Sustained Positive Inotropic Effect Research Articles

Acute effects of nonexcitatory electrical stimulation during systole in isolated cardiac myocytes and perfused heart.

Application of electrical field to the heart during the refractory period of the beat has been shown to increase the force of contraction both in animal models and in heart failure patients (cardiac contractility modulation, or CCM). A direct increase in intracellular calcium during CCM has been suggested to be the mechanism behind the positive inotropic effect of CCM. We studied the effect of CCM on isolated rabbit cardiomyocytes and perfused whole rat hearts. The effect of CCM was observed in single cells via fluorescent measurements of intracellular calcium concentration ([Ca2+]i) and cell length (L). Cells were paced once per second throughout these recordings, and CCM stimulation was delivered via biphasic electric fields of 20 ms duration applied during the refractory period. CCM increased the peak amplitude of both [Ca2+]i and L for the first beat during CCM compared to control, but then [Ca2+]i and L decayed to levels lower than the control. During CCM, all contractions had a faster time to peak for both [Ca2+]i and L; after stopping CCM the rise times returned to control levels. In the whole rat heart, the positive inotropic effect of CCM stimulation on left ventricular pressure was completely abolished in the presence of metoprolol, a beta‐1 adrenergic blocker. In summary, the CCM‐induced changes in intracellular calcium handling by cardiomyocytes did not explain the sustained positive inotropic effect in the whole heart and the β‐adrenergic pathway may be involved in the CCM mechanism of action.

Reduced contractile response to α 1-adrenergic stimulation in atria from mice with chronic cardiac calmodulin kinase II inhibition

The sustained positive inotropic effect of α-adrenoceptor agonists in the heart is associated with a small increase in intracellular Ca 2+ transients together with a larger sensitization of myofilaments to Ca 2+. The multifunctional Ca 2+ and calmodulin-dependent protein kinase II (CaMKII) could contribute to this effect, either by affecting the Ca 2+ release (ryanodine receptor) or by an uptake mechanism (via phospholamban [PLB] and SR Ca 2+ ATPase). Here we examined the role of CaMKII in the positive inotropic effect of the α-adrenoceptor agonist phenylephrine in left atria isolated from a genetic mouse model of cardiac CaMKII inhibition (AC3-I). Compared to atria from wild-type (WT) or AC3-C (scrambled peptide), AC3-I atria showed the following abnormalities. PLB phosphorylation at Thr17, a known CaMKII target, was significantly lower (∼ 20%). Post-rest (30 s, 1 Hz, 37 °C) potentiation of force was absent (AC3-C, 190% of pre-rest amplitude). Basal force was ∼ 20% lower at 1.8 mM Ca 2+, but normal at high Ca 2+ concentration (> 4.5 mM). The maximal positive inotropic effect of phenylephrine, which was more pronounced at low frequencies in WT and AC3-C atria, lost its frequency dependence (1 Hz to 8 Hz). Thus, the effect of phenylephrine was reduced by ∼ 50% at 1 Hz, but was normal at 8 Hz. All three groups showed a negative force–frequency relation, and did not differ in the frequency-dependent acceleration of relaxation. Our data indicate a role of CaMKII in post-rest potentiation and the positive inotropic effect of α-adrenergic stimulation at low frequencies.

α 1‐Adrenoceptor‐mediated Negative Inotropic Effect Caused by Intracellular Ionic Activities in Guinea‐pig Papillary Muscle

