Effect of stellate ganglionectomy on basal cardiovascular function and responses to beta1-adrenoceptor blockade in the rat

Cardiac sympathetic nerve activity is an important short-term controller of cardiac function and arterial pressure. Studies also suggest that long-term increases in cardiac sympathetic nerve activity may contribute to hypertension, coronary artery disease, and cardiac remodeling in heart failure. However, our understanding of the role of cardiac sympathetic nerves in chronic models of cardiovascular disease has been limited by inadequate experimental approaches. The present study was conducted to develop a surgical method to surgically denervate the sympathetic nerves of the rat heart for long-term cardiovascular studies. We characterized the effect of cardiac sympathetic denervation on basal levels of mean arterial pressure (MAP) and heart rate (HR) and the responses to a chronic administration of atenolol, a beta1-adrenoceptor antagonist. Rats were instrumented with telemetry transmitters for continuous recording of MAP and HR. After a 4-day baseline period, the rats were subjected to bilateral stellate ganglionectomy (SGX; n=9) or sham surgery (Sham; n=8). Seven days following SGX or Sham, the rats were administered atenolol for 5 days, followed by a 7-day recovery period. Following a transient decrease, SGX had no effect on basal MAP but decreased HR compared with baseline and Sham rats. Five days of atenolol treatment decreased MAP similarly in SGX and Sham rats. Atenolol resulted in a marked bradycardia in Sham rats but had a neglible effects on HR in SGX rats. The measurement of the content of cardiac catecholamines in all cardiac chambers at the end of the study verified a successful sympathetic denervation. This study confirms that bilateral SGX is a useful method to study the contribution of cardiac sympathetic nerves on the regulation of cardiac function. Moreover, these results suggest that cardiac sympathetic nerves are relatively unimportant in maintaining the basal level of MAP or the depressor response to atenolol in conscious, unrestrained rats.     

Association of an oral formulation of angiotensin-(1–7) with atenolol improves lipid metabolism in hypertensive rats

The β-adrenergic blockers and antagonists of the renin-angiotensin system (RAS) are among the drugs that present better results in the control of cardio-metabolic diseases. The aim of the present study was to evaluate the effect of the association of the β-blocker, atenolol, and an oral formulation of Ang-(1–7) on lipid metabolism in spontaneously hypertensive rats (SHR). The main results showed that SHR treated with oral formulation of Ang-(1–7) in combination to atenolol have an improvement of lipid metabolism with a reduction of total plasma cholesterol, improvement of oral fat load tolerance and an increase in the lipolytic response stimulated by the β-adrenergic agonist, isoproterenol, without modification of resting glucose or insulin sensitivity in adipocytes. In conclusion, we showed that administration of an Ang-(1–7) oral formulation in association with a β-blocker induces beneficial effects on dyslipidemia treatment associated with hypertension.     

Rate-dependent shortening of action potential duration increases ventricular vulnerability in failing rabbit heart

Congestive heart failure (CHF) predisposes to ventricular fibrillation (VF) in association with electrical remodeling of the ventricle. However, much remains unknown about the rate-dependent electrophysiological properties in a failing heart. Action potential properties in the left ventricular subepicardial muscles during dynamic pacing were examined with optical mapping in pacing-induced CHF (n=18) and control (n=17) rabbit hearts perfused in vitro. Action potential durations (APDs) in CHF were significantly longer than those observed for controls at basic cycle lengths (BCLs)>1,000 ms but significantly shorter at BCLs<400 ms. Spatial APD dispersions were significantly increased in CHF versus control (by 17-81%), and conduction velocity was significantly decreased in CHF (by 6-20%). In both groups, high-frequency stimulation (BCLs<150 ms) always caused spatial APD alternans; spatially concordant alternans and spatially discordant alternans (SDA) were induced at 60% and 40% in control, respectively, whereas 18% and 82% in CHF. SDA in CHF caused wavebreaks followed by reentrant excitations, giving rise to VF. Incidence of ventricular tachycardia/VFs elicited by high-frequency dynamic pacing (BCLs<150 ms) was significantly higher in CHF versus control (93% vs. 20%). In CHF, left ventricular subepicardial muscles show significant APD shortenings at short BCLs favoring reentry formations following wavebreaks in association with SDA. High-frequency excitation itself may increase the vulnerability to VF in CHF.     

