Spontaneously active cells isolated from the sino-atrial and atrio-ventricular nodes of the rabbit heart

Multiple ion channel block by the cation channel inhibitor SKF-96365 in myocytes from the rabbit atrioventricular node

The atrioventricular node (AVN) of the cardiac conduction system coordinates atrial-ventricular excitation and can act as a subsidiary pacemaker. Recent evidence suggests that an inward background sodium current, IB,Na, carried by nonselective cation channels (NSCCs), contributes to AVN cell pacemaking. The study of the physiological contribution of IB,Na has been hampered, however, by a lack of selective pharmacological antagonists. This study investigated effects of the NSCC inhibitor SKF-96365 on spontaneous activity, IB,Na, and other ionic currents in AVN cells isolated from the rabbit. Whole-cell patch-clamp recordings of action potentials (APs) and ionic currents were made at 35-37°C. A concentration of 10 μmol/L SKF-96365 slowed spontaneous action potential rate by 13.9 ± 5.3% (n = 8) and slope of the diastolic depolarization from 158.1 ± 30.5 to 86.8 ± 30.5 mV sec(-1) (P < 0.01; n = 8). Action potential upstroke velocity and maximum diastolic potential were also reduced. Under IB,Na-selective conditions, 10 μmol/L SKF-96365 inhibited IB,Na at -50 mV by 36.1 ± 6.8% (n = 8); however, effects on additional channel currents were also observed. Thus, the peak l-type calcium current (ICa,L) at +10 mV was inhibited by 38.6 ± 8.1% (n = 8), while the rapid delayed rectifier current, IKr, tails at -40 mV following depolarization to +20 mV were inhibited by 55.6 ± 4.6% (n = 8). The hyperpolarization-activated current, If, was unaffected by SKF-96365. Collectively, these results indicate that SKF-96365 exerts a moderate inhibitory effect on IB,Na and slows AVN cell pacemaking. However, additional effects of the compound on ICa,L and IKr confound the use of SKF-96365 to dissect out selectively the physiological role of IB,Na in the AVN.

Beneficial Effect of Eplerenone on Cardiac Remodelling and Electrical Properties of the Failing Heart

Introduction. The effect of chronic administration of eplerenone on cardiac remodelling and electrical properties was investigated in the failing heart of cardiomyopathic hamsters (TO-2) at five months of age.

Materials and Methods. Two-month-old hamsters were treated with eplerenone (200 mg/kg/day) administered into the chow for a period of three months. Measurements of membrane potential were performed with intracellular microelectrodes connected to a high impedance DC amplifier. The thickness of the ventricular wall as well as the area of fibrosis were measured. To investigate the influence of eplerenone on the electrogenic sodium pump myocytes were isolated from the ventricle and the pump current density was whole cell clamp configuration. measured in voltage clamped cells using the whole cell clamp configuration

Results. The results indicated that: 1) the width of the left and right ventricular wall was significantly reduced; 2) the heart weight/body weight ratio was decreased by 38±2.4% (n=24 P ) (p<0.05); 3) the fibrotic area in the left ventricle (LV) was reduced by 12.6±2% (n=25) (p<0.05); 4) the incidence of cardiac arrhythmias was decreased from 58±3.8% (n=20) in the control to 40±4.1% (n=20) (p<0.05) in animals treated with eplerenone. Moreover, a significant reduction in the dispersion of the QT interval was found with the drug; 5) eplerenone increased the resting potential of ventricular fibres from 64.3±1.5 mV to 73.4±1.4 mV (n=30) (p<0.05), an effect related to the activation of an electrogenic sodium pump. The conduction velocity, in longitudinal direction, was enhanced from 50±2.2 cm/s (n=10) in the controls to 59±2.4 cm/s (n=13) (p<0.05) in animals treated with eplerenone.

Conclusions. Eplerenone reduces cardiac remodelling, the incidence of cardiac arrhythmias and improves impulse propagation, an effect in part related to the antifibrotic effect of the drug but also to the activation of the electrogenic sodium pump.

Angiotensin (1-7) re-establishes impulse conduction in cardiac muscle during ischaemia-reperfusion. The role of the sodium pump

Introduction

The effect of angiotensin (1-7) (Ang 1-7) on membrane potential and excitability of rat heart muscle under ischaemia/reperfusion was investigated.

