Human Papillary Muscle Strips Research Articles

Na+-channel modulating effect of the inotropic compound S(-)BDF 9196 in human myocardium.

S(-)BDF 9196, the active enantiomer of racemic (+/-)BDF 9148, has been shown to increase force of contraction in myocardium from different species including humans. The present study aimed to investigate the mechanism of the positive inotropic action of the active enantiomer S(-)BDF 9196 in human myocardium. In electrically driven human left ventricular papillary muscle strips (dilated cardiomyopathy, NYHA IV, cardiac transplantation, n=9), S(-)BDF 9196 increased force of contraction concentration-dependently. The maximal positive inotropic effect remained unchanged after the addition of carbachol (1 mmol/l, indicating a cAMP-independent mode of action of S(-)BDF 9196. While [3H]ouabain binding in human myocardial membranes was not influenced by S(-)BDF 9196 up to 10 micromol/l, the inward Na(+)-current in isolated human left ventricular myocytes was increased significantly by S(-)BDF 9196 (1 micromol/l, n=5). These results provide evidence that S(-)BDF 9196 increases force of contraction in human myocardium primarily by enhancing Na(+)-influx, while cAMP-dependent or Na(+),K(+)-ATPase blocking effects do not seem to play a role.

Positive inotropic effects of the novel Na+-channel modulator BDF 9198 in human nonfailing and failing myocardium.

The aim of this study was to investigate the inotropic properties of the novel Na+-channel modulator BDF 9198 in human nonfailing and failing myocardium. For comparison the Na+-channel modulator BDF 9148, the beta-adrenoceptor-agonist isoprenaline, and calcium were studied. Concentration-response curves for BDF 9198 (0.01-30 microM), BDF 9148 (0.01-30 microM), isoprenaline (0.001-1 microM), and calcium (1.8-15 mM) were obtained in electrically driven left ventricular human papillary muscle strips (1 Hz, 37 degrees C; dilated cardiomyopathy, NYHA IV, heart transplantation; nonfailing, donor hearts). Whereas isoprenaline was significantly less effective and less potent in increasing the force of contraction in failing human myocardium than in nonfailing myocardium (p < 0.01), BDF 9198 and BDF 9148 were (in NYHA IV) as effective as in nonfailing human tissue. In both tissues, BDF 9198 and BDF 9148 exerted similar positive inotropic effects as calcium, with the novel Na+-channel modulator BDF 9198 being more potent in increasing force of contraction than was the preceding agent BDF 9148. The potencies of both Na+-channel modulators, BDF 9198 and BDF 9148, were enhanced in human failing myocardium when compared with nonfailing myocardium. In summary, the novel Na+-channel modulator BDF 9198 increases force of contraction to the same extent as calcium and with a higher potency than BDF 9148. The sensitivity of failing human myocardium to Na+-channel modulators is increased when compared with nonfailing myocardium, which might be the result of an altered Na+ homeostasis in human heart failure.

Increase in force of contraction by activation of the Na+/Ca(2+)-exchanger in human myocardium.

The aim of the present study was to investigate whether agents which enhance force of contraction via increasing intracellular Na+, i.e. cAMP-independently, remain effective in failing human myocardium. Cumulative concentration-response curves with (+/-)BDF 9148 (0.01-10 mumol l-1), a Na(+)-channel activator, and ouabain (0.01-0.1 mumol l-1), a Na+/K(+)-ATPase inhibitor, were performed on electrically driven left ventricular human papillary muscle strips (1 Hz, 37 degrees C; dilative cardiomyopathy, NYHA IV, heart transplantation, n = 16; nonfailing, donor hearts, n = 5). The beta-adrenoceptor agonist isoprenaline (0.001-1 mumol l-1) and Ca2+ (1.8-15 mmol l-1) were studied for control. In addition, Ca2+ response curves were obtained on skinned fibre preparations from left ventricular myocardium (NYHA IV, n = 7) in the presence of BDF 9148 (1 mumol l-1) or a high Na+ concentration (50 mmol l-1) to investigate a possible direct or indirect interaction of (+/-)BDF 9148 with the myofilaments. While isoprenaline was significantly less effective in increasing force of contraction in failing human myocardium than in nonfailing myocardium (P < 0.01), in NYHA IV, (+/-)BDF 9148 and ouabain were as effective as in nonfailing human tissue. In failing and nonfailing myocardium, (+/-)BDF 9148 and ouabain exerted positive inotropic effects similar to those of Ca2+. However, the potency for (+/-)BDF 9148 to increase force of contraction was higher in NYHA IV than in nonfailing human myocardium (P < 0.05). Neither (+/-)BDF 9148 (1 mumol l-1) nor an increased concentration of Na+ (50 mmol l-1) altered the Ca2+ sensitivity or maximal developed tension of the contractile apparatus in experiments on chemically skinned left ventricular fibres. The enhanced sensitivity of the failing human myocardium towards Na(+)-channel modulation is not due to a direct or indirect interaction of (+/-)BDF 9148 with cardiac myofilaments but may be due to an altered Na(+)-homeostasis in human heart failure.

