Differential effects of amrinone on contractility and taurine influx in rat and guinea pig hearts

Differential effects of phosphodiesterase inhibitors on accumulation of cyclic AMP in isolated ventricular cardiomyocytes

The intracellular actions of phosphodiesterase (PDE) inhibitors on the accumulation of cyclic nucleotides were studied in isolated ventricular cardiomyocytes from adult Sprague-Dawley rats. Elevated levels of cyclic AMP, due to the effects of selective PDE inhibitors, were detected only when the levels of cyclic nucleotide were enhanced with forskolin (10 microM). The time course for the elevation of cyclic AMP levels was similar for all the PDE inhibitors tested, following the pattern of an initial rise in the first 2-4 min, proceeded by a steady state at 67 +/- 6% of the maximum stimulation. HN-10200 (2-[3-methoxy-5-methylsulfinyl-2-thienyl]-1H-imidazo-[4,5-c]- pyridine hydrochloride), a new imidazopyridine derivative, had a similar concentration-dependent profile to the structurally related compound, sulmazole (AR-L 115 BS, 2-[2-methoxy-4-methylsulfinyl)phenyl]-1H- imidazo-[4,5-b]-pyridine). Both the non-selective inhibitor, 3-isobutyl-1-methylxanthine (IBMX), and the selective PDE IV inhibitor, Ro 20-1724 (4-[(3-butoxy-4-methoxyphenyl)methyl]-2- imidazolidinone), potentiated the forskolin-stimulated levels of cyclic AMP with a much greater efficacy than sulmazole or HN-10200. The concentrations of forskolin required by IBMX, sulmazole and HN-10200 (10(-3) M) to increase levels of cyclic AMP by 4 pmol/mg protein were 3.2 x 10(-6) M, 1.32 x 10(-5) M and 1.46 x 10(-5) M, respectively. Enoximone failed to cause an increase in the levels of cyclic AMP, even when stimulated with maximal concentrations of forskolin. Furthermore, in the presence of forskolin, enoximone attenuated the response of Ro 20-1724 and IBMX in a concentration-dependent manner. Enoximone, similarly to HN-10200, sulmazole, Ro 20-1724 and IBMX did not produce any significant effect on levels of cyclic GMP under elevated conditions in the presence of sodium nitroprusside. The combined action of Ro 20-1724, with either HN-10200, sulmazole, or IBMX (10(-4) M), on intracellular levels of cyclic AMP, was not greater than the response to Ro 20-1724 alone. These data demonstrate the differential actions of PDE III and PDE IV inhibitors in rat ventricular cardiomyocytes. It is suggested that enoximone has a high selectivity for the PDE III isoenzyme so that hydrolysis of cyclic AMP by the PDE IV isoenzyme is not inhibited, in accordance with the lack of increase in cyclic AMP by enoximone in rat cardiomyocytes. HN-10200 and sulmazole, producing small increases in intracellular levels of cyclic AMP, are less selective PDE III inhibitors than enoximone.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular calcium and mechanical function in isolated perfused hearts from rats and guinea pigs

We tested the hypothesis that the variable functional properties of rat versus guinea pig hearts are due to differences in intracellular Ca2+ handling. Hearts isolated from rats and guinea pigs were perfused with physiological saline, and isovolumic left ventricular (LV) pressure as well as coronary perfusion pressure were recorded simultaneously with Ca2+ transients from aequorin-loaded cells. Guinea pig hearts developed 47% less LV pressure than rat hearts, and the time to peak pressure was prolonged by 71% at similar heart rates. Diastolic and systolic levels of myoplasmic Ca2+ were approximately the same in both species at normal external Ca2+ concentration (1 mM); however, at low Ca2+ concentration (0.5 mM), guinea pig hearts maintained a higher level of myoplasmic Ca2+ than rat hearts, and the relative depression of LV systolic pressure was less. Guinea pig hearts also exhibited higher resistance to the negative inotropic effect of caffeine and did not respond to increments in perfusion pressure with increases in LV-developed pressure and systolic Ca2+ levels as did rat hearts. These contrasting findings with regard to intracellular Ca2+ handling may be attributed to a different organization of the ionic transport system with higher dependence of rat cardiomyocytes on normal function of the sarcoplasmic reticulum.