β-AR blockers suppresses ER stress in cardiac hypertrophy and heart failure

BACKGROUND: Long-term β-adrenergic receptor (β-AR) blockade reduces mortality in patients with heart failure. Chronic sympathetic hyperactivity in heart failure causes sustained β-AR activation, and this can deplete Ca(2+) in endoplasmic reticulum (ER) leading to ER stress and subsequent apoptosis. We tested the effect of β-AR blockers on ER stress pathway in experimental model of heart failure. METHODS AND DISCUSSIONS: ER chaperones were markedly increased in failing hearts of patients with end-stage heart failure. In Sprague-Dawley rats, cardiac hypertrophy and heart failure was induced by abdominal aortic constriction or isoproterenol subcutaneous injection. Oral β-AR blockers treatment was performed in therapy groups. Cardiac remodeling and left ventricular function were analyzed in rats failing hearts. After 4 or 8 weeks of banding, rats developed cardiac hypertrophy and failure. Cardiac expression of ER chaperones was significantly increased. Similar to the findings above, sustained isoproterenol infusion for 2 weeks induced cardiac hypertrophy and failure with increased ER chaperones and apoptosis in hearts. β-AR blockers treatment markedly attenuated these pathological changes and reduced ER stress and apoptosis in failing hearts. On the other hand, β-AR agonist isoproterenol induced ER stress and apoptosis in cultured cardiomyocytes. β-AR blockers largely prevented ER stress and protected myocytes against apoptosis. And β-AR blockade significantly suppressed the overactivation of CaMKII in isoproterenol-stimulated cardiomyocytes and failing hearts in rats. CONCLUSIONS: Our results demonstrated that ER stress occurred in failing hearts and this could be reversed by β-AR blockade. Alleviation of ER stress may be an important mechanism underlying the therapeutic effect of β-AR blockers on heart failure.     

Neural and humoral control of regional vascular beds via A1 adenosine receptors located in the nucleus tractus solitarii

Our previous studies showed that stimulation of adenosine A(1) receptors located in the nucleus of the solitary tract (NTS) exerts counteracting effects on the iliac vascular bed: activation of the adrenal medulla and β-adrenergic vasodilation vs. sympathetic and vasopressinergic vasoconstriction. Because NTS A(1) adenosine receptors inhibit baroreflex transmission in the NTS and contribute to the pressor component of the HDR, we hypothesized that these receptors also contribute to the redistribution of blood from the visceral to the muscle vasculature via prevailing sympathetic and vasopressinergic vasoconstriction in the visceral (renal and mesenteric) vascular beds and prevailing β-adrenergic vasodilation in the somatic (iliac) vasculature. To test this hypothesis, we compared the A(1) adenosine-receptor-mediated effects of each vasoactive factor triggered by NTS A(1) adenosine receptor stimulation [N(6)-cyclopentyladenosine (CPA), 330 pmol in 50 nl] on the regional vascular responses in urethane/chloralose-anesthetized rats. The single-factor effects were separated using adrenalectomy, β-adrenergic blockade, V(1) vasopressin receptor blockade, and sinoaortic denervation. In intact animals, initial vasodilation was followed by large, sustained vasoconstriction with smaller responses observed in renal vs. mesenteric and iliac vascular beds. The initial β-adrenergic vasodilation prevailed in the iliac vs. mesenteric and renal vasculature. The large and sustained vasopressinergic vasoconstriction was similar in all vascular beds. Small sympathetic vasoconstriction was observed only in the iliac vasculature in this setting. We conclude that, although A(1) adenosine-receptor-mediated β-adrenergic vasodilation may contribute to the redistribution of blood from the visceral to the muscle vasculature, this effect is overridden by sympathetic and vasopressinergic vasoconstriction.     

Role of ATP-sensitive K+ channels in electrophysiological alterations during myocardial ischemia: a study using Kir6.2-null mice

