Mechanisms contributing to the arrhythmogenic influences of alpha 1-adrenergic stimulation in the ischemic heart

The Antiarrhythmic Activity of Novel Pyrrolidin-2-one Derivative S-75 in Adrenaline-Induced Arrhythmia

Arrhythmia is a quivering or irregular heartbeat that can often lead to blood clots, stroke, heart failure, and other heart-related complications. The limited efficacy and safety of antiarrhythmic drugs require the design of new compounds. Previous research indicated that pyrrolidin-2-one derivatives possess an affinity for α₁-adrenergic receptors. The blockade of α₁-adrenoceptor may play a role in restoring normal sinus rhythm; therefore, we aimed to verify the antiarrhythmic activity of novel pyrrolidin-2-one derivative S-75. In this study, we assessed the influence on sodium, calcium, potassium channels, and β₁-adrenergic receptors to investigate the mechanism of action of S-75. Lack of affinity for β₁-adrenoceptors and weak effects on ion channels decreased the role of these adrenoceptors and channels in the pharmacological activity of S-75. Next, we evaluated the influence of S-75 on normal ECG in rats and isolated rat hearts, and the tested derivative did not prolong the QT_c interval, which may confirm the lack of the proarrhythmic potential. We tested antiarrhythmic activity in adrenaline-, aconitine- and calcium chloride-induced arrhythmia models in rats. The studied compound showed prophylactic antiarrhythmic activity in the adrenaline-induced arrhythmia, but no significant activity in the model of aconitine- or calcium chloride-induced arrhythmia. In addition, S-75 was not active in the model of post-reperfusion arrhythmias of the isolated rat hearts. Conversely, the compound showed therapeutic antiarrhythmic properties in adrenaline-induced arrhythmia, reducing post-arrhythmogen heart rhythm disorders, and decreasing animal mortality. Thus, we suggest that the blockade of α₁-adrenoceptor might be beneficial in restoring normal heart rhythm in adrenaline-induced arrhythmia.

High vulnerability of the heart and liver to 3-hydroxypalmitic acid-induced disruption of mitochondrial functions in intact cell systems

Patients affected by long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency predominantly present severe liver and cardiac dysfunction, as well as neurological symptoms during metabolic crises, whose pathogenesis is still poorly known. In this study, we demonstrate for the first time that pathological concentrations of 3-hydroxypalmitic acid (3HPA), the long-chain hydroxyl fatty acid (LCHFA) that most accumulates in LCHAD deficiency, significantly decreased adenosine triphosphate-linked and uncoupled mitochondrial respiration in intact cell systems consisting of heart fibers, cardiomyocytes, and hepatocytes, but less intense in diced forebrain. 3HPA also significantly reduced mitochondrial Ca²⁺ retention capacity and membrane potential in Ca²⁺ -loaded mitochondria more markedly in the heart and the liver, with mild or no effects in the brain, supporting a higher susceptibility of the heart and the liver to the toxic effects of this fatty acid. It is postulated that disruption of mitochondrial energy and Ca²⁺ homeostasis caused by the accumulation of LCHFA may contribute toward the severe cardiac and hepatic clinical manifestations observed in the affected patients.

Disorders of mitochondrial long-chain fatty acid oxidation and the carnitine shuttle

Mitochondrial fatty acid oxidation is an essential pathway for energy production, especially during prolonged fasting and sub-maximal exercise. Long-chain fatty acids are the most abundant fatty acids in the human diet and in body stores, and more than 15 enzymes are involved in long-chain fatty acid oxidation. Pathogenic mutations in genes encoding these enzymes result in a long-chain fatty acid oxidation disorder in which the energy homeostasis is compromised and long-chain acylcarnitines accumulate. Symptoms arise or exacerbate during catabolic situations, such as fasting, illness and (endurance) exercise. The clinical spectrum is very heterogeneous, ranging from hypoketotic hypoglycemia, liver dysfunction, rhabdomyolysis, cardiomyopathy and early demise. With the introduction of several of the long-chain fatty acid oxidation disorders (lcFAOD) in newborn screening panels, also asymptomatic individuals with a lcFAOD are identified. However, despite early diagnosis and dietary therapy, a significant number of patients still develop symptoms emphasizing the need for individualized treatment strategies. This review aims to function as a comprehensive reference for clinical and laboratory findings for clinicians who are confronted with pediatric and adult patients with a possible diagnosis of a lcFAOD.

Sexual dimorphism in myocardial acylcarnitine and triglyceride metabolism

Background

Cardiovascular disease is the leading cause of death among diabetic patients. Importantly, recent data highlight the apparent sexual dimorphism in the pathogenesis of cardiovascular disease in diabetics with respect to both frequency- and age-related risk factors. The disposition to cardiovascular disease among diabetic patients has been attributed, at least in part, to excess lipid supply to the heart culminating in lipotoxicity of the heart and downstream derangements. A confounding factor in obese animal models of diabetes is that increased peripheral lipid availability to the heart can induce cardio-metabolic remodeling independent of the underlying pathophysiology of diabetes, thus masking the diabetic phenotype. To that end, we hypothesized that the use of non-obese diabetic (NOD) animal models will reveal metabolic signatures of diabetes in a sex-specific manner.

Methods

To test this hypothesis, male and female NOD Goto-Kakizaki (GK) rats were used to assess the expression profile of 84 genes involved in lipid metabolism. In parallel, targeted lipidomics analysis was performed to characterize sex differences in homeostasis of non-esterified fatty acids (NEFA), acylcarnitines (AC), and triglycerides (TG).

