Myocardial Membrane Research Articles

Perinatal cocaine exposure reduces myocardial norepinephrine transporter function in the neonatal rat

Norepinephrine transporter (NET) mediates the active removal of norepinephrine (NE) released from sympathetic nerve terminals via reuptake, and NET function and expression can be regulated by cocaine. NET expression and its regulation by cocaine in the developing sympathetic nervous system during early postnatal period, however, have not been examined. We quantified immunodetectable NET protein expression in the neonatal rat heart to examine the developmental pattern of myocardial NET during the first 2 weeks after birth. To assess sympathetic innervations, we simultaneously quantified the expression of myocardial tyrosine hydroxylase (TH). Timed pregnant rats received daily intragastric treatment with saline (CTL) or cocaine at 60 mg/kg (Coc) from Gestational Day 2 until parturition. After birth, nursing mothers continued to receive the same treatment. The expression of myocardial TH and NET in neonatal rats were then studied at 1 day (Postnatal Day 1, PD1), 7 days (PD7) or 14 days (PD14) of age. We observed a similar age-dependent increase in the expression for myocardial NET and TH during the first 2 weeks of postnatal life, in both CTL and Coc animals. While myocardial TH was significantly up-regulated following perinatal cocaine exposure, no significant change in immunodetectable myocardial NET protein was evident. To further examine whether NET function might be affected by perinatal cocaine exposure, we performed NE uptake in myocardial membranes from PD14 CTL and Coc rats. We found that NE uptake was reduced at PD14 in the cocaine-treated group. Our results indicate that myocardial NET and TH are both developmentally regulated. Furthermore, our results indicate that perinatal exposure to cocaine did not change NET protein expression but impaired myocardial NET function in the neonatal rat.

On minimum cyclic AMP formation rates associated with positive inotropic effects mediated through ?1-adrenoceptors in kitten myocardium

The agonist (-)-denopamine was used as a tool to study relationships between pharmacological effects and adenylate cyclase stimulation mediated through beta 1-adrenoceptors. 1. (-)-Denopamine was a full agonist in kitten papillary muscles (force), kitten left atria (force) and kitten and guinea-pig atria (sinoatrial frequency). (-)-Denopamine was a strong partial agonist in guinea-pig tracheae (relaxation). None of these effects was influenced by blockade of beta 2-adrenoceptors. beta 1-Adrenoceptors mediated all effects of (-)-denopamine in atria and effects of low (-)-denopamine concentrations in papillary muscles and tracheae, as assessed with beta 1-selective antagonists. 2. High (-)-denopamine concentrations caused positive inotropic effects in papillary muscles and tracheorelaxant effects that were resistant to blockade by beta 1-, beta 2- and alpha-adrenoceptor antagonists (non-adrenergic effects). 3. (-)-Denopamine stimulated the adenylate cyclase of membranes derived from kitten ventricle and calf tracheal cells with an intrinsic activity of 0.3 and 0.2, respectively, compared to catecholamines. The contribution of beta 1- and beta 2-adrenoceptors to cyclase stimulation was assessed by selective blockade. Cyclase stimulation through beta 2-adrenoceptors by (-)-denopamine was 12% in ventricle and 82% in trachea but is not associated with positive inotropic effects and tracheal relaxation. 4. (-)-Denopamine exhibited only a 2- to 5-fold selectivity for beta 1-adrenoceptors compared to beta 2-adrenoceptors, as estimated consistently from binding assays and blockade of cyclase stimulation in myocardial and tracheal cell membranes. 5. Desensitization of kitten ventricular tissues, caused by a 3 h exposure to 30 mumol/l (-)-isoprenaline followed by 5 h washout, reduced the inotropic sensitivity of papillary muscles without decreasing the maximum inotropic effects of (-)-denopamine. In desensitized tissues, the nonadrenergic effect contributed by 30% to the maximum inotropic effect of (-)-denopamine. 6. In membranes, derived from desensitized tissues, the maximum adenylate cyclase stimulation induced by (-)-isoprenaline, (-)-denopamine and xamoterol was reduced to 1/2 of the corresponding stimulations observed in membranes from sham desensitized tissues. The density of beta-adrenoceptors, assessed with 3H-(-)-CGP 12,177, was not changed by the desensitization procedure suggesting that part of the receptors was uncoupled from the adenylate cyclase. The partial inotropic agonist xamoterol, which has an intrinsic activity of 0.5 in non-desensitized tissues, failed to cause positive inotropic effects in desensitized papillary muscles suggesting that not all cyclic AMP possesses inotropic relevance.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardioprotective effects of carvedilol on acute autoimmune myocarditis: anti-inflammatory effects associated with antioxidant property.

