Impaired beta-adrenergic response and decreased L-type calcium current of hypertrophied left ventricular myocytes in postinfarction heart failure

Infarct-induced heart failure is usually associated with cardiac hypertrophy and decreased beta-adrenergic responsiveness. However, conflicting results have been reported concerning the density of L-type calcium current (I Ca(L)), and the mechanisms underlying the decreased beta-adrenergic inotropic response. We determined I Ca(L) density, cytoplasmic calcium ([Ca2+]i) transients, and the effects of beta-adrenergic stimulation (isoproterenol) in a model of postinfarction heart failure in rats. Left ventricular myocytes were obtained by enzymatic digestion 8-10 weeks after infarction. Electrophysiological recordings were obtained using the patch-clamp technique. [Ca2+]i transients were investigated via fura-2 fluorescence. beta-Adrenergic receptor density was determined by [ H]-dihydroalprenolol binding to left ventricle homogenates. Postinfarction myocytes showed a significant 25 percent reduction in mean I Ca(L) density (5.7 + or - 0.28 vs 7.6 + or - 0.32 pA/pF) and a 19 percent reduction in mean peak [Ca2+]i transients (0.13 + or - 0.007 vs 0.16 + or - 0.009) compared to sham myocytes. The isoproterenol-stimulated increase in I Ca(L) was significantly smaller in postinfarction myocytes (Emax: 63.6 + or - 4.3 vs 123.3 + or - 0.9 percent in sham myocytes), but EC50 was not altered. The isoproterenol-stimulated peak amplitude of [Ca2+]i transients was also blunted in postinfarction myocytes. Adenylate cyclase activation through forskolin produced similar I Ca(L) increases in both groups. beta-Adrenergic receptor density was significantly reduced in homogenates from infarcted hearts (Bmax: 93.89 + or - 20.22 vs 271.5 + or - 31.43 fmol/mg protein in sham myocytes), while Kd values were similar. We conclude that postinfarction myocytes from large infarcts display reduced I Ca(L) density and peak [Ca2+]i transients. The response to Beta-adrenergic stimulation was also reduced and was probably related to Beta-adrenergic receptor down-regulation and not to changes in adenylate cyclase activity

Impaired beta-adrenergic response and decreased L-type calcium current of hypertrophied left ventricular myocytes in postinfarction heart failure.

Infarct-induced heart failure is usually associated with cardiac hypertrophy and decreased -adrenergic responsiveness. However, conflicting results have been reported concerning the density of L-type calcium current (I Ca(L)), and the mechanisms underlying the decreased -adrenergic inotropic response. We determined I Ca(L) density, cytoplasmic calcium ([Ca2+]i) transients, and the effects of -adrenergic stimulation (isoproterenol) in a model of postinfarction heart failure in rats. Left ventricular myocytes were obtained by enzymatic digestion 8-10 weeks after infarction. Electrophysiological recordings were obtained using the patch-clamp technique. [Ca2+]i transients were investigated via fura-2 fluorescence. -Adrenergic receptor density was determined by [ H]-dihydroalprenolol binding to left ventricle homogenates. Postinfarction myocytes showed a significant 25% reduction in mean I Ca(L) density (5.7 0.28 vs 7.6 0.32 pA/pF) and a 19% reduction in mean peak [Ca2+]i transients (0.13 0.007 vs 0.16 0.009) compared to sham myocytes. The isoproterenol-stimulated increase in I Ca(L) was significantly smaller in postinfarction myocytes (Emax: 63.6 4.3 vs 123.3 0.9% in sham myocytes), but EC50 was not altered. The isoproterenol-stimulated peak amplitude of [Ca2+]i transients was also blunted in postinfarction myocytes. Adenylate cyclase activation through forskolin produced similar I Ca(L) increases in both groups. -Adrenergic receptor density was significantly reduced in homogenates from infarcted hearts (Bmax: 93.89 20.22 vs 271.5 31.43 fmol/mg protein in sham myocytes), while Kd values were similar. We conclude that postinfarction myocytes from large infarcts display reduced I Ca(L) density and peak [Ca2+]i transients. The response to -adrenergic stimulation was also reduced and was probably related to -adrenergic receptor down-regulation and not to changes in adenylate cyclase activity.

Sera from chronic chagasic patients depress cardiac electrogenesis and conduction.

