Alpha 1-adrenergic system and arrhythmias in ischaemic heart disease.

Several lines of experimental evidence obtained over the last decade indicate that alterations in the alpha 1-adrenergic receptor system may contribute significantly to arrhythmogenesis in the ischaemic heart. Under normal physiological conditions, alpha 1-adrenergic stimulation of myocytes elicits a modest increase in inotropy, a lengthening of repolarization secondary to a decrease in IK through activation of protein kinase C, and a decrease in automaticity in Purkinje cells due to an increase in Na+/K+ ATPase activity. These findings suggest that alpha 1-adrenergic stimulation of the myocardium would elicit an antiarrhythmic effect. However, during both early ischaemia and reperfusion there is an enhanced responsivity to alpha-adrenergic stimulation and a potent antiarrhythmic effect of alpha 1-adrenergic blockade in several species including the conscious dog. This enhanced alpha-adrenergic responsivity may be due to an increase in alpha 1-adrenergic receptors in ischaemic myocardium originating from a site distinct from the intracellular site for trafficking of beta-adrenergic receptors, possibly within or near the sarcolemma. Recently, we developed an isolated adult canine ventricular myocyte preparation which also exhibits a 2- to 3-fold reversible increase in alpha 1-adrenergic receptors in response to severe hypoxia (PO2 = less than 15 mmHg) associated with marked sarcolemmal accumulation of long-chain acylcarnitines (LCA) secondary to hypoxia-induced inhibition of beta-oxidation of fatty acids. The increase in alpha 1-adrenergic receptors is prevented by inhibition of carnitine acyltransferase I which precludes accumulation of LCA. The sarcolemmal accumulation of LCA increases membrane fluidity, suggesting that the alpha 1-adrenergic receptor may be latent within or near the sarcolemma and becomes accessible to a surface ligand only as membrane fluidity is altered. This conclusion is also supported by our findings that hypoxia elicits a marked increase in the coupling of the alpha 1-adrenergic receptor to inositol 1,4,5-trisphosphate (IP3) production in canine myocytes exposed to norepinephrine. IP3 has been shown to mobilize Ca2+ from the sarcoplasmic reticulum, thereby modulating the levels of intracellular Ca2+. Stimulation of hypoxic myocytes with norepinephrine also results in the appearance of delayed after-depolarizations and triggered rhythms, probably in response to an increase in intracellular Ca2+. In conclusion, these findings indicate that the alpha 1-adrenergic system can contribute to arrhythmogenesis in the ischaemic heart and that approaches to reduce the incidence of sudden cardiac death should include blockade of alpha 1-adrenergic receptors.

Differential turnover of polyunsaturated fatty acids in plasmalogen and diacyl glycerophospholipids of isolated cardiac myocytes.

To investigate the relative turnover of esterified polyunsaturated fatty acids in diacylglycerophospholipids and plasmalogens in isolated cardiac myocytes, we characterized the phospholipid composition and distribution of radiolabel in different phospholipid classes and in individual molecular species of diradyl choline (CGP) and ethanolamine (EGP) glycerophospholipids after incubation of isolated cardiac myocytes with [3H]arachidonate or [14C]linoleate. Plasmalogens in CGP (55%) and EGP (42%) quantitatively accounted for the total plasmalogen content (39%) of cardiac myocyte phospholipids. Plasmalogens comprised 86% and 51% of total arachidonylated CGP and EGP mass, respectively, and [3H]arachidonate was primarily incorporated into plasmalogens in both CGP (65%) and EGP (61%) classes. The specificity activity of [3H]arachidonylated diacyl-CGP was approximately 2- to 5-fold greater than that of [3H]arachidonylated choline plasmalogen, whereas comparable specific activities were found in the [3H]arachidonate-labeled ethanolamine plasmalogen and diacyl-EGP pools. Of the total linoleate-containing CGP and EGP mass, 54% and 57%, respectively, was esterified to plasmalogen molecular species. However, [14C]linoleate was almost exclusively incorporated into diacyl-CGP (96%) and diacyl-EGP (86%). The specific activities of [14C]linoleate-labeled diacyl-CGP and diacyl-EGP were 5- to 20-fold greater than that of the [14C]linoleate-labeled plasmalogen pools. The differential incorporation of polyunsaturated fatty acids in plasmalogens and diacylglycerophospholipids demonstrates that the metabolism of the sn-2 fatty acyl moiety in these phospholipid subclasses is differentially regulated, possibly fulfilling separate and distinct physiologic roles.