In guinea-pig papillary muscle, phenylephrine (PE), an agonist of alpha1-adrenoceptor (alpha1-AR), led to a transient negative inotropic effect (NIE) and a subsequent sustained positive inotropic effect (PIE). To clarify the ionic mechanisms underlying the NIE, we measured the [Na+]i or [pH]i by ion-selective microelectrodes. PE produced a decrease in the intracellular Na+ concentration ([Na+]i) and an increase in intracellular pH ([pH]i). During the phase of NIE, PE produced only a (-) change of [Na+]i (Delta[Na+]i). With a decrease in extracellular Na+ or an increase in extracellular Ca2+, the PE-induced NIE was attenuated and PE produced (+)Delta[Na+]i. The PE-induced NIE and (-)Delta[Na+]i were definitely strengthened by lowering the bath temperature or increasing the stimulation frequency. 2-(2,6-di-methoxyphenoxyethyl)amino-methyl-1,4-benzidioxane HCl, an antagonist of alpha1A-AR, completely abolished the PE-induced NIE and (-)Delta[Na+]i. Phorbol 12,13-dibutyrate, an activator of protein kinase C (PKC), decreased the baseline [Na+]i and twitch force and increased the baseline [pH]i in mimicry of PE. Pretreatment with 1-5(isoquinolinesulphonyl)-2-methylpiperazine, an inhibitor of PKC, abolished the PE-induced NIE and (-)Delta[Na+]i. During pretreatment with benzamil, an inhibitor of Na+/Ca2+ exchange, we found that the PE-induced NIE and (-)Delta[Na+]i were reversibly abolished. Our results indicate the PE-induced NIE may be elicited upon the activation of Na+/Ca2+ exchange which can be attributed to the (-)Delta[Na+]i. (-)Delta[Na+]i is mediated through a PKC-dependent pathway via an activation of alpha1A-AR subtype and its effect could be strengthened remarkably at high [Na+]i and [Ca2+]i values.

Key role of myosin light chain (MLC) kinase-mediated MLC2a phosphorylation in the ?-adrenergic positive inotropic effect in human atrium

Mechanisms of the positive inotropic response to alpha(1)-adrenergic stimulation in the heart remain poorly understood, but recent evidence in rat papillary muscle suggests an important role of regulatory myosin light chain (MLC2) phosphorylation. This study investigated alpha(1)-adrenergic contractile effects and the role of MLC kinase (MLCK)-dependent phosphorylation of MLC2 in human atrial muscle strips. Force measurement was performed on electrically stimulated atrial muscle strips (n=140; 20 hearts) in the presence of the beta-blocker nadolol. MLC2a phosphorylation was determined by 2D-polyacrylamide gel electrophoresis and Western blotting of muscle strips that were immediately freeze-clamped following force measurements. The alpha(1)-agonist phenylephrine (PE; 0.3-100 microM) exerted a concentration-dependent, monophasic, sustained positive inotropic effect (86% above basal) that was accompanied by an 80% increase in MLC2a phosphorylation. Desinhibition of MLC phosphatase by the Rho kinase inhibitor Y-27632 (10 microM) reduced the effect of PE by 16%. The MLCK inhibitor wortmannin (10 microM) completely abolished both the PE-induced increase in force and MLC2a phosphorylation. The structurally unrelated MLCK inhibitor ML-7 (10 microM) had similar effects. Neither Y-27632 nor wortmannin or ML-7 affected beta-adrenergic force stimulation. In contrast to our findings in atrial muscle strips, we observed no increase in MLC2v phosphorylation after PE in human ventricular muscle strips and wortmannin failed to inhibit PE-induced force of contraction. alpha(1)-Adrenergic receptors mediate a prominent increase in contractile force in human atria that depends on MLCK activity and is accompanied by an increase in MLC2 phosphorylation.

Receptor Subtypes Mediating the Inotropic Effects and Ca2+ Signaling Induced by Endothelin-1 Through Crosstalk With Norepinephrine in Canine Ventricular Myocardium

In canine ventricular myocardium, endothelin-1 (ET-1) alone induced only a weak transient negative inotropic effect (NIE). However, ET-1 induced a marked sustained positive inotropic effect (PIE) subsequent to a transient NIE in the presence of norepinephrine (NE) at low concentrations (0.1 – 1 nM) and elicited a pronounced sustained NIE in the presence of NE at high concentrations (around 100 nM). Thus, the extent of β-adrenoceptor stimulation induced by NE played a crucial role in determining the characteristics of the inotropic effects of ET-1. The characteristics of ET receptor subtypes involved in contractile regulation and Ca2+ signaling induced by ET-1 were determined. The ET-1-induced transient NIE and decrease in Ca2+ transients were abolished by the selective ETA-receptor antagonist FR319317, but not by the selective ETB-receptor antagonist BQ-788. The sustained PIE and the increase in Ca2+ transients induced by ET-1 were abolished by FR319317, but not inhibited by BQ-788. In contrast, the sustained NIE of ET-1 was abolished by the non-selective ET antagonist TAK-044, markedly attenuated by FR319317, and partially inhibited by BQ-788. ET-1 alone elicited a PIE in the presence of BQ-788, which indicates that the activation of ETB-receptors counteracts the development of the PIE of ET-1. The current findings indicate that both ETA and ETB receptors are involved in the regulation of Ca2+ signaling and contractility in canine ventricular myocardium.

