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E Marban

Publications and source records attributed to E Marban.

At least 73 records · Page 4Linked to original sources
PubMed1995

Metabolic oscillations in heart cells.

Oscillatory rhythms underlie biological processes as diverse and fundamental as neuronal firing, secretion, and muscle contraction. We have detected periodic changes in membrane ionic current driven by intrinsic oscillations of energy metabolism in guinea pig heart cells. Withdrawal of exogenous substrates initiated oscillatory activation of ATP-sensitive potassium current and cyclical suppression of depolarization-evoked intracellular calcium transients. The oscillations in membrane current were not driven by pacemaker currents or by alterations in intracellular calcium and thus represent a novel cytoplasmic cardiac oscillator. The linkage to energy metabolism was demonstrated by monitoring oscillations in the oxidation state of pyridine nucleotides. Interventions which altered the rate of glucose metabolism modulated the oscillations, suggesting that the rhythms originated at the level of glycolysis. The metabolic oscillations produced cyclical changes in electrical excitability, underscoring the potential importance of this intrinsic oscillator in the genesis of cardiac arrhythmias.

Animals↗
PubMed1995

Prospects for genetic manipulation of cardiac excitability.

Despite impressive advances in the therapy of a number of types of heart disease in the last two decades, sudden cardiac death remains a public health problem of staggering dimensions. Current treatment options include antiarrhythmic drugs that have higher than desired failure rates and implantable defibrillators that incur significant costs to the patient and society. The development of therapies that better suppress the cardiac arrhythmias responsible for sudden cardiac death requires a broad and comprehensive understanding of the basic mechanisms underlying electrical instability in the heart. This study explores the scientific basis for a molecular genetic approach to modify cardiac excitability and thereby to create animal models of sudden cardiac death. The availability of such models will open up new avenues of research in arrhythmogenesis and facilitate the development of novel antiarrhythmic agents.

Animals↗
PubMed1995

Transcriptional regulation of the neuronal L-type calcium channel alpha 1D subunit gene.

1. The transcriptional regulation of the rat brain L-type calcium channel alpha 1D subunit (RB alpha 1D) gene was investigated using NG108-15 neuroblastoma-glioma cells. 2. Differentiation of NG108-15 cells in the presence of prostaglandin E1 or retinoic acid resulted in the appearance of mRNA encoding the RB alpha 1D subunit detected using Northern blot analysis. 3. A rat genomic DNA library was screened, and a 15.2-kb clone was isolated and partially sequenced which included part of the 5' upstream sequence through the initial part of intron 2 of the RB alpha 1D gene. 4. Deletion analysis, using a CAT reporter gene and transfected NG108-15 cells, revealed that the 1.2-kb 5'-upstream sequence from the RB alpha 1D gene contains cis-acting positive and negative regulatory elements. A deletion of the 3' end of exon 1 also suggested the presence of regulatory elements in the first exon. 5. DNase footprinting of exon 1 of the RB alpha 1D gene revealed two regions protected from digestion by specific protein binding, and the second region included an (ATG)7 trinucleotide repeat sequence. Electrophoretic mobility shift assays confirmed nuclear protein(s) binding to the (ATG)7 sequence. 6. The (ATG)7 sequence functions as a enhancer when linked to a thymidine kinase promoter and a CAT reporter gene. 7. These results provide the initial description of the transcriptional regulation of the RB alpha 1D gene and identify a novel enhancer that consists of an (ATG)7 trinucleotide repeat sequence.

Alprostadil↗
PubMed1995

Alterations of excitation-contraction coupling in stunned myocardium and in failing myocardium.

Although both myocardial stunning and chronic heart failure are characterized by contractile dysfunction, there are profound differences in their underlying mechanisms. Changes in cardiac contractile force can be effected by modulation of intracellular [Ca2+] or by alteration of the contractile protein response to intracellular Ca2+. New evidence suggests that the principal lesion in the stunned myocardium resides at the level of the contractile proteins, which may be injured by proteases activated early during reperfusion. In contrast, failing myocardium is known to display abnormal intracellular Ca2+ handling, indicative of dysfunction of the sarcoplasmic reticulum. Alterations of gene expression and isoform switching of the myofilaments also occur in failing myocardium, consistent with an observed shift of the kinetics of crossbridge cycling. In conclusion, changes in both intracellular Ca2+ homeostasis and myofilament function occur in failing myocardium, while stunned myocardium primarily reflects an uncoupling between Ca2+ and contractile force.

