Maximal Inotropic Effect Research Articles

Cardiac glycoside-induced elevation of intracellular Na+ ion concentration in human erythrocytes studied by 23Na NMR spectroscopy: relationship between inotropy speed and elevation rate of intracellular Na+ ion concentration.

Elevation of intracellular sodium ion concentration in human erythrocyte induced by the cardiac glycoside, proscillaridin, and its four derivatives was measured using 23Na NMR spectrometry. In this examination, there was a significant correlation between the time to half maximum inotropic effect and the time to maximum of Na+ concentrations in human erythrocyte, determined by 23Na NMR.

An analysis of the mechanism of the inotropic action of some milrinone analogues in guinea‐pig isolated atria

1. It has been reported previously that the milrinone analogues, ethyl 5-cyano-1,6-dihydro-2-methyl-6-oxo-3 pyridine carboxylate (I) and ethyl 5-cyano-1,6-dihydro-2-ethyl-6-oxo-3 pyridine carboxylate (II) exert a positive inotropic effect (EC50 = 15.6 +/- 0.2 microM and 40.3 +/- 0.1 microM) both on spontaneously beating and on electrically driven atria from reserpine-treated guinea-pigs. In the present study the mechanism of the inotropic action of these two agents was investigated. 2. In electrically driven left atrium from reserpine-treated guinea-pigs the EC50 values for inotropic activity for compounds (I) and (II) corresponded to that of milrinone (EC50 = 25 +/- 0.1 microM) but compound (I) induced a greater maximum effect. This corresponded to a percentage increase in developed tension over control of 63 +/- 0.3 whereas the maximum inotropic effect of milrinone was 48 +/- 0.3 and that of compound (II) was 47 +/- 0.2. 3. The inotropic activity of compounds (I) and (II) (10-100 microM) was resistant to propranolol (0.1 microM), thus excluding the involvement of beta-adrenoceptors. 4. Since the inotropism induced by compounds (I) and (II) was not reduced by carbachol (1 nM-0.5 microM), an action involving changes in adenosine 3':5'-cyclic monophosphate (cyclic AMP) can be excluded. 5. The inotropic action of compounds (I) and (II) was blocked selectively by 8-phenyltheophyline (10 microM) or adenosine deaminase (2 u ml-1). 6. Both (I) and (II) inhibited, in an apparently competitive manner, the negative inotropic effect induced by N6-(L-phenylisopropyl) adenosine (L-PIA), a stable adenosine agonist. The pA2 values for (I) and (II) were 4.79 and 4.36, respectively.7. In rat brain compounds (I) and (II) inhibited the specific binding of N6-cyclohexyl[3H]-adenosine- ([3H]-CHA) with an IC50 of 0.18 + 0.01 mM and 0.25 + 0.02 mm, respectively, which were similar to their IC50 values for blocking the PIA-induced negative inotropic effect and which are also in the range of concentrations that are effective in inducing positive inotropism in guinea-pig atria.8. The results from the present study suggest that antagonism of endogenous purines causes positive inotropism without affecting intracellular cyclic AMP levels.

Functional link between myocardial α1‐ and β1‐adrenoceptors in neonatal rabbits

AbstractOntogeny of inotropic responses of the left atria and chronotropic responses of the right atria of rabbits was studied in vitro. The maximal inotropic effect (efficacy) of isoproterenol on atria from neonatal (1.5 weeks old) rabbits was significantly less than that of norepinephrine and epinephrine. The inotropic efficacy of isoproterenol increased with age and reached adult levels at 6 weeks. There was no difference in the potency and efficacy of inotropic effects of epinephrine and norepinephrine on neonatal and adult (∼40 weeks old) rabbit atria. At threshold concentrations, phenylephrine synergistically potentiated the inotropic effects of isoproterenol on atria from neonatal but not on atria from adult rabbits. The potentiation of the inotropic effects of isoproterenol by phenylephrine on neonatal rabbit atria was abolished by prazosin. There was no difference in the chronotropic effects of isoproterenol on atria from neonatal and adult rabbits. It is speculated that α1‐ and β1‐myocardial adrenoceptors are functionally linked in early life and a full expression of inotropic responses requires a concomitant activation of both receptor types.