The role of cardiac ATP-sensitive K(+) (K(ATP)) channels in ischemia-induced electrophysiological alterations has not been thoroughly established. Using mice with homozygous knockout (KO) of Kir6.2 (a pore-forming subunit of cardiac K(ATP) channel) gene, we investigated the potential contribution of K(ATP) channels to electrophysiological alterations and extracellular K(+) accumulation during myocardial ischemia. Coronary-perfused mouse left ventricular muscles were stimulated at 5 Hz and subjected to no-flow ischemia. Transmembrane potential and extracellular K(+) concentration ([K(+)](o)) were measured by using conventional and K(+)-selective microelectrodes, respectively. In wild-type (WT) hearts, action potential duration (APD) at 90% repolarization (APD(90)) was significantly decreased by 70.1 +/- 5.2% after 10 min of ischemia (n = 6, P < 0.05). Such ischemia-induced shortening of APD(90) did not occur in Kir6.2-deficient (Kir6.2 KO) hearts. Resting membrane potential in WT and Kir6.2 KO hearts similarly decreased by 16.8 +/- 5.6 (n = 7, P < 0.05) and 15.0 +/- 1.7 (n = 6, P < 0.05) mV, respectively. The [K(+)](o) in WT hearts increased within the first 5 min of ischemia by 6.9 +/- 2.5 mM (n = 6, P < 0.05) and then reached a plateau. However, the extracellular K(+) accumulation similarly occurred in Kir6.2 KO hearts and the degree of [K(+)](o) increase was comparable to that in WT hearts (by 7.0 +/- 1.7 mM, n = 6, P < 0.05). In Kir6.2 KO hearts, time-dependent slowing of conduction was more pronounced compared with WT hearts. In conclusion, the present study using Kir6.2 KO hearts provides evidence that the activation of K(ATP) channels contributes to the shortening of APD, whereas it is not the primary cause of extracellular K(+) accumulation during early myocardial ischemia.     

Dysfunction of supramedullary alpha-adrenergic mechanisms following sino-aortic denervation in Kyoto Wistar rats

Central α-adrenergic mechanisms of blood pressure regulation were investigated by injecting norepinephrine or bradykinin into the carotid input of the cross-circulated head preparations of normotensive Wistar Kyoto rats (WKY). Rats were divided into three groups: sham-operated (sham), carotid sinuses denervated (SD) and carotid sinuses and aortic nerves debuffered (SAD). Norepinephrine, 5 μg, produced vasodepression in all rats, accompanied by corresponding decreases in sympathetic nerve activity recorded in some rats. Magnitude of vasodepression was largest in SAD rats. In sham rats, bradykinin, 1 μg, produced a biphasic response:initial vasodepression followed by a sustained pressor phase. This was accompanied by corresponding changes in peripheral sympathetic nerve activity recorded in some rats. In both SAD and SD rats bradykinin-induced vasodepression was abolished, while the magnitude of the pressor phase became more prominent. The increase in the pressor phase was greater in SAD than in SD rats. In similar studies of spontaneously hypertensive rats (SHR), responses to both α-adrenergic agonist and bradykinin are augmented, suggesting a dysfunction of hypothalamic α-adrenergic mechanisms. Since in the present study it has been shown that sino-aortic denervation produces effects similar to those seen in SHR, dysfunction of buffer nerves may account for the deficient central α-adrenergic mechanisms in SHR.     

Effect of beta-adrenergic blockade on dynamic electrical restitution in vivo

The slope of the action potential duration (APD) restitution curve may be a significant determinant of the propensity to develop ventricular fibrillation, with steeper slopes associated with a more arrhythmogenic substrate. We hypothesized that one mechanism by which beta-blockers reduce sudden cardiac death is by flattening the APD restitution curve. Therefore, we investigated whether infusion of esmolol modulates the APD restitution curve in vivo. In 10 Yorkshire pigs, dynamic APD restitution curves were determined from measurements of APD at 90% repolarization with a monophasic action potential catheter positioned against the right ventricular septum during right ventricular apical pacing in the basal state and during infusion of esmolol. APD restitution curves were fitted to the three-parameter (a, b, c) exponential equation, APD = a.[1 - e((-b.DI))] + c, where DI is the diastolic interval. Esmolol decreased the maximal APD slope, 0.68 +/- 0.14 vs. 0.94 +/- 0.24 (baseline), P = 0.002, and flattened the APD restitution curve at shorter DIs, 75 and 100 ms (P < 0.05). To compare the slopes of the APD restitution curves at similar steady states, slopes were also computed at points of intersection between the restitution curve and the lines representing pacing at a fixed cycle length (CL) of 200, 225, 250, 275, and 300 ms using the relationship CL = APD + DI. Esmolol decreased APD restitution slopes at CLs 200-275 ms (P < 0.05). Esmolol flattens the cardiac APD restitution curve in vivo, particularly at shorter CLs and DIs. This may represent a novel mechanism by which beta-blockers prevent sudden cardiac death.     

