Carbon disulfide exposure attenuates adrenergic inotropic response in rats

Catecholamine-induced myocardial necrosis is enhanced in carbon disulfide exposed rats. We investigated whether the reported morphological findings after carbon disulfide exposure are accompanied by functional disturbances of the adrenergic inotropic response as well as by biochemical alterations. The cardiac effects of epinephrine and norepinephrine were studied in urethane-anesthetized rats which had been subacutely exposed to carbon disulfide. Compared with not exposed control animals left ventricular inotropic response was diminished and the transient T-wave elevation of the ECG due to ischemic episodes was enhanced in carbon disulfide-exposed rats. Activity of LDH-isozymes in the myocardium was shifted toward LDH-M. Our results support the hypothesis that carbon disulfide causes disturbances of energy supply in the heart.     

Pulse pressure response to the strain of the Valsalva maneuver in humans with preserved systolic function

Arterial pulsepressure response during the strain phase of the Valsalva maneuver hasbeen proposed as a clinical tool for the diagnosis of left heartfailure, whereas responses of subjects with preserved systolic functionhave been poorly documented. We studied the relationship between theaortic pulse amplitude ratio (i.e., minimum/maximum pulse pressure)during the strain phase of the Valsalva maneuver and cardiachemodynamics at baseline in 20 adults (42 ± 14 yr) undergoingroutine right and left heart catheterization. They were normal subjects(n = 5) and patients withvarious forms of cardiac diseases(n = 15), and all had a leftventricular ejection fraction 40%. High-fidelity pressures wererecorded in the right atrium and the left ventricle at baseline and atthe aortic root throughout the Valsalva maneuver. Aortic pulseamplitude ratio 1) did not correlatewith baseline left ventricular end-diastolic pressure, cardiac index(thermodilution), or left ventricular ejection fraction(cineangiography) and 2) waspositively related to total arterial compliance (area method) (r = 0.59) and to basal mean rightatrial pressure (r = 0.57) (eachP < 0.01). Aortic pulse pressureresponses to the strain were not related to heart rate responses duringthe maneuver. In subjects with preserved systolic function, the aorticpulse amplitude ratio during the strain phase of the Valsalva maneuver relates to baseline total arterial compliance and right heart fillingpressures but not to left ventricular function.

     

Peroxynitrite irreversibly decreases diastolic and systolic function in cardiac muscle

Much of the damaging action of nitric oxide in heart may be due to its diffusion-limited reaction with superoxide to form peroxynitrite. Direct infusion of peroxynitrite into isolated perfused hearts fails to model the effects of in situ formation because the bulk of peroxynitrite decomposes before reaching the myocytes. To examine the direct effects of peroxynitrite on the contractile apparatus of the heart, we exposed intact and skinned rat papillary muscles to a steady state concentration of 4-microM peroxynitrite for 5 min, followed by a 30-min recovery period to monitor irreversible effects. In intact muscles developed force fell immediately to 26% of initial force, recovering to 43% by 30 min. Resting tension increased by 600% immediately, and was still elevated 500% by 30 min. Nitrotyrosine immunochemistry showed that peroxynitrite can induce tyrosine nitration at low concentrations and is capable of penetrating 200-380 microm into the papillary muscle after a 5-min infusion. Decomposed peroxynitrite had no effect on either intact or skinned muscle developed force or resting tension. Our results show that peroxynitrite directly damages both developed force and resting tension of isolated heart muscle, which can be extrapolated to systolic and diastolic injury in intact hearts.     