Animals↗
PubMed1995

A mutation in the pore of the sodium channel alters gating.

Ion permeation and channel gating are classically considered independent processes, but site-specific mutagenesis studies in K channels suggest that residues in or near the ion-selective pore of the channel can influence activation and inactivation. We describe a mutation in the pore of the skeletal muscle Na channel that alters gating. This mutation, I-W53C (residue 402 in the mu 1 sequence), decreases the sensitivity to block by tetrodotoxin and increases the sensitivity to block by externally applied Cd2+ relative to the wild-type channel, placing this residue within the pore near the external mouth. Based on contemporary models of the structure of the channel, this residue is remote from the regions of the channel known to be involved in gating, yet this mutation abbreviates the time to peak and accelerates the decay of the macroscopic Na current. At the single-channel level we observe a shortening of the latency to first opening and a reduction in the mean open time compared with the wild-type channel. The acceleration of macroscopic current kinetics in the mutant channels can be simulated by changing only the activation and deactivation rate constants while constraining the microscopic inactivation rate constants to the values used to fit the wild-type currents. We conclude that the tryptophan at position 53 in the domain IP-loop may act as a linchpin in the pore that limits the opening transition rate. This effect could reflect an interaction of I-W53 with the activation voltage sensors or a more global gating-induced change in pore structure.

Amino Acid Sequence↗
PubMed1995

The relationship between contractile force and intracellular [Ca2+] in intact rat cardiac trabeculae.

The control of force by [Ca2+] was investigated in rat cardiac trabeculae loaded with fura-2 salt. At sarcomere lengths of 2.1-2.3 microns, the steady state force-[Ca2+]i relationship during tetanization in the presence of ryanodine was half maximally activated at a [Ca2+]i of 0.65 +/- 0.19 microM with a Hill coefficient of 5.2 +/- 1.2 (mean +/- SD, n = 9), and the maximal stress produced at saturating [Ca2+]i equalled 121 +/- 35 mN/mm2 (n = 9). The dependence of steady state force on [Ca2+]i was identical in muscles tetanized in the presence of the Ca(2+)-ATPase inhibitor cyclopiazonic acid (CPA). The force-[Ca2+]i relationship during the relaxation of twitches in the presence of CPA coincided exactly to that measured at steady state during tetani, suggesting that CPA slows the decay rate of [Ca2+]i sufficiently to allow the force to come into a steady state with the [Ca2+]i. In contrast, the relationship of force to [Ca2+]i during the relaxation phase of control twitches was shifted leftward relative to the steady state relationship, establishing that relaxation is limited by the contractile system itself, not by Ca2+ removal from the cytosol. Under control conditions the force-[Ca2+]i relationship, quantified at the time of peak twitch force (i.e., dF/dt = 0), coincided fairly well with steady state measurements in some trabeculae (i.e., three of seven). However, the force-[Ca2+]i relationship at peak force did not correspond to the steady state measurements after the application of 5 mM 2,3-butanedione monoxime (BDM) (to accelerate cross-bridge kinetics) or 100 microM CPA (to slow the relaxation of the [Ca2+]i transient). Therefore, we conclude that the relationship of force to [Ca2+]i during physiological twitch contractions cannot be used to predict the steady state relationship.

Animals↗
PubMed1995

Functional consequences of sulfhydryl modification in the pore-forming subunits of cardiovascular Ca2+ and Na+ channels.

The structure and function of many cysteine-containing proteins critically depend on the oxidation state of the sulfhydryl groups. In such proteins, selective modification of sulfhydryl groups can be used to probe the relation between structure and function. We examined the effects of sulfhydryloxidizing and -reducing agents on the function of the heterologously expressed pore-forming subunits of the cloned rabbit smooth muscle L-type Ca2+ channel and the human cardiac tetrodotoxin-insensitive Na+ channel. The known sequences of the channels suggest the presence of three or four cysteine residues within the putative pores of Ca2+ or Na+ channels, respectively, as well as multiple other cysteines in regions of unknown function. We determined the effects of sulfhydryl modification on Ca2+ and Na+ channel gating and permeation by using the whole-cell and single-channel variants of the patch-clamp technique. Within 10 minutes of exposure to 2,2'-dithiodipyridine (DTDP, a specific lipophilic oxidizer of sulfhydryl groups), Ca2+ current was reduced compared with the control value, with no significant change in the kinetics and no shift in the current-voltage relations. The effect could be readily reversed by 1,4-dithiothreitol (an agent that reduces disulfide bonds). Similar results were obtained by using the hydrophilic sulfhydryl-oxidizing agent thimerosal. The effects were Ca(2+)-channel specific: DTDP induced no changes in expressed human cardiac Na+ current. Single-channel Ba2+ current recordings revealed a reduction in open probability and mean open time by DTDP but no change in single-channel conductance, implying that the reduction of macroscopic Ca2+ current reflects changes in gating and not permeation. In summary, the pore-forming (alpha 1) subunit of the L-type Ca2+ channel contains functionally important free sulfhydryl groups that modulate gating. These free sulfhydryl groups are accessible from the extracellular side by an aqueous pathway.