Sympathetic blockade prevents the decrease in cardiac VEGF expression and capillary supply in experimental renal failure

Uremic cardiomyopathy of men and rodents is characterized by lower myocardial capillary supply that in rats could be prevented by central and peripheral blockade of the sympathetic nervous system. The underlying pathomechanisms remain largely unknown. We investigated whether alterations of cardiac vascular endothelial growth factor (VEGF) gene and protein expression were involved. In our long-term experiment, we analyzed whether VEGF gene and protein expression was altered in the heart of male Sprague-Dawley rats with either sham operation (sham, n=10) or subtotal nephrectomy (SNX, n=10). In our short-term experiment (17 sham, 24 SNX), the effect of a putative downregulation of sympathetic nervous activity by surgical renal denervation (interruption of renal afferent pathways) on cardiac gene expression of VEGF, flt-1, and flk-1 and on myocardial capillary supply was analyzed. In the long-term study, cardiac capillary supply and vascular endothelial growth factor gene and protein expression were significantly lower in SNX than in sham. In the short-term experiment, cardiac VEGF mRNA expression was significantly lower in untreated SNX (4,258±2,078 units) than in both sham groups (11,709±4,169 and 8,998±4,823 units); this decrease was significantly prevented by renal denervation (8,190±3,889, P<0.05). We conclude that cardiac VEGF gene and protein expression is reduced in experimental renal failure, and this may be considered as one potential reason for impaired myocardial adaptation under the situation of cardiac hypertrophy. The beneficial effect of sympathetic downregulation on cardiac structure and function in renal failure may be at least in part explained by increased cardiac VEGF gene expression.     

压力负荷性心衰小鼠左心室跨壁L-型钙电流的变化

为了观察正常和心衰时心内膜下和心外膜下心肌细胞L-型钙电流(ICa-L)的差别,我们采用主动脉弓狭窄的方法建立小鼠压力超负荷性心衰模型,采用全细胞膜片钳技术记录了正常、主动脉狭窄(band)及假手术对照(sham)组动物左心室游离壁内、外膜下心肌细胞的动作电位时程(action potential duration,APD)和ICa-L。结果显示:(1)与sham组同龄的正常小鼠左心室心内膜下细胞动作电位复极达90％的时程(APD90)为(38．2±6．44)ms,较心外膜下细胞的APD90(15.67±5.31)ms明显延长,二者的比值约为2．5:1;内膜下细胞和外膜下细胞ICa-L密度没有差异,峰电流密度分别为(-2.7±0.49)pA/pF和(-2.54±0．53)pA/pF;(2)Band组内、外膜下细胞的动作电位复极达50％的时程(APD50)、APD90均较sham组显著延长,尤以内膜下细胞延长突出,分别较sham组延长了400％和360％,内、外膜下细胞APD90的比值约为4.2:1;(3)与sham组相比, band组内膜下细胞ICa-L密度显著减小,在+10 mV～+40 mV的4个电压下分别降低了20．2％、21．4％、21.6％和25.7％(P< 0．01),但其激活电位、峰电位和翻转电位没有改变;band组外膜下细胞的ICa-L密度与同期sham组相比无明显变化;band组钙通道激活、失活及复活的动力学特征与sham组相比没有改变。以上结果提示,生理状态下小鼠左心室内、外膜下细胞ICa-L密度不存在明显差别,提示ICa-L与APD跨壁异质性的产生无关;心衰时左心室内、外膜下细胞APD明显延长,以内膜下细胞延长尤为突出,内膜下细胞ICa-L密度明显减少,而外膜下细胞ICa-L密度无明显改变,这种ICa-L的非同步变化在心衰时可能起到对抗APD延长、减少复极离散度的有益作用。     

Increased oxidative stress, the renin-angiotensin system, and sympathetic overactivation induce hypertension in kidney androgen-regulated protein transgenic mice