Effects of amrinone on cardiac contraction and relaxation in isolated, perfused rat heart

The effects of amrinone on cardiac contraction and relaxation were assessed in isolated, perfused rat hearts. It was found that the left ventricular developed pressure (LVDP), dp/dtmax and -da/dtmax did not significantly increase, and the time constant (tau) did not markedly shorten with perfusion of low concentration (1 nmol/L, 100 nmol/L) of amrinone. The perfusion with higher concentration (1000 nmol/L) of amrinone reduced LVDP (P < 0.01), dp/dtmax (P < 0.01), -dp/dtmax (P < 0.01), and prolonged tau (P < 0.05) significantly. It was assumed that amrinone has no direct positive inotropic effect, and can not improve cardiac relaxation. On the contrary, the cardiac contraction and relaxation will be inhibited at higher concentration of amrinone.

Effects of Amrinone on shortening velocity, force development and ATPase activity of demembranated preparations of rat ventricular myocardium

This study analyses the effects of Amrinone (bipyridine derivative with phosphodiesterase inhibitor properties) on the myofibrillar apparatus of rat myocardium. Thin trabeculae were isolated from the right ventricle and chemically demembranated. Force development and shortening velocity were measured during maximal calcium activations (pCa = 4.45) in control conditions and in the presence of 1-3 mM Amrinone. Maximum shortening velocity was obtained both from extrapolation of the force-velocity curve and with the slack test method. Amrinone was found to significantly reduce maximum shortening velocity and force development. Myofibrils and myosin were prepared from rat ventricular myocardium and their ATPase activity was assessed in control conditions and in the presence of Amrinone (0.3-6 mM). Ca-Mg dependent myofibrillar ATPase activity which was determined at low ionic strength was depressed by Amrinone in a dose-dependent way. Ca-stimulated ATPase activity determined at high ionic strength in myofibril or myosin preparations was not affected. Furthermore, Amrinone did not influence the pCa-ATPase activity curve of the myofibrillar preparations. A comparison between the inhibitory effects of Amrinone on myofibrils prepared from euthyroid rats and myofibrils prepared from hypothyroid rats was carried out. The ATPase activity was significantly less depressed in myofibrils prepared from hypothyroid rats than in those prepared from euthyroid rats. These results provide the first evidence of an effect of Amrinone on ATP splitting and force generation in the myofilament system of cardiac muscle.

Antagonism towards endogenous adenosine and inhibition of cGI-PDE in the cardiac effects of amrinone, milrinone and related analogues

1. The effect of amrinone, milrinone and of three milrinone analogues was tested on spontaneous chronotropic and inotropic activity of guinea-pig isolated atria, on the activity of cGMP-inhibited phosphodiesterase (cGI-PDE) from guinea-pig heart and on specific binding of N6-cyclohexyl[3H]adenosine ([3H]CHA) to Ri adenosine receptors in guinea-pig atria. 2. The Ki-values towards [3H]CHA binding to Ri receptors were linearly related to the EC50S for the increase in force of contraction but not to the EC50S for the increase in frequency of the atria. The Ki values towards cGI-PDE were linearly related to the EC50S for the positive chronotropic effect.

The efficacy and safety of milrinone for treating heart failure in dogs

Milrinone has been studied in a variety of situations. In experimental dogs it has been documented to increase contractility to a similar degree as beta-receptor agonists and to produce mild arteriolar dilation in dogs. In canine patients with heart failure, milrinone produces demonstrable improvement in echocardiographic ventricular function and hemodynamic variables. In addition, it improves clinical signs in these patients, improving their quality of life. Milrinone is superior to digoxin as evidenced by the improvement in clinical signs noted in dogs that were unresponsive or no longer responding to digoxin administration. There is no doubt that milrinone improves short-term prognosis and in so doing prolongs life. Many of the patients that the author has observed would not have gone home without the benefits of milrinone. Milrinone's effects on long-term survival cannot be assessed, but its effects on survival time are certainly not dramatic enough to be evident without a comparison population. Therefore, milrinone administration should be considered palliative, as is administration of all other cardiovascular medications for heart failure. In addition to its beneficial effects, milrinone also appears to be relatively safe when compared with the alternative of digoxin administration. Fatal events attributable to milrinone administration are rare, and those directly attributable to enhanced ventricular arrhythmia can generally be avoided by monitoring an electrocardiogram after initial milrinone administration commences. Milrinone does not increase the incidence of sudden death in Doberman Pinschers. It is possible that a small number of dogs with mitral regurgitation may develop mitral chordal rupture. For this reason and possibly others, milrinone probably will not be indicated in early heart failure due to mitral regurgitation when heart failure is readily responsive to diuretic administration. The risk-to-benefit ratio turns markedly in the favor of milrinone administration in the dog with mitral regurgitation that is partially or completely refractory to other cardiovascular drugs. Milrinone appears to be a more effective and safer positive inotrope for long-term treatment of dogs with congestive heart failure than drugs currently available. The author and all the investigators involved in the milrinone clinical trials hope that it will soon be available for use by the veterinary community.