Results

Our analysis revealed significant sex differences in the expression of a broad range of genes involved in transport, activation, and utilization of lipids. Furthermore, NOD male rats exhibited enhanced oxidative metabolism and accumulation of TG, whereas female NOD rats exhibited reduced TG content coupled with accumulation of AC species. Multi-dimensional statistical analysis identified saturated AC16:0, AC18:0, and AC20:0 as dominant metabolites in mediating sex differences in AC metabolism. Confocal microscopy of rat cardiomyocytes exposed to AC14:0, AC16:0, and AC18:0 confirmed induction of ROS with AC18:0 being more potent followed by AC14:0.

Conclusion

Overall, we demonstrate sex differences in myocardial AC and TG metabolism with implications for therapy and diagnosis of diabetic cardiovascular disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13293-016-0077-7) contains supplementary material, which is available to authorized users.

Can moderate elevations of parathyroid hormone acutely increase risk for ischemic cardiac arrhythmias?

There is suggestive evidence that chronic elevations of parathyroid hormone (PTH), associated with poor vitamin D status or low calcium intake, can increase risk for insulin resistance, weight gain, hypertension, and left ventricular hypertrophy, while stimulating production of acute phase reactants. New evidence that elevated PTH is prognostic for increased vascular mortality in very elderly subjects, prompts an examination of the possible impact of PTH on risk for arrhythmias. The cardiac effects of PTH are mediated by G protein-coupled receptors that activate phospholipase C (PLC). Catecholamines, angiotensin II, and endothelin have been shown to be arrhythmogenic for ischemic myocardium in animal studies; the receptors mediating this effect are all likewise linked to activation of PLC. Thus, it is reasonable to presume that a sufficient concentration of PTH can be arrhythmogenic in the ischemic heart. The extent to which this effect can be evoked by the high-normal PTH levels prevalent in many elderly subjects, can be assessed in epidemiological studies.

Alpha-2 adrenergic antagonism enhances risk of ventricular tachycardia during acute ischemia

OBJECTIVE

In this study we tested the hypothesis that alpha-2 adrenergic antagonism could facilitate induction of previously non-inducible ventricular tachycardia (VT) during acute ischemia. Previous reports suggest that VT during ischemia may be modulated by (alpha-2 adrenergic agonists.

DESIGN

The left anterior descending artery was occluded after instrumentation of the ischemic risk zone with 21 multipolar plunge needles, each recording 6 bipolar electrograms. Three dimensional mapping characterized the mechanism of VT induced with extrastimuli.

RESULTS

Of 16 non-inducible dogs included, eight which were given the alpha-2 adrenergic antagonist yohimbine all had inducible VT, while all eight in the control group remained non-inducible (p < 0.05). Six of the VTs were of focal Purkinje origin. The cycle length of the VTwas 119 +/- 4 ms. Mean arterial pressure (81+/- 8 to 82 +/- 8 mmHg, p = ns), ventricular effective refractory period (146 +/- 6 to 144 +/- 5 ms, p = ns) and ischemic zone size (55 +/-6% vs. 61 +/- 4%, p = 0.45) were not altered by yohimbine indicating minimal central or pre-junctional effects of the drug.

CONCLUSIONS

Yohimbine facilitates induction of VT, especially those with focal Purkinje fiber origin, suggestive of an effect mediated through antagonism of post-junctional alpha-2 adrenoceptors on Purkinje fibers.

Synthesis and Pharmacological Evaluation of New 1-[3-(4-Arylpiperazin-1-yl)-2-hydroxypropyl]-pyrrolidin-2-one Derivatives with Anti-arrhythmic, Hypotensive, and α-Adrenolytic Activity

A series of novel arylpiperazines bearing a pyrrolidin-2-one fragment was synthesized and evaluated for the binding affinity of the alpha(1)- and alpha(2)-adrenoceptors (AR) and for the antiarrhythmic and hypotensive activities of the compounds. The most potent and selective compound 1-[2-hydroxy-3-[4-[(2-hydroxyphenyl)piperazin-1-yl]propyl]pyrrolidin-2-one 8 binds with pK(i) = 6.71 for alpha(1)-AR. Derivative 8 was also the most active in the prophylactic antiarrhythmic test in adrenaline-induced arrhythmia in anaesthetized rats. Its ED(50 )value equals 1.9 mg/kg (i.v.). Compounds with substituents such as a fluorine atom 4, a methyl 5, or a hydroxyl 8 group, or two substituents such as fluorine/chlorine atoms and methoxy groups in the phenyl ring, significantly decreased the systolic and diastolic pressure in normotensive anesthetized rats at a dosages of 5-10 mg/kg (i.v.). It was found that the presence of the piperazine ring and a hydroxy group in the second position of the propyl chain are critical structural features in determining the affinity of the compounds tested.

Arrhythmogenesis in isolated rat hearts with enhanced alpha-adrenoceptor-mediated responsiveness

1. It has been postulated that stimulation of myocardial alpha-adrenoceptors is one of the primary mediators of the dysrhythmias which occur during periods of myocardial ischaemia and reperfusion. This study examines arrhythmogenesis during coronary artery occlusion and reperfusion in isolated perfused rat hearts from control animals and from rats with enhanced myocardial alpha-adrenoceptor responsiveness. 2. Rats were administered propylthiouracil (PTU) in their drinking water for 8 weeks. This treatment resulted in an enhanced responsiveness of isolated left atria to the alpha-adrenoceptor agonist phenylephrine compared with atria from control animals. 3. In Langendorff-perfused isolated hearts, the spontaneous rate of contraction was significantly lower in the PTU-pretreated group than in either age-matched or weight-matched controls. Occlusion of the left anterior descending artery (LAD) for 25 min resulted in ventricular tachycardia (VT) of similar incidence and duration in all groups and ventricular fibrillation (VF) in both control groups but not the PTU-pretreated group. 4. Following the 25-min ischaemic period the myocardium was reperfused for 10 min. The incidence and duration of VT and VF during this period was similar in all groups except that the duration of VF in the PTU-pretreated group was significantly lower than in controls. 5. In perfused hearts paced at 4 Hz, the incidence and duration of dysrhythmias during ischaemia and reperfusion was again similar in all groups, only the duration of VF being affected (reduced) by PTU-pretreatment. 6. In conclusion, this study does not lend support to the hypothesis that myocardial alpha-adrenoceptors have a primary role in arrhythmogenesis, but the data would support a role for these receptors in myocardial protection.