Interaction of Halothane with Inhibitory G-proteins in the Human Myocardium

Halothane has been reported to possess a catecholamine-sensitizing effect in laboratory animals and in anesthetized patients and to enhance the positive inotropic effect of isoproterenol in human papillary muscle strips. The current study was designed to investigate further the underlying subcellular mechanisms on human myocardium, in particular the mechanism of action of halothane on G-proteins. To investigate the effect of halothane on adenylyl cyclase activity, isoproterenol-, guanylylimidodiphosphate (Gpp(NH)p)-, and forskolin-activated enzyme activities were studied alone and in the presence of halothane in native and manganese-treated membranes. The mechanisms of halothane interaction with inhibitory G-proteins (G1) were studied in adenosine diphosphate-ribosylation studies with pertussis toxin and immunochemical techniques. Halothane (1%) augmented isoproterenol- and Gpp(NH)p-stimulated adenylyl cyclase activity but had no effect on forskolin-stimulated enzyme activity. Manganese ions inhibited the stimulating effect of isoproterenol and Gpp(NH)p on adenylyl cyclase activity, but the effect of forskolin remained unchanged in control and halothane-treated membranes. In the presence of pertussis toxin, the effect of isoproterenol and Gpp(NH)p on adenylyl cyclase activity was enhanced, but further stimulation by halothane was abolished. Halothane did not influence the attachment of Gi alpha to the membrane. No effect of halothane on adenosine diphosphate-ribosylation of Gi alpha by pertussis toxin was observed. Halothane stimulates adenylyl cyclase activity by inhibiting the function of the inhibitory G-proteins by interfering with the effects of the alpha subunits or beta gamma subunits with the effector. Decreased membrane attachment of Gi alpha in the presence of halothane does not occur. The interaction of alpha and beta gamma subunits is not affected by halothane. Halothane does not impair the binding of pertussis toxin to the Gi alpha-protein.

Studies of the nucleoside transporter inhibitor, draflazine, in the human myocardium.

1. The aim of the present study was to determine the effect of the nucleoside transporter inhibitor, draflazine, on the force of contraction in human myocardium and the affinity of the compound for the nucleoside transporter. Nucleoside transport inhibitors, like draflazine, are of potential importance for cardiopreservation of donor hearts for heart transplantation. 2. Functional experiments were performed in isolated electrically driven (1 Hz, 1.8 mmol l-1 Ca2+) human atrial trabeculae and ventricular papillary muscle strips. The affinity of draflazine for the myocardial nucleoside transporter was studied in isolated membranes from human ventricular myocardium and human erythrocytes in radioligand binding experiments using [3H]-nitrobenzylthioinosine ([3H]-NBTI). Dipyridamole was studied for comparison. 3. In membranes from human myocardium and erythrocytes, [3H]-NTBI labelled 1.18 pmol mg-1 protein and 23.0 pmol mg-1 protein, respectively, nucleoside transporter molecules with a KD value of 0.8 nmol l-1. Draflazine concentration-dependently inhibited binding of [3H]-NBTI to myocardial and erythrocyte membranes with a K(i)-value of 4.5 nmol l-1. The potency as judged from the K(i) values was ten times greater than that of dipyridamole in both myocardial and erythrocyte membranes. 4. Draflazine, at concentrations up to 100 mumol l-1, did not produce negative inotropic effects in atrial and ventricular myocardium. (-)-N6-phenylisopropyladenosine (R-PIA) and carbachol did not reduce force of contraction in ventricular myocardium, but exerted concentration-dependent direct negative inotropic effects in atrial myocardium. 5. The data provide evidence that draflazine specifically binds to the nucleoside transporter of the human heart and erythrocytes with high affinity. The compound does not produce negative inotropic effects at concentrations as high as 100 micromol 1-1.6. Draflazine could be a useful agent for cardio preservation because it does not produce cardio depressant effects. Thus, it may be possible to perfuse explanted hearts directly with this agent without the hazard of cardiodepression.