Dietary taurine deficiency and dilated cardiomyopathy in the fox

Taurine deficiency has been implicated as a potential cause of dilated cardiomyopathy. However, the relationship between taurine and myocardial function is presently unclear. The purpose of this study was to determine whether dilated cardiomyopathy in the fox is associated with dietary taurine deficiency. A total of 68 foxes from farms with a history of death caused by dilated cardiomyopathy and 14 foxes from a farm with no history of dilated cardiomyopathy were studied. Dilated cardiomyopathy was diagnosed by echocardiography in 48% of the foxes from one farm with a positive history and in none of the foxes from the control farm. Foxes less than 9 months of age were more commonly affected than older foxes (p = 0.03). Plasma taurine concentrations were significantly less (p less than 0.01) in foxes that had dilated cardiomyopathy (26.8 +/- 16.4 nmol/ml) than in the control foxes (99.3 +/- 60.2 nmol/ml). A significantly higher (p less than 0.01) incidence of dilated cardiomyopathy was present in foxes with a history of a sibling or offspring that died of dilated cardiomyopathy than in foxes without a family history of cardiac death. In one fox with dilated cardiomyopathy that was tested, the myocardial taurine concentration was lower (1.7 mumol/gm wet weight) than that of control foxes (7.3 +/- 1.6 mumol/gm wet weight). Hepatic cysteinesulfinic acid decarboxylase activity was significantly less (p less than 0.001) in foxes with dilated cardiomyopathy (0.97 +/- 0.2 nmol/mm.mg protein) than in control foxes (2.11 +/- 0.07 nmol CO2/mm.mg protein).(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of purine compounds in the inotropic action of milrinone

In spontaneously beating atria from reserpine-treated guinea pigs, milrinone (1-100 micrograms/ml) induced a positive inotropic and chronotropic effect but was ineffective in preparations preincubated with adenosine deaminase (1 U/ml). Both in spontaneously beating and in electrically driven atria, ATP and adenosine evoked a dual effect: a first negative phase characterized by a reduction in contractile force, followed by a positive phase of increased inotropism. In these preparations milrinone inhibited the early negative influence exerted by purine compounds and amplified the following positive phase. These data suggest that the positive inotropic and chronotropic effect of milrinone may originate from its interference with endogenous purines.

Taurine: effect on myocardial relaxation

1. The effects of taurine on cardiac inotropic state and relaxation were studied in the isolated rabbit heart using solutions containing three concentrations of calcium: 0.7, 1.8 and 3.0 mmol/l. 2. Increases in Ca2+ caused increases in inotropic indices, but had no effect on rate constant of relaxation. 3. Taurine had a positive inotropic effect at Ca2+ = 0.7 mmol/l, an equivocal effect at Ca2+ = 1.8 mmol/l and a negative inotropic effect at Ca2+ = 3.0 mmol/l. 4. Taurine had no effect on the rate constant of relaxation at any Ca2+ concentration. 5. Taurine modulates the positive inotropic effect of Ca2+ but neither taurine nor Ca2+ affect the rate constant of relaxation.

Milrinone enhances cytosolic calcium transient and contraction in rat cardiac myocytes during beta-adrenergic stimulation