2,2'-Dipyridyl↗
PubMed1995

Relationship between intracellular calcium and contractile force in stunned myocardium. Direct evidence for decreased myofilament Ca2+ responsiveness and altered diastolic function in intact ventricular muscle.

To elucidate the abnormalities of excitation-contraction coupling in stunned myocardium, we measured [Ca2+]i and force in thin fura 2-loaded ventricular trabeculae from control or stunned (20 minutes ischemia followed by 20 minutes reflow at 37 degrees C) rat hearts. At any given [Ca2+]o, force development was significantly lower in the stunned trabeculae than in control trabeculae. In contrast, there was no difference in the amplitude of Ca2+ transients between the two groups. The steady state force-[Ca2+]i relationship, assessed by tetanization in the presence of ryanodine, revealed both a decrease in maximal Ca(2+)-activated force and an increase in the [Ca2+]i required for 50% activation in stunned trabeculae. Postischemic myocardium also exhibited an accelerated rate of diastolic relaxation that was not due to changes in the rate of Ca2+ transient decay. Destabilization of attached cross-bridges in a quantitative model of cardiac myofibrils accurately reproduced the salient systolic and diastolic features of the stunned phenotype, suggesting an abnormality of the thin filaments. In response to supraphysiological increases in [Ca2+]o, diastolic [Ca2+]i and diastolic tone increased much more in stunned trabeculae than in controls, with the frequent occurrence of aftercontractions. This novel experimental model lends further support to the hypothesis that the primary lesion of excitation-contraction coupling resides at the level of the contractile proteins. The finding of enhanced susceptibility to calcium overload helps to rationalize the functional deterioration of stunned myocardium during intense inotropic stimulation and additionally suggests that stunned myocardium may represent a favorable substrate for triggered arrhythmias.

Actin Cytoskeleton↗
PubMed1995

Adenovirus-mediated expression of a voltage-gated potassium channel in vitro (rat cardiac myocytes) and in vivo (rat liver). A novel strategy for modifying excitability.

Excitability is governed primarily by the complement of ion channels in the cell membrane that shape the contour of the action potential. To modify excitability by gene transfer, we created a recombinant adenovirus designed to overexpress a Drosophila Shaker potassium channel (AdShK). In vitro, a variety of mammalian cell types infected with AdShK demonstrated robust expression of the exogenous channel. Spontaneous action potentials recorded from cardiac myocytes in primary culture were abbreviated compared with noninfected myocytes. Intravascular infusion of AdShK in neonatal rats induced Shaker potassium channel mRNA expression in the liver, and large potassium currents could be recorded from explanted hepatocytes. Thus, recombinant adenovirus technology has been used for in vitro and in vivo gene transfer of ion channel genes designed to modify cellular action potentials. With appropriate targeting, such a strategy may be useful in gene therapy of arrhythmias, seizure disorders, and myotonic muscle diseases.

3T3 Cells↗
PubMed1994

Electrophysiological properties of neonatal mouse cardiac myocytes in primary culture.

