activity. Compounds were obtained by a palladium-catalyzed cross-coupling reaction between a heteroarylzinc chloride and a 7-iodo-1,2,3,5-tetrahydroimidazo [2,1-b]quinazolin-2(1H)-one or by cyclization of an N-[(2-aminophenyl)methyl]glycinate with cyanogen bromide. Compared to the parent ring system (3), introduction of a 2,6-dimethylpyridin-3-yl (6), 2,4-dimethylimidazol-1-yl (7), or 1,2,4-triazol-1-yl (8) moiety at the 7-position led to a 13-17-fold increase in positive inotropic activity (percentage increase in dP/dtmax) in anesthetized dogs. Potency could be further enhanced with a 9-methyl substituent (10-12). The most potent member of the series, 7-(2,4-dimethylimidazol-1-yl)-9-methyl-1,2,3,5-tetrahydroimidaz o [2,1-b]quinazolin-2(1H)-one (11) (23% increase in dP/dtmax, 2 micrograms/kg), was 80 times more active than 3 and displayed a 5-fold advantage over milrinone. In conscious dogs, 6 elicited marked and sustained positive inotropic activity (decrease in QA interval) after oral administration (1 mg/kg), whereas 10-12 were 10 times more potent. 11 produced an obvious increase in cardiac contractility (20% increase in dP/dtmax) at low dose levels (25 micrograms/kg) while, after 100 micrograms/kg, the marked response (50% increase in dP/dtmax) was maintained for the whole 7-h test period. In these experiments, 11 had no effect on heart rate, and the compound also displayed exceptional selectivity for increasing the force rather than the rate of cardiac contraction (greater than 150% increase in dP/dtmax) in the Starling heart-lung preparation. These studies demonstrate that the tetrahydroimidazoquinazolinone nucleus is an effective bioisostere for the 2(1H)-quinolinone system and that 11 displays improved cardiac stimulant activity and duration of action when compared to milrinone.

2(1H)-Quinolinones with cardiac stimulant activity. 3. Synthesis and biological properties of 6-imidazol-1-yl derivatives

A series of 6-imidazol-1-yl-8-methyl-2(1H)-quinolinones was synthesized and evaluated for cardiac stimulant activity in dogs. The majority of compounds were prepared from an appropriate 6-imidazol-1-yl-2(1H)-quinolinone precursor or by sulfuric acid catalyzed cyclization of an N-(4-heteroarylphenyl)-3-ethoxypropenamide. Introduction of a range of 5-substituents into 6-(2,4-dimethylimidazol-1-yl)-8-methyl-2(1H)-quinolinone (1) reduced inotropic activity in anesthetized dogs (percentage increase in dP/dtmax) although replacement of the 2-methyl group by iodo (10) or cyano (11) substituents was well tolerated. The 2-methyl-4-chloro (15) and 2-methyl-4-(methylthio) (22) derivatives displayed similar potency to 1 (40-50% increase in dP/dtmax, 10-12.5 micrograms/kg) and these compounds were 3-5 times more potent than milrinone. Introduction of iodo (14), cyano (16), or acetyl (17) substituents into the 4-position approximately halved inotropic activity. In conscious dogs, 11 (0.25 mg/kg) and 16 and 17 (0.125 mg/kg) produced similar increases in cardiac contractility (decrease in the QA interval) to 1 (0.125 mg/kg) and maximum responses were maintained for at least 3 h. Dose-related (25, 125, 250 micrograms/kg) cardiac stimulant activity was demonstrated by 17 and after the higher doses a marked response (approximately 30% increase in dP/dtmax) was still observed after 7 h, in contrast to milrinone. The substantial increases in cardiac contractility observed with 16 and 17 in the conscious dog were not accompanied by any tachycardia. These compounds also displayed an overwhelming selectivity for increasing the force of cardiac contraction (greater than 120% increase in dP/dtmax) rather than heart rate (5-10 beats/min decrease) in the Starling heart-lung preparation. As a result of this beneficial pharmacological profile, 6-(4-acetyl-2-methylimidazol-1-yl)-8-methyl-2(1H)-quinolinone (17, UK-66,838) was selected for preclinical development studies.

Characterization and pharmacological relevance of high affinity binding sites for [3H]LY186126, a cardiotonic phosphodiesterase inhibitor, in canine cardiac membranes.