Amphipathic lipid metabolites and their relation to arrhythmogenesis in the ischemic heart.

Myocardial ischemia is associated with profound electrophysiologic derangements which occur within minutes and are rapidly reversible with reperfusion, suggesting that subtle and reversible biochemical alterations within or near the sarcolemma contribute. Our efforts have concentrated on two structurally similar amphipathic metabolites, long-chain acylcarnitine and lysophosphatidylcholine. Studies performed in vitro in isolated tissue indicate that incorporation of either metabolite into the sarcolemma at concentrations of 1-2 mole %, as verified using electron microscopic (EM) autoradiography, elicits profound electrophysiologic derangements analogous to those seen in the ischemic heart in vivo. In isolated myocytes in vitro, the electrophysiologic derangements elicited by hypoxia are associated with a marked 70-fold increase in the endogenous sarcolemmal accumulation of long-chain acylcarnitine. Inhibition of carnitine acyltransferase I (CAT-I) not only prevents the accumulation of long-chain acylcarnitine in isolated myocytes exposed to severe hypoxia, but also markedly attenuates the electrophysiologic alterations. Several lines of experimental evidence, including measurements in venous effluents as well as cardiac lymph, indicate that lysophosphatidylcholine (LPC) accumulates to a large extent in the extracellular space during ischemia. This extracellular accumulation may be secondary to release from vascular endothelium, smooth muscle or blood cell elements. In crude homogenates of myocardial tissue, the total enzymic activity for catabolism of LPC far exceeds the total activity for synthesis of LPC mediated by phospholipase A2 (PLA2) catalyzed hydrolysis of phosphatidylcholine (PC). Therefore, inhibition of catabolism would be required for net accumulation of LPC to occur. Three enzymes responsible for the catabolism of LPC are inhibited by either long-chain acylcarnitine or acidic pH. Thus, accumulation of long-chain acylcarnitine and acidosis contribute to the increase in LPC observed in ischemic tissue. In this report, we provide evidence that accumulation of long-chain acylcarnitine occurs very rapidly in ischemic myocardium in vivo, coincident with the development of electrophysiologic alterations leading to malignant arrhythmias as verified using 3-dimensional cardiac mapping procedures. Following a brief, 2-min period of ischemia, long-chain acylcarnitine content increased four-fold in the ischemic region, concomitant with the development of electrophysiologic abnormalities observed during this period. Additionally, we demonstrate that modification of intracellular lipolysis by beta-adrenergic receptor stimulation or blockade does not influence long-chain acylcarnitine accumulation following this 2-min interval of ischemia. These results suggest that production of long-chain acylcarnitine is not limited by the intracellular free fatty acid concentration early in ischemia.(ABSTRACT TRUNCATED AT 400 WORDS)

Flecainide acetate treatment of paroxysmal supraventricular tachycardia and paroxysmal atrial fibrillation: dose-response studies. The Flecainide Supraventricular Tachycardia Study Group.

The dose-response relations for efficacy and tolerance of the antiarrhythmic drug flecainide acetate were studied in 28 patients with paroxysmal supraventricular tachycardia (Group 1) and 45 patients with paroxysmal atrial fibrillation or flutter (Group 2). Recurrent symptomatic tachycardia was documented with use of transtelephonic electrocardiographic recording. Patients received flecainide in doses of 25, 50, 100 and 150 mg twice daily and placebo for 1 month treatment periods. Among 14 patients in Group 1 who qualified for efficacy analysis, 4 (29%) had no tachycardia while taking placebo. The number with no tachycardia increased with progressively larger flecainide doses; with the 150 mg twice daily dose, 12 (86%) of 14 patients had no tachycardia (p less than 0.01 for overall differences among all treatments). Among 28 patients in Group 2, 2 (7%) had no tachycardia while taking placebo. The number with no tachycardia also increased with progressively larger flecainide doses; with the 150 mg twice daily dose, 17 (61%) of 28 patients had no tachycardia (p less than 0.01 for overall differences among all treatments). Noncardiac adverse experiences were the leading cause of premature study discontinuation during flecainide treatment periods (five patients in Group 1 and six patients in Group 2).