1. The increasing utility of transgenic mice in molecular studies of the cardiovascular system has motivated us to characterize the ionic currents in neonatal mouse ventricular myocytes. 2. Cell capacitance measurements (30 +/- 1 pF, n = 73) confirmed visual impressions that neonatal mouse ventricular myocytes in primary culture are considerably smaller than freshly isolated adult ventricular myocytes. With the use of electron microscopy, mitochondria and sarcoplasmic reticulum were found in close association with myofibrils, but transverse tubules were not observed. 3. Action potential durations, measured at 50 and 90% repolarization, were 23 +/- 1 and 42 +/- 2 ms respectively (n = 46). Application of 4-aminopyridine (4-AP; 5 mM) prolonged action potential duration at 50% repolarization by 26 +/- 5% (n = 3). The brevity of the action potential is explained by the rapid activation of a transient outward K+ current upon voltage-clamp depolarization to plateau potentials. 4. Potassium currents identified include an inward rectifier, a large 4-AP-sensitive transient outward, a slowly inactivating 4-AP-insensitive outward, a slowly activating delayed rectifier and a small rapidly activating E-4031 (10 microM)-sensitive delayed rectifier K+ current. 5. Sodium currents (-305 +/- 50 pA pF-1, n = 21) were recorded in 40 mM Na+ with Ni2+ (1 mM) to block Ca2+ currents and with K+ replaced by Cs+. The relative insensitivity of the Na+ current to block by tetrodotoxin (IC50 = 2.2 +/- 0.3 microM, n = 4) is distinctive of the cardiac Na+ channel isoform. 6. Nitrendipine-insensitive (10 microM) Ba2+ currents elicited during steps from -90 to -30 mV measured -25 +/- 5 pA pF-1 (n = 7, 30 mM Ba2+). Decay of these currents was complete during 180 ms depolarizations, even with Ba2+ as the charge carrier. These currents were not present when the holding potential was set at -50 mV. These data support the presence of a low threshold, T-type Ca2+ current. 7. The maximal nitrendipine-sensitive L-type Ca2+ current density was -10 +/- 2 pA pF-1 (n = 8) in 2 mM Ca2+ and -38 +/- 5 pA pF-1 (n = 9) in 30 mM Ba2+. Exposure to isoprenaline (1 microM) resulted in an 82% increase (n = 3) in the amplitude of the Ba2+ currents elicited at 0 mV. 8. Neonatal mouse cardiac myocytes in primary culture possess surprisingly large inward currents given the brevity of their action potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1994

Oscillations of membrane current and excitability driven by metabolic oscillations in heart cells.

Periodic changes in membrane ionic current linked to intrinsic oscillations of energy metabolism were identified in guinea pig cardiomyocytes. Metabolic stress initiated cyclical activation of adenosine triphosphate-sensitive potassium current and concomitant suppression of depolarization-evoked intracellular calcium transients. The oscillations in membrane current and excitation-contraction coupling were linked to oscillations in the oxidation state of pyridine nucleotides but were not driven by pacemaker currents or alterations in the concentration of cytosolic calcium. Interventions that altered the rate of glucose metabolism modulated the oscillations, suggesting that the rhythms originated at the level of glycolysis. The energy-driven oscillations in potassium currents produced cyclical changes in the cardiac action potential and thus may contribute to the genesis of arrhythmias during metabolic compromise.

Adenosine Triphosphate↗
PubMed1994

Simultaneous expression of cardiac and skeletal muscle isoforms of the L-type Ca2+ channel in a rat heart muscle cell line.

1. We have investigated the identity of the L-type Ca2+ channels present in the H9c2 myoblast line derived from embryonic rat ventricle. To this end, we characterized macroscopic and unitary Ba2+ currents through Ca2+ channels, and looked for specific genetic messages encoding different L-type Ca2+ channel isoforms. 2. The macroscopic Ba2+ current (recorded in 10 mM BaCl2) revealed two components with different time courses of activation. The fast component (IBa,fast) activates with a time constant of 23 +/- 12 ms (at +10 mV), while the slow component activates with a time constant of 125 +/- 12 ms (at +10 mV). 3. Single-channel recordings revealed the presence of two independent channels with conductance values of 11 and 25 pS (in 70 mM Ba2+). These values are identical to those reported previously for skeletal muscle and cardiac Ca2+ channels, respectively. 4. The mean ensemble current from the 11 pS channel reproduced the time course of the slow component observed at the macroscopic level, while the 25 pS ensemble time course paralleled that of the fast component. 5. Reverse transcriptase polymerase chain reaction (PCR) with alpha 1-isoform-specific primers revealed the presence of two distinct transcripts in H9c2 cells. The sequences of the PCR products showed a high degree of homology with the corresponding segments of the rabbit cardiac and skeletal muscle L-type Ca2+ channel isoforms. Adult rat skeletal and cardiac muscle expressed only one type of transcript. 6. H9c2 cells appear to be unique in that they simultaneously express both skeletal muscle and cardiac isoforms of the L-type Ca2+ channel alpha 1-subunit. Thus, the H9c2 cell line may prove to be useful when studying the regulation of subtype-specific Ca2+ channel gene expression.