Pertussis toxin-sensitive and -insensitive mechanisms of α 1-adrenoceptor-mediated inotropic responses in rat heart

In rat left ventricular papillary muscle, phenylephrine, an α1-adrenoceptor agonist, induced a triphasic inotropic response; an initial transient, small, positive inotropic effect followed by a transient chloroethylclonidine-sensitive negative inotropic effect and a sustained 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB4101)-sensitive positive inotropic effect. Treatment with pertussis toxin for 2 days significantly inhibited only the transient negative inotropic effect without changing the sustained positive inotropic effect. This treatment also prevented the acetylcholine (1 μM)-induced negative inotropic effect. Further, phenylephrine-induced transient negative inotropic effect was attenuated in the presence of ouabain. These results suggest that pertussis toxin-sensitive or-insensitive G-protein may be responsible for α1-adrenoceptor subtype-mediated negative inotropic effect or positive inotropic effect, respectively, in which the transient negative inotropic effect was produced via the stimulation of Na +, K + pump, presumably through pertussis toxin-sensitive G-protein-dependent pathway.

Biphasic inotropic response to endothelin-1 in the presence of various concentrations of norepinephrine in dog ventricular myocardium.

The present study was undertaken to investigate the interaction between endothelin-1 (ET-1) and norepinephrine (NE) on contractile regulation in dog ventricular myocardium. ET-1 alone did not elicit any inotropic response in isolated dog ventricular trabeculae (37 degrees C, 0.5 Hz). In the presence of NE at a high concentration (10(-7) M), ET-1 (10(-8) M) elicited a long-lasting negative inotropic effect, while in the presence of NE at a moderate concentration (3 x 10(-8) M) it produced a biphasic inotropic effect: a sustained positive inotropic effect subsequent to a short-lasting negative inotropic effect. In the presence of a lower concentration (10(-9) M) that affected scarcely the basal force of contraction, ET-1 produced a pronounced positive inotropic effect in association with negative lusitropic and negative clinotropic effects in a concentration-dependent manner subsequent to a small transient negative inotropic effect. The presented results indicate that not only the extent, but also the quality of the inotropic response to ET-1 is determined by the level of NE in the biophase. The crosstalk of ET-1 with NE may play a crucial role in pathophysiological regulation of cardiac contractility in intact dog ventricular myocardium.

Role of α1-adrenoceptor subtypes in production of the positive inotropic effects in mammalian myocardium: Implications for the α1-adrenoceptor subtype distribution

Both α 1A- and α 1B-adrenoceptor subtypes are present in mammalian myocardium. α 1-Adrenoceptor activation can enhance myocardial contractility, and two possible inotropic mechanisms are proposed: an increase in myofilament Ca 2+ sensitivity and an increase in transsarcolemmal Ca 2+ influx during action potential prolongation that accompanies the transient outward current inhibition. We suggest that the former is mediated by the α 1B-adrenoceptor subtype and the latter is by the α 1A-adrenoceptor subtype. The α 1B-adrenoceptor subtype may be located on a space more proximal to the sympathetic nerve endings than the α 1A-adrenoceptor subtype, because the positive inotropic effect of endogenous norepinephrine was mediated entirely by the α 1B-adrenoceptor subtype. In some species, the sustained positive inotropic effect develops following the transient negative inotropic effect, which is mediated by the α 1A-adrenoceptor subtype.

Hypoxia-induced activation of KATP channels limits energy depletion in the guinea pig heart.