Effects of 5-day hypoxia on cardiac adrenergic neurotransmission in rats

Chronic hypoxiainduces an overall sympathetic hyperactivation associated with amyocardial -receptor desensitization. The mechanismsinvolved in this desensitization were evaluated in 32 male Wistar ratskept in a hypobaric pressure chamber(PO₂ = 40 Torr, atmospheric pressure = 450 Torr) for 5 days. In hypoxic compared with normoxic conditions,plasma norepinephrine (NE) levels were higher (2.1 ± 0.7 vs. 0.6 ± 0.2 ng/ml) with no difference in the plasma epinephrine levels(2.2 ± 0.7 vs. 1.8 ± 0.3 ng/ml). In hypoxia neuronal NE uptakemeasured by [³H]NE wasdecreased by 32% in the right ventricle (RV) and by 35% in the leftventricle (LV), and[³H]mazindol in vitrobinding showed a decrease in uptake-1 carrier protein density by 38%in the RV and by 41% in the LV. In vitro binding assays with[³H]CGP-12177 indicate-adrenoceptor density reduced by 40% in the RV and by 32% in theLV, and this was due to reduced₁-subtype fraction (competitionbinding experiments with practolol). Hypoxia reduced the production ofcAMP induced by isoproterenol (36% decrease in the RV and 41%decrease in the LV), 5'-guanylylimododiphosphate (40% decreasein the RV and 42% decrease in the LV), and forskolin (39% decrease inthe RV and 41% decrease in the LV) but did not alter the effect ofMnCl₂ and NaF. Quantitation ofinhibitory G-protein -subunit by immunochemical analysis showed a46% increase in the cardiac-specific isoformG_i2 inhypoxic hearts. The present data demonstrate that in rats 5-day hypoxialeads to changes in pre- and postsynaptic myocardial adrenergicfunction. The myocardial desensitization associated with both areduction in externalized ₁-adrenoceptor and an increasein inhibitory G-protein subunit may be caused by increased synaptic NElevels due to impaired uptake-1 system.

     

The adrenergic regulation of cardiomyocyte electrophysiological activity, cardiac biochemical function and coronary blood flow in rats with incorporated 131I]

The study was made of the rat heart isolated according to Langendorf and right auricles. Intraperitoneal injections of 2.5 MBq/kg 131I were performed. Electrophysiological and biomechanical myocardium response to stimulation of alpha and beta-adrenoreceptors with isoprenalin and phenylephrine hydrochloride was analyzed. Postradiation modification of adrenergic control in animals with incorporated 131I appeared as reduced functional response of cardiomyocytes and intact heart to beta-adrenoagonist and enhanced response to alpha-agonist was detected.     

Telmisartan ameliorates cardiac fibrosis and diastolic function in cardiorenal heart failure with preserved ejection fraction

Chronic kidney disease (CKD) is a major contributor to the development of heart failure with preserved ejection fraction (HFpEF), whereas the underlying mechanism of cardiorenal HFpEF is still elusive. The aim of this study was to investigate the role of cardiac fibrosis in a rat model of cardiorenal HFpEF and explore whether treatment with Telmisartan, an inhibitor of renin-angiotensin-aldosterone system (RAAS), can ameliorate cardiac fibrosis and preserve diastolic function in cardiorenal HFpEF. Male rats were subjected to 5/6 subtotal nephrectomy (SNX) or sham operation (Sham), and rats were allowed four weeks to recover and form a stable condition of CKD. Telmisartan or vehicle was then administered p.o. (8 mg/kg/d) for 12 weeks. Blood pressure, brain natriuretic peptide (BNP), echocardiography, and cardiac magnetic resonance imaging were acquired to evaluate cardiac structural and functional alterations. Histopathological staining, real-time polymerase chain reaction (PCR) and western blot were performed to evaluate cardiac remodeling. SNX rats showed an HFpEF phenotype with increased BNP, decreased early to late diastolic transmitral flow velocity (E/A) ratio, increased left ventricular (LV) hypertrophy and preserved ejection fraction (EF). Pathology revealed increased cardiac fibrosis in cardiorenal HFpEF rats compared with the Sham group, while chronic treatment with Telmisartan significantly decreased cardiac fibrosis, accompanied by reduced markers of fibrosis (collagen I and collagen III) and profibrotic cytokines (α-smooth muscle actin, transforming growth factor-β1, and connective tissue growth factor). In addition, myocardial inflammation was decreased after Telmisartan treatment, which was in a linear correlation with cardiac fibrosis. Telmisartan also reversed LV hypertrophy and E/A ratio, indicating that Telmisartan can improve LV remodeling and diastolic function in cardiorenal HFpEF. In conclusion, cardiac fibrosis is central to the pathology of cardiorenal HFpEF, and RAAS modulation with Telmisartan is capable of alleviating cardiac fibrosis and preserving diastolic dysfunction in this rat model.     