Species differences in the negative inotropic effect of acetylcholine and soman in rat, guinea pig, and rabbit hearts

1. Acetylcholine reduced atrial contractions by 82.5% in guinea pig, 50.8% in rat, and 41.5% in rabbit. 2. The EC 50 values for the negative inotropic effect of acetylcholine were 3.3 × 10 −7 M in rat and guinea pig atria and 4.1 × 10 −6 M in rabbit atria. 3. There was no correlation between the species differences in the negative inotropic effect of acetylcholine in atria and the density or affinity of acetylcholinesterase or muscarinic receptors. 4. Inhibition of atrial acetylcholinesterase with soman reduced the ec 50of acetylcholine three-fold in all species, but did not change the maximal inotropic effect of acetylcholine. 5. Species differences in the negative inotropic effect of acetylcholine may be caused by differences in the coupling between myocardial muscarinic receptors and the ion channels that mediate negative inotropy.

Adrenoceptor-mediated changes of action potential and force of contraction in human isolated ventricular heart muscle.

1. The effects of alpha-adrenoceptor stimulation on the action potential and force of contraction were investigated in human isolated ventricular heart muscle and compared with those of beta-adrenoceptor stimulation. 2. The maximal stimulation by isoprenaline of beta-adrenoceptors produced large changes in the force of contraction, which were accompanied by moderate increases in the height of the action potential. The maximal inotropic effect produced by stimulation of alpha-adrenoceptors with phenylephrine, in the presence of propranolol (1 mumol 1(-1)) was much smaller (about 10% of that seen in response to beta-adrenoceptor stimulation), and no significant changes of the action potential configuration were observed. 3. The effects of noradrenaline and adrenaline on the force of contraction were not affected by prazosin. 4. It is concluded that the adrenoceptor-mediated changes of the force of contraction (in the presence of either noradrenaline or adrenaline) in the human ventricle are due virtually exclusively to the stimulation of beta-adrenoceptors.

Hearts from diabetic rats are more resistant to in vitro ischemia: possible role of altered Ca2+ metabolism.

The effects of whole heart ischemia were studied in isolated perfused rat hearts from control and diabetic animals. When whole heart ischemia was maintained for 30 minutes at 37 degrees C, diabetic hearts recovered 100% whereas hearts from normal animals recovered 30% of their preischemic function. Reperfusion Ca2+ uptake was about 2.5 microM/g dry wt in diabetic hearts compared with 10 microM/g dry wt in control hearts. When the ischemic period was extended to 40, 50, and 60 minutes, diabetic hearts had depressed recovery of ventricular function, and greater Ca2+ overload but reperfusion function was still significantly higher and Ca2+ overload significantly less than in control hearts. Depressed function and increased Ca2+ uptake were both linearly related to low tissue levels of residual high energy phosphates and inversely related to the amount of lactate that accumulated in the tissue during ischemia. However, regression lines relating these metabolic changes to depressed function and increased Ca2+ uptake showed that for any level of residual high energy phosphate or ischemic lactate, diabetic hearts performed much better and had less Ca2+ uptake than control hearts. These effects of diabetes were due to the diabetogenic action of the drugs used since both streptozotocin and alloxan had the same effect and in vivo insulin treatment reversed the effect. Diabetic hearts had a reduced maximum inotropic effect to increased extracellular Ca2+ under control aerobic perfusion conditions. The improved recovery of ventricular function during reperfusion of ischemic hearts from diabetic animals was highly correlated with reduced Ca2+ uptake, and regression lines relating depressed ventricular function to Ca2+ overload showed that data from control and diabetic hearts fell on the same line; that is, when depressed function occurred it was related to increased Ca2+ uptake to the same extent in both control and diabetic hearts. The resistance to ischemia in diabetic hearts was not related to higher tissue levels of high energy phosphates during reperfusion nor to lactate accumulation during ischemia. The observations suggest a role of increased reperfusion Ca2+ influx in ischemic damage and that alterations of sarcolemmal Ca2+ transport systems in diabetic myocardium may account for the greater resistance of these hearts to ischemia.