Suppression of inositol phosphate release by cardiac myocytes isolated from fish oil-fed pigs

Fatty acid composition of cardiac myocytes and release of inositol phosphates in pigs fed a fish oil supplemented diet was examined. Two groups of female pigs were fed diets supplemented with either 50 g/kg diet beef tallow (as control) or 50 g/kg diet fish oil (MaxEPA) rich in n-3 fatty acids. After 6 weeks of supplementation, the pigs were anesthetized and hearts were removed. Cardiac myocytes were isolated, lipid extracted and separated into non-polar and polar lipids by thin-layer chromatography. Fatty acid composition of individual neutral and polar lipid classes were examined by gas chromatography. To study the effect of membrane phospholipid modification on the phospholipase C (PLC) mediated release of inositol phosphates, cardiac myocytes were labelled with 4 microCi/mL myo-[2-(3)H]inositol for 48 h. After stimulation with epinephrine and phenylephrine, the water soluble [3H]inositol products were extracted, separated from [3H]inositol and [3H]glycerophosphoinositol by chromatography on Dowex AG 1-X8 and quantitated by scintillation counting. Cardiac myocytes isolated from fish oil-fed pigs had higher levels of n-3 polyunsaturated fatty acid in the non-esterified fatty acid and phospholipid fraction. Similarly, these cardiac myocytes had increased level of n-3 fatty and decreased n-6 fatty acids in all the phospholipid fractions, PE, PC, P1 and PS (p < 0.05). After stimulation, the levels of [3H]inositol trisphosphate (IP3) and [3H]inositol tetrakisphosphate (IP4) in cardiac myocytes isolated from fish oil-fed pigs were significantly reduced (p < 0.05) compared to myocytes isolated from beef tallow fed-pigs. This study for the first time has utilised adult cardiac myocytes to demonstrate the effect of n-3 PUFA supplementation on cardiac myocyte phospholipid fatty acid composition and release of second messengers.

Adrenoceptor subclassification: an approach to improved cardiovascular therapeutics

The subdivision of alpha adrenoceptors into the alpha 1 and alpha 2 classes was the impetus for the design of the selective alpha 1-adrenoceptor antagonists, which remain useful antihypertensives. alpha 2-Adrenoceptor agonists also have application as antihypertensive drugs, based on their ability to reduce sympathetic outflow. Likewise, subdivision of the beta adrenoceptors has lead to the development of selective beta 1-adrenoceptor antagonists as antihypertensive and selective beta 2 agonists as bronchodilators. In the past decade, both the alpha 1 and alpha 2 adrenoceptors have been further subdivided, each into three subclasses. In addition, there is strong functional evidence to suggest the presence of additional adrenoceptor subtypes, such as the "alpha 1L" adrenoceptor and "beta 4" adrenoceptor. alpha 1A (or alpha 1L)-Adrenoceptor antagonists have been evaluated for benign prostatic hyperplasia (BPH), and selective alpha 1A agonists for stress incontinence. Gene knockout experiments in mice suggest an important role for the alpha 1B adrenoceptor in the control of vascular tone. Hence, selective alpha 1B antagonists may offer a new approach toward hypertension. Although targeting of specific adrenoceptors can be used to optimize the therapeutic profile of a drug, there are also cases where blockade of multiple adrenoceptors is desirable, as with the alpha/beta-adrenoceptor antagonist carvedilol in congestive heart failure. It is possible that combination of affinities for selected adrenoceptor subtypes within a single molecule may be desirable for certain applications.

Two distinct mechanisms mediate a differential regulation of protein kinase C isozymes in acute and prolonged myocardial ischemia

An activation of protein kinase C (PKC) in acute myocardial ischemia has been shown previously using its translocation to the plasma membrane as an indirect parameter. However, whether PKC remains activated or whether other mechanisms such as altered gene expression may mediate an isozyme-specific regulation in prolonged ischemia have not been investigated. In isolated perfused rat hearts, PKC activity and the expression of PKC cardiac isozymes were determined on the protein level using enzyme activities and Western blot analyses and on the mRNA level using reverse transcriptase-polymerase chain reaction after various periods of global ischemia (1 to 60 minutes). As early as 1 minute after the onset of ischemia, PKC activity is translocated from the cytosol to the particulate fraction without change in total cardiac enzyme activity. This translocation involves all major cardiac isozymes of PKC (ie, PKCalpha, PKCdelta, PKCepsilon, and PKCzeta). This rapid, nonselective activation of PKCs is only transient. In contrast, prolonged ischemia (>/=15 minutes) leads to an increased cardiac PKC activity (119+/-7 versus 190+/-8 pmol/min per mg protein) residing in the cytosol. This is associated with an augmented, subtype-selective isozyme expression of PKCdelta and PKCvarepsilon (163% and 199%, respectively). The specific mRNAs for PKCdelta (948+/-83 versus 1501+/-138 ag/ng total RNA, 30 minutes of ischemia) and PKCepsilon (1597+/-166 versus 2611+/-252 ag/ng total RNA) are selectively increased. PKCalpha and PKCzeta remain unaltered. In conclusion, two distinct activation and regulation processes of PKC are characterized in acute myocardial ischemia. The early, but transient, translocation involves all constitutively expressed cardiac isozymes of PKC, whereas in prolonged ischemia an increased total PKC activity is associated with an isozyme-selective induction of PKCepsilon and PKCdelta. Whether these fundamentally different activation processes interact remains to be elucidated.