Inotropic and lusitropic dysfunction in myocardium from patients with dilated cardiomyopathy

Isometric force of contraction (DT), peak rate of tension increase (+T), peak rate of tension decrease (−T), time to peak tension (TPT), and time to half-relaxation (T12T) were measured in electrically driven human papillary muscle strips (New York Heart Association [NYHA] class IV heart transplants, dilated cardiomyopathy; nonfailing (NF) donor hearts, brain dead) (1 Hz, 37° C) under basal conditions (1.8 mmol/L Ca2+) and after stimulation with isoprenaline, ouabain, and Ca2+. There was no difference in the isometric contraction (+T, −T, TPT, and T12T) between NYHA IV hearts and NF hearts under basal conditions. Inotropic stimulation above 300% of basal DT increased −T significantly more in NF hearts (p < 0.05) compared with NYHA IV hearts. The effectiveness of ouabain and Ca2+ to increase DT was not significantly changed in NYHA IV hearts compared with NF hearts. The isoprenaline-mediated increase in DT was reduced (p < 0.05) in NYHA IV hearts to a similar extent (70%) as β-adrenoceptors were downregulated. When the rate of stimulation was increased to 3 Hz (force-frequency relationship), force of contraction increased only in NF preparations, whereas it decreased in NYHA IV myocardium (p < 0.05). It was concluded that the contractile apparatus in terminally failing human myocardium is sufficient to maximally increase DT. During inotropic stimulation, abnormalities in diastolic rather than systolic contraction become evident. This may indicate abnormal intracellular Ca2+ handling.

Evidence for a sustained effectiveness of sodium-channel activators in failing human myocardium

Elevation of cytosolic sodium is thought to be correlated with an increase in force of contraction due to an activation of sodium-calcium exchange. We investigated the inotropic response mediated by the new sodium-channel activator BDF 9148 (0.01–100 μmol/l) on failing human myocardium. Force of contraction was studied using electrically driven human papillary muscle strips from moderately (NYHA II–III, mitral valve replacement) and terminally (NYHA IV, heart transplantation) failing hearts. We also investigated the effects in auricular trabeculae from non-failing hearts (aortocoronary bypass operation). Results were compared with inotropic responses to DPI 201-106 (DPI, 0.1–3 μmol/l), Ca 2+ (1.8–15 mmol/l) and isoprenaline (0.001–1 μmol/l). Carbachol (100 μmol/l) and adenosine (1000 μmol/l) were examined in the presence of BDF 9148 and isoprenaline. Both sodium-channel activators, BDF 9148 and DPI 201-106, increased force of contraction in a dose-dependent manner in papillary muscle strips as well as in auricular trabeculae. BDF 9148 and DPI 201-106 were more effective (max. PIE NYHA II–III 1.6 ± 0.2 mN, NYHA IV 5.9 ± 0.7 mN, P<0.05) and more potent (EC 50 (in μmol/l): NYHA IV 0.35, 0.19–0.66; NYHA II–III 1.85, 1.37–2.41) in terminally failing as compared to moderately failing left ventricular myocardium. Moreover, the positive inotropic effects of BDF 9148 were greater than those of DPI 201-106 in NYHA IV (max. PIE 2.7 ± 0.3 mN, P<0.05). In NYHA IV, BDF 9148 was as effective as CA 2+ (max. PIE 5.1 ± 0.4 mN). In the same hearts, the positive inotropic effects of isoprenaline were reduced in NYHA IV (max. PIE 2.1 ± 0.3 mN) compared to NYHA II–III (max. PIE 3.4 ± 0.4 mN, P<0.05). Adenosine as well as carbachol did not affect the positive inotropic response of BDF 9148 or DPI 201-106 but reduced the effectiveness of isoprenaline ( P<0.05). In myocardial membranes, BDF 9148 was 1000-fold less effective in competition experiments with 3H-ouabain than ouabain. We conclude that (1) sodium-channel activators may produce a significant cAMP-independent positive inotropic effect in left ventricular myocardium from failing human hearts; (2) the inotropic effect of sodium-channel activators were more potent and more effective in NYHA IV as compared to NYHA II–III. The degree of myocardial failure does not reduce the effectiveness of the sodium-channel activator BDF 9148.