We have investigated the mechanism that underlies the absence of a positive inotropic effect of milrinone on rat myocardium. The twitch characteristics of enzymatically dissociated left ventricular myocytes from the adult rat and guinea pig were assessed by edge tracking during field stimulation. In some rat myocytes loaded with the ester derivative of the Ca2+ probe Indo-1 we simultaneously measured changes in cell length and in the associated cytosolic Ca2+ (Cai) transient. Our results show that in guinea pig myocytes bathed in 0.5 mM [Ca2+] and field stimulated at 1 Hz, milrinone (10 microM) had a positive inotropic effect. In contrast milrinone had no effect on the contractile properties of rat myocytes studied under similar conditions and field stimulated at 0.2 Hz. In rat myocytes bathed in 0.5 mM [Ca2+] and stimulated at 0.2 Hz isoproterenol (1 nM) increased the amplitude and shortened the duration of the contraction and of the associated Cai transient; these effects of beta-adrenergic stimulation were further enhanced by the addition of milrinone (10 microM) in the presence of isoproterenol. Under conditions of higher cell Ca2+ loading achieved by raising bathing [Ca2+] to 1 mM and isoproterenol to 3 nM the positive inotropic effect of milrinone (10 microM) in rat myocytes saturated when spontaneous oscillatory Ca2+ release appeared in the diastolic intervals between electrically stimulated twitches. Our results suggest that an enhancement in the baseline beta-adrenergic stimulation is required for milrinone to exercise a positive inotropic action on rat myocardial tissue.

Beneficial effect of taurine in rabbits with chronic congestive heart failure

To examine the effect of daily treatment with taurine on improving the status of congestive heart failure (CHF), we used rabbits with artificially induced aortic regurgitation. Ten rabbits were treated daily with taurine (100 mg/kg by mouth) and eight with guanidinoethyl sulfonate (GES) (100 mg/kg by mouth) immediately after induction of aortic regurgitation. The cumulative mortality rate at 8 weeks in the taurine-treated CHF group was 10% (1 of 10) compared with 53% (16 of 30) in the nontreated CHF group and 75% (6 of 8) in the GES-treated CHF group (p less than 0.05). Although cardiac function (max dP/dt) in CHF rabbits was significantly decreased (p less than 0.001), taurine-treated CHF rabbits maintained the same values as control rabbits. Taurine content of the left ventricular tissue of the CHF rabbits was significantly increased (p less than 0.01). Administration of taurine and GES to control rabbits for 8 weeks affected neither the hemodynamics nor the taurine content of the heart. It was concluded that taurine slowed the rapid progression of heart failure and consequently prolonged life expectancy.

Myocardial pharmacodynamics of dopamine, dobutamine, amrinone and isoprenaline compared in the isolated rabbit heart

The isolated spontaneously beating rabbit heart was used for comparing the myocardial effects of isoprenaline, dobutamine, dopamine and amrinone. Both isoprenaline and dobutamine produced a progressive concentration-dependent increase in contractility from 100% to a maximum of about 200% (pD2 7.81 and 7.01, respectively) as measured by the increase in isotonic contraction rate. The simultaneous augmentations in contraction amplitude reached maxima of about 127 and 143% (pD2 7.83 and 7.05) for each of the drugs and the heart frequency rose to 202 and 162% (pD2 7.80 and 6.63), respectively. The accompanying oxygen consumption increased from 100 to 194% (pD2 7.70) for isoprenaline and to only 177% (pD2 6.36) for dobutamine. Coronary flow rate rose to 153 and 134%, respectively. Dopamine increased the contraction rate to 181% (pD2 6.26), contraction amplitude to about 122% (pD2 6.25) and heart rate to 162% (pD2, 5.85), while oxygen consumption rose to a maximum of 202% (pD2 5.69). Coronary flow rate rose to 156%. In contrast amrinone produced an unexpected slowly progressing decrease in contraction rate and contraction amplitude to about 66% (pD2 4.45 and 4.01, respectively). Oxygen consumption increased to 159% (pD2 4.10) and coronary flow rate to 210%. The positive inotropic effect of dobutamine thus equalled that of isoprenaline but with a distinct lower concomitant increase in heart frequency and oxygen consumption which may reflect a better myocardial efficiency during the action of dobutamine.