Functional effects of protein kinase C activation on the human cardiac Na+ channel

The cardiac Na+ current plays an important role in determining normal and abnormal impulse propagation in the heart. We have investigated the effects of protein kinase C (PKC) activation on the recombinant human cardiac Na+ channel (hH1) following heterologous expression in Xenopus laevis oocytes. Phorbol 12-myristate 13-acetate (PMA), which directly activates PKC, reduced current amplitude at all test potentials (43 +/- 12% at -10 mV). In contrast to the rat brain IIA (rBIIA) channel, there was no apparent change in either macroscopic Na+ current decay or the voltage dependence of channel gating. Further experiments indicate that the effects of PMA were mediated by PKC activation: (1) an inactive stereoisomer, 4 alpha-PMA, had no effect; (2) preincubation with the protein kinase inhibitor chelerythrine prevented the PMA effects; and (3) a hydrolyzable diacylglycerol analogue, 1-oleoyl-2-acetyl-glycerol, also reduced current (22 +/- 5%). In addition, when the alpha 1B-adrenergic receptor was coexpressed with hH1, the alpha-receptor agonist methoxamine reduced hH1 current (45 +/- 10%), an effect that could be eliminated by chelerythrine preincubation. When a conserved consensus PKC site (serine 1503) in the III-IV interdomain linker thought to be responsible for the PKC effects on rBIIA was mutated, PMA still reduced Na+ current, but the magnitude of the effect was smaller compared with that for the wild-type channel. Similar findings were obtained with alpha 1-receptor stimulation following receptor coexpression with the mutant channel. We conclude that activation of PKC modulates the human cardiac Na+ channel by at least two mechanisms, one similar to that seen with rat brain channels, involving a conserved putative PKC site, and a second more specific to the cardiac isoform.

Adrenergic modulation of ultrarapid delayed rectifier K+ current in human atrial myocytes

The ultrarapid delayed rectifier K+ current (IKur) in human atrial cells appears to correspond to Kv1.5 cloned channels and to play an important role in human atrial repolarization. Kv1.5 channels have consensus sites for phosphorylation by protein kinase A and C, suggesting possible modulation by adrenergic stimulation. The present study was designed to assess the adrenergic regulation of IKur in human atrial myocytes. Isoproterenol increased IKur in a concentration-dependent manner, with significant effects at concentrations as low as 10 nmol/L. The effects of isoproterenol were reversible by washout or by the addition of propranolol (1 mumol/L). Isoproterenol's effects were mimicked by the direct adenylate cyclase stimulator, forskolin, and by the membrane-permeable form of cAMP, 8-bromo cAMP. Isoproterenol had no effect on IKur when the protein kinase A inhibitor peptide, PKI(6-22)amide, was included in the pipette solution; in a separate set of experiments in which isoproterenol alone increased IKur by 45 +/- 9% relative to control, subsequent superfusion with isoproterenol in the presence of the protein kinase inhibitor H-7 failed to alter IKur. In contrast to isoproterenol, phenylephrine (in the presence of propranolol to block beta-adrenegic effects) induced a concentration-dependent inhibition of IKur, with significant effects observed at concentrations as low as 10 mumol/L. The inhibitory actions of phenylephrine were reversed by the addition of prazosin and prevented by coadministration with a highly selective inhibitor of protein kinase C, bisindolylmaleimide. These results indicate that beta-adrenergic stimulation enhances, whereas alpha-adrenergic stimulation inhibits, IKur and suggest that these actions are mediated by protein kinase A and protein kinase C, respectively. The modulation of IKur by adrenergic influences is a potentially novel control mechanism for human atrial repolarization and arrhythmias.

Antiarrhythmic effect of the selective I1-imidazoline receptor modulator moxonidine on ouabain-induced cardiac arrhythmia in guinea pigs

Moxonidine is a centrally acting antihypertensive agent with potent action on I1-imidazoline receptors. Moxonidine as an SIR modulator elicits a persistent reduction in circulating levels of epinephrine, demonstrating a reduction in sympathetic tone. In the first experiment the threshold dose of ouabain needed to induce ventricular arrhythmia and asystole was determined in guinea pigs, and the influence of moxonidine was tested. In a dose range of 0.1-0.4 mg/kg body weight i.v., moxonidine increased the threshold dose needed to induce ventricular tachycardia, premature ventricular beats, ventricular flutter, ventricular fibrillation, and asystole. The effect was dose-dependent and statistically significant. Clonidine, in a dose range of 0.2-0.8 mg/kg body weight i.v., also increased the threshold dose of ouabain necessary to induce different cardiac rhythm disturbances. Moxonidine was more effective than clonidine. Pretreatment with the alpha 2-receptor and I1-receptor-influencing substances efaroxan, idazoxan, and SKF 86466 attenuated the effect of moxonidine and clonidine. Efaroxan, idazoxan, or SKF 86466 alone reduced the threshold dose of ouabain necessary to induce cardiac arrhythmia as a sign for arrhythmogenic effects. The alpha 1-receptor antagonist prazosin had no influence on ouabain-induced arrhythmia. Pretreatment with prazosin reduced the moxonidine but not the clonidine effect. In the second experiment the influence of moxonidine on aconitine-induced extrasystoles (ES) in the spontaneously beating guinea pig auricle was investigated. Moxonidine in a dose of 10(-7)-10(-8) M reduced the number of ES. A 10-fold higher dose had no influence on ES number. The beta-blocking agent propranolol showed antiarrhythmic effects in both methods. The ouabain-induced cardiac arrhythmia is associated with increased sympathetic tone on central stimulation. The reduced sympathetic tone by centrally acting moxonidine via imidazoline receptors seems responsible for the antiarrhythmic effect of this drug. Clonidine also reduced the sympathetic tone via imidazoline receptor. The selectivity of clonidine to imidazoline receptors is less pronounced than is that of moxonidine. The interaction of moxonidine with imidazoline receptors is not clear. The possible interaction between imidazoline and alpha-adrenoceptors in relation to the antiarrhythmic effect of moxonidine or clonidine is also unknown. Modulation of imidazoline receptors by moxonidine could be an agonistic effect or an antagonism to an endogenous agonistic or antagonistic substance and vice versa.