Materials and methods

The hearts of adult rats were removed under deep anaesthesia and perfused using the Langendorff method. After 40 minutes of global no-flow ischaemia, the heart was reperfused for five minutes and the right ventricle was dissected out and transferred to a transparent chamber, through which normal oxygenated Krebs solution flowed continuously (37°C). Measurements of membrane potential were performed using an intracellular microelectrode connected to a high impedance preamplifier. The muscle was stimulated with rectangular current pulses (3 ms duration; 0.6 Hz) generated by an electronic stimulator and isolation unit. To study the influence of Ang (1-7) on sodium pump current, isolated myocytes were voltageclamped at -40 mV and the current generated by the pump was recorded before and after the administration of Ang (1-7) (10 M)to the bath.

Results

Results

Ang (1-7) (10-8 M) hyperpolarised the ischaemic heart fibre and re-established impulse propagation. The increment of resting potential was related to the activation of the sodium pump. Indeed, Ang (1-7) (10-8 M) enhanced the transient outward current generated by an electrogenic sodium pump. Both effects of Ang (1-7) on membrane potential and pump current were abolished by ouabain (10-7 M). The cardiac refractoriness was also increased by Ang (1-7) (10-8 M).

Conclusions

Ang (1-7) activates the sodium pump, hyperpolarises the heart cell and re-establishes the impulse conduction during ischaemia/reperfusion. These effects of Ang (1-7), and the increment of cardiac refractoriness, provide an explanation for the reduced incidence of arrhythmias during ischaemia/reperfusion in the presence of Ang (1-7).

Beneficial versus harmful effects of Angiotensin (1-7) on impulse propagation and cardiac arrhythmias in the failing heart

Introduction. The presence of Angiotensin (1-7) (Ang 1-7) and ACE 2 in the ventricle of cardiomyopathic hamsters as well as the influence of Ang (1-7) on membrane potential, impulse propagation and cardiac excitability were investigated.

Methods. Histology and immunochemistry were used to demonstrate the presence of Ang (1-7) and ACE 2 in the ventricle of cardiomyopathic hamsters. Measurements of transmembrane potentials, conduction velocity and refractoriness were made using conventional intracellular microelectrodes. The influence of Ang (1-7) on sodium pump current was investigated in voltageclamped myocytes isolated from the ventricle.

Results. The results indicated the presence of Ang (1-7) and ACE 2 in myocytes of cardiomyopathic hamsters. Moreover, Ang (1-7) (10-8 M) hyperpolarised the heart cell, increased the conduction velocity, and I reduced transiently the action potential duration. The cardiac refractoriness was also increased by the heptapeptide, an effect in part reduced by an inhibitor of mas receptor. These findings indicate that Ang (1-7) has important antiarrhythmic properties. However, the beneficial effects of Ang (1-7) are dose-dependent because at higher concentration (10-7 M) the heptapeptide elicited an appreciable increase of action potential duration and early-after depolarisations. Since losartan (10-7 M) did not counteract this effect of the high dose of the heptapeptide, it is possible to conclude that activation of AT1-receptors is not involved in this effect of Ang (1-7).To investigate the mechanism of the hyperpolarising action of Ang (1-7) the influence of the heptapeptide on the sodium potassium pump current was studied in myocytes isolated from the ventricle of cardiomyopathic hamsters. The peak pump current density was measured under voltage clamp using the whole cell configuration. The results indicated that Ang (1-7) (10—8 M) enhanced the electrogenic sodium pump, an effect suppressed by ouabain (10—7 M).

Conclusions. Ang (1-7) has beneficial effects on the failing heart by activating the sodium pump, hyperpolarising the cell membrane and increasing the conduction velocity. These effects as well as the increment of refractoriness indicate that Ang (1-7) has antiarrhythmic properties. At higher concentrations (10—7 M), however, the heptapeptide induced early-after depolarisations which leads to the conclusion that an optimal generation of Ang (1-7) must be achieved to permit a protective role of Ang (1-7) on cardiac arrhythmias.