We report results obtained with sera from 58 chronic chagasic patients that were evaluated for effects on heart rate and atrioventricular (AV) conduction in isolated rabbit hearts and screened for the presence of muscarinic and beta-adrenergic activity. We show that sera from 26 patients decreased heart rate, while 10 increased it and 22 had no effect. Additionally, sera from 20 of the 58 patients blocked AV conduction. Muscarinic activation seems to be involved in both effects, but is not the only mechanism, since atropine did not antagonize the decrease in heart rate in 23% of sera or AV block in 40%. Sera from patients with complex arrhythmias were significantly more effective in depressing both heart rate and AV conduction. Sera that induce increases in heart rate seem to operate exclusively through beta-adrenergic activation. Two of these sera, evaluated with respect to intercellular communication in primary cultures of embryonic cardiomyocytes were able to block gap junction conductance evaluated by a dye injection technique after 24-h exposure. The mechanisms underlying this uncoupling effect are currently being investigated.

Sera from chronic chagasic patients depress cardiac electrogenesis and conduction

We report results obtained with sera from 58 chronic chagasic patients that were evaluated for effects on heart rate and atrioventricular (AV) conduction in isolated rabbit hearts and screened for the presence of muscarinic and beta-adrenergic activity. We show that sera from 26 patients decreased heart rate, while 10 increased it and 22 had no effect. Additionally, sera from 20 of the 58 patients blocked AV conduction. Muscarinic activation seems to be involved in both effects, but is not the only mechanism, since atropine did not antagonize the decrease in heart rate in 23 percent of sera or AV block in 40 percent. Sera from patients with complex arrhythmias were significantly more effective in depressing both heart rate and AV conduction. Sera that induce increases in heart rate seem to operate exclusively through beta-adrenergic activation. Two of these sera, evaluated with respect to intercellular communication in primary cultures of embryonic cardiomyocytes were able to block gap junction conductance evaluated by a dye injection technique after 24-h exposure. The mechanisms underlying this uncoupling effect are currently being investigated

Trypanosoma cruzi-cardiomyocytes: new contributions regarding a better understanding of this interaction.

The present paper summarizes new approaches regarding the progress done to the understanding of the interaction of Trypanosoma cruzi-cardiomyocytes. Mannose receptors localized at the surface of heart muscle cell are involved in binding and uptake of the parasite. One of the most striking events in the parasite-heart muscle cells interaction is the disruption of the actin cytoskeleton. We have investigated the regulation of the actin mRNA during the cytopathology induced in myocardial cells by the parasite. T. cruzi invasion increases calcium resting levels in cardiomyocytes. We have previously shown that Ca2+ ATPase of the sarcoplasmic reticulum (SERCA) is involved in the invasion of T. cruzi in cardiomyocytes. Treating the cells with thapsigargin, a drug that binds to all SERCA ATPases and causes depletion of intracellular calcium stores, we found a 75% inhibition in the T. cruzi-cardiomyocytes invasion.

Functionally active cardiac antibodies in chronic Chagas' disease are specifically blocked by Trypanosoma cruzi antigens.

Antibodies of chronic chagasic patients have been shown to interfere with electric and mechanical activities of cardiac embryonic myocytes in culture and with whole mammalian hearts. A mechanism proposed for this effect involves interaction of the antibodies with G-protein-linked membrane receptors, thus leading to activation of beta adrenergic and muscarinic receptors; more specifically, IgG of chagasic patients would interact with the negatively charged regions of the second extracellular loop of these receptors. We performed competition experiments to test this hypothesis. We evaluated the effect of sera/IgG from patients previously known to depress electrogenesis and/or atrioventricular conduction in isolated rabbit hearts after incubation with live and lysed parasites, the peptide corresponding to the second extracellular loop (O2) of the M2 receptor, and different peptides derived from two ribosomal proteins of T. cruzi: P0 and P2beta. Our results indicate that 1) the antigenic factor inducing the functionally active IgGs in the chagasic patients is probably an intracellular T. cruzi antigen; 2) IgG/serum is interacting with the O2 region of the M2 receptor in the rabbit heart; and 3) the negative charges present in the ribosomal proteins of T. cruzi are important in mediating the interaction between the patients' serum/IgG and the receptor.