Positive inotropic effects of the calcium channel activator Bay K 8644 on guinea-pig and human isolated myocardium

1. The positive inotropic effects of the dihydropyridine calcium activator Bay K 8644 were studied in guinea-pig isolated contracting myocardium and human papillary muscle strips obtained from patients undergoing mitral valve replacement or cardiac transplantation. 2. Bay K 8644 produced a slowly developing, concentration-dependent positive inotropic response in all cardiac tissues studied. In guinea-pig papillary muscle, the increase in force of contraction was half-maximal at 3.9 x 10(-8) mol/l and the maximal inotropic effect was comparable to that obtained with ouabain, dobutamine or calcium. The guinea-pig left atrium (EC50, 2.1 x 10(-7) mol/l) was fivefold less sensitive than the papillary muscle. 3. The maximal inotropic response to dihydroouabain was significantly increased after preincubation with Bay K 8644 (1 x 10(-6) mol/l) in papillary muscles from both guinea-pig and human. In guinea-pig papillary muscles, the maximal inotropic response to dobutamine was not changed by preincubation with Bay K 8644 whereas in human papillary muscle strips, Bay K 8644 increased the inotropic response to dobutamine. 4. Bay K 8644 increased force of contraction (EC50, 4 x 10(-8) mol/l) in human papillary muscle strips from patients undergoing mitral valve replacement. However, the maximal inotropic response to Bay K 8644 was reduced to 32 +/- 4.4% that of calcium (15 mmol/l) measured in the same muscle strips. 5. A further reduction in maximal inotropic response to Bay K 8644 to 13 +/- 1.2% that of calcium (15 mmol/l) with no change in potency was measured in human papillary muscle strips taken from terminally failing hearts of cardiac transplant recipients.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 2,3,7,8-tetrachlorodibenzo- p-dioxin on guinea pig heart muscle

Cardiac effects of TCDD (2,3,7,8-tetrachlorodibenzo- p-dioxin) were examined using atrial muscle isolated from young or mature guinea pigs fed ad libitum, from young animals 10 or 20 days after a single ip injection of 1 μg TCDD/kg body wt, and from control guinea pigs pair-fed to the TCDD-treated animals. Basal contraction force of left atrial muscle obtained from TCDD-treated and pair-fed control guinea pigs 10 days after treatment was significantly increased compared to atrial muscle obtained from ad libitum control animals. Atrial muscle from TCDD-treated animals 20 days after treatment had a significantly lower basal force of contraction while that from pair-fed animals was significantly higher relative to atrial muscle from ad libitum control animals. These results suggest that the TCDD-induced change in basal atrial contraction force is in part due to undernutrition and that prolonged exposure to TCDD inhibits this response observed in pair-fed animals. Furthermore, prolonged TCDD treatment shifted the dose-response curve for the positive inotropic effect of isoproterenol to the right with no statistically significant change in the maximal inotropic effect obtained with high concentrations of isoproterenol. The TCDD-induced changes in force of contraction and the dose-response curve for the positive inotropic effect of isoproterenol were similar in young TCDD-treated and mature control guinea pigs. TCDD did not alter the basal contraction frequency or the dose-response curve for the positive chronotropic effect of isoproterenol. There was no effect on cardiac lipoprotein lipase activity at 1 μg/kg, but in animals administered 4 μg TCDD/kg activity was reduced. It was concluded that TCDD adversely affects atrial muscle of guinea pig heart.

Additive positive inotropic effects of milrinone, ouabain and calcium in diseased human ventricular myocardium.