Carvedilol, a new beta-blocker with antioxidant properties, has been shown to be cardioprotective in experimental models of myocardial damage. We investigated whether carvedilol protects against experimental autoimmune myocarditis (EAM) because of its suppression of inflammatory cytokines and its antioxidant properties. We orally administered a vehicle, various doses of carvedilol, racemic carvedilol [R(+)-carvedilol, an enantiomer of carvedilol without beta-blocking activity], metoprolol, or propranolol to rats with EAM induced by porcine myosin for 3 wk. Echocardiographic study showed that the three beta-blockers, except R(+)-carvedilol, suppressed left ventricular fractional shortening and decreased heart rates to the same extent. Carvedilol and R(+)-carvedilol, but not metoprolol or propranolol, markedly reduced the severity of myocarditis at the two different doses and suppressed thickening of the left ventricular posterior wall in rats with EAM. Only carvedilol suppressed myocardial mRNA expression of inflammatory cytokines and IL-1beta protein expression in myocarditis. In addition, carvedilol and R(+)-carvedilol decreased myocardial protein carbonyl contents and myocardial thiobarbituric acid-reactive substance products in rats with EAM. The in vitro study showed that carvedilol and R(+)-carvedilol suppressed IL-1beta production in LPS-stimulated U937 cells and that carvedilol and R(+)-carvedilol, but not metoprolol or propranolol, suppressed thiobarbituric acid-reactive substance products in myocardial membrane challenged by oxidative stress. It was also confirmed that probucol, an antioxidant, ameliorated EAM in vivo. Carvedilol protects against acute EAM in rats, and the superior cardioprotective effect of carvedilol compared with metoprolol and propranolol may be due to suppression of inflammatory cytokines associated with the antioxidant properties in addition to the hemodynamic modifications.

A CAV3 microdeletion differentially affects skeletal muscle and myocardium.

Caveolin-3 is the muscle-specific protein product of the caveolin gene family and an integral membrane component of caveolae. Mutations in the gene encoding caveolin-3 (CAV3) underlie four distinct disorders of skeletal muscle: the autosomal dominant form of limb-girdle muscular dystrophy type 1C (LGMD-1C), rippling muscle disease (RMD), sporadic and familial forms of hyperCKemia, and distal myopathy. To characterize a multigenerational Italian family affected by an autosomal dominant myopathic disorder and to assess the expression of caveolin-3, dystrophin, dystrophin-associated glycoproteins, and neuronal nitric oxide synthase in the myocardium of an affected patient. Clinical analysis involved 15 family members. Skeletal muscle expression of sarcolemmal proteins was evaluated by immunohistochemistry and western blot analysis in three affected individuals. Caveolar structures were analyzed through electron microscopy in muscle biopsies and in one heart biopsy. CAV3 genetic analysis showed a heterozygous 3-bp microdeletion (328-330del) in affected individuals, resulting in the loss of a phenylalanine (Phe97del) in the transmembrane domain. In the skeletal muscle, the mutation was associated with severe caveolin-3 deficiency and caveolar disorganization, whereas the expression of the other analyzed muscle proteins was unaltered. Remarkably, caveolin-3 was expressed in myocardium at a level corresponding to about 60% of that of control individuals and was correctly localized at the myocardial cell membranes, with preservation of cardiac myofiber caveolar structures. Clinical analysis revealed the concomitant presence in this family of the following phenotypes: RMD, LGMD, and hyperCKemia. Intrafamilial phenotypic heterogeneity is associated with caveolin-3 Phe97 microdeletion. The molecular network interacting with caveolin-3 in skeletal muscle and heart may differ.

MODELING EXCITATION AND PROPAGATION OF ACTION POTENTIALS ACROSS INHOMOGENEOUS VENTRICULAR TISSUE

Heterogeneity in the electrical activity across the ventricular wall might result from transmural differences in myocardial membrane ionic current densities. Computational models of endo- and epi-cardial action potentials of guinea-pig myocytes were developed, assuming transmural differences in the ionic current densities and kinetics of i Kr and i Ks . The cell models were able to reproduce the characteristics of action potentials of endo and epi cells. One- and two-dimensional models of ventricular tissue were also developed to study the effects of transmural heterogeneity on the propagation of excitation waves across the ventricle wall. It was shown that intercellular coupling reduces the transmural heterogeneity across the ventricle wall.