Gender differences in the incidence and severity of hypertension have suggested the involvement of a sex-dependent mechanism. Transgenic (Tg) mice overexpressing kidney androgen-regulated protein (KAP) specifically in kidney showed hypertension associated with oxidative stress. Reactive oxygen species (ROS) are strongly implicated in the pathological signaling leading to hypertension in a framework that includes renin-angiotensin system (RAS) activation, increased sympathetic activity, and cardiac remodeling. In this report, we observed that plasma levels of angiotensin II and catecholamines were increased in KAP Tg mice, compared with wild-type animals. Systemic administration of Tempol, a membrane-permeative superoxide dismutase mimetic, reduced arterial pressure as well as urinary excretion of oxidative stress markers and reduced both angiotensin II and norepinephrine plasma levels in KAP Tg mice. Intracerebroventricular administration of Tempol also reduced arterial pressure in Tg mice. Moreover, administration of apocynin and DPI, inhibitors of NADPH oxidase, a major source of ROS, also reduced arterial pressure and both angiotensin II and norepinephrine plasma levels in Tg mice. Thus, we analyzed the involvement of the RAS and sympathetic nervous system in KAP Tg mouse hypertension. Both captopril and losartan reduced arterial blood pressure in Tg mice, as also occurred after β-adrenergic blockade with atenolol. Also, intracerebroventricular losartan administration reduced arterial pressure in KAP Tg mice. Our data demonstrate that hypertension in male KAP Tg mice is based on increased oxidative stress, increased sympathetic activity, and RAS activation. Moreover, our results suggest a role for increased oxidative stress in the CNS as a major cause of hypertension in these animals.     

Beta-adrenergic signals regulate cardiac differentiation of mouse embryonic stem cells via mitogen-activated protein kinase pathways

As embryonic stem cell-derived cardiomyocytes (ESC-CMs) have the potential to be used in cell replacement therapy, an understanding of the signaling mechanisms that regulate their terminal differentiation is imperative. In previous studies, we discovered the presence of adrenergic and muscarinic receptors in mouse embryonic stem cells (ESCs). However, little is known about the role of these receptors in cardiac differentiation and development, which is critically important in cardiac physiology and pharmacology. Here, we demonstrated that a β-adrenergic receptor (β-AR) agonist significantly enhanced cardiac differentiation as indicated by a higher percentage of beating embryoid bodies and a higher expression level of cardiac markers. Application of β1-AR and β2-AR antagonists partly abolished the effect of the β-AR agonist. In addition, by administering selective inhibitors we found that the effect of β-AR was driven via p38 mitogen-activated protein kinase and extracellular-signal regulated kinase pathway. These findings suggest that ESCs are also a target for β-adrenergic regulation and β-adrenergic signaling plays a role in ESC cardiac differentiation.     

The role of PP5 and PP2C in cardiac health and disease

Protein phosphatases are important, for example, as functional antagonists of β-adrenergic stimulation of the mammalian heart. While β-adrenergic stimulations increase the phosphorylation state of regulatory proteins and therefore force of contraction in the heart, these phosphorylations are reversed and thus force is reduced by the activity of protein phosphatases. In this context the role of PP5 and PP2C is starting to unravel. They do not belong to the same family of phosphatases with regard to sequence homology, many similarities with regard to location, activation by lipids and putative substrates have been worked out over the years. We also suggest which pathways for regulation of PP5 and/or PP2C described in other tissues and not yet in the heart might be useful to look for in cardiac tissue. Both phosphatases might play a role in signal transduction of sarcolemmal receptors in the heart. Expression of PP5 and PP2C can be increased by extracellular stimuli in the heart. Because PP5 is overexpressed in failing animal and human hearts, and because overexpression of PP5 or PP2C leads to cardiac hypertrophy and KO of PP5 leads to cardiac hypotrophy, one might argue for a role of PP5 and PP2C in heart failure. Because PP5 and PP2C can reduce, at least in vitro, the phosphorylation state of proteins thought to be relevant for cardiac arrhythmias, a role of these phosphatases for cardiac arrhythmias is also probable. Thus, PP5 and PP2C might be druggable targets to treat important cardiac diseases like heart failure, cardiac hypertrophy and cardiac arrhythmias.     

Adrenergic regulation of AMP-activated protein kinase in brown adipose tissue in vivo

