Mechanisms of preserved baseline cardiac systolic function in rats with adrenergic inotropic downregulation

Despite reductions in beta-adrenoreceptor (beta-AR)-mediated inotropic effects induced by sustained sympathetic activation in cardiac disease, whether these changes necessarily result in reductions in systolic function under resting conditions (baseline function) is not clear. Moreover, possible compensatory mechanisms which might contribute to maintaining the baseline systolic function despite reductions in beta-AR-mediated inotropic effects have not been systematically sought. In the present study, 1 month of daily administration of the beta-AR agonist, isoproterenol (0.05 mg/kg/day, i.p.), to rats resulted in an attenuation of left ventricular inotropic responses to isoproterenol over a wide range of concentrations (10(-8)-10(-4) M), whereas a decline of inotropic responses to norepinephrine, an endogenous inotrope, occurred only at high concentrations (10(-5)-10(-4) M). However, chronic isoproterenol administration failed to modify baseline systolic chamber and myocardial function, as determined in vivo using echocardiography (endocardial and midwall fractional shortening), and in isolated, perfused heart preparations (end-systolic chamber and myocardial elastance) Sustained baseline chamber function despite profound beta-AR-mediated inotropic downregulation was not attributed to alterations in cardiac loading conditions, resting heart rate, chamber remodeling, increased myocardial norepinephrine release, or enhanced contractile responses to alternative receptor/signal transduction pathways mediating positive inotropy (as assessed from histamine, serotonin, forskolin, angiotensin II or phenylephrine responsiveness). These findings indicate that baseline cardiac contractile function might be unaltered despite a profound impairment of beta-AR-induced responsiveness, an effect related to a preserved stimulatory influence of low physiological concentrations of endogenous norepinephrine constituting adrenergic tone at rest.     

The role of superoxide anions in regulation of coronary vessel tone and myocardial contractile function in changed redox-state of the myocardium

The study relates to the character of tirone effect (chemical trap of superoxide--anions) on regulation of coronary vessel tone and myocardial contractile function in normal and changed cell redox-state of coronary and cardiac vessels. The experiments were performed in 64 female Wistar rats (180-320 g). The coronary blood flow and myocardial contractile junction were studied in isolated heart preparations. To determine the role of superoxide-anions in regulation of coronary vessel tone, tirone was added to the perfusion solution (4,5-dihydroxy-1,3-benzene disulfonic acid, 10 mm, Sigma USA). Preliminary injection of N-acetyl-L-cysteine evoked a 16 % increase, whereas injection of L-buthionine-(S,R)-sulfoximine reduced concentration of nonprotein thiol group in the myocardium and erythrocytes of experimental animals by 37%. The influence of superoxide anions on the cardiac vessel tone and myocardial contractile function was due to nitric monoxide participation the concentration of which increased in binding of superoxide anions and was directly dependent on concentration of sulfhydrilis groups in the cardiac cells. The oxygen active forms and cellular redox-state seem to play an important role in the regulation mechanisms of the coronary vessel tone and myocardial contractile function.     

Carvedilol Prevents Ovariectomy-Induced Myocardial Contractile Dysfunction in Female Rat

Carvedilol has beneficial effects on cardiac function in patients with heart failure but its effect on ovariectomy-induced myocardial contractile dysfunction remains unclear. Estrogen deficiency induces myocardial contractile dysfunction and increases cardiovascular disease risk in postmenopausal women. Our aim was to investigate whether carvedilol, a beta receptor blocker, would prevent ovariectomy-induced myocardial contractile dysfunction. Female rats (8 weeks old) that underwent bilateral ovariectomy were randomly assigned to receive daily treatment with carvedilol (OVX+CAR, 20 mg/kg), placebo (OVX) and SHAM for 58 days. Left ventricle papillary muscle was mounted for isometric tension recordings. The inotropic response to Ca²⁺ (0.62 to 3.75 mM) and isoproterenol (Iso 10⁻⁸ to 10⁻² M) were assessed. Expression of calcium handling proteins was measured by western blot analysis. Carvedilol treatment in the OVX animals: prevented weight gain and slight hypertrophy, restored the reduced positive inotropic responses to Ca²⁺ and isoproterenol, prevented the reduction in SERCA2a expression, abolished the increase in superoxide anion production, normalized the increase in p22^phox expression, and decreased serum angiotensin converting enzyme (ACE) activity. This study demonstrated that myocardial contractile dysfunction and SERCA2a down regulation were prevented by carvedilol treatment. Superoxide anion production and NADPH oxidase seem to be involved in this response.     