Protein kinase C epsilon induces systolic cardiac failure marked by exhausted inotropic reserve and intact Frank-Starling mechanism

Myofilament dysfunction is a common point of convergence for many forms of heart failure. Recently, we showed that cardiac overexpression of PKC epsilon initially depresses myofilament activity and then leads to a progression of changes characteristic of human heart failure. Here, we examined the effects of PKC epsilon on contractile reserve, Starling mechanism, and myofilament activation in this model of end-stage dilated cardiomyopathy. Pressure-volume loop analysis and echocardiography showed that the PKC epsilon mice have markedly compromised systolic function and increased end-diastolic volumes. Dobutamine challenge resulted in a small increase in contractility in PKC epsilon mice but failed to enhance cardiac output. The PKC epsilon mice showed a normal length-dependent tension development in skinned cardiac muscle preparations, although Frank-Starling mechanism appeared to be compromised in the intact animal. Simultaneous measurement of tension and ATPase demonstrated that the maximum tension and ATPase were markedly lower in the PKC epsilon mice at any length or Ca2+ concentration. However, the tension cost was also lower indicating less energy expenditure. We conclude 1) that prolonged overexpression of PKC epsilon ultimately leads to a dilated cardiomyopathy marked by exhausted contractile reserve, 2) that PKC epsilon does not compromise the Frank-Starling mechanism at the myofilament level, and 3) that the Starling curve excursion is limited by the inotropic state of the heart. These results reflect the significance of the primary myofilament contractilopathy induced by phosphorylation and imply a role for PKC epsilon-mediated phosphorylation in myofilament physiology and the pathophysiology of decompensated cardiac failure.     

Mechanisms contributing to the cardiac inotropic effect of Na pump inhibition and reduction of extracellular Na

Reduction of the transsarcolemmal [Na] gradient in rabbit cardiac muscle leads to an increase in the force of contraction. This has frequently been attributed to alteration of Ca movements via the sarcolemmal Na/Ca exchange system. However, the specific mechanisms that mediate the increased force at individual contractions have not been clearly established. In the present study, the [Na] gradient was decreased by reduction of extracellular [Na] or inhibition of the Na pump by either the cardioactive steroid acetylstrophanthidin or by reduction of extracellular [K]. Contractile performance and changes in extracellular Ca (sensed by double-barreled Ca-selective microelectrodes) were studied in order to elucidate the underlying basis for the increase in force. In the presence of agents that inhibit sarcoplasmic reticulum (SR) function (10 mM caffeine, 100-500 nM ryanodine), reduction of the [Na] gradient produced increases in contractile force similar to that observed in the absence of caffeine or ryanodine. It is concluded that an intact, functioning SR is not required for the inotropic effect of [Na] gradient reduction (at least in rabbit ventricle). However, this does not exclude a possible contribution of enhanced SR Ca release in the inotropic response to [Na] gradient reduction in the absence of caffeine or ryanodine. Acetylstrophanthidin (3-5 microM) usually leads to an increase in the magnitude of extracellular Ca depletions associated with individual contractions. However, acetylstrophanthidin can also increase extracellular Ca accumulation during the contraction, especially at potentiated contractions. This extracellular Ca accumulation can be suppressed by ryanodine and it is suggested that this apparent enhancement of Ca efflux is secondary to an enhanced release of Ca from the SR. Under conditions where Ca efflux during contractions is minimized (after a rest interval in the presence of ryanodine), acetylstrophanthidin increased both the rate and the extent of extracellular Ca depletions. Thus, acetylstrophanthidin can increase both Ca influx and Ca efflux during the cardiac muscle contraction. These results can be explained by a simple model where the direction of net Ca flux via Na/Ca exchange during the action potential is determined by the changes in reversal potential of the Na/Ca exchange. Reduction of the [Na] gradient may well lead to net cellular Ca uptake (via Na/Ca exchange) and may also elevate the resting intracellular [Ca].(ABSTRACT TRUNCATED AT 400 WORDS)     