Effect of lidocaine on alpha 1-adrenoceptors in cultured neonatal rat cardiocytes

The effect of lidocaine on alpha 1-adrenoceptors in cultured neonatal rat ventricular cardiocytes was studied by binding assay. When the cells were cultured in the presence of lidocaine, the binding of (+/-)-beta-([125I]iodo-4-hydroxyphenyl)-ethyl-aminomethyl-tetralone to the cells increased in a concentration- and time-dependent manner. The effect was due to an increase in maximum binding but not due to a change in the dissociation constant for the ligand. The increased number of alpha 1-adrenoceptors returned to the control level after culturing the cells without lidocaine. The stimulating effect on the number of alpha 1-adrenoceptors of lidocaine was also observed in the presence of norepinephrine. Other class I antiarrhythmic drugs such as procainamide. disopyramide, mexiletine and flecainide increased the number of alpha 1-adrenoceptors in the cells. These results suggest that cardiac responsiveness mediated by alpha 1-adrenoceptors is increased by class I antiarrhythmic drugs.

Adenosine-sensitive alpha 1-adrenoceptor effects on reperfused ischaemic hearts: comparison with phorbol ester

1. We have examined the effects of the alpha 1-adrenoceptor agonists, phenylephrine or methoxamine, on contractility in rat and rabbit isolated hearts as well as their effects on postischaemic ventricular recovery. We compared these effects to those of 12-phorbol 13-myristate acetate (PMA), a direct activator of protein kinase C (PKC). 2. The positive inotropic effect of alpha 1-receptor agonists was significantly attenuated in the presence of the Na/H exchange inhibitor, methylisobutyl amiloride (MIA, 1 microM), whereas the positive inotropic effect of PMA was unaffected. 3. Reperfusion of rat hearts subjected to either 30 or 60 min of zero-flow ischaemia, resulted in recovery of contractility to 91 +/- 2% and 57 +/- 7% of the preischaemic values, respectively which was unaffected by phenylephrine. In contrast, PMA at a concentration (10 pM) devoid of direct depressant effects, significantly decreased recovery following 60 min of ischaemia to 31 +/- 4% of pre-ischaemic value (P < 0.05 from control); an effect which was completely prevented by the PKC inhibitor, bisindolylmaleimide. A similar inhibitory effect of PMA and lack of effect of phenylephrine were seen in reperfused rabbit hearts. 4. As alpha 1-receptor activation has been shown previously to stimulate cardiac adenosine production, we assessed whether blockade of adenosine A1 receptors with the specific antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 0.5 microM) would unmask the actions of phenylephrine in hearts subjected to 30 min ischaemia and reperfusion. In the presence of DPCPX, phenylephrine reduced recovery to 44 +/- 9% compared to 82 +/- 10% recovery in the absence of phenylephrine (P < 0.05). Identical results were observed in rabbit hearts treated with DPCPX in which recovery was reduced from 57.1 +/- 11.2% to 17.8 +/- 6.8% by phenylephrine (P < 0.05). Another A1 receptor antagonist, (+/-)-N6-endonorbornan-2-yl-9-methyladenine (N-0861, 0.5 microM) produced virtually identical results to those observed with DPCPX. 5. MIA failed to modulate the inhibition of postischaemic recovery by phenylephrine. Bisindolylmaleimide, on the other hand, partially prevented the effects of phenylephrine on postischaemic contractile dysfunction. The inhibitory effect of either PMA or phenylephrine on postischaemic recovery of both rat and rabbit hearts was generally dissociated from alterations in energy metabolism, although in the case of rat hearts, inhibition by phenylephrine was associated with diminished high energy phosphate content. 6. Our results demonstrate that both alpha 1-receptor activation as well as direct activation of PKC with phorbol ester can attenuate post-ischaemic ventricular recovery. Moreover, our results strongly suggest that endogenous adenosine protects the heart against the deleterious effects of alpha 1-receptor activation during ischaemia and reperfusion.

Propranolol unmasks class III like electrophysiological properties of norepinephrine

Isolated perfused spontaneously beating rabbit hearts were treated with increasing concentrations of norepinephrine (0.01, 0.1, 0.5 mumol/l) either alone or in presence of propranolol (0.1 mumol/l). For analysis of the epicardial activation and repolarization process and epicardial mapping (256 unipolar leads) was performed. For each electrode the activation and repolarization time was determined. From these data the "breakthrough-points" (BTP) of epicardial activation were determined. At each electrode an activation vector (VEC) was calculated giving direction and velocity of the local excitation wave. The beat similarity of various heart beats (under NE) compared to control was evaluated by determination of the percentage of identical BTP and of similar VEC (deviation < or = 5 degrees). Moreover at each electrode the local activation recovery interval (ARI) and its standard deviation (of 256 leads, dispersion, DISP) were determined. Norepinephrine alone (0.01, 0.1, 0.5 mumol/l) led to an increase in left ventricular pressure, heart rate and DISP with concomitant frequency dependent reduction in ARI, and to changes in the epicardial activation pattern (reduction in BTP, VEC). We found that in the presence of propranolol (0.1 mumol/l) norepinephrine prolonged ARI and reduced ARI-dispersion. This effect was not due to changes in heart rate. The disturbing effects on the activation pattern were diminished. These effects could be prevented by pretreatment with 1 mumol/l prazosin. From these results we conclude, that norepinephrine prolongs the relative action potential duration via stimulation of alpha 1-adrenoceptor and enhances cellular coupling.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of amosulalol, a combined alpha 1- and beta-adrenoceptor-blocking agent, on ischemic myocardial energy metabolism in dogs