Ethacrynic acid induced inotropism.

Ethacrynic acid (ECA), a sulfhydryl group inhibiting diuretic was examined for positive inotropic effects. These were found to be present in isolated guinea pig left atria studied in 0.9 and 1.8 mM Ca bathing solutions and were partially dependent upon adrenergic mechanisms (presumably secondary to norepinephrine release from sympathetic nerve endings) and partly independent of such mechanisms as demonstrated by propranolol induced beta-blockade and reserpine-induced catecholamine depletion. The mechanism of the non-beta adrenergic inotropism is unclear but may relate to the ability of ECA to inhibit the sarcolemmal Na-K-Mg-dependent ATPase. ECA-induced premature contractile failure occurred in all atria as well as a late increase in diastolic tension, the latter being comparable to that described for toxic doses of cardiac glycosides in similar preparations.     

Interaction between glucose utilization and left ventricular heart function in type I-diabetics

The aim of the present study was to check whether equal, therapeutically relevant, positively inotropic doses of different adrenergic agents elicit equal inotropic and metabolic effects in 6 type I-diabetics as in 6 matched nondiabetic subjects. The effects of increasing doses of norepinephrine (NE)- and orciprenaline (0.12, 0.20, 0.33 microgram/kg min) on heart function (systolic time interval, heart rate, blood pressure) and on serum fatty acid (NEFA), glucose, lactate, pyruvate and insulin concentrations were recorded. In the therapeutic dose range, NE, and orciprenaline elicited in diabetics without clinical signs of any cardiovascular disease a diminished myocardial inotropic response (20-40%), less marked vascular effects (vasoconstriction, vasodilatation), but greater metabolic changes in right atrial blood (NEFA, pyruvate, lactate) compared to matched controls (p less than 0.05). The smaller increase of cardiac performance in diabetics to exogenous catecholamines cannot be explained by sympathetic cardiac denervation, since chronotropic beta 1-beta 2-stimulation with orciprenaline provoked nearly equal dose-dependent changes in diabetics and controls. It is suggested that the smaller positive inotropic effect during NE and orciprenaline infusion in type I-diabetics is a result first of all of alterations in myocardial energy turnover in diabetes due to reduced myocardial glucose utilization. It seems necessary to secure continuous myocardial glucose utilization and subnormal NEFA concentrations in the serum during the therapeutic application of inotropic adrenergic agents in severe cardiac failure and cardiogenic shock in diabetics.     

Neuronostatin, a novel peptide encoded by somatostatin gene, regulates cardiac contractile function and cardiomyocyte survival

Neuronostatin, a recently discovered peptide encoded by somatostatin gene, is involved in regulation of neuronal function, blood pressure, food intake, and drinking behavior. However, the biological effects of neuronostatin on cardiac myocytes are not known, and the intracellular signaling mechanisms induced by neuronostatin remain unidentified. We analyzed the effect of neuronostatin in isolated perfused rat hearts and in cultured primary cardiomyocytes. Neuronostatin infusion alone had no effect on left ventricular (LV) contractile function or on isoprenaline- or preload-induced increase in cardiac contractility. However, infusion of neuronostatin significantly decreased the positive inotropic response to endothelin-1 (ET-1). This was associated with an increase in phosphorylation of p38 mitogen-activated protein kinase and c-Jun N-terminal kinase (JNK). Treatment of both neonatal and adult cardiomyocytes with neuronostatin resulted in reduced cardiomyocyte viability. Inhibition of JNK further increased the neuronostatin-induced cell death. We conclude that neuronostatin regulates cardiac contractile function and cardiomyocyte survival. Receptors for neuronostatin need to be identified to further characterize the biological functions of the peptide.     