Outward potassium current oscillations in macrophage polykaryons: extracellular calcium entry and calcium-induced calcium release

Outward current oscillations associated with transient membrane hyperpolarizations were induced in murine macrophage polykaryons by membrane depolarization in the absence of external Na+. Oscillations corresponded to a cyclic activation of Ca2+ -dependent K+ currents (IKCa) probably correlated with variations in intracellular Ca2+ concentration. Addition of external Na+ (8mM) immediately abolished the outward current oscillations, suggesting that the absence of the cation is necessary not only for their induction but also for their maintenance. Oscillations were completely blocked by nisoldipine. Ruthenium red and ryanodine reduced the number of outward current cycles in each episode, whereas quercetin prolonged the hyperpolarization 2- to 15-fold. Neither low molecular weight heparin nor the absence of a Na+ gradient across the membrane had any influence on oscillations. The evidence suggests that Ca+ entry through a pathway sensitive to Ca2+ channel blockers is elicited by membrane depolarization in Na+ -free medium and is essential to initiate oscillations, which are also dependent on the cyclic release of Ca2+ from intracellular Ca2+ -sensitive stores; Ca2+ ATPase acts by reducing intracellular Ca2+, thus allowing slow deactivation of IKCa. Evidence is presented that neither a Na+/Ca2+ antiporter nor Ca2+ release from IP3 -sensitive Ca2+ stores participate directly in the mechanism of oscillation.

Sera from chronic chagasic patients with complex cardiac arrhythmias depress electrogenesis and conduction in isolated rabbit hearts.

BACKGROUND: Immune dysfunction has long been proposed as a mechanism for the etiopathogenesis of the chronic phase of Chagas' disease. Antibodies of chagasic patients have been shown to interfere with electric and mechanical activity of embryonic myocardial cells in culture. Here, we demonstrate that antibodies derived from a group of chronic chagasic patients are able to induce disturbances in the electrogenesis and conduction in isolated adult rabbit hearts. METHODS AND RESULTS: Sera from chronic chagasic patients with complex cardiac arrhythmias (ChA+) decreased heart rate (from 131+/-26 to 98+/-37 bpm [mean+/-SD]; n=6; P<.05) in isolated rabbit hearts when perfused at a dilution of 1:100 (vol:vol) by the Langendorff method. Sera from another experimental group of four chronic chagasic patients without complex arrhythmias (ChA-) and two control groups composed of five Wolff-Parkinson-White (WPW) syndrome patients and five orthopedic surgery patients did not affect heart rate when tested under similar conditions. In addition, sera from five of six ChA+ patients and from one WPW patient induced AV conduction blockade. Effects of the sera from ChA+ patients are due to their IgG fractions. Both serum and IgG effects are blocked by atropine (10 micromol/L). CONCLUSIONS: Antibodies of ChA+ patients decrease heart rate and induce AV conduction block in isolated adult rabbit hearts through activation of muscarinic receptors.

Outward potassium current oscillations in macrophage polykaryons: extracellular calcium entry and calcium-induced calcium release.

Outward current oscillations associated with transient membrane hyperpolarizations were induced in murine macrophage polykaryons by membrane depolarization in the absence of external Na+. Oscillations corresponded to a cyclic activation of Ca(2+)-dependent K+ currents (IKCa) probably correlated with variations in intracellular Ca2+ concentration. Addition of external Na+ (8 mM) immediately abolished the outward current oscillations, suggesting that the absence of the cation is necessary not only for their induction but also for their maintenance. Oscillations were completely blocked by nisoldipine. Ruthenium red and ryanodine reduced the number of outward current cycles in each episode, whereas quercetin prolonged the hyperpolarization 2- to 15-fold. Neither low molecular weight heparin nor the absence of a Na+ gradient across the membrane had any influence on oscillations. The evidence suggests that Ca2+ entry through a pathway sensitive to Ca2+ channel blockers is elicited by membrane depolarization in Na(+)-free medium and is essential to initiate oscillations, which are also dependent on the cyclic release of Ca2+ from intracellular Ca(2+)-sensitive stores; Ca2+ ATPase acts by reducing intracellular Ca2+, thus allowing slow deactivation of IKCa. Evidence is presented that neither a Na+/Ca2+ antiporter nor Ca2+ release from IP3-sensitive Ca2+ stores participate directly in the mechanism of oscillation.

Ventricular action potential and L-type calcium channel in infarct-induced hypertrophy in rats.