The interactions of milrinone, ouabain and calcium on force of contraction in isolated, contracting human papillary muscle strips were measured. Milrinone (EC50, 8 X 10(-5)M) increased force of contraction maximally by 2.8 +/- 0.8 mN at 5 X 10(-4)M; significantly less than either ouabain (1 X 10(-7)M; 4.8 +/- 0.5 mN increase) or calcium (15 mM; 6.2 +/- 0.6 mN increase). A submaximal, but not a maximal, inotropic effect of ouabain could be increased by the addition of milrinone; in contrast, both ouabain and calcium increased the maximal inotropic effect of milrinone by 1.7 +/- 0.2 mN and 2.7 +/- 0.3 mN, respectively. The combined inotropic effect of milrinone with either ouabain of 4.2 +/- 0.3 mN or calcium of 5.6 +/- 0.4 mN was not different from that with calcium or ouabain alone. We conclude that further positive inotropic effects should be expected when digitalis is given to patients with congestive heart failure who are already optimally treated with milrinone.

The interaction of polypeptide neurotoxins with tetrodotoxin-resistant Na + channels in mammalian cardiac cells. Correlation with inotropic and arrhythmic effects

This paper describes the interaction of several polypeptide neurotoxins isolated from sea anemone toxins and scorpion venom with the tetrodotoxin-resistant Na + channel of rat cardiac cells. The 22Na + flux and tension development were measured to examine in parallel the cardiotonic and cardiotoxic effects of these polypeptides. Inotropic effects and arrhythmias were seen in the concentration range in which an action of the toxins on the Na + channel was observed. The maximal inotropic effect was systematically observed at toxin concentrations below the concentration value observed for half-maximal stimulation of 22Na + flux through the Na + channel. Arrhythmias began at concentrations near the value for half-maximal stimulation of 22Na + flux by the toxins. Toxins extracted from the sea anemones Anemonia sulcata and Anthopleura xanthogrammica were more active than scorpion toxins and sea anemone Radianthus paumotensis toxins. The most interesting among all the toxins tested for potential use in cardiotherapy was toxin II from Anthopleura xanthogrammica.

Myocardial uptake of drugs and clinical effects.

The process of uptake of cardioactive drugs into the myocardium is a major determinant of the efficacy and potential toxicity of such agents. Evaluation of responses to anti-arrhythmic and positive inotropic agents is best performed with reference to their concentration in the myocardium, and the potential toxicity of drugs such as tricyclic antidepressants and anthracycline antineoplastics is likely to be related to peak myocardial drug concentrations. Although it has been appreciated for many years that even during long term drug administration the myocardial drug content may not be readily predictable on the basis of estimation of plasma drug concentrations, methodology for direct assessment of myocardial drug content has remained limited. The results of in vitro experiments, utilising tissue culture preparations of myocardial cells or isolated atria, have shed some light on the role of local factors as determinants of myocardial drug uptake. For many agents, attainment of maximal cardiac drug content in vitro is a very slow process (taking up to 3 hours), although maximal inotropic and electrophysiological effects may occur more rapidly. The prolonged time course of drug washout from these preparations also reflects their extensive and slow intracellular accumulation. Mechanical activity of the myocardium appears to accelerate drug uptake, particularly for otherwise slowly equilibrating agents, but the major determinant of the extent of drug uptake into isolated myocardial preparations is lipophilicity, perhaps reflecting the passage of drugs through the sarcolemma. In intact animals, assessment of myocardial drug content after acute drug administration has been performed utilising serial myocardial biopsy or sacrifice of animals. Studies in open-chested dogs suggest that acute accumulation of agents may be most closely predicted from the second compartment of a 3-compartment pharmacokinetic model, and that there is a variable correlation between changes in plasma and myocardial drug concentrations. For example, bretylium concentrations within the myocardium continue to increase for up to 6 hours after drug administration. Factors which may influence drug uptake into the myocardium in intact animals include ischaemia, which usually results in a delay in both drug uptake and subsequent clearance. This change can also be inferred from the time course of onset of antiarrhythmic drug effects in some models of myocardial ischaemia. Anoxia may also inhibit myocardial drug uptake.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiotonic activity of a new inotropic agent, 3,4-dihydro-6-[4-(3,4-dimethoxybenzoyl)-1-piperazinyl]-2(1H)- quinolinone (OPC-8212), in the dog with and without beta-blocker and Ca++-antagonist pretreatment.