The negative inotropic action of catecholamines: role of beta3-adrenoceptors

There is now evidence for the involvement of four beta-adrenoceptor populations in the regulation of cardiac function by catecholamines. Beta1- and beta2-adrenoceptor stimulation classically produces an increase in contractility. A fourth beta-adrenoceptor, as yet uncloned and designated provisionally as a beta4-adrenoceptor, also mediates a positive inotropic effect. Beta3-adrenoceptors, which had been cloned at the end of the eighties, has been extensively studied as a potential target for antiobesity and antidiabetic drugs. Its characterization in the heart has opened new fields of investigations for the understanding of the cardiac adrenergic regulation. This review describes the cardiac electrical and mechanical effects induced by Beta3-adrenoceptor stimulation in different species (including human), as well as the signaling pathway. It also analyzes the role of these receptors in the abnormal responsiveness of catecholamines in heart failure.     

Compromised myocardial energetics in hypertrophied mouse hearts diminish the beneficial effect of overexpressing SERCA2a

The sarcoplasmic reticulum calcium ATPase (SERCA) plays a central role in regulating intracellular Ca(2+) homeostasis and myocardial contractility. Several studies show that improving Ca(2+) handling in hypertrophied rodent hearts by increasing SERCA activity results in enhanced contractile function. This suggests that SERCA is a potential target for gene therapy in cardiac hypertrophy and failure. However, it raises the issue of increased energy cost resulting from a higher ATPase activity. In this study, we determined whether SERCA overexpression alters the energy cost of increasing myocardial contraction in mouse hearts with pressure-overload hypertrophy using (31)P NMR spectroscopy. We isolated and perfused mouse hearts from wild-type (WT) and transgenic (TG) mice overexpressing the cardiac isoform of SERCA (SERCA2a) 8 weeks after ascending aortic constriction (left ventricular hypertrophy (LVH)) or sham operation. We found that overexpressing SERCA2a enhances myocardial contraction and relaxation in normal mouse hearts during inotropic stimulation with isoproterenol. Energy consumption was proportionate to the increase in contractile function. Thus, increasing SERCA2a expression in the normal heart allows an enhanced inotropic response with no compromise in energy supply and demand. However, this advantage was not sustained in LVH hearts in which the energetic status was compromised. Although the overexpression of SERCA2a prevented the down-regulation of SERCA protein in LVH hearts, TG-LVH hearts showed no increase in inotropic response when compared with WT-LVH hearts. Our results suggest that energy supply may be a limiting factor for the benefit of SERCA overexpression in hypertrophied hearts. Thus, strategies combining energetic support with increasing SERCA activity may improve the therapeutic effectiveness for heart failure.     

Adrenergic regulation of cardiac myocyte apoptosis.

The direct effects of catecholamines on cardiac myocytes may contribute to both normal physiologic adaptation and pathologic remodeling, and may be associated with cellular hypertrophy, apoptosis, and alterations in contractile function. Norepinephrine (NE) signals via alpha- and beta-adrenergic receptors (AR) that are coupled to G-proteins. Pharmacologic studies of cardiac myocytes in vitro demonstrate that stimulation of beta1-AR induces apoptosis which is cAMP-dependent and involves the voltage-dependent calcium influx channel. In contrast, stimulation of beta2-AR exerts an anti-apoptotic effect which appears to be mediated by a pertussis toxin-sensitive G protein. Stimulation of alpha1-AR causes myocyte hypertrophy and may exert an anti-apoptotic action. In transgenic mice, myocardial overexpression of either beta1-AR or G(alpha)s is associated with myocyte apoptosis and the development of dilated cardiomyopathy. Myocardial overexpression of beta2-AR at low levels results in improved cardiac function, whereas expression at high levels leads to dilated cardiomyopathy. Overexpression of wildtype alpha1B-AR does not result in apoptosis, whereas overexpression of G(alpha)q results in myocyte hypertrophy and/or apoptosis depending on the level of expression. Differential activation of the members of the mitogen-activated protein kinase (MAPK) superfamily and production of reactive oxygen species appear to play a key role in mediating the actions of adrenergic pathways on myocyte apoptosis and hypertrophy. This review summarizes current knowledge about the molecular and cellular mechanisms involved in the regulation of cardiac myocyte apoptosis via stimulation of adrenergic receptors and their coupled effector pathways.     