INTRODUCTION: The present investigation was aimed at characterization of: (1) action potential parameters; and (2) L-type calcium channels in the hypertrophied ventricular tissue surviving an extensive healed myocardial infarction in the rat. METHODS AND RESULTS: Myocardial infarction was produced in Wistar rats by ligation of the left coronary artery. One to 2 months later, their hearts were subjected to electrophysiologic study. The main difference in subendocardial transmembrane potentials recorded with intracellular microelectrodes was an increase in action potential duration (APD). In the left ventricle, the infarcted/sham-operated APD ratio ranged from 2.7 to 7.2, whereas in the right ventricle it ranged from 1.6 to 2.3 in different regions. When compared with control cells, ventricular myocytes from infarcted hearts were found to be larger (P < 0.01) and showed a reduction (P < 0.05) in L-type calcium current (LCa,L) density obtained by whole cell, patch clamp (at 0 mV: 4.44 +/- 0.41 in infarcted vs 8.03 +/- 1.22 pA/pF in normal). The time course of decay of the currents could be fitted by two exponential functions in both normal and infarcted hearts. There was a tendency toward an increase in the time constant of the slower component of inactivation, tau 2, significant only at +20 mV (215 +/- 25 vs 151 +/- 15 msec). CONCLUSIONS: Cardiac hypertrophy of healed infarction in rats is associated with lengthening of the action potential in both ventricles. The main alteration observed in ICa,L was a decrease in the current density. Thus, alteration of the calcium channel is not the determinant factor of APD increase.

Conduction defects and arrhythmias in Chagas' disease: possible role of gap junctions and humoral mechanisms.

The protozoan parasite Trypanosoma cruzi causes Chagas' disease, a major cause of cardiac dysfunction in Latin Americans. Chagas' disease exhibits both acute and chronic phases, and each may be characterized by cardiac conduction disturbances. In acutely infected cultures of rodent heart cells, synchronized spontaneous beating becomes less regular, and coupling between cells is reduced. The basis of this decreased conduction is apparently in localization of the gap junction protein (Cx43) inside infected cells. Although total Cx43 is normal in infected cells, little is recognizable at appositional membranes. Electrophysiological properties are also altered by this infection. Action potentials are shortened, resting Ca2+ levels are elevated, and response to alpha-adrenergic agonists was altered, compared to controls. Humoral factors may contribute to the conduction defects in chronic Chagas' disease. Sera from chronically infected rabbits produced ECG abnormalities in Langendorff-perfused rabbit hearts. These findings indicate that chagasic infection may modify ion channel function in the heart, and we suggest that these changes may be manifested in the conduction disturbances that characterize this disease.

Effects of extracellular sodium, quinidine, and ryanodine on slow response excitability in rabbit atrial trabeculae.

1. We investigated Na(+)-Ca2+ exchange and the involvement of the sarcoplasmic reticulum in frequency-dependent slow response excitability enhancement in rabbit atrial trabeculae. 2. Slow responses were induced in a modified Tyrode solution containing high K+ and Ba2+ and conventional electrophysiological techniques were used for stimulating and recording membrane potentials. 3. Under these conditions, the frequency-dependence of slow response excitability can be demonstrated with excitability enhancement as stimulation frequency is increased (0.25 to 1.0 Hz). 4. The frequency-dependent excitability enhancement depends on external Na+, increasing in high-[Na+]o (173.8 mM) and decreasing in low-[Na+]o (103.8 mM) media. 5. Quinidine (10 microM) and ryanodine (10 microM) decrease frequency-dependent slow response excitability enhancement. 6. These results indicate that the Na(+)-Ca2+ exchange might have an important role in frequency-dependent excitability enhancement of slow responses. Moreover, we suggest that the control of internal Ca2+ by the sarcoplasmic reticulum might have an additional role in regulating the excitability enhancement process in depolarized atrial trabeculae.

Effects of extracellular sodium, quinidine, and ryanodine on slow response excitability in rabbit atrial trabeculae

1. We investigated Na(+)-Ca2+ exchange and the involvement of the sarcoplasmic reticulum in frequency-dependent slow response excitability enhancement in rabbit atrial trabeculae. 2. Slow responses were induced in a modified Tyrode solution containing high K+ and Ba2+ and conventional electrophysiological techniques were used for stimulating and recording membrane potentials. 3. Under these conditions, the frequency-dependence of slow response excitability can be demonstrated with excitability enhancement as stimulation frequency is increased (0.25 to 1.0 Hz). 4. The frequency-dependent excitability enhancement depends on external Na+, increasing in high-[Na+]o (173.8 mM) and decreasing in low-[Na+]o (103.8 mM) media. 5. Quinidine (10 microM) and ryanodine (10 microM) decrease frequency-dependent slow response excitability enhancement. 6. These results indicate that the Na(+)-Ca2+ exchange might have an important role in frequency-dependent excitability enhancement of slow responses. Moreover, we suggest that the control of internal Ca2+ by the sarcoplasmic reticulum might have an additional role in regulating the excitability enhancement process in depolarized atrial trabeculae

Heart muscle cells acutely infected with Trypanosoma cruzi: characterization of electrophysiology and neurotransmitter responses.