A sensitive method for the quantification of the mass of inositol phosphates using gas chromatography-mass spectrometry.

A sensitive method to directly measure the mass of inositol phosphates from biologic samples is described. The procedure uses ammonium sulfate gradient elution anion exchange column chromatography to isolate inositol monophosphate, bisphosphate, trisphosphate, and tetrakisphosphate. The isolated fractions are dephosphorylated and subsequently desalted by a novel approach using solid barium hydroxide in a 1:1 stoichiometric ratio to the amount of ammonium sulfate present in the dephosphorylated sample. The myo-inositol derived from each inositol phosphate species was quantified by stable isotope dilution gas chromatography-mass spectrometry of the hexakis(trimethylsilyl) derivative using hexadeutero-myo-inositol as the internal standard. The applicability and sensitivity of this method are illustrated by measuring the mass of individual inositol phosphates in isolated adult canine cardiac myocytes.

Modulation of alpha-adrenergic receptors and their intracellular coupling in the ischemic heart.

The alpha 1-adrenergic receptor exists as at least two distinct subtypes, alpha 1a and alpha 1b. Based on hydrophobic exclusion studies and limited proteolysis of the cloned receptor, it appears to possess characteristics analogous to other membrane-bound receptors including seven membrane spanning domains, three extracellular, and three intracellular loops, with extensive glycosylation near the extracellular amino terminus. Although the receptor is coupled to phospholipase C in cardiac myocytes, with activation resulting in the production of inositol trisphosphate (IP3) and diacylglycerol, recent findings suggest that the receptor may also be linked to phospholipase A2, phospholipase D, and cyclic nucleotide phosphodiesterase. The alpha 1-adrenergic receptor has been shown to increase in response to myocardial ischemia in a number of different species and to mediate not only positive inotropic effects, but also to contribute substantially to arrhythmogenesis. The increase in alpha 1-adrenergic receptors can also occur in isolated adult ventricular myocytes in response to hypoxia, a mechanism which appears to be secondary to the sarcolemmal accumulation of long-chain acylcarnitines. This increase in alpha 1-adrenergic receptors in hypoxic myocytes is also linked to an enhanced increase in IP3 in response to receptor stimulation. These and other findings obtained in vivo during ischemia suggest that alpha 1-adrenergic mechanisms can become prominent in myocardium under pathophysiologic conditions in which a depressed contractile state exists and may therefore serve as a secondary inotropic system. However, the arrhythmogenic effects of stimulation of the alpha 1-adrenergic receptor in the ischemic heart in man may contribute substantially to arrhythmogenesis and, thereby, to the incidence of sudden cardiac death.

Increased pacing threshold after an automatic defibrillator shock in dogs: effects of class I and class II antiarrhythmic drugs.

A high energy shock delivered by an automatic defibrillator may interfere with pacemaker function. To provide insight into the changes that occur in the threshold for ventricular pacing after the shock from an automatic defibrillator, we measured the time to capture during asynchronous ventricular pacing in dogs from endocardial or epicardial sites, after a 30 joule shock was delivered via conventional automatic defibrillator (AICD) patch electrodes. After a 30 joule shock, there was a transient loss of ventricular capture. The duration of capture loss was related to current strength. During endocardial pacing at threshold current, the time to capture was 4.9 +/- 1.2 s, whereas at current values twice threshold the time to capture from endocardial pacing was 2.2 +/- 0.9 s. No difference was found between endocardial and epicardial pacing sites in the time to capture. To ascertain the mechanism of capture loss we: (1) examined the effects of converting the pacing catheter to a current sink (transiently shunting to ground); (2) altered excitability by an infusion of flecainide; (3) blocked sympathetic input (propranolol). No change in time to capture was noted by shunting the pacer to ground. After an infusion of flecainide the time to capture from endocardial pacing was significantly prolonged to 14.9 +/- 2.2 s at the threshold value (P less than .01) and 5.6 +/- 2.1 s at twice threshold (P less than .05). Conversely, intravenous propranolol had no effect on the time to capture after shock from endocardial pacing. These data indicate that there is a transient increase in pacing threshold after the shock from an automatic defibrillator.(ABSTRACT TRUNCATED AT 250 WORDS)