The functional role of ATP-dependent potassium (KATP) in hypoxic cardiac failure was investigated in isolated guinea pig hearts with glibenclamide and rimalkalim as inhibitor and activator, respectively. Monophasic action potential duration at 90% of repolarization (MAP50), left ventricular function, and cardiac energy status (31P nuclear magnetic resonance spectroscopy) were measured during normotoxic (95% O2) and hypoxic (20% O2) perfusion. In normoxic hearts, 1 microM glibenclamide did not affect MAP50, left ventricular function, and coronary flow (n = 4). In contrast, rimalkalim rapidly shortened MAP50 and left ventricular pressure (LVP) in a dose-dependent fashion (e.g., by 60.2 +/- 3.5 and 80.8 +/- 8.2%, respectively, with 0.6 microM rimalkalim). This latter effect was reversed by 1 microM (glibenclamide (n = 4). With hypoxic perfusion, a reduction in LVP was observed, along with a shortening of the action potential (MAP90; 202 +/- 13 vs. 164 +/- 9 ms) and an increase in coronary flow. Glibenclamide (1 microM) reversed the MAP90 shortening and the increase in coronary flow. In addition, glibenclamide increased LVP transiently (n = 4). When coronary flow of hypoxic hearts was kept constant, however, glibenclamide elicited a sustained positive inotropic effect (n = 7). After glibenclamide, an increase in LVP from 54 +/- 4 to 64 +/- 3 mmHg was observed, along with a reduction in the free energy change of ATP hydrolysis from -54.5 +/- 1.9 to -52.9 +/- 0.2 nJ/mol and a further increase in the coronary venous adenosine from 269 +/- 48 to 1,680 +/- 670 nmol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 1-adrenergic effects on intracellular pH and calcium and on myofilaments in single rat cardiac cells.

1. The cellular effects of alpha 1-adrenoceptor stimulation by phenylephrine were studied in the presence of propranolol in single cells isolated from the ventricles of rat hearts. 2. Phenylephrine (10-100 microM) induced a biphasic pattern of inotropism in these cells: a transient negative followed by a sustained positive inotropic effect as usually observed in cardiac tissues. 3. In Snarf-1-loaded cells, phenylephrine induced an alkalinization. This effect was reversible on wash-out and inhibited by prazosin, an alpha 1-adrenoceptor antagonist. 4. The alpha 1-adrenoceptor-mediated increase in intracellular pH (pHi) was 0.1 pH unit in HEPES buffer containing 4.4 mM-NaHCO3 and in Krebs buffer containing 25 mM-NaHCO3. 5. The alkalinization was blocked by the Na(+)-H+ antiport blocker, ethylisopropylamiloride (EIPA). 6. The recovery from an acidosis induced by a NH4Cl pre-pulse was accelerated by phenylephrine. The phenylephrine-induced alkalinization was attributed to activation of the Na(+)-H+ antiport. 7. Despite its ability to increase pHi, phenylephrine did not alter Ca2+ current amplitude and kinetics. 8. Ca2+ transients recorded in Indo-1-loaded cells were not augmented by phenylephrine. Diastolic calcium level was decreased. 9. In single skinned cells, the Ca2+ sensitivity of the contractile proteins was increased by a pre-treatment with phenylephrine even when the alpha 1-adrenoceptor-mediated alkalinizing effect had been prevented by EIPA. 10. These results lead us to propose that the alpha 1-adrenergic-induced positive inotropic response of heart muscle could result from an increased sensitivity of the myofilaments to Ca2+ ions. This alpha 1-adrenoceptor-mediated Ca2+ sensitization could result both from an intracellular alkalinization and from a direct effect on contractile proteins.

Opposite modulation of ouabain cardiotoxicity by hexamethyleneamiloride and phenylephrine.

To see whether the Na/H antiporter plays a role in digitalis cardiotoxicity, we investigated the influence of modulators of Na/H exchange on the toxic effects of ouabain in isolated, paced (0.4 Hz) rat left atria. Ouabain (1 mmol/l) caused a transient positive inotropic effect followed by toxic events, including a complete loss of developed force and a gradual increase in resting force. In the presence of hexamethyleneamiloride (3 and 10 mumol/l), an inhibitor of Na/H exchange, ouabain (1 mmol/l) caused a sustained positive inotropic effect without toxicity. By contrast, phenylephrine (100 mumol/l), an alpha-adrenoceptor agonist reported to stimulate the antiporter, hastened the development of ouabain's toxicity. Neither ouabain, at a subtoxic concentration (650 mumol/l), nor phenylephrine (100 mumol/l) affected diastolic force, but in their combined presence, a substantial contracture developed and twitch contractions disappeared. Phenylephrine (30 or 100 mumol/l) or adrenaline (30 mumol/l), in the presence of a beta-adrenoceptor antagonist, increased the intracellular pH by up to 0.15 pH unit, as measured using ion-selective microelectrodes in quiescent preparations. This effect on pHi was prevented by hexamethyleneamiloride (10 mumol/l). Consistent with phenylephrine's ability to stimulate Na+ influx via the Na/H antiporter, phenylephrine (100 mumol/l) increased intracellular Na+ activity by about 3 mmol/l in ouabain (650 mumol/l)-treated atria. These findings indicate that modulators of Na/H exchange affect the cardiotoxicity of digitalis glycosides and imply that the stimulation of myocardial alpha-adrenoceptors may aggravate digitalis toxicity.