Primary cell cultures of mouse ventricular myocardium were infected with Trypanosoma cruzi, to study the consequences of T. cruzi-muscle cell interaction on the rate of spontaneous contractions, on the responses to norepinephrine, and on action potential parameters. Single cells or small cell groups of infected cultures were subjected to pharmacological and electrophysiological experiments. In concentrations ranging from 1 nM to 100 microM, norepinephrine exerted positive or negative chronotropic effects mediated by alpha-adrenergic receptors. A significant number of infected cells (25%) did not respond to the agonist. Two days after infection the cultures exhibited a higher frequency of spontaneous contractions (20%), paralleled by an increase in firing rate and a decrease in the action potential duration without significant changes in maximum diastolic potential and action potential amplitude. A decrease in alpha-adrenergic receptor-mediated positive chronotropic response to norepinephrine was also observed in 2-day infected cells. Cells made to phagocyte ferritin particles showed an increase in the rate of spontaneous contractions, but no changes in the positive chronotropic responses to norepinephrine. In conclusion, these observations show that during acute infection with T. cruzi, there are alterations in automaticity and in the chronotropic responses to norepinephrine, whose mechanisms are related to the process of parasite endocytosis by the cardiac cells.

Electrophysiological effects of somatostatin at the supraventricular level of isolated guinea pig hearts.

1. The objective of the present study was to evaluate the electrophysiological effects of the peptide somatostatin (SST) at the supraventricular level in isolated guinea pig hearts. 2. ECG recording from isolated hearts perfused by the Langendorff method indicated that 1.0 microM SST induced a decrease in heart rate from 174 +/- 15 to 157 +/- 9 bpm (N = 6, P < 0.05), blocked AV conduction (the PR interval increased from 92 +/- 11 ms to 106 +/- 5 ms, N = 5, P < 0.05) and increased the QTc interval from 210 +/- 0 to 232 +/- 4 ms (N = 5, P < 0.05). The supraventricular effects of SST, particularly upon the AV conduction, were potentiated by a reduction in calcium concentration from 2.5 to 0.5 mM in the perfusing solution. Thus, 1.0 microM SST induced 2nd degree AV conduction block progressing to AV dissociation in 75% of the hearts in the low calcium medium instead of the first degree conduction block observed in all hearts in normal calcium medium. 3. His bundle electrogram evidenced a complete A-H dissociation without significant change in the H-V interval and microelectrode studies showed a complete abolition of the AV node action potential in the presence of 1.0 microM SST. Both results demonstrate that the site of AV conduction block induced by SST is at the AV node. 4. All the supraventricular effects of SST were transitory, subsiding within about 10 min of hormone exposition, showing desensitization. 5. The effects of somatostatin here described were not blocked by 10 microM atropine, indicating that they are not mediated by muscarinic receptors. 6. These data provide a direct electrophysiological demonstration of the supraventricular effects of SST, and suggest that this peptide decreases calcium influx during the action potential.

Electrophysiological effects of somatostatin at the supraventricular level of isolated guinea pig hearts

The objective of the present study was to evaluate the electrophysiological effects of the peptide somatostatin (SST) at the supraventricular level in isolated guinea hearts. ECG recording from isolated hearts perfused by the Langendorff method i9ndicating that 1.0 uM SST induced a decrease in heart rate from 174 ñ 15 to 157 ñ 9 bpm (N=6, P<0.05), blocked AV conductio9n (the PR interval increased from 92 ñ 11 ms to 106 ñ 5 ms, N+5, P<0.05) and increased the QTc interval from 210 ñ 0 to 232 ñ 4 ms (N+5, P<0.05). The supraventricular effect of SST, particulary upon the AV conduction , were potentiated by a reduction in calcium concentration from 2.5 to 0.5 mM in the perfusing solution. Thus, 1.0 uM SST induced 2nd degree AV conduction block progressing to AV dissociation in 75% of thye hearts in the low calcium medium instead of the first degree conduction block observed in all hearts in normal calcium medium. His bundle electrogram evidence a complete A-H dissociation withouth significant change in the H-V interval and microelectrode studies showed a complete abolition of the AV node action potential in the presence of 1.0 uM SST. Both results demonstrate that the site of AV conduction block induced by SST is at the AV node. All the supraventricular effects of SST were transitory, subsiding within abouth 10 min of hormone exposition, showing desensitization. The effects of somatostatin here described were not blocked by 10 uM atropine, indicating that they are not mediated by muscarinic receptors. These data provide a direct electrophysiological demonstration of the supraventricular effects of SST, and suggest that this peptide decreases calcium influx during the action potential

The electrocardiogram of rats with an old extensive myocardial infarction.