Mesenteric lymph from rats with trauma-hemorrhagic shock causes abnormal cardiac myocyte function and induces myocardial contractile dysfunction

Myocardial contractile dysfunction develops following trauma-hemorrhagic shock (T/HS). We have previously shown that, in a rat fixed pressure model of T/HS (mean arterial pressure of 30-35 mmHg for 90 min), mesenteric lymph duct ligation before T/HS prevented T/HS-induced myocardial contractile depression. To determine whether T/HS lymph directly alters myocardial contractility, we examined the functional effects of physiologically relevant concentrations of mesenteric lymph collected from rats undergoing trauma-sham shock (T/SS) or T/HS on both isolated cardiac myocytes and Langendorff-perfused whole hearts. Acute application of T/HS lymph (0.1-2%), but not T/SS lymph, induced dual inotropic effects on myocytes with an immediate increase in the amplitude of cell shortening (1.4 ± 0.1-fold) followed by a complete block of contraction. Similarly, T/HS lymph caused dual, positive and negative effects on cellular Ca2? transients. These effects were associated with changes in the electrophysiological properties of cardiac myocytes; T/HS lymph initially prolonged the action potential duration (action potential duration at 90% repolarization, 3.3 ± 0.4-fold), and this was followed by a decrease in the plateau potential and membrane depolarization. Furthermore, intravenous infusion of T/HS lymph, but not T/SS lymph, caused myocardial contractile dysfunction at 24 h after injection, which mimicked actual T/HS-induced changes; left ventricular developed pressure (LVDP) and the maximal rate of LVDP rise and fall (±dP/dt(max)) were decreased and inotropic response to Ca2? was blunted. However, the contractile responsiveness to β-adrenergic receptor stimulation in the T/HS lymph-infused hearts remained unchanged. These results suggest that T/HS lymph directly causes negative inotropic effects on the myocardium and that T/HS lymph-induced changes in myocyte function are likely to contribute to the development of T/HS-induced myocardial dysfunction.     

Intermedin elicits a negative inotropic effect in rat papillary muscles mediated by endothelial-derived nitric oxide

Intermedin (IMD) is a novel vasoactive peptide from the calcitonin gene-related peptide (CGRP) implicated in cardiac regulation, yet the contractile effects of IMD remain controversial, since previous studies in vivo and isolated cardiomyocytes documented contradictory results. We hypothesized cardiac endothelial cells involvement in IMD modulation of cardiac function as an explanation for these opposing observations. With this in mind, we investigated the direct action of increasing concentrations of IMD (10(-8) to 10(-6)M) on myocardial performance parameters in rat left ventricular (LV) papillary muscles with and without endocardial endothelium (EE) and in presence of receptor antagonists and intracellular pathways inhibitors. In LV papillary muscles with intact EE, IMD induced a concentration-dependent negative inotropic action (%decrease relative to baseline, at IMD concentration of 10(-6)M, active tension of 14 ± 4%, and maximum velocity of tension rise of 10 ± 4%). These effects were blunted by EE removal, AM receptor antagonist (AM(22-52)), and CGRP receptor antagonist (CGRP(8-37)). Additionally, nitric oxide (NO) synthase inhibition with N(G)-nitro-l-arginine (l-NAME) in muscles with and without EE and guanylyl cyclase inhibition with {1H-[1,2,4]oxadiazole-[4,4-a]-quinoxalin-1-one} not only blunted the negative inotropic action of IMD but also unmasked IMD-positive inotropic effect dependent on CGRP receptor PKA activation. Western blot quantification of phosphorylated cardiac troponin I (P-cTnI) in IMD-treated papillary muscles revealed a significant increase in P-cTnI when compared with untreated muscles, while in l-NAME-pretreated papillary muscles IMD failed to increase P-cTnI. Finally, we found that stimulation of both EE and microvascular endothelial cells with IMD significantly increased NO production by 40 ± 3 and 38 ± 3%, respectively, suggesting the role of cardiac endothelial cells in NO production upon IMD stimulation. Our findings establish IMD negative inotropic effect in isolated myocardium due to NO/cGMP pathway activation with concomitant thin myofilament desensitization by increase in cTnI phosphorylation and provide a coherent explanation for the previously reported contradictory results.     