Pericardial fluid level of heart-type cytoplasmic fatty acid-binding protein (H-FABP) is an indicator of severe myocardial ischemia

Background: Heart-type cytoplasmic fatty acid-binding protein (H-FABP) has been reported as a sensitive and specific marker for the early diagnosis of acute myocardial infarction. Our hypothesis was that serum or pericardial fluid levels of H-FABP can reflect not only myocardial infarction but also myocardial ischemia. Methods: A total of 34 patients with unstable angina, who had anginal symptoms and/or ST-changes in ECG monitoring within 24 h before operation, were classified into group A (n=17), and those without these symptoms and changes into group B (n=17). Blood and pericardial fluid samples were obtained immediately after median sternotomy, and serum and pericardial fluid levels of creatine kinase-MB, cardiac troponin-T, and H-FABP were measured. Results: Serum H-FABP levels were slightly elevated compared with their normal values in both groups. While they showed no difference between groups A and B (group A vs. B: 8.5±1.0 vs. 7.1±0.7 ng/ml, P=0.25), pericardial fluid levels of H-FABP were significantly higher in group A than in group B (16.3±2.0 vs. 9.6±1.0 ng/ml, P=0.0046). H-FABP showed a weak correlation between its serum levels and pericardial fluid levels (r=0.40). Conclusions: Pericardial fluid levels of H-FABP reflect myocardial ischemia occurring within 24 h of their measurements. H-FABP may be secreted into the interstitial space by increased permeability of the myocardial cell membrane associated with severe myocardial ischemia. Thus, pericardial fluid reflects pathophysiological conditions of cardiomyocytes more sensitively than circulating blood.

Depressed contractile function and adrenergic responsiveness of cardiac myocytes in an experimental model of Parkinson disease, the MPTP-treated mouse

Radiotracer and biochemical studies have shown that patients with Parkinson disease lack functional sympathetic innervation to the heart. The same observation was made in mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), an experimental model of Parkinson disease. This study examined the mechanical properties, adrenergic receptor level and intracellular Ca 2+ handling in cardiac myocytes isolated from C57/BL6 mice that received either MPTP (30 mg/kg, i.p., twice in 24 h) or vehicle. Mechanical properties were evaluated using an IonOptix MyoCam® system. Myocytes were electrically stimulated at 0.5 Hz. The contractile properties analyzed included peak shortening (PS), time-to-PS (TPS), time-to-90% relengthening (TR 90), and maximal velocities of shortening and relengthening (±d L/d t). Intracellular Ca 2+ handling was evaluated with fura 2. Myocytes from MPTP-treated mice exhibited a depressed PS (85% of normal), normal TPS, prolonged TR 90 (147% of normal), and reduced ±d L/d t (both 79% of normal). These results were correlated with a 67% reduction of β-adrenergic receptor expression in myocardial membranes from MPTP-treated mice when compared to normal. Myocytes from MPTP-treated mice also exhibited a reduced peak of intracellular Ca 2+ sequestration and sarcoplasmic reticulum (SR) Ca 2+ load (55 and 38% of normal, respectively). The resting intracellular Ca 2+ and Ca 2+-transient decay were comparable to the values seen in myocytes from untreated mice. Myocytes from MPTP-treated and untreated mice were equally responsive over a range of stimulation frequencies (0.1, 0.5, 1, 3 and 5 Hz). Response to norepinephrine (1 μM) and isoproterenol (1 μM) was reduced in myocytes from MPTP-treated mice. These results demonstrate substantial cardiac dysfunctions in this model of experimental Parkinson disease, probably due to reduced adrenergic responsiveness and SR Ca 2+ load.

Pinacidil-induced opening, like glibenclamide-induced closure of cardiac KATP channels, protects cardiac function against ischemia in isolated, working, erythrocyte perfused rat hearts.

Glibenclamide-induced closure of ATP-dependent potassium (KATP) channels decreases coronary blood flow during normoxic and post-ischemic conditions. We have found that post-ischemic cardiac function is improved after glibenclamide treatment. Our theory was that this is a result of higher intracellular calcium concentrations due to reduction in ischemia-mediated hyperpolarization of the myocardial cell membrane. We hypothesized therefore that opening KATP channels would reduce post-ischemic function in our isolated, erythrocyte perfused, working rat heart model. During treatment with 1 or 12 mumol.L-1 pinacidil (protein unbound concentration) both before and after 12 minutes global ischemia coronary blood flow increased 2-3 fold compared with vehicle, while cardiac functional recovery post-ischemically was improved with both concentrations. Because closing and opening cardiac KATP channels both improve post-ischemic function, our calcium theory above can be discounted. The protective effect of glibenclamide may possibly be ascribed to metabolic effects such as preservation of ATP levels during ischemia.