AMP-activated protein kinase (AMPK), an evolutionarily conserved serine-threonine kinase that senses cellular energy status, is activated by stress and neurohumoral stimuli. We investigated the mechanisms by which adrenergic signaling alters AMPK activation in vivo. Brown adipose tissue (BAT) is highly enriched in sympathetic innervation, which is critical for regulation of energy homeostasis. We performed unilateral denervation of BAT in wild type (WT) mice to abolish neural input. Six days post-denervation, UCP-1 protein levels and AMPK α2 protein and activity were reduced by 45%. In β(1,2,3)-adrenergic receptor knock-out mice, unilateral denervation led to a 25-45% decrease in AMPK activity, protein expression, and Thr(172) phosphorylation. In contrast, acute α- or β-adrenergic blockade in WT mice resulted in increased AMPK α Thr(172) phosphorylation and AMPK α1 and α2 activity in BAT. But short term blockade of α-adrenergic signaling in β(1,2,3)-adrenergic receptor knock-out mice resulted in decreased AMPK activity in BAT, which strongly correlated with enhanced phosphorylation of AMPK on Ser(485/491), a site associated with inhibition of AMPK activity. Both PKA and AKT inhibitors attenuated AMPK Ser(485/491) phosphorylation resulting from α-adrenergic blockade and prevented decreases in AMPK activity. In vitro mechanistic studies in BAT explants showed that the effects of α-adrenergic blockade appeared to be secondary to inhibition of oxygen consumption. In conclusion, adrenergic pathways regulate AMPK activity in vivo acutely via alterations in Thr(172) phosphorylation and chronically through changes in the α catalytic subunit protein levels. Furthermore, AMPK α Ser(485/491) phosphorylation may be a novel mechanism to inhibit AMPK activity in vivo and alter its biological effects.     

Neurochemical approaches to the prevention of ventricular fibrillation

Current evidence indicates that susceptibility to ventricular fibrillation (VF) can be reduced by decreasing cardiac sympathetic activity and by increasing vagal tone. Pharmacological agents that favor such a pattern of autonomic outflow protect the heart against fibrillation. These include morphine sulfate, clonidine, digitalis drugs, and bromocriptine. An intriguing new approach involves changing the serum concentration of amino acid precursors of the central neurotransmitters that modulate autonomic traffic. Considerable evidence indicates that accumulation of serotonin in the brain reduces sympathetic neural activity. When L-tryptophan or 5-hydroxytryptophan is administered with phenelzine (a monoamine oxidase inhibitor) and carbidopa (a selective peripheral L-amino acid decarboxylase inhibitor) to raise brain serotonin, a significant increase in myocardial electrical stability is observed. This effect results from a decrease in cardiac sympathetic tone as indicated by selective denervation and nerve recording studies. Enhancing serotoninergic neurotransmission can also significantly reduce vulnerability to VF during acute coronary artery occlusion. The effect of augmenting serotoninergic activity without the use of enzyme inhibitors has been investigated by administering agents such as melatonin, 5-methoxytryptophol, and 6-chloro-2-(1-piperazinyl) pyrazine (MK-212). Each of these substances increases cardiac electrical stability. The protective influence is unaffected by bilateral vagotomy but is blocked by the specific serotonin antagonist methergoline. Diminution of cardiac sympathetic drive appears to be the main mechanism of action. Thus, neurochemical interventions can exert a profound effect on cardiac electrical stability. Recent advances in neurochemistry and psychopharmacology promise new insights into the problem of sudden death and suggest a fresh approach for the management of life-threatening arrhythmias.     

Simulation and mechanistic investigation of the arrhythmogenic role of the late sodium current in human heart failure

Heart failure constitutes a major public health problem worldwide. The electrophysiological remodeling of failing hearts sets the stage for malignant arrhythmias, in which the role of the late Na(+) current (I(NaL)) is relevant and is currently under investigation. In this study we examined the role of I(NaL) in the electrophysiological phenotype of ventricular myocytes, and its proarrhythmic effects in the failing heart. A model for cellular heart failure was proposed using a modified version of Grandi et al. model for human ventricular action potential that incorporates the formulation of I(NaL). A sensitivity analysis of the model was performed and simulations of the pathological electrical activity of the cell were conducted. The proposed model for the human I(NaL) and the electrophysiological remodeling of myocytes from failing hearts accurately reproduce experimental observations. The sensitivity analysis of the modulation of electrophysiological parameters of myocytes from failing hearts due to ion channels remodeling, revealed a role for I(NaL) in the prolongation of action potential duration (APD), triangulation of the shape of the AP, and changes in Ca(2+) transient. A mechanistic investigation of intracellular Na(+) accumulation and APD shortening with increasing frequency of stimulation of failing myocytes revealed a role for the Na(+)/K(+) pump, the Na(+)/Ca(2+) exchanger and I(NaL). The results of the simulations also showed that in failing myocytes, the enhancement of I(NaL) increased the reverse rate-dependent APD prolongation and the probability of initiating early afterdepolarizations. The electrophysiological remodeling of failing hearts and especially the enhancement of the I(NaL) prolong APD and alter Ca(2+) transient facilitating the development of early afterdepolarizations. An enhanced I(NaL) appears to be an important contributor to the electrophysiological phenotype and to the dysregulation of [Ca(2+)](i) homeostasis of failing myocytes.     