Differential effect of R 56865 on ouabain binding to isolated sarcolemma and intact atrial tissue of guinea‐pig

1. R 56865 (N-[1-[4-(4-fluorophenoxy)-butyl]-4-piperidinyl]-N-methyl- 2-benzothiazolamine) is a compound known to antagonize cardiac glycoside intoxication. Therefore, the effect of the compound on ouabain binding to intact cardiac tissue as well as cardiac membrane preparations was investigated. 2. The binding of ouabain to highly purified sarcolemmal membranes was not influenced by R 56865 1 x 10(-6) mol l-1 (ouabain: KD = 1.3 x 10(-7) mol l-1, Bmax = 160 pmol mg-1; ouabain + R 56865: KD = 1.4 x 10(-7) mol l-1, Bmax = 168 pmol mg-1). 3. In contrast to the results in purified membranes, the binding of ouabain (10(-8) mol l-1 to 5 x 10(-7) mol l-1) to intact atria was significantly reduced. 4. Ouabain, 5 x 10(-7) mol l-1, led to a transient positive inotropic effect of about 220% followed by a developing negative inotropic effect after 3 h. R 56865, 10(-7) mol l-1, led to a maximal positive inotropic effect of about 290% also followed by a delayed decline of contractile force. A tenfold higher concentration of R 56865 led to sustained positive inotropic effect of about 250% in the same time interval. 5. The different effects of R 56865 on ouabain binding in subcellular preparations and intact tissue do not support the view that R 56865 interferes directly with the action of ouabain on Na/K-ATPase. An indirect effect, which may be mediated by a lowered intracellular sodium load is discussed.

Effects of inhibitors of protein kinase C and Na +/H + exchange on α1‐adrenoceptor‐mediated inotropic responses in the rat left ventricular papillary muscle

1. Alpha 1-adrenoceptor stimulation of rat left ventricular papillary muscle produced a triphasic inotropic response: an initial transient positive inotropic effect (PIE) followed by a transient negative inotropic effect (NIE) and a sustained PIE. 2. The protein kinase C inhibitor, staurosporine, at concentrations ranging from 30 nM to 100 nM inhibited the sustained PIE, but had no significant effect on the transient PIE and NIE. 3. H-7, 1-(5-isoquinoline sulphonyl)-2-methylpiperazine, a less specific inhibitor of protein kinase C than staurosporine, at a concentration of 100 microM inhibited both the transient NIE and the sustained PIE without affecting the transient PIE. 4. Amiloride, an inhibitor of Na+/H+ exchange, at concentrations ranging from 0.1 mM to 1 mM inhibited the sustained PIE and, at higher concentrations, also inhibited the transient NIE. 5. An amiloride analogue, 5-(N-methyl-N-isobutyl)amiloride (MIBA), inhibited only the sustained PIE with an IC50 of 0.3 microM which is approximately two orders of magnitude lower than amiloride. 6. The receptor-linked stimulation of Na+/H+ exchange through protein kinase C activation may be a mechanism for alpha 1-adrenoceptor-mediated sustained PIE.

Myocardial response to alpha-agonist (phenylephrine) in relation to age.

We studied the inotropic effect of the alpha-agonist phenylephrine on human and rat myocardium in relation to age. Strips of human atrial myocardium, discarded during cardiac surgery, were obtained from pediatric (2 to 18 months) and adult (40 to 60 yr) patients, and their electrically driven contractions were studied. The inotropic response of pediatric myocardium to phenylephrine was five times larger than that of the adult myocardium (p less than .05). The beta-agonist, isoproterenol, was also administered to the same myocardial specimens, and the average pediatric myocardial response was three times larger than that of the adult myocardium (p less than .05). Strips of right ventricular myocardium were obtained from four age groups of rats: 14 days, 3 months, 6 months, and 18 months, and exposed to phenylephrine. We demonstrated two age-related phenomena: a) a biphasic response with an early short negative inotropic effect preceding a sustained positive inotropic effect was registered in most of the rats aged 3 months and older, but in none of the youngest age group. b) A high mean positive inotropic response (211% of basal response) was generated in the youngest age group, whereas the oldest age group of rats showed only a slight response (8% of basal response, p less than .05). We conclude that loss of myocardial responsiveness to alpha and beta-agonist with increasing age is a prominent feature. The pattern of the ventricular responsiveness to phenylephrine also changes during aging as it turns from monophasic to biphasic.