Quick Review: Hemodynamics

Before discussing basic hemodynamics, we should remind ourselves of the systemic circuit: 1. Cardiac Anatomy 2. Circulatory Pathways This brief review discusses the basics of hemodynamics. CARDIAC ANATOMY The Heart: 2 Separate Volume Pumps ! The in-series nature of these two systems implies that the output of the Right Heart becomes the input of the Left Heart, and therefore, the output of the Left Heart becomes the input of the Right Heart FLOW VIA SERIES: DEMONSTRATED BY WILLIAM HARVEY, 1628 Desaturated Blood returns from the Systemic Vessels via the SVC & IVC Saturated Blood is then returned to the Left Atrium via the Pulmonary Veins ! MYOCARDIAL PERFUSION OCCURS PRIMARILY DURING DIASTOLE MYOCARDIAL BLOOD FLOW: MYOCYTE CONTRACTION At the cellular level, electrical depolarization of the myocardial cell membrane allows ionized calcium flux into the cytoplasm leading to hydrolysis of ATP by Myosin. This leads to a conformational change in the Actin-Myosin Cross Bridge producing sliding of myosin filaments relative to actin & overall shortening of the sarcomere [Sliding Filament Theory] Calcium is then removed from the cell by Active Transport in the Sarcoplasmic Reticulum allowing Relaxation, while ATP is regenerated by Metabolic Processes Over the physiologic range of sarcomere length (1.6 2.0 um), the amount of metabolic energy converted to mechanical work is dependent on the available Surface Area of Cross-Bridge Interactions ! Work is directly proportional to End-Diastolic Sarcomere Length This “Length Dependency” is the fundamental basis for the Frank-Starling Law ! Otto Frank, 1885 (Frog Heart Preparations): “the output of a normal heart is influenced primarily by the volume of blood in the ventricle at the end of diastole” Figure 1 Quick Review: Hemodynamics 2 of 3 CARDIAC OUTPUT CO = HR x SV “the amount of blood pumped by the heart per unit time” NORMAL C.O. : 3.5 8.5 L/MIN Manipulation of the factors can lead to augmentation of CO at the lowest possible energy cost ! DETERMINANTS OF CARDIAC PERFORMANCE & OUTPUT Preload: EDV (the load that stretches a muscle prior to contraction) Afterload: SVR (the load that must be moved during muscle contraction) Contractility: the velocity of muscle shortening at a constant preload and afterload Compliance: the length that a muscle is stretched by a given preload. Determined by the inherent Elasticity ! Heart Rate: several effects on overall Cardiac Function : Tachycardia/Bradycardia PRELOAD PCWP = LAP = LVEDP (best approximation) But Remember, the relationship between LVEDP & LVEDVis NOT Linear!! PCWP is by definition an ESTIMATE of EDV& thus, an ESTIMATE of Preload Elevation of CVP to Equal PAD& PCWP Square Root Sign : characteristic RA waveform in patients with Constrictive Pericarditis AFTERLOAD The impedance to LV Ejection and is usually estimated by the Systemic Vascular Resistance. Remember: changes in afterload have no effect on the contractility of a normal heart SVR = {(MABP CVP)/CO} x 80 MBAP (Mean Arterial Blood Pressure) = DBP + [1/3(SBP DBP)] SVR units: dynes-second/cm Decreasing Afterload exchanges Pressure Work for Flow Work and serves to increase vital organ perfusion ! Pressure Work Flow Work Plus, since pressure work is more costly than flow work in terms of myocardial oxygen consumption, by decreasing afterload you also decrease the overall energy requirement PVR = {(MPAP PWP)/CO} x 80 Remember: CONTRACTILITY the inotropic state: an intrinsic property of myocardial muscle which is manifested as a greater force of contraction for a given preload By increasing intropic state, you increase both Pressure Work & Flow Work thus, the cost in myocardial oxygen consumption may be high !! COMPLIANCE & ELASTICITY “Compliance”: the tendency of an object to return to it's original shape when it has been deformed or altered (Compliance = change in Volume / change in Pressure) HEART RATE Heart Rate can Influence Cardiac Function in Several Ways: Cardiac Physiology is based on a thorough understanding of the underlying mechanics ! References Quick Review: Hemodynamics 3 of 3 Author Information Tisha K. Fujii, DO Dept. of Trauma & Critical Care , Boston University School of Medicine , Boston Medical Center Bradley J. Phillips, MD Dept. of Trauma & Critical Care , Boston University School of Medicine , Boston Medical Center