Negative Inotropic Activity of the Calcium Antagonists Isradipine, Nifedipine, Diltiazem, and Verapamil in Diseased Human Myocardium

The use of calcium antagonists in the treatment of cardiac failure is limited by the negative inotropic effects they exert. The comparative inotropic activity of four calcium antagonists was therefore investigated, using electrically driven human papillary muscle strips and human atrial trabeculae. These inotropic effects were studied with cumulative concentration-response curves. The four calcium antagonists included in this study were all found to significantly (P less than or equal to .05) diminish the force of contraction above 0.01 mumol/L, but with different potencies. On the basis of the relationship between therapeutic vasoactive plasma concentrations and the negative inotropic effective concentrations in our system in vitro, the ranking order of potential negative inotropism in the treatment of cardiac failure with calcium antagonists was verapamil greater than nifedipine greater than diltiazem greater than isradipine. Thus, to minimize cardiodepressant risk, the use of calcium antagonists with weak negative inotropic effects at relevant concentrations should be preferred, especially in the case of patients with compromised cardiac function.

Different negative inotropic activity of Ca2+-antagonists in human myocardial tissue

To evaluate the negative inotropic effect of various Ca2(+)-antagonists in human myocardium without additional influences of preload, afterload, or frequency, we examined their effects on isometric force of contraction in isolated human papillary muscle strips and in auricular trabeculae. The 1,4-dihydropyridines isradipine, nitrendipine, and nifedipine, the phenylalkylamine verapamil, and the benzothiazepine diltiazem exerted concentration-dependent negative inotropic effects. The potency of the investigated Ca2(+)-antagonists was identical in papillary muscle strips of patients with only moderate clinical signs of heart failure undergoing mitral valve replacement-operation (NYHA II-III) and in terminally failing (heart transplantation, NYHA IV) human hearts. The IC50 values were lower in auricular trabeculae than in papillary muscle strips. The difference was significant for nifedipine, nitrendipine, and verapamil. The restorative effects of external Ca2+ after pretreatment with Ca2(+)-antagonists were significantly less strong after pretreatment with 1,4-dihydropyridine than with non-dihydropyridines in papillary muscle strips. It is concluded that 1,4-dihydropyridines and verapamil and diltiazem did differently influence Ca2(+)-mediated increase in force of contraction. Moreover, a relation between the therapeutically active free plasma concentration in vivo and the negative inotropic potency in vitro can be found. This relation follows a rank order of potency for negative inotropism (isradipine less than or equal to nitrendipine less than diltiazem less than nifedipine less than verapamil) and might have clinical relevance in the treatment of patients with compromised cardiac function.

Negative Inotropic Properties of Isradipine, Nifedipine, Diltiazem, and Verapamil in Diseased Human Myocardial Tissue

We investigated the inotropic responses to Ca2+ antagonists using electrically driven human papillary muscle strips and human auricular trabeculae. Specimens were obtained during cardiac surgery for mitral valve replacement [New York Heart Association (NYHA) Class II-III] or heart transplantation (NYHA IV) and during aortocoronary bypass operations. The inotropic effects were studied with cumulative concentration-response curves. All Ca2+ antagonists tested significantly (p less than 0.05) depressed force of contraction at concentrations above 0.01 mumol/L, but their potencies were different. A 50% reduction of the initial force of contraction occurred at the following concentrations (NYHA II-III): nifedipine (mean IC50) 0.09 mumol/L isradipine 0.12 mumol/L, diltiazem 0.69 mumol/L, and verapamil 0.79 mumol/L. There were no significant differences in the negative inotropic effects of any tested Ca2+ antagonist between NYHA II-III and NYHA IV. When the initial force of contraction was reduced by 90%, addition of Ca2+ increased force of contraction significantly less after diltiazem (2.76 +/- 0.4 mN), isradipine (1.82 +/- 0.23 mN), and nifedipine (1.68 +/- 0.25 mN) compared to control (4.63 +/- 0.56 mN) (NYHA II-III). The negative inotropic potencies of nifedipine and verapamil were significantly greater in human auricular trabeculae compared to papillary muscle strips (p less than 0.05). However, on the relation between therapeutic vasoactive plasma concentrations and IC25 values, an entirely different rank order of potential negative inotropism could be observed: verapamil greater than nifedipine greater than diltiazem greater than isradipine.