[3H]LY186126, an analogue of the cardiotonic agent indolidan, was shown to bind reversibly and with high affinity (Kd = 4 nM) to a single class of binding sites within canine myocardial vesicles. Binding site density measured in various cardiac membrane fractions correlated well with Ca2+-ATPase activity (r = 0.94; p less than 0.01), but not with Na+,K+-ATPase or azide sensitive ATPase, indicating a localization of these sites within sarcoplasmic reticulum membranes. Divalent cations were required for binding and displayed the following order of activation: Zn2+ greater than Mn2+ greater than Mg2+ greater than Ca2+. Differential activation of [3H]LY186126 binding by various divalent cations was due to alterations in binding site density, rather than affinity. cGMP and selective inhibitors of type IV membrane-bound phosphodiesterase (SR-PDE), for example, indolidan, milrinone, imazodan, and enoximone, selectively displaced bound [3H]LY186126 caffeine, theophylline, and rolipram were relatively impotent as inhibitors of radiolabel binding. Kd values from displacement curves were highly correlated with IC50 values for inhibition of SR-PDE (r = 0.92; p less than 0.001). In addition, Kd values correlated well with published ED50 values for increases in cardiac contractility in pentobarbital-anesthetized dogs (r = 0.94; p less than 0.001). The results support the hypothesis that [3H]LY186126 labels the pharmacological receptor for the class of positive inotropic agents characterized as isozyme-selective phosphodiesterase inhibitors. Furthermore, the data suggest that the identity of the site labeled by [3H]LY186126 is SR-PDE, the type IV isozyme of cardiac phosphodiesterase located in the sarcoplasmic reticulum.

2(1H)-Quinolinones with cardiac stimulant activity. 2. Synthesis and biological activities of 6-(N-linked, five-membered heteroaryl) derivatives

A series of 6-(N-linked, five-membered heteroaryl)-2(1H)-quinolinone derivatives was synthesized and evaluated for cardiotonic activity. Most compounds were prepared by sulfuric acid catalyzed cyclization of an N-(4-heteroarylphenyl)-3-ethoxypropenamide or by condensation of a 2-amino-5-heteroarylbenzaldehyde or -acetophenone derivative with the ylide derived from triethyl phosphonoacetate. In anesthetized dogs, 6-imidazol-1-yl-8-methyl-2(1H)-quinolinone (3; 25 micrograms/kg) produced a greater increase in cardiac contractility (percentage increase in dP/dt max) than alternative 6-(five-membered heteroaryl)-substituted analogues (4-8). Introduction of 4-methyl (10) or 2,4-dimethyl (13) substituents into the imidazole ring of 3 produced a marked increase in inotropic activity, and these compounds were some 10 and 5 times more potent than milrinone. Most of these quinolinones also displayed positive inotropic effects (decrease in QA interval) in conscious dogs after oral administration (0.0625-1 mg/kg) and in many cases (3, 5-7, 9, 11, 13, 16) there was little difference in activities at both the 1- and 3-h time points. Compound 13 (62.5, 125, 250 micrograms/kg po) demonstrated dose-related cardiac stimulant activity which, in contrast to milrinone, was maintained over the whole 7-h test period. No changes in heart rate were detected at any dose level and compounds 3, 9, 10, and 13 also displayed high selectivity for the stimulation of cardiac contractile force rather than heart rate in the Starling dog heart-lung preparation. Increases in dP/dt max of approximately 50% were accompanied by heart rate changes of less than 10 beats/minute. Physicochemical measurements gave a log P of 1.64 for 13 with pKa values of 7.13 +/- 0.04 and 11.5 +/- 0.2 for the imidazole and quinolinone moieties, respectively. X-ray structural analysis of 13 showed the imidazole and quinolinone rings at 52 degrees to one another in close agreement with the minimum-energy conformation (30 degrees) suggested by PCILO calculations. 6-(2,4-Dimethylimidazol-1-yl)-8-methyl-2-(1H)-quinolinone (13, UK-61,260) is currently undergoing phase II clinical evaluation in congestive heart failure patients.

Cardiovascular Properties of MS-857, a New and Potent Cardiotonic Agent, on Normal and Failing Hearts

The cardiovascular properties of MS-857 [4-acetyl-1-methyl-7-(4-pyridyl)-5,6,7,8-tetrahydro-3(2H)-isoquinolinone ], a novel cardiotonic agent, were investigated in anesthetized and conscious dogs. MS-857 (1-100 micrograms/kg i.v.) produced a significant and dose-dependent increase in cardiac contractility with relatively small changes in heart rate and blood pressure. This indicates a sizable separation between positive inotropic and other effects of MS-857. Oral administration of MS-857 to conscious dogs (0.1-1 mg/kg) also produced a sustained increase in cardiac contractility in a dose-dependent manner. The total duration of action was longer than 7 h at a dose of 1 mg/kg p.o. There occurred no arrhythmias and no changes in animal behavior. After chronic oral administration, MS-857 completely retained its activities, indicating the lack of tachyphylaxis. In the acute heart failure models induced by either propranolol or pentobarbital, MS-857 reversed the cardiac depressant effects of these drugs. Moreover, MS-857 also significantly improved the pentobarbital-induced heart failure in the heart-lung preparation. MS-857 did not inhibit the Na+, K+-ATPase, but inhibited the phosphodiesterase (PDE) III selectively, both of which were prepared from the dog ventricular muscle. Thus, MS-857 can be characterized as a potent nonsympathomimetic, nonglycoside cardiotonic drug with a selective inhibitory activity on PDE III. The cardiovascular properties revealed by this study strongly suggest that MS-857 will exert a beneficial effect in the treatment of congestive heart failure.