Chronic inhibition of Rho kinase blunts the process of left ventricular hypertrophy leading to cardiac contractile dysfunction in hypertension-induced heart failure

The Gq–RhoA–Rho kinase pathway, activated by neurohormonal factors such as angiotensin II (Ang II), has been proposed to be one of the important signaling pathways involved in the progression of left ventricular (LV) hypertrophy to heart failure. We tested the hypothesis that chronic inhibition of Rho kinase prevents this process. Heart failure was induced in Dahl salt-sensitive (DS) rats fed an 8% NaCl diet from 8 until 17 weeks of age. Y-27632 (5 mg/kg per day), a selective Rho kinase inhibitor, was applied orally to DS rats starting at 10 weeks of age for 7 weeks (DS/Y+). DS rats without Y-27632 (DS/Y–) and Dahl salt-resistant (DR) rats fed the 8% NaCl diet were regarded as non-therapeutic and normotensive controls, respectively. At 17 weeks of age, there was no significant difference in the blood pressure of DS/Y– and DS/Y+ rats. DS/Y– rats exhibited: (1) increases in LV mass, cross-sectional area (CSA) of cardiomyocytes, and interstitial fibrosis; (2) contractile dysfunction, i.e. decreases in LV ejection fraction and % fractional shortening, and prolongation of time to peak tension as well as to 50% relaxation in the twitch contraction of isolated papillary muscle; and (3) increases in the protein expression of Gαq and Rho kinase in the myocardial membrane fraction. In DS/Y+ rats, the degree of myocardial hypertrophy was significantly inhibited in association with improved contractile function, without a decrease in the degree of interstitial fibrosis. Our results suggest the possibility that the Gq–Rho kinase pathway plays an important role in the process of hypertension-induced LV hypertrophy leading to contractile dysfunction.

Modulation of myocardial contractility by lysophosphatidic acid (LPA)

Lysophosphatidic acid (LPA) is a phospholipid messenger, which is released from activated platelets and leukocytes. This study examined the effects of LPA on myocardial contractility and characterized the signal transduction pathway involved in these effects. Functional effects of LPA were determined in isolated, electrically driven human myocardial preparations and rat cardiac myocytes. In human atrial and ventricular myocardial preparations, LPA (100 μmol/l) decreased isoprenaline (0.03 μmol/l) enhanced force of contraction by 17 ± 2% and 28 ± 3%, respectively. The effect of LPA was attenuated by suramin (1 mmol/l). In isolated rat cardiomyocytes, LPA (1–100 μmol/l) concentration dependently abolished isoprenaline (0.03 μmol/l) induced increase in cell shortening. This antiadrenergic effect was blunted after pretreatment with pertussis toxin (5 μg/ml, 12 h). Forskolin (10 μmol/l) stimulated adenylyl cyclase activity was inhibited by LPA in human myocardial membranes. PCR analysis of human atrial and ventricular cDNAs revealed the expression of two cognate LPA receptors: EDG-2 and EDG-7. Our results suggest that LPA exerts antiadrenergic effects on force of contraction in human and rodent myocardium via a Gα i/o protein-mediated mechanism, most probably by LPA binding to the mammalian LPA receptors EDG-2 and/or EDG-7. This newly discovered action of LPA might be of pathophysiological importance in conditions like myocardial ischemia or inflammatory disorders when LPA release is enhanced.

β 1-adrenoceptor selectivity of nebivolol and bisoprolol. A comparison of [ 3H]CGP 12.177 and [ 125I]iodocyanopindolol binding studies