Alterations in cardiac adrenergic terminal function and beta-adrenoceptor density in pacing-induced heart failure

Congestive heart failure is associated with cardiac adrenergic nerve terminal changes and beta-adrenoceptor density downregulation. To study the temporal sequence of these changes, we performed studies in rabbits at 2, 4, and 8 wk of cardiac pacing (360 beats/min) and at 1, 2, and 4 wk after cessation of pacing. Rapid pacing produced left ventricular (LV) dysfunction and an increase in plasma norepinephrine (NE) in 1-2 wk. At week 2, NE uptake activity, NE uptake-1 density, and adenylyl cyclase responses to isoproterenol, 5'-guanylyl imidodiphosphate [Gpp(NH)p], and forskolin reduced. However, immunostained tyrosine hydroxylase profile, beta-adrenoceptor density, and NE histofluorescence did not reduce until 4-8 wk of pacing. After cessation of cardiac pacing, LV function normalized quickly, followed by return of tyrosine hydroxylase and NE profiles in 1 wk and adenylyl cyclase responses to agonists and NE uptake activity in 2 wk. Myocardial beta-adrenoceptor density returned to normal by 4 wk after cessation of pacing. Our results suggest that there is no permanent structural neuronal damage in the myocardium within the first 8 wk of rapid cardiac pacing. Abnormal myocardial NE reuptake mechanism may play an important pathophysiological role in heart failure.     

Cardiac mechanical efficiency is preserved in primary cardiac hypertrophy despite impaired mechanical function

Increased heart size is a major risk factor for heart failure and premature mortality. Although abnormal heart growth subsequent to hypertension often accompanies disturbances in mechano-energetics and cardiac efficiency, it remains uncertain whether hypertrophy is their primary driver. In this study, we aimed to investigate the direct association between cardiac hypertrophy and cardiac mechano-energetics using isolated left-ventricular trabeculae from a rat model of primary cardiac hypertrophy and its control. We evaluated energy expenditure (heat output) and mechanical performance (force length work production) simultaneously at a range of preloads and afterloads in a microcalorimeter, we determined energy expenditure related to cross-bridge cycling and Ca²⁺ cycling (activation heat), and we quantified energy efficiency. Rats with cardiac hypertrophy exhibited increased cardiomyocyte length and width. Their trabeculae showed mechanical impairment, evidenced by lower force production, extent and kinetics of shortening, and work output. Lower force was associated with lower energy expenditure related to Ca²⁺ cycling and to cross-bridge cycling. However, despite these changes, both mechanical and cross-bridge energy efficiency were unchanged. Our results show that cardiac hypertrophy is associated with impaired contractile performance and with preservation of energy efficiency. These findings provide direction for future investigations targeting metabolic and Ca²⁺ disturbances underlying cardiac mechanical and energetic impairment in primary cardiac hypertrophy.     

Effects of systolic overload and swim training on cardiac mechanics and biochemistry in rats