The effect of amosulalol, a combined alpha 1- and beta-adrenoceptor-blocking agent, on changes in myocardial energy and carbohydrate metabolism during ischemia was studied. Ischemia was induced by ligating the left anterior descending coronary artery for 3 or 30 min during open-chest surgery in anesthetized dogs. The myocardial energy stores were depleted, and the levels of glycolytic intermediates were altered by 3 and 30 min of ischemia, indicating that anaerobic myocardial metabolism had occurred. Amosulalol (0.3 or 1 mg.kg-1) was injected intravenously 5 min before ischemia. Pretreatment with amosulalol, particularly at a dose of 0.3 mg.kg-1, reduced the myocardial energy depletion and the alteration of carbohydrate metabolism induced by ischemia. This result indicates that amosulalol can reduce ischemic influences on the myocardium.

The use of antiarrhythmics in advanced cardiac life support

Antiarrhythmic agents have been used to treat malignant ventricular arrhythmias in the setting of acute myocardial ischemia with proven efficacy for many years. Thus, it has been presumed that these agents would be efficacious for the treatment of cardiac arrest. Unfortunately, hard data supporting this contention are unavailable to date. Furthermore, some of the experimental data in this area are conflicting, especially regarding the relative effects of lidocaine and bretylium. Thus, little definitive can be said based on experimental information. In two randomized patient studies, lidocaine and bretylium performed comparably. Because of the frequent use of lidocaine and thus the familiarity of most health care professionals with its use, it makes educational sense to utilize lidocaine as the antiarrhythmic drug of first choice during the cardiac arrest sequence. Recent data suggesting that amiodarone may be efficacious in patients with recurrent arrhythmias require additional confirmation. Although antiarrhythmic agents have been shown to be effective in the treatment of malignant arrhythmias in patients with acute myocardial infarction, their use prophylactically for patients with suspected infarction (advocated in the past) has recently undergone reevaluation. It is now clear that despite a reduction in ventricular fibrillation, overall mortality may be increased. This may be because the prophylactic treatment of patients with suspected infarction includes a large number of patients not at risk for ventricular fibrillation who still may be at risk for drug toxicity. Thus, prophylactic administration of lidocaine to all patients with suspected acute myocardial infarction can no longer be recommended. There are inadequate data upon which to base a recommendation concerning the use of lidocaine in patients receiving thrombolytic therapy. The group most likely to benefit from lidocaine are patients with ST segment elevation who present early after the onset of acute myocardial infarction. The use of lidocaine in this group requires additional study. At present, despite enthusiasm for the prophylactic use of magnesium for the treatment of arrhythmias, data are inadequate to support its routine administration. However, given the importance of magnesium and potassium levels in the genesis of malignant arrhythmias, their levels in plasma should be assessed, and abnormalities should be promptly corrected. The potential uses of antiarrhythmic agents during advanced cardiac life support span a remarkably diverse number of applications. For the purpose of this review, only the use of these agents during CPR and during the early hours of acute or suspected acute myocardial infarction will be considered.

Hypoxia-induced coronary flow changes in the perfused rat heart: effects of high L-carnitine concentrations

1. Hemodynamic effects of physiological (0.04 and 0.07 mM) and high (8, 15 and 25 mM) L-carnitine (LC) concentrations were tested on the normally oxygenated and hypoxic perfused rat heart. 2. No effect was detected on aerobic hearts, whereas a dose-dependent rise in coronary flow (CF) during both the hyperemic and constrictive phases was observed in hypoxic hearts with 8, 15 and 25 mM LC. This action was apparently unrelated to a resting tension (RT) improvement, which was observed only with 25 mM LC. 3. When 11 mM glucose was replaced by 11 mM mannitol in the perfusion buffer, LC effects on the hyperemic phase were abolished; however, 25 mM LC resulted in CF-values still significantly higher than those detected without the drug, though RT was similar in these glucose-free groups. 4. It may be concluded that LC is ineffective on the perfused rat heart in aerobic conditions, whereas high LC concentrations can enhance CF only during hypoxia, these effects being independent of heart function improvements and partly unrelated to glucose presence.

Canine left ventricular hypertrophy predisposes to ventricular tachycardia induction by phase 2 early afterdepolarizations after administration of BAY K 8644

OBJECTIVES

The purpose of this study was to test the hypothesis that the longer duration of ventricular action potentials in hypertrophied hearts predisposes to the development of early after-depolarizations and triggered ventricular tachyarrhythmias.

BACKGROUND

For unknown reasons, the incidence of sudden death is greater in patients with myocardial hypertrophy.

METHODS

We measured left ventricular monophasic action potentials in normal dogs and dogs with left ventricular hypertrophy before and after administration of the calcium agonist BAY K 8644 and the potassium channel blocker cesium.