There is an ongoing discussion on whether or not high β 1-adrenoceptor selectivity of β-adrenoceptor antagonists may be favorable in the treatment of patients with heart failure. The present study compared the β 1-adrenoceptor selectivity of nebivolol and bisoprolol with that of carvedilol in the human myocardium, using a binding assay in conjunction with either the hydrophilic ligand (±)-[ 3H]4-(3-tertiarybutylamino-2-hydroxypropoxy)-benzimidazole-2-on HCl ([ 3H]CGP 12.177) or the lipophilic ligand [ 125I]iodocyanopindolol as radiolabeled compound. Measurements were made using membrane preparations obtained from identical nonfailing donor hearts. β-adrenoceptor density was found to be slightly higher when [ 125I]iodocyanopindolol was used compared to [ 3H]CGP 12.177 (256±15 and 213±18 fmol/mg protein, respectively). When the highly β 1-adrenoceptor-selective compound 2-hydroxy-5-(2-(hydroxy-3-(4((1-methyl-4-trifluoromethyl)-1- H-imidazol-2-yl)-phenoxy)-propyl)-aminoethoxyl)-benzamide (CGP 20.712A) and the highly β 2-adrenoceptor-selective compound erythro-(±)-1-(7-methylindan-4-yloyl)-3-isopropylaminobutan-2-ol HCl (ICI 118.551) were used in competition experiments, a similar proportion of β 1-adrenoceptors was seen for [ 3H]CGP 12.177 (69.3±1.6%) and for [ 125I]iodocyanopindolol (67.0±2.1%). K i(β 1) and K i(β 2) were obtained in the presence of 50 nM ICI 118.551 and 300 nM CGP 20.712A. The rank order of β 1-adrenoceptor selectivity ( K i(β 2)/ K i(β 1) ratio) was nebivolol (for [ 3H]CGP 12.177 46.1 and for [ 125I]iodocyanopindolol 22.5)>bisoprolol (13.1 and 6.4)>carvedilol (0.65 and 0.41). To investigate whether in vivo metabolized nebivolol retains high β 1-adrenoceptor selectivity, serum specimens were collected before and 2 h after oral administration of 5 mg nebivolol. The samples were used for [ 125I]iodocyanopindolol binding studies with the myocardial membrane preparations. In these samples, the binding of [ 125I]iodocyanopindolol to β 1-adrenoceptors was inhibited by 46.4±5.3%, whereas the binding to β 2-adrenoceptors was inhibited by 20.5±1.1% compared to that of control samples. It is concluded that nebivolol is approximately 3.5 times more β 1-adrenoceptor-selective than bisoprolol in the human myocardium. Furthermore, in vivo metabolized nebivolol retains β 1-adrenoceptor selectivity.

Cardiac pacemaker mechanisms in the ostracod crustacean Vargula hilgendorfii

The heart of the ostracod crustacean Vargula hilgendorfii has a single intrinsic neuron that morphologically appears to innervate the myocardium. We, therefore, examined the heart activity electrophysiologically to determine whether the heartbeat is neurogenic. Each heartbeat is associated with a myocardial action potential composed of a spike potential followed by a plateau potential. The frequency of the action potential is not stable but changes successively over a wide range. The action potential is not preceded by a pacemaker potential and has an inflection in its rising phase. The myocardial cells couple electrically and fire almost simultaneously. The frequency of the action potential was unchanged by injection of depolarizing or hyperpolarizing current into the myocardium. However, slow oscillatory potentials appeared during the depolarization and its frequency was higher with increasing current intensity. Application of 1-μM tetrodotoxin (TTX) depolarized the myocardial membrane and completely prevented the action potential. During this depolarization, slow oscillatory potentials often appeared spontaneously. These results suggest that, although the myocardium has a property of conditional oscillator, the heartbeat is driven by the single cell cardiac ganglion that has both pacemaker and motor functions.

QT prolongation and torsades de pointes associated with concurrent use of cisapride and erythromycin

Prolongation of QT interval may lead to serious, potentially life-threatening, ventricular tachyarrhythmia, such as torsades de pointes. The cause may be an inherited or an acquired malfunction of ion channels at the myocardial cell membrane. Metabolic abnormalities, starvation, nervous system injury, and drug administration cause the much more frequent acquired long QT syndrome (LQTS). Types Ia and III antiarrhythmic drugs account for the majority of these life-threatening events, whereas a number of drugs widely used in otolaryngology, such as antibiotics and antihistamines, have been recently implicated. A case of a life-threatening ventricular tachyarrhythmia after the concurrent administration of cisapride and erythromycin is presented. Reviewed are drugs commonly prescribed in otolaryngology, as well as the associated risk factors that potentially lead to LQTS. (Am J Otolaryngol 2002;23:303-307. Copyright 2002, Elsevier Science (USA). All rights reserved.)

Density and binding characteristics of beta-adrenoceptors in the normal and failing equine myocardium.