Investigations of Molecular Mechanisms of Actin–Myosin Interactions in Cardiac Muscle

The functional characteristics of cardiac muscle depend on the composition of protein isoforms in the cardiomyocyte contractile machinery. In the ventricular myocardium of mammals, several isoforms of contractile and regulatory proteins are expressed–two isoforms of myosin (V1 and V3) and three isoforms of tropomyosin chains (α, β, and κ). Expression of protein isoforms depends on the animal species, its age and hormonal status, and this can change with pathologies of the myocardium. Mutations in these proteins can lead to cardiomyopathies. The functional significance of the protein isoform composition has been studied mainly on intact hearts or on isolated preparations of myocardium, which could not provide a clear comprehension of the role of each particular isoform. Present-day experimental techniques such as an optical trap and in vitro motility assay make it possible to investigate the phenomena of interactions of contractile and regulatory proteins on the molecular level, thus avoiding effects associated with properties of a whole muscle or muscle tissue. These methods enable free combining of the isoforms to test the molecular mechanisms of their participation in the actin–myosin interaction. Using the optical trap and the in vitro motility assay, we have studied functional characteristics of the cardiac myosin isoforms, molecular mechanisms of the calcium-dependent regulation of actin–myosin interaction, and the role of myosin and tropomyosin isoforms in the cooperativity mechanisms in myocardium. The knowledge of molecular mechanisms underlying myocardial contractility and its regulation is necessary for comprehension of cardiac muscle functioning, its disorders in pathologies, and for development of approaches for their correction.     

Determination of mechanisms of myocardial ischemic injury by 31P-MRS effect of catecholamine on ischemic hearts

To investigate mechanisms of development in ischemic myocardial injury, intracellular pH and high energy phosphates in perfused guinea-pig hearts were monitored by 31P-MRS. Intracellular ATP content decreased to 1.2% and 26.4% of control during 60 minutes global ischemia, respectively with and without preischemic administration of isoproterenol. Intracellular pH declined to 6.48 and 6.03 respectively. Postischemic cardiac function was severely impaired by isoproterenol. ATP breakdown had little influence on intracellular pH in ischemic hearts. It was verified that inotropic agents can progress ischemic myocardial injury, and that contractile recovery is more correlated with the residual ATP level than intracellular pH.     

Developmental changes in dopamine modulation of the heart in the isopod crustacean Ligia exotica: reversal of chronotropic effect

Developmental changes in dopamine modulation of the heart were examined in the isopod crustacean Ligia exotica. The Ligia cardiac pacemaker is transferred from the myocardium to the cardiac ganglion during juvenile development and the heartbeat changes from myogenic to neurogenic. In the myogenic heart of early juveniles, dopamine affected the myocardium and caused a decrease in the frequency and an increase in the duration of the myocardial action potential, resulting in negative chronotropic (decrease in beat frequency) and positive inotropic (increase in contractile force) effects on the heart. Contrastingly, in the heart of immature adults just after juvenile development, dopamine caused effects of adult type, positive chronotropic and positive inotropic effects on the heart affecting the cardiac ganglion and myocardium. During the middle and late juvenile stages, dopamine caused individually a negative or a positive chronotropic effect on the heart. These results suggest that the chronotropic effect of dopamine on the Ligia heart is reversed from negative to positive in association with the cardiac pacemaker transfer from the myocardium to the cardiac ganglion during juvenile development.     