The effect of xamoterol in failing human myocardium

As the dual pharmacological action of partial beta 1-adrenoceptor agonists should improve left ventricular function while also protecting the myocardium against excessive sympathetic stimulation they may be useful in the treatment of heart failure. Therefore, we studied the pharmacological effects of xamoterol (Corwin, ICI 118, 587), a compound with mixed agonist and antagonistic properties at cardiac beta-adrenoceptors in electrically driven human papillary muscle strips from failing human hearts. Specimens were obtained from patients with different grades of myocardial failure who underwent mitral valve replacement (NYHA II-III) or heart transplantation (NYHA IV). Xamoterol (0.0001-100 mumol l-1) produced only negative inotropic effects, as measured by changes in isometric force of contraction in diseased human papillary muscle strips. However, isoprenaline (0.0001-10 mumol l-1) and ouabain (0.01-0.3 mumol l-1) enhanced force of contraction in the same hearts. Prestimulation with noradrenaline (3 mumol l-1) augmented the negative inotropic effect of xamoterol. It is concluded that xamoterol exerts primarily beta-adrenoceptor antagonistic activity in the failing human myocardium.

Screening of positive inotropic agents in isolated cardiac preparations from different sources

The present study was designed to elucidate whether or not the positive inotropic effects of isoprenaline, milrinone, and DPI 201-106 in myocardial tissue from various sources reveal reliable results for the failing human heart. Therefore, in vitro positive inotropic responses were studied in human papillary muscle strips from patients with moderate heart failure (NNYHA II–III), human atrial trabeculae (HAT), isolated papillary muscles from Wistar-Kyoto rats (WK), and from spontaneously hypertensive rats (SHR). Results were compared with the effects of the compounds in papillary muscle strips from patients with severe heart failure (NYHA IV). In NYHA IV, positive inotropic responses were smaller for isoprenaline and milrinone than in NYHA II–III. The response to DPI 201-106 was more pronounced. In HAT, the effects of isoprenaline and milrinone were greater than in NYHA IV. The positive inotropic effect of DPI 201-106 was similar. In SHR, only the positive inotropic effect of isoprenaline was smaller than in WK. The effects of DPI 201-106 and milrinone did not differ. These data show that inotropic responses in NYHA II–III, HAT, WK, and SHR differ from the severely failing myocardium. It is concluded that new positive inotropic agents should be screened in human myocardial tissue from patients with heart failure, as experimental results from other sources may be irrelevant in this respect.

Enhancement of the effectiveness of milrinone to increase force of contraction by stimulation of cardiac beta-adrenoceptors in the failing human heart.

The positive inotropic effect of milrinone was investigated in isolated, electrically driven (1 Hz) human papillary muscle strips from nonfailing myocardium (control) and from patients with moderate (NYHA II-III) and severe (NYHA IV) heart failure. In the control hearts, milrinone increased force of contraction to about the same degree as Ca2+ at 15 mmol/l. In NYHA II-III, the positive inotropic effect was significantly reduced compared with the control hearts. In NYHA IV, the effectiveness (maximal increase in force of contraction) and potency (as judged from the EC50 values) were significantly less than in NYHA II-III and controls. Preexposure of the preparations to isoprenaline in NYHA II-III and NYHA IV shifted the concentration-response curve for milrinone to the left and, moreover, restored the effectiveness of the compound (i.e., similar positive inotropic effect as Ca2+). No difference in the cAMP-phosphodiesterase inhibition by milrinone could be detected between controls, NYHA II-III, and NYHA IV. It is concluded that diminished basal cAMP production is responsible for reducing the effectiveness of milrinone in the failing human heart. A diminished inhibition of cAMP-phosphodiesterase does not play a role. The fact that stimulation of cardiac beta-adrenoceptors increased the effectiveness of milrinone provides evidence that combined application of catecholamines and phosphodiesterase inhibitors may be useful in the treatment of patients with terminal heart failure.