RESULTS

We demonstrated longer action potential durations in dogs with than in those without left ventricular hypertrophy. Also, BAY K 8644 produced phase 2 early afterdepolarizations and ventricular tachyarrhythmias more frequently in the dogs with than in those without left ventricular hypertrophy. Phenylephrine, an alpha agonist, further increased the action potential duration in hypertrophied hearts and the propensity to develop early afterdepolarizations and ventricular tachyarrhythmia after administration of BAY K 8644. Control and hypertrophied hearts developed early afterdepolarizations and ventricular tachyarrhythmia equally when exposed to cesium.

CONCLUSIONS

Although in vitro studies have shown that fibers of hypertrophied ventricular myocardium can develop triggered activity as a result of both early and late afterdepolarizations, the present study is the first to show in vivo that the hypertrophied ventricular myocardium compared with the normal ventricle is predisposed to develop phase 2 early afterdepolarizations that appear to trigger ventricular tachyarrhythmia. It is possible that such a mechanism contributes to the development of ventricular tachyarrhythmia and sudden cardiac death in patients with left ventricular hypertrophy. If this is shown to be true, specific pharmacologic interventions can be suggested.

Mechanisms for the positive inotropic effect of alpha 1-adrenoceptor stimulation in rat cardiac myocytes

alpha 1-Adrenoceptor activation can enhance myocardial contractility, and two possible inotropic mechanisms are an increase in myofilament Ca2+ sensitivity and action potential prolongation, which can increase net Ca2+ entry into cells. In adult rat ventricular myocytes (bath Ca2+, 1 mM; stimulated at 0.2-0.5 Hz), the drug 4-aminopyridine and the whole-cell voltage clamp have been used to control Ca2+ entry and differentiate between the two mechanisms. At 22-23 degrees C the specific alpha 1-adrenoceptor agonist methoxamine (100 microM) prolonged action potential duration at 50% repolarization from 55 +/- 2 to 81 +/- 5 msec, delayed time to peak contraction, and increased shortening amplitude from 5.3 +/- 0.6 to 7.8 +/- 1 microns (n = 18). Reduction of the transient outward current and other K+ currents by methoxamine was the major cause of action potential prolongation in rat myocytes with little change in the L-type calcium current. Block of the transient outward current with 2 mM 4-aminopyridine prolonged action potential duration from 52 +/- 6 to 98 +/- 12 msec and increased unloaded cell shortening from 2.9 +/- 0.4 to 6.6 +/- 0.6 microns (n = 4). Subsequently, methoxamine no longer increased cell shortening, although significant potentiation of twitch amplitude was still seen after a brief rest interval. In voltage-clamp experiments, with 70-500-msec pulses, although membrane currents were reduced, methoxamine had no positive inotropic effect and reduced cell shortening from 5.3 +/- 0.7 to 4.97 +/- 0.8 microns at pulse potentials positive to -40 mV. Similar alpha 1-adrenoceptor responses were observed at 35 degrees C during action potential and voltage-clamp experiments, which could be blocked by 10 microM prazosin. In myocytes loaded with the Ca2+ indicator indo-1, alpha 1-adrenoceptor stimulation or 4-aminopyridine both increased cell contraction and intracellular Ca2+ transients by similar amounts. As in unloaded cells, prior exposure to 4-aminopyridine prevented any inotropic effect of methoxamine without changing the systolic intracellular Ca2+ transient. The results indicated that under our experimental conditions positive inotropy in rat cardiomyocytes on exposure to alpha 1-adrenoceptor agonists was strongly correlated with the action potential prolongation that accompanied K+ current reduction. In addition, modulation of K+ channels could occur independent of changes in contractility and/or [Ca2+]i.

Effects of alpha-adrenoceptor and of combined sympathetic and parasympathetic blockade on cardiac performance and vascular resistance

1. Cardiac performance and vascular resistance was studied in seven healthy men by radionuclide cardiography and venous plethysmography before and after alpha-adrenoceptor blockade with phentolamine and after combined alpha-adrenoceptor, beta-adrenoceptor (propranolol) and parasympathetic (atropine) blockade. 2. During alpha-adrenoceptor blockade heart rate and cardiac output increased considerably and left ventricular ejection fraction increased because of increased contractility. Systemic vascular resistance fell both during alpha-adrenoceptor blockade alone and during combined blockade. The increase in calf blood flow was of the same magnitude after combined blockade and after alpha-adrenoceptor blockade alone, and was considerably higher than the fall in systemic vascular resistance. Plasma catecholamine concentrations increased after phentolamine, but the changes were blunted when propranolol and atropine were added. 3. These results indicate that peripheral vasoconstriction especially that exerted by alpha-adrenoceptor nervous tone in skeletal muscle restricts left ventricular emptying of the intact heart. During pharmacologic blockade of the sympathetic and parasympathetic nervous system at rest the chronotropic state is augmented, whereas preload and inotropy are unaffected.

Effects of bunazosin, a selective alpha 1-adrenergic blocking agent, on myocardial energy metabolism in ischemic dog heart

Effects of a selective alpha 1-adrenergic blocking agent, bunazosin, on myocardial energy metabolism in the ischemic heart were studied. Ischemia was induced by ligating the left anterior descending coronary artery of the dog heart. Bunazosin was injected intravenously either 5 or 20 min before coronary artery ligation. Hearts were removed 3 min after coronary ligation and used for determination of the levels of cardiac tissue metabolites. Ischemia decreased the levels of ATP, creatine phosphate, glycogen and glucose, and increased the levels of ADP, AMP, hexose monophosphates and lactate. The energy charge potential (ECP) calculated was decreased by ischemia. Pretreatment with bunazosin inhibited the decrease in ATP and the increase in AMP caused by ischemia, resulting in the high value of ECP in the ischemic myocardium. Bunazosin also prevented the changes in carbohydrate metabolism caused by ischemia. It is concluded that bunazosin may reduce the influence of ischemia on the myocardium.