Acetoacetate augments beta-adrenergic inotropism of stunned myocardium by an antioxidant mechanism

Blunted beta-adrenergic inotropism in stunned myocardium is restored by pharmacological (N-acetylcysteine) and metabolic (pyruvate) antioxidants. The ketone body acetoacetate is a natural myocardial fuel and antioxidant that improves contractile function of prooxidant-injured myocardium. The impact of acetoacetate on postischemic cardiac function and beta-adrenergic signaling has never been reported. To test the hypothesis that acetoacetate restores contractile performance and beta-adrenergic inotropism of stunned myocardium, postischemic Krebs-Henseleit-perfused guinea pig hearts were treated with 5 mM acetoacetate and/or 2 nM isoproterenol at 15-45 and 30-45 min of reperfusion, respectively, while cardiac power was monitored. The myocardium was snap frozen, and its energy state was assessed from phosphocreatine phosphorylation potential. Antioxidant defenses were assessed from GSH/GSSG and NADPH/NADP(+) redox potentials. Stunning lowered cardiac power and GSH redox potential by 90 and 70%, respectively. Given separately, acetoacetate and isoproterenol each increased power and GSH redox potential three- to fivefold. Phosphocreatine potential was 70% higher in acetoacetate- vs. isoproterenol-treated hearts (P < 0.01). In combination, acetoacetate and isoproterenol synergistically increased power and GSH redox potential 16- and 7-fold, respectively, doubled NADPH redox potential, and increased cAMP content 30%. The combination increased cardiac power four- to sixfold vs. the individual treatments without a coincident increase in phosphorylation potential. Potentiation of isoproterenol's inotropic actions endured even after acetoacetate was discontinued and GSH potential waned, indicating that temporary enhancement of redox potential persistently restored beta-adrenergic mechanisms. Thus acetoacetate increased contractile performance and potentiated beta-adrenergic inotropism in stunned myocardium without increasing energy reserves, suggesting its antioxidant character is central to its beneficial actions.     

Synergistic activation of salmon cardiac function by endothelin and beta-adrenergic stimulation

The aim was to find out the effects of endothelin-1 (ET-1) in salmon (Salmo salar) cardiac contractile and endocrine function and its possible interaction with beta-adrenergic regulation. We found that ET-1 has a positive inotropic effect in salmon heart. ET-1 (30 nM) increased the contraction amplitude 17+/-4.7% compared with the basal level. beta-Adrenergic activation (isoprenaline, 100 nM) increased contraction amplitude 30+/-13.1%, but it did not affect the contractile response to ET-1. ET-1 (10 nM) stimulated the secretion of salmon cardiac natriuretic peptide (sCP) from isolated salmon ventricle (3.3+/-0.14-fold compared with control) but did not have any effect on ventricular sCP mRNA. Isoprenaline alone (0.1-1,000 nM) did not stimulate sCP release, but ET-1 (10 nM) together with isoprenaline (0.1 nM) caused a significantly greater increase of sCP release than ET-1 alone (5.4+/-0.07 vs. 3.3+/-0.14 times increase compared with control). The effects on the contractile and secretory function could be inhibited by a selective ETA-receptor antagonist BQ-610 (1 microM), whereas ETB-receptor blockage (by 100 nM BQ-788) enhanced the secretory response. Thus ET-1 is a phylogenetically conserved regulator of cardiac function, which has synergistic action with beta-adrenergic stimulation. The modulatory effects of ET-1 may therefore be especially important in situations with high beta-adrenergic tone.     

Nitric oxide-dependent and -independent mechanisms are involved in TNF-alpha -induced depression of cardiac myocyte contractility

Previous studies have demonstrated the presence of myocardial depression in clinical and experimental septic shock. This response is mediated, in part, through circulating TNF-alpha-induced, nitric oxide-dependent, depression of basal myocyte contractility. Other mechanisms of early myocardial dysfunction involving decreased response to adrenergic stimulation may exist. This study evaluated the presence and nitric oxide dependence of impaired adrenergic response to TNF-alpha in in vitro cardiac myocytes. The contraction of electrically paced neonatal rat cardiac myocytes in tissue culture was quantified using a closed-loop video tracking system. TNF-alpha induced depression of baseline contractility over the first 20 min of cardiac myocyte exposure. This effect was blocked by N-methyl-arginine (NMA), a nitric oxide synthase inhibitor, in all studies. Contractile and cAMP response to increasing concentrations of isoproterenol was deficient in cardiac myocytes exposed to TNF-alpha regardless of the presence of NMA. In contrast, increasing concentrations of forskolin (a direct stimulant of adenylate cyclase) and dibutyryl cAMP (a metabolically active membrane-soluble analog of cAMP) completely reversed TNF-alpha-mediated depression, though only in the presence of NMA. Forskolin-stimulated cAMP generation remained intact regardless of NMA. Increasing concentrations of exogenous calcium chloride, unlike other inotropic agents, corrected TNF-alpha-mediated defects of contractility independent of the presence of NMA. These data suggest that TNF-alpha exposure is associated with a second nitric oxide-independent but calcium-dependent early depressant mechanism that is manifested by reduced contractile and cAMP response to beta-adrenergic stimulation.     