Hemodynamic effects of a new inotropic agent, OPC-8212 (2(1H)-quinolinone derivative) were studied in anesthetized open chest dogs pretreated with propranolol and diltiazem. Three doses (1, 3 and 10 mg/kg) of OPC-8212 were administered intravenously and the net hemodynamic effect (% change) was obtained by subtraction of the effect of the solvent from the gross effect, since the vehicle has a transient, but significant hemodynamic effect. The maximal inotropic effect occurred 3 minutes after administration: LV dP/dt max and cardiac output (CO) increased by 19 +/- 2.5% and 28 +/- 8.5%, respectively, at 3 mg/kg. These cardiotonic effects were dose-dependent, whereas heart rate, peak LV pressure (PLVP) and mean aortic pressure were minimally changed at any dose. Accordingly, systemic vascular resistance (SVR) decreased in a dose-dependent manner although the decrease was much less than that in administration of isoproterenol. The inotropic effect was not blocked by beta-adrenoceptor blockade (propranolol 1 mg/kg), indicating that the cardiotonic action of this agent is not due to beta-adrenergic stimulation. Thus, this agent could reverse beta-blocker-induced heart failure. During infusion of diltiazem (0.1 mg/kg/min following bolus intravenous administration of 0.5 mg/kg), the increases in LV dP/dt max and CO due to OPC-8212 were similar to those in the control study. In contrast to the effects under beta-adrenoceptor blockade, however, decreased PLVP was restored by OPC-8212. Neither chronotropic nor rrhythmogenic effects were observed in the control or with either pharmacological intervention. These results indicate that OPC-8212 has a potent inotropic action with modest vasodilatory effect even with propranolol or diltiazem pretreatment.(ABSTRACT TRUNCATED AT 250 WORDS)

The inotropic effects of dopamine and its precursor levodopa on isolated human ventricular myocardium.

The direct positive inotropic effects of dopamine and its precursor, levodopa, were measured using isolated, contracting human papillary muscle strips taken from patients during mitral valve replacement. Levodopa did not produce any positive inotropic effect at concentrations up to 3 X 10(-3) M. The positive inotropic effects of dopamine were observed at concentrations above 1 X 10(-5) M with the maximal effect at 3 X 10(-3) M - concentrations higher than those observed in therapy. This inotropic effect was reduced by the beta 1-selective antagonist, 1-practolol (1 X 10(-6) M); the beta 2-selective antagonist, ICI 118,551 HCl (1 X 10(-6) M); the dopamine antagonist, haloperidol (3 X 10(-6) M); the neuronal uptake inhibitor, cocaine (3 X 10(-5) M), but not by the alpha 1-antagonist, prazosin (1 X 10(-7) M). This indicates that dopamine exerts its positive inotropic effects on human heart muscle mainly through release of noradrenaline, together with possible interactions at beta- and dopamine-receptors. The maximal inotropic effect of dopamine was about 50% that of calcium (15 mM, 6.2 +/- 0.7 mN) or ouabain (1 X 10(-7) M, 5.0 +/- 0.8 mN) when measured in the same muscle strips, possibly due to the reduced cardiac noradrenaline content together with the reduced beta-receptor number in congestive heart failure. This concentration of ouabain (1 X 10(-7) M) gave almost maximal inotropy without marked toxicity; when dopamine was then added, only toxicity developed without any further increases in force of contraction. Any haemodynamic benefits of dopamine therapy in optimally digitalis-treated patients are probably due to other cardiovascular effects such as vasodilatation.