We have previously shown that swim conditioning corrects the depressed mechanical function and myosin adenosinetriphosphatase (ATPase) activities associated with renovascular hypertension (HTN) in the rat. The present study was designed to assess the effects of swim conditioning on another form of systolic overload, subdiaphragmatic suprarenal aortic stenosis. Cardiac mechanics in an isolated working heart apparatus and myosin enzymology were studied in four groups of rats: controls (C), animals with chronic systolic overload secondary to aortic constriction (St), swim-conditioning animals (Sw), and animals exposed to a combined load (St-Sw). Heart weight was increased by 23% in St, 27% in Sw, and 36% in St-Sw. In contrast to HTN, cardiac pump and muscle function were not depressed in St. Sw was associated with improved cardiac output, stroke work, and velocity of circumferential fiber shortening. St-Sw showed improved mechanical cardiac performance relative to both C and St. The percent of ventricular myosin of the V1 type and Ca2+-activated myosin ATPase activity relative to C was unchanged in Sw but was depressed in St and St-Sw. These data demonstrate that the salutory mechanical effects of Sw can be superimposed on the systolic overload of St. However, the dissociation between mechanics and myosin enzymology suggests that factors in excitation-contraction coupling other than myosin isoenzyme shifts are responsible for this finding.     

Overexpression of ornithine decarboxylase decreases ventricular systolic function during induction of cardiac hypertrophy

Ornithine decarboxylase (ODC), the first enzyme of polyamine metabolism, is rapidly upregulated in response to agents that induce a pathological cardiac hypertrophy. Transgenic mice overexpressing ODC in the heart (MHC-ODC mice) experience a much more dramatic left ventricular hypertrophy in response to β-adrenergic stimulation with isoproterenol (ISO) compared to wild-type (WT) controls. ISO also induced arginase activity in transgenic hearts but not in controls. The current work studies the cooperation between the cardiac polyamines and L-arginine (L-Arg) availability in MHC-ODC mice. Although ISO-induced hypertrophy is well-compensated, MHC-ODC mice administered L-Arg along with ISO showed a rapid onset of systolic dysfunction and died within 48 h. Myocytes isolated from MHC-ODC mice administered L-Arg/ISO exhibited reduced contractility and altered calcium transients, suggesting an alteration in [Ca(2+)] homeostasis, and abbreviated action potential duration, which may contribute to arrhythmogenesis. The already elevated levels of spermidine and spermine were not further altered in MHC-ODC hearts by L-Arg/ISO treatment, suggesting alternative L-Arg utilization pathways lead to dysregulation of intracellular calcium. MHC-ODC mice administered an arginase inhibitor (Nor-NOHA) along with ISO died almost as rapidly as L-Arg/ISO-treated mice, while the iNOS inhibitor S-methyl-isothiourea (SMT) was strongly protective against L-Arg/ISO. These results point to the induction of arginase as a protective response to β-adrenergic stimulation in the setting of high polyamines. Further, NO generated by exogenously supplied L-Arg may contribute to the lethal consequences of L-Arg/ISO treatment. Since considerable variations in human cardiac polyamine and L-Arg content are likely, it is possible that alterations in these factors may influence myocyte contractility.     

Insensitivity of cardiac phosphofructokinase to adrenergic activation in Zucker rats. A post-receptor defect.

Glyoxal bis(guanylhydrazone), the parent compound of methylglyoxal bis(guanylhydrazone), was synthesized and tested for its ability to inhibit the biosynthesis of polyamines. It was found to be a powerful competitive inhibitor of adenosylmethionine decarboxylase (EC 4.1.1.50), yet the lack of the methyl group at the glyoxal portion increased the apparent Ki value for the enzyme by about 30-fold in comparison with methylglyoxal bis(guanylhydrazone). Glyoxal bis(guanylhydrazone) inhibited diamine oxidase (EC 1.4.3.6) activity as effectively as did methylglyoxal bis(guanylhydrazone). The cellular accumulation curves of glyoxal bis(guanylhydrazone) in L1210 cells were practically superimposable with those of methylglyoxal bis(guanylhydrazone), and the uptake of both compounds was distinctly stimulated by a prior treatment with 2-difluoromethylornithine. The drug decreased the concentration of spermidine in a dose-dependent manner and, in contrast with methylglyoxal bis(guanylhydrazone), without a concomitant accumulation of putrescine. The fact that putrescine concentrations were decreased in cells exposed to glyoxal bis(guanylhydrazone) was, at least in part, attributable to an inhibition of ornithine decarboxylase (EC 4.1.1.17) activity in cells treated with the compound. Under these experimental conditions equivalent concentrations of methylglyoxal bis(guanylhydrazone) [1,1'-[(methylethanediylidine)dinitrilo]diguanidine] elicited large increases in the enzyme activity. When combined with difluoromethylornithine, glyoxal bis(guanylhydrazone) potentiated the growth-inhibitory effect of that drug. Taking into consideration the proven anti-leukaemic activity of glyoxal bis(guanylhydrazone), its effectiveness to inhibit spermidine biosynthesis (without raising the concentration of putrescine) as well as its suitability for combined use with inhibitors of ornithine decarboxylase, this drug is apparently worthy of further testing in tumour-bearing animals, especially in combination with difluoromethylornithine or related inhibitors of ornithine decarboxylase.     