The electrocardiographic alterations of old murine myocardial infarction have not been well characterized. In the present study, adult Wistar rats of both sexes were infarcted by left coronary artery ligation and the electrocardiogram was recorded 1 to 11 months later. When compared to sham-operated rats, animals with large infarcts, identified on the basis of extensive transmural scars, showed (P less than 0.01) a marked rightward deviation of the QRS axis (+125.3 degrees +/- 34.3 degrees vs +59.9 degrees +/- 15.9 degrees), a high incidence of Q waves (88% vs 0% in classic lead 1), a decrease in QRS amplitude index (0.66 +/- 0.31 mV vs 1.00 +/- 0.23 mV), a discrete increase in PR interval (58 +/- 7 ms vs 53 +/- 5 ms) and greater P wave amplitude. The present results show that the electrocardiogram (EKG) is a reliable tool for diagnosis of old extensive infarctions in rats.

Frequency-dependent excitability enhancement in isolated ventricular myocardial cells.

The understanding of the mechanisms underlying the frequency-dependent slow response excitability enhancement has been hindered by the problems inherent in multicellular preparations. These include ion accumulation/depletion in intercellular spaces and difficulties in the spatial control of transmembrane voltage. In the present communication we show that isolated ventricular cells exposed to a depolarizing (high potassium-barium containing) solution present electrophysiological properties similar to those of multicellular preparations: stable resting potential of -45.2 +/- 0.7 mV (mean +/- SEM, N = 57) in 75% of the cells and spontaneous activity in the remaining 25% (maximum diastolic potential of -41.9 +/- 1.2 mV, N = 19); high input resistance and slow response, under current clamp conditions. Under whole cell voltage clamp conditions with -45 mV holding potential, transient outward and delayed potassium currents as well as typical L type calcium channel are present. These cells also present the frequency-dependent excitability enhancement of the slow response, with the threshold stimulus at 1 Hz corresponding to about 50% of that obtained at 0.1 Hz. Thus, isolated ventricular cells constitute a suitable model for the study of frequency-dependent excitability enhancement of the slow response.

The electrocardiogram of rats with an old extensive myocardial infarction

The electrocardiographic alterations of old murine myocardial infarction have not been well characterized. In the present study, adult Wistar rats, of both sexes were infarcted by left coronary artery ligation and the electrocardiogram was recorded 1 to 11 months later. When compared to sham-operated rats, animals with large infarcts, identified on the basis of extensive transmural scars, showed (P<0.01) a marked rightward deviation of the QRS axis (+125.3- ñ 34.3- vs 59.99- ñ 15.9-), a high incidence of Q waves (88% vs 0% in classic lead 1), a decrease in QRS amplitude index (0.66 ñ 0.31 mV vs 1.00 ñ 0.23 mV), a discrete increse in PR interval (58 ñ 7 ms vs 53 ñ 5ms) and greater P wave amplitude. The present results show that the electrocardiogram (EKG) is a reliable tool for diagnosis of old extensive infarctions in rats

Frequency-dependent excitability enhancement in isolated ventricular myocardial cells

The understanding of the mechanisms underling the frquency-dependent slow response ecitability enhancement has been hndered by the problem inhyerent in multicellular preparations. These include ion acdcumulation/depletion in intercellular space and difficulties in the spatial control of transmembrane voltage. In the present communication we show that isolated ventricular cells exposed to a depolarizing (high potassium-barium containing) solution present electrophysiological properties similar to those of mulcellular preparations: stable resting potential of -45.2 ñ 0.7 mV (mean ñ SEM, N = 57) in 75% of the cells and spontaneous activity in the remaining 25% (maximum diatolic potential of -41.9 ñ 1.2 mV, N=19)ñ high input resistance and slow response, under current clamp conditions. Under whole cell voltage clamp conditions with -45 mV holding potential, transient outward and delayed potassium currents as well as typical L type calcium channel are present. These cells also present thye frequency-dependent excitability enhancement of the slow response, with the threshold stimulus at 1 Hz corresponding to about 50% of that obtained at 0.1 Hz. Thus, isolated ventricular cells constitute a suitable model for the study of frequency-dependent exitability enhancement of the slow response