Characterization and influence of cardiac background sodium current in the atrioventricular node

Background inward sodium current (IB,Na) that influences cardiac pacemaking has been comparatively under-investigated. The aim of this study was to determine for the first time the properties and role of IB,Na in cells from the heart's secondary pacemaker, the atrioventricular node (AVN). Myocytes were isolated from the AVN of adult male rabbits and mice using mechanical and enzymatic dispersion. Background current was measured using whole-cell patch clamp and monovalent ion substitution with major voltage- and time-dependent conductances inhibited. In the absence of a selective pharmacological inhibitor of IB,Na, computer modelling was used to assess the physiological contribution of IB,Na. Net background current during voltage ramps was linear, reversing close to 0mV. Switching between Tris- and Na(+)-containing extracellular solution in rabbit and mouse AVN cells revealed an inward IB,Na, with an increase in slope conductance in rabbit cells at -50mV from 0.54±0.03 to 0.91±0.05nS (mean±SEM; n=61 cells). IB,Na magnitude varied in proportion to [Na(+)]o. Other monovalent cations could substitute for Na(+) (Rb(+)>K(+)>Cs(+)>Na(+)>Li(+)). The single-channel conductance with Na(+) as charge carrier estimated from noise-analysis was 3.2±1.2pS (n=6). Ni(2+) (10mM), Gd(3+) (100μM), ruthenium red (100μM), or amiloride (1mM) produced modest reductions in IB,Na. Flufenamic acid was without significant effect, whilst La(3+) (100μM) or extracellular acidosis (pH6.3) inhibited the current by >60%. Under the conditions of our AVN cell simulations, removal of IB,Na arrested spontaneous activity and, in a simulated 1D-strand, reduced conduction velocity by ~20%. IB,Na is carried by distinct low conductance monovalent non-selective cation channels and can influence AVN spontaneous activity and conduction.

Electrophysiological properties of myocytes isolated from the mouse atrioventricular node: L-type ICa, IKr, If, and Na-Ca exchange

The atrioventricular node (AVN) is a key component of the cardiac pacemaker-conduction system. This study investigated the electrophysiology of cells isolated from the AVN region of adult mouse hearts, and compared murine ionic current magnitude with that of cells from the more extensively studied rabbit AVN. Whole-cell patch-clamp recordings of ionic currents, and perforated-patch recordings of action potentials (APs), were made at 35-37°C. Hyperpolarizing voltage commands from -40 mV elicited a Ba(2+)-sensitive inward rectifier current that was small at diastolic potentials. Some cells (Type 1; 33.4 ± 2.2 pF; n = 19) lacked the pacemaker current, If, whilst others (Type 2; 34.2 ± 1.5 pF; n = 21) exhibited a clear If, which was larger than in rabbit AVN cells. On depolarization from -40 mV L-type Ca(2+) current, IC a,L, was elicited with a half maximal activation voltage (V0.5) of -7.6 ± 1.2 mV (n = 24). IC a,L density was smaller than in rabbit AVN cells. Rapid delayed rectifier (IK r) tail currents sensitive to E-4031 (5 μmol/L) were observed on repolarization to -40 mV, with an activation V0.5 of -10.7 ± 4.7 mV (n = 8). The IK r magnitude was similar in mouse and rabbit AVN. Under Na-Ca exchange selective conditions, mouse AVN cells exhibited 5 mmol/L Ni-sensitive exchange current that was inwardly directed negative to the holding potential (-40 mV). Spontaneous APs (5.2 ± 0.5 sec(-1); n = 6) exhibited an upstroke velocity of 37.7 ± 16.2 V/s and ceased following inhibition of sarcoplasmic reticulum Ca(2+) release by 1 μmol/L ryanodine, implicating intracellular Ca(2+) cycling in murine AVN cell electrogenesis.