Effects of several newer cardiotonic drugs on cardiac cyclic AMP metabolism

The purpose of this study was to investigate the possible roles of selective inhibition of cyclic nucleotide phosphodiesterase (PDE) isozymes, adenylate cyclase activation, and tissue cyclic 3',5'-adenosine monophosphate (cyclic AMP) elevation in the positive inotropic action of five new cardiotonic drugs. Three PDE isozymes (PDE I, II and III), homogenates, and slices of guinea pig ventricles were used. The inotropics amrinone, milrinone, AR-L 115BS, MDL 17,043, and RMI 82,249 all inhibited cyclic AMP hydrolysis by PDE III in a concentration-dependent manner, as did the PDE inhibitors aminophylline and 1-methyl-3-isobutylxanthine (MIX). All drugs except for AR-L 115BS inhibited PDE III at concentrations lower than those producing a standard inotropic response. A significant correlation (r = 0.80, P less than 0.05) was observed between PDE III inhibition and inotropic activity for six of the drugs. Only aminophylline and MIX, but none of the cardiotonic drugs, inhibited cyclic AMP hydrolysis by PDE I and II and cyclic 3',5'-guanosine monophosphate (cyclic GMP) hydrolysis (amrinone not tested) by PDE I. Further, none of the cardiotonic drugs inhibited the calmodulin-stimulated cyclic AMP hydrolysis by PDE I, indicating their lack of calmodulin antagonist activity. These drugs also did not stimulate adenylate cyclase activity but all increased net cyclic AMP formation from ATP in guinea pig ventricular homogenates through inhibition of cyclic AMP breakdown. Amrinone, milrinone, MDL 17,043 and RMI 82,249, but not AR-L 115BS, raised cyclic AMP levels significantly (P less than 0.05) in guinea pig ventricular slices. Also, amrinone, MDL 17,043 and RMI 82,249, but not AR-L 115BS, potentiated forskolin-induced cyclic AMP increase. These data taken together suggest that the specific inhibition of cyclic AMP PDE III isozyme and the consequent elevation of tissue cyclic AMP levels in cardiac tissue are an important mechanism of action of amrinone, milrinone, MDL 17,043 and RMI 82,249. Because AR-L 115BS did not increase cyclic AMP levels, it is likely that another mechanism may participate in the inotropic response to AR-L 115BS.

Interaction of amrinone with endogenous adenosine in guinea-pig atria

In spontaneously beating atria from reserpine-treated guinea-pigs, amrinone (10 microM to 2 mM) induced a positive inotropic and chronotropic effect that was preceded by a transient reduction in contractile force and in frequency. Both the positive and negative effects were concentration-dependent. The inotropic action of amrinone was antagonized by low concentrations of 8-phenyltheophylline that compete with adenosine at R-receptors on plasma membrane without significantly influencing phosphodiesterase activity. Cumulative concentrations of amrinone (1 mM) antagonized the reduction of rate of contraction and amplitude induced by dipyridamole 1 microM in spontaneously beating atria and restored the maximum contractile effect reached in the absence of dipyridamole. In spontaneously beating preparations incubated in the presence of adenosine deaminase (1 u ml-1), amrinone lost its positive effects on the atria and only reduction of rate and contractile force was evident. Both effects were antagonized by scopolamine 1 mM thus indicating their cholinergic nature. Adenosine at 0.1 microM and 0.5 microM significantly inhibited the inotropic effect induced by amrinone (0.03 to 3 mM) and the concentration-effect curves of amrinone obtained in the absence and presence of adenosine clearly indicate a competitive antagonism between the two drugs. Thus the contractile activity of amrinone in spontaneously beating atria from reserpine-treated guinea-pigs originates from a displacement of adenosine from its R-receptor sites in the cardiac cell.

Echocardiographic evaluation of cardiovascular effects of amrinone

Amrinone, a new inotropic drug, was infused at a dosage of 2.5 mg/kg body weight in 14 patients affected by dilatative cardiomyopathy in New York Heart Association (NYHA) functional class III and IV. Cardiac index, mean arterial pressure, and some echocardiographic parameters were evaluated. Cardiac index (CI) increased from 2.03 +/- 0.24 to 2.82 +/- 0.43 1/min/m2 (p less than 0.001). Fractional shortening (FS) increased from 16.4 +/- 5.2 to 21.5 +/- 5.3% (p less than 0.05). End-diastolic and end-systolic diameters showed a significant reduction. Mean arterial pressure decreased from 90.7 +/- 88 to 87.3 +/- 8.4 mmHg (p less than 0.001), the end-systolic stress (ESS) decreased from 5.8 +/- 1 to 5.2 +/- 1 g/cm (p less than 0.001). Analyzing the relationship between FS and ESS, it was possible in some cases to suppose the presence of an important vasodilator effect of the drug. The afterload in 7 patients was therefore modified before and after infusion of the drug to analyze FS at the same levels of afterload. This was done to evaluate the vasodilator effect of amrinone. Examining the regression line of FS/ESS ratio it was possible to observe a predominant vasodilator effect in some patients, but in most, a sinergic action was noted. This may be useful for chronic treatment of congestive heart failure, reducing amrinone doses, and using it in association with other vasodilator drugs.