Comparison of ouabain receptors in sheep myocardium and purkinje fibres

The conducting system of the heart has been reported to be more sensitive to the toxic effects of digitalis than the working myocardium. To investigate the molecular basis of these observations, we have characterized the ouabain receptor in Purkinje fibres and ventricular muscle of the digitalis-sensitive sheep heart using cell membrane preparations, crude homogenates and contracting heart tissues. [ 3H]-Ouabain binding has the following characteristics: (1) in sheep left ventricular cell membranes, specific binding was of high affinity ( K D 1.9 × 10 −9M at 37); was co-incident with an inhibition of (Na + + K +-ATPase activity; and was inhibited by K + and unlabelled cardiotonic steroids; (2) in crude homogenates, the maximal binding capacity but not the affinity for ouabain varied in different parts of the sheep heart with Purkinje fibres containing markedly fewer binding sites (0.33 × 10 14/g wet weight; left ventricle, 1.3 × 10 14/g wet weight) and (3) in isolated, contracting Purkinje fibres and right ventricular moderator band strips, concentration-response curves for [ 3H]-ouabain binding, increase in force of contraction and inhibition of [ 86Rb +]-uptake were co-incident. In both contracting tissues, a ouabain concentration of 3 × 10 −7 M occupied about 50% of the specific binding sites, gave the maximal inotropic effect without toxicity and inhibited [ 86Rb +]-uptake by about 50%. The maximal binding capacity was lower in contracting Purkinje fibres (2 × 10 14 binding sites/g wet weight) than in contracting moderator band strips (3.9 × 10 14 binding ites/g wet weight). The maximal inotropic effects were reached slightly faster in Purkinje fibres but toxicity also occurred faster in these fibres. We conclude that the specific ouabain binding site is the receptor mediating positive inotropy and inhibition of (Na + + K +-ATPase in the sheep heart. Further, this receptor is identical in both the conducting system and working myocardium but the conducting system contains many fewer receptors. This change in receptor number, rather than affinity, may underlie the increased ouabain toxicity observed in Purkinje fibres.

Erythrosin B inhibits high affinity ouabain binding in guinea-pig heart Na+-K+-ATPase without influence on cardiac glycoside induced contractility.

Binding of [3H]-ouabain to guinea-pig heart membranes enriched in Na+-K+-ATPase revealed two different cardiac glycoside binding sites. High affinity binding was obtained at a KD = 2.2 X 10(-7) mol 1(-1) (Bmax = 16.8 pmol ouabain mg-1 protein) whereas low affinity ouabain binding occurred at a KD much greater than 10(-6) mol 1(-1). To discover whether the two ouabain binding sites are functional in guinea-pig heart muscle, erythrosin B, an inhibitor of the high affinity ouabain binding in rat brain tissue, was tested in guinea-pig isolated heart muscle preparations. Erythrosin B proved to be a potent inhibitor of the Mg2+ (Na+)-dependent-, as well as Na+-K+-activated ATPase (ID50 = 9 X 10(-6) mol 1(-1). Contractility of guinea-pig isolated papillary muscles, however, was not influenced by erythrosin B in concentrations up to 1 X 10(-5) mol 1(-1). Only very high concentrations (4 X 10(-4) mol 1(-1) resulted in a slightly negative inotropic effect (about 20%). Erythrosin B dose-dependently inhibited [3H]-ouabain binding to the Na+-K+-ATPase (KD = - 3.6 X 10(-6) mol 1(-1). In a concentration of 1 X 10(-5) mol 1(-1) the dye abolish high affinity [3H]-ouabain binding without affecting the low affinity binding sites. In contrast, in guinea-pig isolated atria, no functional antagonism between erythrosin B (5 X 10(-5) mol 1(-1) and ouabain was observed. 5 As there is a coincidence between the high affinity binding (KD = 2.2 x 10-7moll ') and the concentration for half maximum inotropic effects of ouabain (EDIo = 1.6 x 10-7 mol I), the lack of effect oferythrosin B on ouabain-induced inotropy may be caused by an inaccessibility of the dye to the (internal) ATP-site of the Na+-K+-ATPase.

A dihydropyridine (Bay k 8644) that enhances calcium currents in guinea pig and calf myocardial cells. A new type of positive inotropic agent.