Beta-adrenoceptors are important regulators of cardiac function and their characteristics are known to change in human and canine diseased myocardium. This study aimed to determine the density and subtypes of beta-adrenoceptors in the normal and failing equine ventricular myocardium. Membrane preparations of the left papillary muscles were incubated with increasing concentrations of the nonselective beta-adrenoceptor antagonist [3H]-CGP12177. Saturable and reversible binding of [3H]-CGP12177 to myocardial membranes was demonstrated with Kd values (+/- s.d.) of 0.49 +/- 0.40 and 0.43 +/- 0.22 nmol/l and Bmax values of 93.4 +/- 20.5 and 110.0 +/- 21.2 and fmol/mg protein for normal (n = 19) and heart failure (n = 10) tissues, respectively. Heart failure had no significant effect on the density of ventricular beta-adrenoceptors. The cardiac beta-adrenoceptors were further characterised by studying displacement of [3H]-CGP12177 (0.6 nmol/l) with the beta1-selective antagonists CGP20712A and the beta2-selective antagonist ICI118.551. In normal ventricular muscle, CGP20712A was 26 times more potent than ICI118.551 (Ki values 30.4 +/- 24.8 and 814.1 +/- 485.2 nmol/l, respectively). In heart failure cases, CGP 20712A curves were monophasic with a Ki value of 45.6 +/- 39.7 nmol/l. ICI 118.551 curves were biphasic in 5 horses where 11-31% of the cardiac beta-adrenoceptors had a high affinity for ICI 118.551. These data suggest that the normal equine ventricular myocardium possesses predominately beta1-adrenoceptors, with no evidence for co-existence of a significant population of beta2-adrenoceptors. The density of beta-adrenoceptors did not appear to change in heart failure, but the appearance of receptors with a high affinity for ICI118.551 may suggest that, in some cases, heart failure increases the expression of beta2-adrenoceptors in equine ventricular myocardium. This study provides an insight into the role of the adrenergic system in heart disease in the horse. Further studies in this area are warranted.

Hyperkalaemia: a dangerous electrolyte disturbance

INTRODUCTION The most important electrolytes are sodium, potassium, calcium and magnesium. The flow of these electrolytes into and out of cardiac cells, as currents, creates the energy needed for depolarisation and repolarisation and allows contractile mechanisms to function. The levels of electrolytes in the fluid bathing the cardiac cells has an effect on these currents and the appearance of the complexes seen on the electrocardiograph (ECG). Of all the electrolyte changes that can occur, hyperkalaemia is the most dangerous. Hyperkalaemia can not only kill, but it can kill in minutes, or even seconds, and it prevents response to the drugs used in resuscitation (Garcia & Holtz, 2001). Hyperkalaemia causes changes in the appearance of the ECG, symbolising the changes in cell function, and can cause any and all arrhythmias. Immediate recognition and action to stabilise the myocardial membrane and reverse the pathological processes are the keys to combatting effectively the complications of hyperkalaemia (Garcia & Holtz, 2001). The nurse within the critical care area is in the optimum position to diagnose and direct such management.

6-Sulfonylchromenes as highly potent K(ATP)-channel openers.

We synthesized K(ATP)-channel openers (KCOs) composed of the 4-pyridonechromene moiety of bimakalim (1) and a variety of sulfonyl-containing 6-substituents 4-29. Dilator potencies were measured in rat aorta and trachea. In both test systems the KCOs exhibit potency ranges of roughly 3 log units. The 6-N-phenyl-N-methylsulfonamido derivative 24 shows the highest potency. In rat aorta the potency spectrum ranges from a pEC(50) value of 8.76 to 5.68; in rat trachea it ranges from 8.01 to 4.99. On average, the dilator activity is about 0.8 log units stronger in the aorta. Aortic relaxation by chromene 13 is markedly retarded, the clinical relevance of which (e.g., preventing tachycardia) remains to be clarified. Binding affinities were determined in myocardial membranes and aortic smooth muscle cells of the rat. The affinity spectrum in myocardial membranes ranges from a pK(D) of 7.83 to 5.18; the highest affinity in aortic smooth muscle cells is measured for compound 28 (pK(D) = 8.55), whereas the lowest affinity is measured for 4 (pK(D) = 4.51). Significant selectivities discriminating between K(ATP)-channels of different organs could not be detected. PLS analysis yielded no significant correlation between vasodilator activity in aorta and chemical descriptors (GRIND). Compounds 13, 24, and 28 represent the most potent KCOs of the 4-pyridonechromene type published so far. Their 6-substituents exhibit a phenyl ring with a congruent conformational orientation in relation to the sulfonylchromene. From SAR data and conformational analysis we postulate that these new 6-substituents extend the binding site for chromene KCOs. Correspondingly, we assume that the receptor area exhibits two separate interaction sites with the capacity to bind 6-substituents: (a) one site interacting with negatively polarized partial structures (e.g., CN, NO(2), SO(2)) and (b) one spatially restricted site enabling favorable pi-interactions.