Animals↗
PubMed1994

Mechanism of force inhibition by 2,3-butanedione monoxime in rat cardiac muscle: roles of [Ca2+]i and cross-bridge kinetics.

1. We investigated the mechanism of force inhibition by 2,3-butanedione monoxime (BDM) on rat cardiac trabeculae. [Ca2+]i was measured by iontophoretic injection of fura-2 salt. Isometric force was recorded at an end-systolic sarcomere length of 2.1-2.2 microns. 2. With an external [Ca2+] of 1 mM, peak twitch force was monotonically reduced with increasing [BMD]; at 5 and 20 mM [BDM], force was 35 and 1% of the control force. In contrast, the mean peak [Ca2+]i during transients was only reduced at [BDM] > or = 10 mM. 3. The duration of the twitch was dramatically reduced by BDM in a dose-dependent fashion with no significant change in the time course of the underlying Ca2+ transients. The abbreviation of twitch force duration was much greater than expected for the observed reduction in peak force by this agent. 4. The mechanism of the inhibition of force by BDM was explored by examining the relationship between twitch force and Ca2+ transients at various values of external [Ca2+]. In the presence of BDM, the steepness of the relationship between peak force and peak [Ca2+]i was reduced compared to control conditions. As a result, significant elevation in the [Ca2+]i transient was unable to reverse the reduction in force observed in the presence of BDM. 5. The direct inhibitory effects of BDM on the contractile system were examined using ryanodine tetani in intact trabeculae to measure the steady-state force-[Ca2+]i relationship. In contrast to the effects on twitch force at 5 mM BDM, maximal force was only reduced to 71% of control. Furthermore, the [Ca2+]i required for half-maximal activation (Ca50) was increased while the Hill coefficient was reduced slightly by BDM. 6. BDM dramatically slowed the rate of rise of tetanic force. At maximal activation, the time required to reach 90% maximal force was prolonged by a factor of 3-8 in the presence of 5 mM BDM. This suggests that the observed reduction in twitch force and steady-state force may result from slowed kinetics of cross-bridge attachment, consistent with recent biochemical studies. 7. The contribution of altered cross-bridge kinetics to the effects of BDM was investigated using a co-operative cross-bridge model of the contractile system. Changing the rate constants for cross-bridge attachment in the model to mimic the reported biochemical effects of BDM reproduced the observed effects of BDM.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

Control of steady-state intracellular pH in intact perfused ferret hearts.

To elucidate the role of bicarbonate-dependent mechanisms and Na+/H+ exchange in maintenance of physiological intracellular pH (pHi) under various steady-state conditions, phosphorus NMR spectra were taken in isovolumically-contracting, perfused ferret hearts. Switching the perfusate from HCO3-/CO2 to HEPES buffer significantly decreased pHi from the control value of 7.14 +/- 0.01 (mean +/- S.E., n = 4) to 7.08 +/- 0.01 (P < 0.01). Exposure to 4-acetamide-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS; 10(-4) M) a blocker of anion exchange (7.12 +/- 0.01 in control, 7.07 +/- 0.02 with SITS, n = 6, P < 0.05), led to acidification of pHi. Ethylisopropylamiloride (EIPA; 10(-6) M), a blocker of the Na+/H+ exchange, induced a decrease in pHi (7.17 +/- 0.01 in control, 7.11 +/- 0.01 with EIPA, n = 5, P < 0.05). Lowering [Cl-] in the perfusate (7.14 +/- 0.02 in control, 7.09 +/- 0.03 with low-[Cl]o perfusate, n = 5, P < 0.01) also decreased pHi, perhaps by a non-bicarbonate-dependent mechanism. These results indicate that both bicarbonate-dependent mechanisms and Na+/H+ exchange contribute significantly and additively to the maintenance of physiological pHi in isovolumically contracting, perfused hearts.

Animals↗
PubMed1994

Sudden cardiac death in heart failure. The role of abnormal repolarization.