Bay k 8644 is a structural analog of nifedipine with positive inotropic activity. The mechanism of drug action was evaluated by measuring the effects of Bay k 8644 on twitch tension, action potential configuration, and calcium channel currents in myocardial cells. Bay k 8644 increases twitch tension in guinea pig atria without changing the time course of tension development. The drug does not occlude the effect of isoproterenol on twitch tension. The effects of Bay k 8644 on atrial twitch tension are highly dependent on the frequency of stimulation. Maximal inotropic effects are observed at approximately 0.5 Hz, but no inotropic effect occurs at 0.003 Hz (a rested-state contraction). Since positive inotropic effects only occur with frequent electrical stimulation, they are not due to an intracellular action or to mechanisms that elevate cell calcium in quiescent muscle, such as inhibition of the Na,K-ATPase. Bay k 8644 increases the action potential duration of calf ventricular muscle and Purkinje fibers. Effects on action potential duration are occluded by 1 microM nisoldipine, which specifically blocks calcium channels. The interaction of Bay k 8644 with calcium channels in calf Purkinje fibers was studied using the two-microelectrode voltage clamp technique. Strontium was used as a charge carrier to minimize current through calcium-activated channels and to avoid changes in calcium conductance due to changes in intracellular calcium. Bay k 8644 increases strontium currents and alters the time- and voltage-dependence of channel opening. The greatest percent increase in strontium current occurs for weak depolarizations. For strong depolarizations, strontium current is increased most at the beginning of a test pulse. The drug-induced changes in calcium channel gating are inconsistent with a calcium- or cyclic adenosine monophosphate-mediated effect, and indicate a novel mechanism of action on calcium channels. Thus, Bay k 8644 is the first positive inotropic agent shown to act specifically and directly on calcium channels.

In Vitro Comparative Studies of the Calcium-Entry Activators YC-170, CGP 28392, and BAY K 8644

Recently, the novel dihydropyridine derivates YC-170, CGP 28392, and BAY K 8644 have been reported to act in the opposite way to Ca2+-entry blockers. We have found that these compounds inhibit the binding of [3H]nitrendipine on guinea pig heart membranes (Ki: 6 nM BAY K 8644, 115 nM CGP 28392 and 690 nM YC-170). Like those of nifedipine (Ki 1 nM), the curves had slopes close to unity, and, unlike those of some nondihydropryridine Ca2+ antagonists, were not altered in the presence of diltiazem, indicating a competitive interaction at dihydropyridine-sensitive sites. In isolated guinea pig atria, these agents exerted positively inotropic effects similar in their potency ratio to those observed in the binding experiments (BAY K 8644 1, CGP 28392 1:17, YC-170 1:600). The maximum inotropic effects of BAY K 8644 and CGP 28392, and of YC-170 corresponded respectively to two-thirds and one-third of those induced by isoprenaline or extracellular Ca2+. In the isolated rat mesenteric artery, perfused with a depolarizing solution, vasoconstrictor Ca2+ dose-response curves are shifted to the right by nifedipine. By contrast, BAY K 8644 and CGP 28392 caused a distinct leftward shift of the Ca2+ dose-response curves, at concentrations of 3-300 nM and 30-300 nM, respectively, and YC-170 a marginal shift at concentrations of 200-2000 nM, i.e., similar ranges to their inhibitory effects on [3H]nitrendipine binding. At higher concentrations, all three compounds produced Ca2+-antagonistic effects. These results indicate that the compounds act at dihydropyridine-sensitive sites and exert partial agonistic activities in vascular and myocardial tissue.

UD-CG 115 ? a cardiotonic pyridazinone which elevates cyclic AMP and prolongs the action potential in guinea-pig papillary muscle