How effective is digitalis in the treatment of congestive heart failure?

The evidence suggests that digitalis glycosides do indeed improve ventricular performance through a sustained but moderate positive inotropic effect. This effect is more marked in failing than in nonfailing myocardium. The clinical studies suggest a moderate salutary effect in patients with chronic CHF who are in sinus rhythm. The drug can be given safely to patients with CAD and in combination with other medications when the physician is aware of those factors leading to increased sensitivity to digitalis.

Alpha 1-adrenoceptor-mediated phosphoinositide breakdown and inotropic response in rat left ventricular papillary muscles.

alpha 1-Adrenoceptor stimulation of rat left ventricular papillary muscles by phenylephrine in the presence of propranolol resulted in rapid breakdown of phosphatidylinositol 4,5-bisphosphate (PI-4,5-P2) and a triphasic inotropic response in a concentration-dependent manner. The release of inositol trisphosphate (IP3) was maximum within 30 seconds and remained high for at least 30 minutes. The IP3 formation was associated with a rapid, but small, increase in contractile force followed by a transient decline in the contractility prior to the development of a sustained and more pronounced positive inotropic response. Inhibition of PI-4,5-P2 hydrolysis by the alpha 1-adrenergic antagonist prazosin or the PI-4,5-P2 phosphodiesterase inhibitor neomycin blocked all components of the inotropic responses. Combined addition of 2,3-diphosphoglyceric acid, a competitive inhibitor of IP3 phosphatase, with phenylephrine doubled the IP3 formation and potentiated the initial phases of inotropic responses but had no effect on the sustained positive inotropic response. Nifedipine and Mn2+ did not block the transient inotropic responses but inhibited the sustained positive inotropic response. alpha 1-Adrenoceptor stimulation resulted in restoration of slow responses in the high K+-depolarized muscles in the time course similar to that of the development in the sustained positive inotropic response. Addition of phorbol-12,13-dibutyrate alone or in combination with caffeine or A23187 failed to produce a sustained positive inotropic effect, but pretreatment with this phorbol ester (1-100 nM) for 30 minutes resulted in dose-dependent potentiation of alpha 1-adrenoceptor-mediated sustained positive inotropic effect associated with enhanced slow responses. These results suggest that the inotropic effects mediated by cardiac alpha 1-adrenoceptor stimulation occur through the phosphodiesteratic cleavage of PI-4,5-P2, such that IP3 may produce transient inotropic effects by mobilizing intracellular Ca2+, while diacylglycerol, along with cofactors that are also generated on alpha 1-adrenoceptor stimulation, may provoke a sustained positive inotropic effect by potentiating slow Ca2+ channels through activation of protein kinase C.

Digoxin Withdrawal After Cardiac Failure in Patients with Sinus Rhythm

A double-blind placebo-controlled crossover trial of digoxin withdrawal was undertaken in 22 patients with sinus rhythm who had a previous history of frank heart failure and were taking therapeutic doses of the drug. During the course of the study, 14 patients showed no clinical change whether taking digoxin or placebo, five patients deteriorated on placebo (four with heart failure and one with supraventricular tachycardia), and three on digoxin (two with heart failure and one with digoxin toxicity). These differences were not statistically significant. Compared to placebo, patients, while taking digoxin, had lower resting heart rates and significant shortening of all the systolic time intervals. The drug appears to exert a sustained positive inotropic effect during maintenance therapy, but this is not of clinical benefit to the majority of patients.

CA2+-free perfusion of rat heart reveals a (Na+ + K+)ATPase form highly sensitive to ouabain.