Electrophysiologic properties and ventricular fibrillation in normal and myopathic hearts.

This study tests the hypothesis that moderate myocardial dysfunction is associated with altered myocardial anisotropic properties and structurally altered ventricular fibrillation (VF). Mongrel dogs were randomized to either a control group or a group that was rapidly paced at 250 beats/min until the left ventricular ejection fraction was < or = 40%. Changes in anisotropic properties and the electrical characteristics of VF associated with the development of moderate myocardial dysfunction were assessed by microminiature epicardial mapping studies. In vivo conduction, refractory periods, and repolarization times were prolonged in both longitudinal and transverse directions in myopathic animals versus controls. VF was different in myopathic versus control animals. There were significantly more conducted deflections during VF in normal hearts compared with myopathic hearts. Propagated deflection-to-deflection intervals during VF were significantly longer in myopathic hearts compared with controls (125.5 +/- 49.06 versus 103.4 +/- 32.9 ms, p = 0.009). There were no abnormalities in cell size, cell shape, or the number of intercellular gap junctions and there was no detectable change in the expression of the gap junction proteins Cx43 and Cx45. Moderate myocardial dysfunction is associated with significant electrophysiological abnormalities in the absence of changes in myocardial cell morphology or intercellular connections, suggesting a functional abnormality in cell-to-cell communication.     

Sex differences and the effects of ovariectomy on the β-adrenergic contractile response

The presence of sex differences in myocardial β-adrenergic responsiveness is controversial, and limited studies have addressed the mechanism underlying these differences. Studies were performed using isolated perfused hearts from male, intact female and ovariectomized female mice to investigate sex differences and the effects of ovarian hormone withdrawal on β-adrenergic receptor function. Female hearts exhibited blunted contractile responses to the β-adrenergic receptor agonist isoproterenol (ISO) compared with males but not ovariectomized females. There were no sex differences in β(1)-adrenergic receptor gene or protein expression. To investigate the role of adenylyl cyclase, phosphodiesterase, and the cAMP-signaling cascade in generating sex differences in the β-adrenergic contractile response, dose-response studies were performed in isolated perfused male and female hearts using forskolin, 3-isobutyl-1-methylxanthine (IBMX), and 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP). Males showed a modestly enhanced contractile response to forskolin at 300 nM and 5 μM compared with females, but there were no sex differences in the response to IBMX or CPT-cAMP. The role of the A(1) adenosine receptor (A(1)AR) in antagonizing the β-adrenergic contractile response was investigated using both the A(1)AR agonist 2-chloro-N(6)-cyclopentyl-adenosine and A(1)AR knockout (KO) mice. Intact females showed an enhanced A(1)AR anti-adrenergic effect compared with males and ovariectomized females. The β-adrenergic contractile response was potentiated in both male and female A(1)ARKO hearts, with sex differences no longer present above 1 nM ISO. The β-adrenergic contractile response is greater in male hearts than females, and minor differences in the action of adenylyl cyclase or the A(1)AR may contribute to these sex differences.     

Functional Morphology of the Heart in Fishes

The systemic heart of fishes consists of four chambers in series,the sinus venosus, atrium, ventricle, and conus or bulbus. Valvesbetween the chambers and contraction of all chambers exceptthe bulbus maintain a unidirectional blood flow through theheart. The heart is composed of typical vertebrate cardiac muscle,although there may be minor differences in the distributionof spontaneously active cells, the rate and nature of spreadof excitatory waves, and the characteristics of resting andaction potentials between different fish and other vertebrates.Cholinergic fibers innervate the heart, except in hagfish whichhave aneural hearts. Fish hearts lack sympathetic innervation.The level of vagal tone varies considerably, and is affectedby many factors. In some fish the heart is essentially aneural(without vagal tone) during exercise and may resemble an isolatedmammalian ventricle with increased venous return causing increasedcardiac output. There are many mechanisms that could increasevenous return in exercising fish. rß-adrenergic receptorshave been located on the hearts of some fish, and changing levelsof catecholamines may play a role in regulating cardiac activity.Changes in cardiac output in fish are normally associated withlarge changes in stroke volume and small cha-nges in heart rate.