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)

H-ANF does not play a role in the regulation of myocardial force of contraction

In the present study, h-ANF failed to reduce myocardial force of contraction alone and in the presence of isoprenaline. This lack of effect was observed in electrically driven, isolated human papillary muscle strips and rat papillary muscles. In contrast, adenosine and carbachol exerted antiadrenergic actions in rat and human myocardium. It is concluded that h-ANF obviously does not play a role in the regulation of human and rat myocardial force of contraction, whereas adenosine and carbachol are capable to produce antiadrenergic effects in the human and rat myocardium.

Dobutamine: positive inotropy by nonselective adrenoceptor agonism in isolated guinea pig and human myocardium.

Positive inotropic responses to dobutamine have been examined using isolated myocardium from guinea pigs und humans. The potency (EC50) of dobutamine was 1.5 X 10(-6) mol/l on guinea pig papillary muscles, 1.8 X 10(-6) mol/l on guinea pig left atria and 2.5 X 10(-6) mol/l on human papillary muscle strips. In guinea pig cardiac muscles, Schild plots for the beta 1-selective antagonist, 1-practolol, using dobutamine as agonist, had slopes of less than unity. This suggested the involvement of other receptors in the inotropic response to dobutamine. The beta 2-selective antagonist, ICI 118,551, but not the alpha 1-selective antagonist, prazosin, attenuated the dobutamine response in guinea pig papillary muscles. Both ICI 118,551 and prazosin shifted the dobutamine concentration-response curve in guinea pig left atria. The positive inotropic response to dobutamine in human papillary muscles was antagonised by l-practolol and ICI 118,551 but not by prazosin. The maximal inotropic response to dobutamine was 90% that of calcium measured in the same guinea pig papillary muscles but only 37% that of calcium in human papillary muscle strips. This reduced maximal effect of dobutamine in human myocardium is probably a disease-induced change but species variations cannot be excluded.

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 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.

Response of isolated human ventricular myocardium to cyclic AMP and its dibutyryl derivative.

The contractile responses to c-AMP and DB-c-AMP were studied in isolated electrically stimulated human papillary muscle strips. C-AMP (1×10−4 to 1×10−3 M) had no effect on contractile force in all of 6 human papillary muscle preparations studied. In contrast, DB-c-AMP (10−4 to 5×10−3 M) produced a concentration-dependent and reversible positive inotropic effect which was associated by a decrease in time to peak force and in relaxation time and which was not inhibited by 10−6 M propranolol. The possibility of a clinical applicability of DB-c-AMP is discussed.

Influence of glucose metabolism on ouabain-induced changes in the transmembrane potential and contraction of human heart in vitro.

Effects of glucose, anoxia, iodoacetate, and 2,4-dinitrophenol on the ouabin-induced changes in the simultaneously recorded transmembrane action potential and contraction of 72 human papillary muscle strips obtained from 36 patients undergoing corrective open heart surgery were investigated. Ouabain (10−8M, 10−9M, 10−10M) produced a shortening of the action potential duration (APD) and an increase in the force of contraction in the muscle. Non-oxygenated glucose-free solution, iodoacetate (10−5M), and 2,4-dinitrophenol (10−6M) produced partial to complete inhibition of the positive inotropic effects of ouabain (10−8M or 10−9M) in the papillary muscles. The shortening of the APD produced by ouabain was further enhanced in the presence of the above metabolic inhibitors. As the glucose level was raised (0, 5, 10, 20, 30 mM), anoxic papillary muscles responded to ouabain by a progressive increase in the force of contraction and a parallel increase in the APD. Glucose (30 mM) was effective in restoring the positive inotropic effects of ouabain in DNP-poisoned muscle while it was ineffective in doing so in the muscle poisoned with both IAA and DNP. These results indicate the requirement of energy for the positive inotropic effects of ouabain in cardiac muscle.