Electroporation in a Model of Cardiac Defibrillation

It is known that high-strength shock disrupts the lipid matrix of the myocardial cell membrane and forms reversible aqueous pores across the membrane. This process is known as "electroporation." However, it remains unclear whether electroporation contributes to the mechanism of ventricular defibrillation. The aim of this computer simulation study was to examine the possible role of electroporation in the success of defibrillation shock. Using a modified Luo-Rudy-1 model, we simulated two-dimensional myocardial tissue with a homogeneous bidomain nature and unequal anisotropy ratios. Spiral waves were induced by the S1-S2 method. Next, monophasic defibrillation shocks were delivered externally via two line electrodes. For nonelectroporating tissue, termination of ongoing fibrillation succeeded; however, new spiral waves were initiated, even with high-strength shock (24 V/cm). For electroporating tissue, high-strength shock (24 V/cm) was sufficient to extinguish ongoing fibrillation and did not initiate any new spiral waves. Weak shock (16 to 20 V/cm) also extinguished ongoing fibrillation; however, in contrast to the high-strength shock, new spiral waves were initiated. Success in defibrillation depended on the occurrence of electroporation-mediated anodal-break excitation from the physical anode and the virtual anode. Some excitation wavefronts following electrical shock used a deexcited area with recovered excitability as a pass-through point; therefore, electroporation-mediated anodal-break excitation is necessary to block out the pass-through point, resulting in successful defibrillation. The electroporation-mediated anodal-break excitation mechanism may play an important role in electrical defibrillation.

Evidence for a short form of RGS3 preferentially expressed in the human heart.

RGS proteins (regulators of G protein signalling) negatively regulate G protein function as GTPase-activating proteins (GAP) for G protein alpha-subunits. The existence of mRNAs of different size for some of the RGS proteins, e.g. RGS3, suggests that these proteins may exist in isoforms due to alternative splicing. We therefore investigated RGS3 mRNA and protein expression in different human tissues. Ribonuclease protection assays and Northern blot analysis showed two specific mRNAs for RGS3 (RGS3L, RGS3S) in human myocardium, suggesting an additional, N-terminally truncated form of approximately 168 aa. When expressed as a recombinant protein RGS3S was recognized at approximately 23 kDa by an antipeptide antiserum originally raised against an RGS2 sequence. In membranes of human tissues this antiserum detected specific signals for RGS3L (approximately 70 kDa), RGS2 (approximately 30 kDa) and a 25-kDa protein, most likely RGS3S. Both RGS3S mRNA and the 25 kDa protein were abundant in human heart, whereas expression in liver, brain and myometrium was much weaker. To characterize RGS3S functionally, single turnover GTPase, adenylyl cyclase (AC) and phospholipase C (PLC) activities were determined. Both recombinant RGS3S and RGS16 increased Pi release from Galphai1 by about 150% and increased GTP- and GTP plus isoprenaline-stimulated AC activity by 20-30% in human left ventricular myocardial membranes. Additionally, both RGS proteins reduced basal and endothelin-stimulated PLC activity in these membranes by about 40%. We conclude that an additional truncated form of RGS3 is expressed in the human heart. As described for the full-length protein, RGS3S negatively regulates the activity of Gi/o- and Gq-, but not Gs-subfamily members.

Impaired Ca2+-sequestration in dilated cardiomyopathy (review).

Excitation-contraction coupling is the process by which depolarisation of the myocardial surface membrane leads to the release of Ca2+-ions from the sarcoplasmic reticulum, inducing cardiac muscle contraction. This process is made possible by an elaborate system of ion-release, uptake and sequestration that controls the contraction and relaxation cycle of heart muscle fibres. The free intracellular Ca2+-concentration determines the contractile state of the myocardium, and the sequestration of Ca2+-ions into the lumen of the sarcoplasmic reticulum by the Ca2+-ATPase pump units represents a critical step towards the maintenance of normal Ca2+-cycling. The Ca2+-ATPase pump activity is regulated by phospholamban, a small 52-amino acid protein whose phosphorylation state dictates its inhibitory action on the pump. A large body of evidence points to the central role of abnormal Ca2+-ATPase-phospholamban interactions in pathophysiological heart conditions, thereby compromising the contractile state of the cardiac muscle cell. It has been shown that alterations in the oligomeric status of the Ca2+-ATPase and modified interactions between the Ca2+-pump and its regulatory subunit phospholamban underlie the contractile dysfunction that characterises certain forms of dilated cardiomyopathy. Hence, elucidation of interactions within physiological Ca2+-ATPase pump units in normal and diseased myocardium is a vital link in the development of improved diagnostic and therapeutic techniques for dealing with this elusive condition.