Changes in cardiac muscle myosin light-chain phosphorylation associated with varying inotropic states

Alterations in the phosphorylation of the 20,000-dalton cardiac muscle myosin light chain (LC₂) have been associated with varying inotropic states of the myocardium exposed to catacholamines. It is not known if LC₂ phosphorylation is altered when contractile function is enhanced or depressed as a result of varying the steady-state calcium activation in the isolated perfused myocardium. In this investigation the level of LC₂ phosphorylation was measured in control (C) cardiac muscle, perfused with a medium containing 2.5 mm calcium; in mechanically depressed heart, perfused with 0.5 mm calcium (L-Ca); and in the myocardium with above normal contractile function, perfused with 4.5 mm calcium (H-Ca). The length-tension relationship was determined for all hearts perfused under control conditions and for those perfused with L-Ca and H-Ca. Subsequently, the myocardium was rapidly frozen, myosin LC₂ isolated, and the phosphate content determined. Mechanical function of the myocardium was directly related to calcium activation (Peak tension C = 30.25 ± 3.27 mm Hg, L-Ca = 21.52 ± 3.92 mm Hg, H-Ca = 46.61 ± 11.79 mm Hg). The phosphorylation of LC₂ varied inversely with the calcium concentration of the perfusing solution (slope = −0.134 (mole P_i/mole LC, mm Ca⁻¹, intercept = 1.909; SEE 0.231; P < 0.05). The phosphate content of LC₂ varied inversely with changes in peak tension (slope = −103 Δ% (mole P_i/mole LC)⁻¹, intercept = 158, SEE = 0.156; P < 0.05).The precise mechanism through which light-chain phosphorylation is associated with the actin-myosin interaction remains to be elucidated. This study provides additional evidence of an association between contractile performance and myosin light-chain phosphorylation in cardiac muscle. It is apparent that the amount of free calcium present is important in this relationship.     

Electrocardiographic activity and cardiac function in copper-restricted rats.

The temporal sequence of events leading to cardiac dysfunction during copper restriction in the Long-Evans rat was studied over a 6-week period. Weanling rats were fed either copper-adequate (6 mg Cu/kg diet, n = 25) or copper-restricted (less than 1 mg Cu/kg diet, n = 25) diets for varying periods of time for up to 6 weeks. Beginning at 2 weeks after weaning and weekly thereafter, five rats from each diet were evaluated for cardiac function, and sacrificed, and indicators of copper deficiency were determined on several tissues. Electrocardiograms began showing indications of cardiac disease at Week 3 in the copper-restricted rats, at which time cardiac hypertrophy and other signs of copper deficiency were apparent. Greater QT intervals and QRS amplitudes were observed in copper-restricted rats at various weeks. Peak + and - dP/dt maxs did not differ by diet copper treatment for any of the time intervals studied, nor was any notable difference in developed left ventricular pressure apparent. Hematocrit and liver copper levels were decreased in copper-restricted rat hearts at all weeks. These results suggest that the onset of cardiac dysfunction in copper deficiency is rapid, with both dysfunction and hypertrophy apparent within 3 weeks after copper restriction and when liver copper levels have declined.