Congestive heart failure is a common, highly lethal cardiovascular disorder claiming over 200,000 lives a year in the United States alone. Some 50% of the deaths in heart failure patients are sudden, and most of these are probably the result of ventricular tachyarrhythmias. Methods designed to identify patients at risk have been remarkably unrewarding, as have attempts to intervene and prevent sudden death in these patients. The failure to impact favorably on the incidence of sudden death in heart failure patients stems largely from a lack of understanding of the underlying mechanisms of arrhythmogenesis. This article explores the role of abnormalities of ventricular repolarization in heart failure patients. We will examine evidence for the hypothesis that alteration of repolarizing K+ channel expression in failing myocardium predisposes to abnormalities in repolarization that are arrhythmogenic. The possible utility of novel electrophysiological and ECG measures of altered ventricular repolarization will be explored. Understanding the mechanism of sudden death in heart failure may lead to effective therapy and more accurate identification of patients at greatest risk.

Action Potentials↗
PubMed1994

Myofilament Ca2+ sensitivity in intact versus skinned rat ventricular muscle.

The Ca2+ sensitivity of myofilaments was compared before and after skinning in the same rat trabeculae at a diastolic sarcomere length of 2.2 to 2.3 microns. Trabeculae from rat right ventricle were loaded with fura-2 salt by iontophoretic microinjection, and [Ca2+]i was determined from the epifluorescence at 510 nm when excited at 340 and 380 nm. Steady-state activation was achieved by stimulating the muscle at 10 Hz after 10 to 20 minutes of application of ryanodine (5 mumol/L). The muscles were then skinned with Triton X-100 (1%) for 15 to 25 minutes and subsequently activated with solutions containing varied [Ca2+]. The intact force-[Ca2+] relation was highly cooperative (Hill coefficient, 4.87 +/- 0.35; n = 10), with a low [Ca2+]i required for half-maximal activation (K1/2) (0.62 +/- 0.03 mumol/L). After skinning, the Hill coefficient fell to 2.72 and the K 1/2 shifted rightward to 2.2 mumol/L in the presence of 1.2 mmol/L free Mg2+. Because of uncertainty regarding the appropriate [Mg2+], we measured [Mg2+]i at 0.72 +/- 0.06 mmol/L (n = 11) with Mg-fura-2 salt. When activating solutions were modified to contain [Mg2+] = 0.5 mmol/L, the force-[Ca2+] relation was shifted to the left (K 1/2 = 0.93 +/- 0.1, n = 10) with a Hill coefficient of 3.75 +/- 0.37, but the changes were not sufficient to superimpose with the intact force-[Ca2+] relation (P < .05 versus intact). These results suggest that, despite the significant effect of Mg2+ on the force-[Ca2+] relation in skinned muscles, the Ca2+ responsiveness of the myofilaments is still altered by skinning.(ABSTRACT TRUNCATED AT 250 WORDS)

Actin Cytoskeleton↗
PubMed1994

Mechanism of the diastolic dysfunction induced by glycolytic inhibition. Does adenosine triphosphate derived from glycolysis play a favored role in cellular Ca2+ homeostasis in ferret myocardium?

Several lines of evidence indicate that glycolysis is especially important for normal diastolic relaxation and for the maintenance of cellular ion homeostasis in myocardium. To elucidate whether the glycolytic flux of ATP contributes to diastolic tone and to the regulation of intracellular Ca2+, myocardial content of sugar phosphates ([SP]) and intracellular Ca2+ concentration ([Ca2+]i) were measured in isolated, perfused ferret hearts using nuclear magnetic resonance. Glucose and acetate were used as substrates for glycolysis and oxidative phosphorylation, respectively. Glycogen was effectively depleted after 15-min perfusion with glucagon (2 mg/liter), as verified by the lack of rise in [SP] during exposure to iodoacetate (100 microM) in substrate-free perfusate. Despite the fact that glycolytic flux had been blocked both by iodoacetate and by absence of substrate, end-diastolic left ventricular pressure (EDP) remained unchanged (P > 0.15, n = 6). The subsequent addition of glucose to the perfusate led to SP accumulation and a marked rise in EDP, with a significant correlation between EDP and [SP] (r = 0.86 +/- 0.04, P < 0.01, n = 6). A similar correlation was observed when glucose in the perfusate was replaced by 2-deoxyglucose (r = 0.78 +/- 0.09, P < 0.01, n = 3). Fluorine nuclear magnetic resonance measurements of [Ca2+]i verified that EDP faithfully reports changes in diastolic [Ca2+]i under the present experimental conditions. Thus, intracellular Ca2+ overload is caused by the accumulation of SP rather than by the inhibition of glycolysis per se. Glycolysis may appear to be important because its by-products are deleterious, and not necessarily because glycolytically derived ATP plays a favored role in ion homeostasis.

Adenosine Triphosphate↗