Alpha-adrenoceptor stimulation and blockade modulates cesium-induced early afterdepolarizations and ventricular tachyarrhythmias in dogs

In 84 open-chest dogs, we studied the effects on early afterdepolarizations (EADs) and ventricular tachyarrhythmias (VTs) induced by cesium chloride (168 mg/kg i.v.) of alpha-adrenoceptor stimulation with phenylephrine (100 micrograms plus 0.25 microgram/kg/min i.v.) and with left ansa subclavia stimulation (LAS; 2 Hz, 4 msec, 2 mA) after propranolol (0.5 mg/kg) administration. We also studied the effects of alpha-adrenoceptor blockade with phentolamine (3 mg/kg), prazosin (25-500 micrograms/kg), yohimbine (10-500 micrograms/kg), WB 4101 (2 mg/kg), and benoxathian (2 mg/kg) during decentralized LAS. EAD amplitude, presented as a percentage of monophasic action potential amplitude, was recorded simultaneously with contact electrodes from the right and left ventricular endocardium. Phenylephrine and LAS plus propranolol increased EAD amplitude (31.5 +/- 8.8% to 47.8 +/- 9.7% and 34.8 +/- 4.1% to 46.1 +/- 6.4%, respectively) and the prevalence of VT (from three to nine of 11 dogs and from the three to five of six dogs, respectively). Prazosin produced a dose-response decrease in EAD amplitude and reduced the prevalence of VT. Yohimbine did not alter the amplitude of EADs or the prevalence of VT. WB 4101 and phentolamine reduced the amplitude of EADs produced by cesium and LAS (from 44.3 +/- 10.2% to 32.6 +/- 9.4% and from 39.8 +/- 6.9% to 30.3 +/- 6.3%, respectively) and the prevalence of VT (from eight to one of 10 dogs and from 13 to 5 of 20 dogs, respectively). Benoxathian did not alter significantly the amplitude of EADs (41.6 +/- 11.4% to 37.5 +/- 9.4%) or the prevalence of VT (from six to five of 10 dogs).(ABSTRACT TRUNCATED AT 250 WORDS)

Structure and function of alpha-adrenergic receptors

Alpha 1- and alpha 2-adrenergic receptors are the initial recognition sites on a wide variety of catecholamine-responsive target cells. This article addresses several major questions related to subtypes, structure, signal transduction mechanisms, and regulation of alpha 1- and alpha 2-adrenergic receptors. The application of biochemical and cell and molecular biologic techniques has provided many new insights regarding alpha-adrenergic receptors. Two (and perhaps three) distinct alpha 2-adrenergic receptor subtypes have been identified, and subtypes may exist for alpha 1-adrenergic receptors as well. These multiple subtypes imply much greater diversity among alpha-adrenergic receptors than among beta-adrenergic receptors. Alpha-adrenergic receptors are membrane glycoproteins with several common structural features (including seven membrane-spanning domains with extracellular amino terminus and intracellular carboxyl terminus) that are shared with other types of membrane receptors linked to guanine nucleotide-binding regulatory (G) proteins. These G proteins appear to link alpha-adrenergic receptors to multiple effector systems, including enzymes such as adenylate cyclase and phospholipases, and ion channels. The receptors themselves are dynamic entities, the number of which is regulated as a consequence of a poorly understood life cycle. Although unproven, it seems likely that several important clinical disorders represent alterations in alpha-adrenergic receptors themselves or in the G proteins or effector systems to which these receptors couple. New tools for studying receptor structure and function should help clarify the numerous, inadequately understood issues regarding alpha-adrenergic receptors and their possible alteration in disease states.

Clinical pharmacotherapeutics of doxazosin

Doxazosin is the latest in a series of highly selective postsynaptic alpha 1-adrenoceptor inhibitors. It is readily absorbed, with high bioavailability and a relatively long plasma half-life, neither of which property is influenced by age. This accounts for the prolonged pharmacologic activity of doxazosin following a single oral dose. Its prime pharmacodynamic activity resides in its ability to counter sympathetic vasoconstriction of the systemic arteriolar resistance vessels and venous capacitance system, which enables the drug to target the major pathophysiologic abnormality in hypertension, i.e., the generalized systemic arteriolar constriction. The widespread vasodilation induced by doxazosin relieves both cardiac preload and afterload and, consequently, reduces left ventricular wall stress and myocardial oxygen consumption. In hypertension, doxazosin reduces blood pressure both at rest and during exercise by reduction of systemic vascular resistance without precipitating substantial reflex cardiac stimulation. The effects are maximal on the standing blood pressure between two and four hours after ingestion; due to doxazosin's relatively slow absorption, postural hypotension is infrequent. Its antihypertensive activity is maintained over 24 hours following a single oral dose, and the optimal dose range is 2 to 8 mg once daily. The antihypertensive efficacy of doxazosin has been shown to be comparable with that of other alpha-adrenoceptor inhibitors, beta-blocking drugs, diuretics, calcium antagonists, and angiotensin-converting enzyme inhibitors. In contrast to other conventional antihypertensive drugs, a unique feature of alpha-adrenoceptor-inhibiting drugs, including doxazosin, is their ability to reduce the plasma concentrations of triglycerides, total cholesterol, and low-density lipoprotein cholesterol and to increase high-density lipoprotein cholesterol concentration. This contrasts with the opposite effect on lipid levels induced by hydrochlorothiazide and atenolol seen in comparative studies. Side effects show no predilection for any organ system, and the overall incidence of such effects compares well with those of other commonly used antihypertensive drugs. This unique combination of antihypertensive efficacy and favorable effect on blood lipid levels indicates that once-daily treatment with doxazosin holds considerable promise in the treatment of hypertension, both from the point of view of its antihypertensive efficacy and also from its primary preventative potential.