Further studies on the effects of intracrine and extracellular angiotensin II on the regulation of heart cell volume. On the influence of aldosterone and spironolactone

The influence of extracellular and intracellular angiotensin II (Ang II) on the cell volume in the failing heart of cardiomyopathic hamsters (TO2) was further investigated as well as the influence of aldosterone and spironolactone on the Ang II action on cell volume. Measurements of cell width and area of quiescent ventricular cardiomyocytes were performed using a video camera and computer analysis and the relative cell volume was calculated. All measurements of cell volume were performed in the same cell before and after the administration of Ang II (10⁻⁸M). The results indicated that: a) the increase in cell volume caused by extracellular Ang II(10⁻⁸ M) was enhanced in cells incubated with aldosterone (100 nM) for 48 h; b) the effect of aldosterone was abolished by spironolactone (10⁻⁸ M); c) the decline in cell volume elicited by intracellular administration of Ang II (10⁻⁸ M) was increased by aldosterone and inhibited by spironolactone; d) the effects of aldosterone and spironolactone were related, in part, to a change in expression of AT1 receptors; and e) the intracellular administration of Ang II reduced the swelling-dependent chloride current (I(Clswell)). The implications of these findings to the failing heart and myocardial ischemia are discussed.

Cell swelling impairs dye coupling in adult rat ventricular myocytes. Cell volume as a regulator of cell communication

The influence of cell swelling on cell communication was investigated in cardiomyocytes isolated from the ventricle of adult rats. Measurements of dye coupling were performed in cell pairs using intracellular dialysis of Lucifer Yellow CH. The pipette was attached to one cell of the pair and after a gig ohm seal was achieved, the membrane was ruptured by a brief suction allowing the dye to diffuse from the pipette into the cell. Fluorescence of the dye in the injected as well as in non-dialyzed cell of the pair was continuously monitored. The results indicate that in cell pairs exposed to hypotonic solution the cell volume was increased by about 60% within 35 min and the dye coupling was significantly reduced by cell swelling. Calculation of gap junction permeability (P(j)) assuming an the intracellular volume accessible to intracellular diffusion of the dye as 12% of total cell volume, showed an average P(j) value of 0.16 ± 0.04 × 10(-4) cm/s (n = 35) in the control and 0.89 ± 1.1 × 10(-5) cm (n = 40) for cells exposed to hypotonic solution (P < 0.05). Similar results were found assuming intracellular volumes accessible to the dye of 20 and 30% of total cell volume, respectively. Cell swelling did not change the rate of intracellular diffusion of the dye. The results which indicate that cell volume is an important regulator of gap junction permeability, have important implications to myocardial ischemia and heart failure as well as to heart pharmacology because changes in cell volume caused by drugs and transmitters can impair cell communication with consequent generation of slow conduction and cardiac arrhythmias.

Prolonged exposure of cardiac cells to renin plus angiotensinogen reduces intracellular renin in the failing heart. On the role of angiotensin II-AT1 complex internalization

To investigate the influence of prolonged exposure of cardiac cells to renin plus angiotensinogen (Ao) on intracellular renin levels, myocytes were isolated from the ventricle of cardiomyopathic hamsters(TO-2) and incubated in Krebs solution containing renin(128 pmol Ang ml/min) plus Ao (110 pmol Ang I generated by renin to exhaustion) for a period of 24 h. Membrane-bound and intracellular AT1 receptors levels as well as intracellular renin were studied using immunological methods and quantified by flow cytometry. The results indicated: a) intracellular renin levels were higher in the failing heart at an advanced stage of the disease (8 months) than in age-matched controls; b) the intracellular renin levels were significantly reduced in cells exposed to renin (128 pmol Ang I.ml/min) plus angiotensinogen (Ao)(110 pmol Ang I generated by renin to exhaustion) for a period of 24 h; c) incubation of the cardiomyocytes with renin (128 pmol Ang I.ml/min) alone did not reduced the intracellular renin levels; d) the fall of the intracellular renin level was related to the formation of angiotensin II (Ang II) at the surface cell membrane and internalization of the Ang II-AT1 complex because losartan (10(-7) M) added to the incubation medium containing renin plus Ao, blocked the internalization of AT1 and suppressed the decline of the intracellular renin levels; e) no internalization of renin or renin secretion was found in these experiments.

IN CONCLUSION

prolonged exposure of cardiac cells to renin plus Ao (24 h) reduced intracellular renin levels through the internalization of Ang II-AT1 complex and inhibition of renin expression.