The mechanism of the positive inotropic effect of a benzimidazole-pyridazinone, UD-CG 115, was analysed in the isolated guinea-pig papillary muscle contracting isometrically at a frequency of 0.2 Hz. UD-CG 115 produced a slowly developing and poorly reversible positive inotropic effect increasing with concentration (3-300 mumol/l). The effect amounted to 30 and 74% of the maximum inotropic effect of a standard, dihydroouabain, at 34 and 300 mumol/l, respectively. Low concentrations shortened and 300 mumol/l UD-CG 115 significantly prolonged the duration of contraction. The enhancement of the maximum rate of relaxation, S2, was intermediate between those produced by isoprenaline and dihydroouabain, respectively. UD-CG 115 prolonged the duration of the transmembrane action potential (90% repol .) by up to 22% at 300 mumol/l, whereas an equieffective concentration of isoprenaline did not consistently alter action potential duration. UD-CG 115 increased Vmax and overshoot, and prolonged the duration, of slow action potentials elicited at 24 mmol/l [K]0. The inotropic potency of UD-CG 115 was not significantly changed by reserpine pretreatment of the guinea pig or by the presence of 1 mumol/l(-)-propranolol, 3 mumol/l phentolamine or 10 mumol/l cimetidine. Neither was it reduced by 10 mumol/l TTX. The inotropic effect of 100 mumol/l UD-CG 115 remained unchanged when [K]0 was elevated from 3.2 to 12.0 mmol/l. A sarcolemmal preparation of guinea- pig ventricular Na,K-ATPase was only slightly inhibited by the highest concentration of UD-CG 115.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased therapeutic range, merely a pharmacokinetic problem?

The difficulty with this topic begins already with the definition of the term “therapeutic range”. In the clinic the limits of the therapeutic range are determined by the dose of a drug necessary for a therapeutic effect and the dose at which toxic effects begin. For digitalis glycosides the therapeutic range lies between the amount of drug required for the beginning of positive inotropic effects and that amount at which toxic side effects (generally arrhythmias) are caused. Here difficulties appear already since at present no methods exist for the definite recognition of the onset of positive inotropic effects. A few weeks ago, a symposium was held on the theme of “Evaluation of Methods for the Measurement of Inotropic Effects”. The conclusion of the workshop discussion was that there are no clinically reliable or sufficiently sensitive methods for the measurement of inotropic changes*. Even in experimental pharmacology the determination of the therapeutic range of digitalis glycosides is problematic. Many experimental investigators using i.v. infusions have defined the therapeutic range as the dose range between onset of arrhythmias and the lethal dose (1). However, this definition does not satisfy practical requirements and for understandable reasons cannot be applied to clinical situations. The onset of the arrhythmic effects of glycosides approximately corresponds to the maximal rather than the minimal increase in inotropy. From the clinical standpoint, the therapeutic range lies between the doses causing minimal and maximal inotropic effects, not between the doses causing arrhythmic and lethal effects, since the latter two effects are both toxic ones which should be avoided in the clinical setting. However, a few investigators measured the dose causing positive inotropic effects and arrhythmic and lethal effects during continuous i.v. infusion. Even this experimental model cannot be compared to the clinical situation since glycosides are usually given orally in the clinic, with the use of increasing doses for initial digitalization, or a constant dose in maintenance therapy.

Secondary AV conduction responses during tonic vagal stimulation.

We gave trains of supramaximal stimuli at frequencies of 2, 4, or 8 Hz to both vagi of anesthetized, open-chest dogs and studied the AV conduction and atrial contractility responses while the atria were paced at different frequencies. The vagal stimulation quickly induced a maximum negative inotropic effect on the atrium. This "initial" response was followed by a "secondary" fading back toward the control level. The negative dromotropic responses to vagal stimulation also rapidly reached an initial value. When this initial response was large, there was then a secondary, relatively rapid fading back toward control. However, when the initial dromotropic response was small, the secondary response instead consisted of a gradual increase in conduction time. At intermediate levels of the initial dromotropic response, there was no appreciable secondary change in AV response. We hypothesize that two opposing mechanisms account for the variable secondary changes in AV conduction time and that the stimulus intensity determines which mechanism will be prepotent. The mechanism responsible for the secondary decline appears to be related to that which causes the fade of the inotropic response; muscarinic receptor desensitization probably plays an important role. The secondary augmentation of the dromotropic response may be related either to a slow diffusion of acetylcholine from the surrounding tissue or to a slow cumulative change in AV nodal refractoriness.