The sensitivities of the cardiac (Na+ + K+)ATPases to inhibition by ouabain have been shown to correlate with the pharmacologically active digitalis concentrations in the various source species1. The rat heart enzyme is generally considered to be insensitive to ouabain with half-maximal inhibition at ∼6 × 10−5 M ouabain2,3 and a KD for binding3–7 of 1–6 × 10−5 M. However, recent data5–7 showed that rat cardiac sarcolemma preparations can bind ouabain with high affinity (KD 1–3 × 10−7M) with no detectable concomitant enzyme inhibition in vitro but producing a sustained positive inotropic effect in vivo. In the present study, we measured the (Na+ + K+)ATPase inhibition by ouabain in sarcolemma-enriched fractions obtained from rat hearts relaxed by perfusion with Ca2+-free solutions. In these conditions, two enzyme forms having high and low sensitivities to ouabain were found (half-maximal inhibitions with 1.2×10−8 and 6×10−5M, respectively). Their relative proportion is close to 1. Confirming previous data2–7, the highly sensitive form was not detected in preparations from hearts maintained at a physiological calcium level. This heterogeneous population of functional (Na+ + K+)ATPases suggests that, in rat heart, the positive inotropic effect of low doses of ouabain in vivo5–9 would act through the inhibition9 of the enzyme activity highly sensitive to ouabain as detected in vitro.

Haemodynamic effects of prenalterol in patients with severe congestive heart failure--NYHA III-IV.

In 12 patients with severe congestive heart failure due to ischaemic heart disease (n = 6) and due to congestive cardiomyopathy (n = 6) the haemodynamic effects of a new beta 1-agonist, prenalterol, were studied. Left ventricular (LV) function was studied before and 20 min after infusion of 12 mg prenalterol. Heart rate was kept constant by atrial pacing at a rate of 100 min-1 unless intrinsic heart rate exceeded it. As a sign of positive inotropic support, prenalterol enhanced peak rate of LV pressure development (dP/dt) from 1160 +/- 100 mm Hg/s to 1590 +/- 190 mm Hg/s (p less than 0.005). In the mean LV end-diastolic and end-systolic volume determined by cineventriculography and two-dimensional echocardiography decreased. LV stroke work index measured with both methods increased with 4 ml/m and 5 ml/m, respectively (p less than 0.02). LV ejection fraction was improved by 6% and 8% (p less than 0.005). Increase of peak fall of left ventricular pressure (dP/dt) (1050 +/- 60 mm Hg/s to 1270 +/- 100 mm Hg/s, p less than 0.005) and shortening of time constant (T) of pressure fall from 64.5 +/- 5.0 ms to 44.5 +/- 6.0 ms (p less than 0.005) demonstrated the improved LV relaxation. Analysis of LV volume and myocardial compliance revealed decrease of left ventricular stiffness. Thus, LV filling pressure was reduced from 22.1 +/- 4 mm Hg to 14 +/- 3.5 mm Hg (p less than 0.001). Pressure volume analysis showed a significant increase of LV power and work, as well as a slight decrease of wall stress. Our study could demonstrate, even in patients with severe heart failure, a sustained positive inotropic effect of prenalterol leading to an improved left ventricular contractility, relaxation and compliance. LV power and work was enhanced. The increase of oxygen demand seemed to be counterbalanced by an improved perfusion of particularly subendocardial layers indicated by an increased transmyocardial pressure gradient.

IS MAINTENANCE DIGOXIN NECESSARY IN PATIENTS WITH SINUS RHYTHM?

Discontinuation of digoxin in 56 patients with sinus rhythm who had been taking it for a long time did not produce clinical deterioration in 33 of 34 patients whose pre-withdrawal steadystate plasma-digoxin concentration was less than 0·8 ng/ml; fast atrial fibrillation developed in the other patient. 22 patients had plasma-digoxin levels between 0·8 and 2.0 ng/ml before withdrawal—of these, 7 deteriorated without digoxin (5 had atrial fibrillation, which was associated with congestive heart-failure in 2, and 2 had congestive heart-failure whilst remaining in sinus rhythm). In the latter 2 patients with heart-failure, measurement of the pre-injection period/left-ventricular ejection time (P.E.P./L.V.E.T.) ratio suggested that digoxin did exert a sustained positive inotropic effect. Thus, successful discontinuation of digoxin was possible in 86% of the total group and was more likely when the plasma-digoxin concentration was below 0·8 ng/ml. Unexpected atrial fibrillation was the commonest development in the 8 patients in whom digoxin withdrawal was unsuccessful.