Cardiac dilatation and pump dysfunction without intrinsic myocardial systolic failure following chronic beta-adrenoreceptor activation

There is no direct evidence to indicate that pump dysfunction in a dilated chamber reflects the impact of chamber dilatation rather than the degree of intrinsic systolic failure resulting from myocardial damage. In the present study, we explored the relative roles of intrinsic myocardial systolic dysfunction and chamber dilatation as mediators of left ventricular (LV) pump dysfunction. Administration of isoproterenol, a beta-adrenoreceptor agonist, for 3 mo to rats (0.1 mg.kg(-1).day(-1)) resulted in LV pump dysfunction as evidenced by a reduced LV endocardial fractional shortening (echocardiography) and a decrease in the slope of the LV systolic pressure-volume relation (isolated heart preparations). Although chronic beta-adrenoreceptor activation induced cardiomyocyte damage (deoxynucleotidyl transferase-mediated dUTP nick-end labeling) as well as beta(1)- and beta(2)-adrenoreceptor inotropic downregulation (attenuated contractile responses to dobutamine and salbutamol), these changes failed to translate into alterations in intrinsic myocardial contractility. Indeed, LV midwall fractional shortening (echocardiography) and the slope of the LV systolic stress-strain relation (isolated heart preparations) were unchanged. A normal intrinsic myocardial systolic function, despite the presence of cardiomyocyte damage and beta-adrenoreceptor inotropic downregulation, was ascribed to marked increases in myocardial norepinephrine release, to upregulation of alpha-adrenoreceptor-mediated contractile effects as determined by phenylephrine responsiveness, and to compensatory LV hypertrophy. LV pump failure was attributed to LV dilatation, as evidenced by increased LV internal dimensions (echocardiography), and a right shift and increased volume intercept of the LV diastolic pressure-volume relation. In conclusion, chronic sympathetic stimulation, despite reducing beta-adrenoreceptor-mediated inotropic responses and promoting myocyte apoptosis, may nevertheless induce pump dysfunction primarily through LV dilatation, rather than intrinsic myocardial systolic failure.     

Augmentation of the inotropic response to insulin in diabetic rat hearts.

Insulin participates in the modulation of myocardial function, but its inotropic action in diabetes mellitus is not fully clear. In the present study, we examined contractile responses to insulin in left-ventricular papillary muscles and ventricular myocytes isolated from hearts of normal or short-term (5-7 days) streptozotocin-induced (65 mg/kg) diabetic rats. Mechanical properties of papillary muscles and ventricular myocytes were evaluated using a force transducer and an edge-detector, respectively. Contractile properties of papillary muscles or cardiac myocytes, electrically stimulated at 0.5 Hz, were analyzed in terms of peak tension development (PTD) or peak twitch amplitude (PTA), time-to-peak contraction (TPT) and time-to-90% relaxation (RT90). Intracellular Ca2+ transients were measured as fura-2 fluorescence intensity change (deltaFFI). Insulin (1-500 nM) had no effect on PTD in normal myocardium, whereas it produced a positive inotropic response in preparations from diabetic animals, with a maximal increase of 11%. Insulin did not modify TPT or RT90 in either group. Further studies revealed that insulin enhanced cell shortening in diabetic but not normal myocytes, with a maximal increase of 21%. Consistent with its action on the mechanical properties of papillary muscles and cardiac myocytes, insulin also induced a dose-dependent increase in the intracellular Ca2+ transient in diabetic but not normal myocytes. Collectively, these data suggest that the myocardial contractile response to insulin may be altered in diabetes.