Intracellular and extracellular renin have opposite effects on the regulation of heart cell volume. Implications for myocardial ischaemia

The influence of intracellular renin plus angiotensinogen (Ao) as well as angiotensin (Ang) II on cell volume was investigated in myocytes isolated from the heart of four-month-old cardiomyopathic hamsters (TO-2) and normal hamsters (F1B). Measurements of cell width and cell length were performed on quiescent cells using a Px-it imaging and computer system. The cell volume was calculated assuming the cells as elliptical cylinders and taking the cell depth equal to one third of cell width. For measurements of sodium pump current, the cells were voltage clamped (holding potential -40 mV) using the whole cell configuration. Cells were exposed to K-free solution to inhibit the pump and then to normal Krebs solution to reactivate the pump. In other experiments the cells were voltage clamped (holding potential -40 mV) and changes in the background current elicited by renin plus Ao or by Ang II were monitored. The results indicated that: a) intracellular dialysis of renin (128 pmol Ang I/ml) plus Ao (110 pmol Ang I generated by renin by exhaustion) decreased the cell volume concurrently with the activation of the sodium pump; b) intracellular losartan (10(-)8 M) or extracellular ouabain (10(-8) M) abolished the effect of renin plus Ao on cell volume; c) intracellular Ang II (10(-8) M), by itself, reduced cell volume and increased the pump current density; d) extracellular administration of renin plus Ao, at the same concentration used intracellularly, increased cell volume and inhibited the sodium pump. This increase of cell volume elicited by extracellular renin plus Ao was related to the activation of the Na-K-2Cl cotransporter; e) intracellular Ang II (10(-8) M) reversed cell swelling induced by hypotonic solutions.

CONCLUSIONS

Intracellular and extracellular renin plus Ao have opposite effects on sodium pump and cell volume regulation in the failing heart. Both effects of renin plus Ao are dependent upon the formation of Ang II. Since intracellular Ang II counteracted the cell swelling induced by hypotonic solution, it is reasonable to think that the activation of the intracrine renin-angiotensin system might play a protective role during myocardial ischaemia by reducing cell volume.

Eplerenone inhibits the intracrine and extracellular actions of angiotensin II on the inward calcium current in the failing heart. On the presence of an intracrine renin angiotensin aldosterone system

The influence of chronic administration of eplerenone on the intracrine as well as on the extracellular action of angiotensin II (Ang II) on L-type inward calcium current was investigated in the failing heart of cardiomyopathic hamsters (TO-2).For this, eplerenone (200 mg/kg/day) was administered orally to 2 month-old cardiomyopathic hamsters for a period of 3 months. Measurements of the peak inward calcium current (I(Ca)) was performed in single cells under voltage clamp using the whole cell configuration. The results indicated that eplerenone suppressed the intracrine action of Ang II (10(-)(8) M) on peak I(Ca) density. Moreover, the intracellular dialysis of the peptide did not change the time course of I(Ca) inactivation in animals treated chronically with eplerenone. The extracellular administration of Ang II (10(-)(8) M) incremented the peak I(Ca) density by only 20+/-8% (n=30) compared with 38+/-4% (n=35) (P<0.05) obtained in age-matched cardiomyopathic hamsters not exposed to eplerenone. Interestingly, the inhibitory of eplerenone (10(-7) M) on the intracrine action of Ang II was also found, in vitro, but required an incubation period of, at least, 24 h. The inhibitory action of eplerenone on the intracellular action of Ang II was partially reversed by exposing the eplerenone-treated cells to aldosterone (10 nM) for a period of 24 h what supports the view that: a) the mineralocorticoid receptor(MR) was involved in the modulation of the intracrine action of the peptide; b) the effect of eplerenone on the intracrine as well as on the extracellular action of Ang II was related ,in part, to a decreased expression of membrane-bound and intracellular AT1 receptors.

IN CONCLUSION

a) eplerenone inhibits the intracrine action of Ang II on inward calcium current and reduces drastically the effect of extracellular Ang II on I(Ca); b) aldosterone is able to revert the effect of eplerenone; c) the mineralocorticoid receptor is an essential component of the intracrine renin angiotensin aldosterone system.

The missing link in the mystery of normal automaticity of cardiac pacemaker cells

Earlier studies of the initiating event of normal automaticity of the heart's pacemaker cells, inspired by classical quantitative membrane theory, focused upon ion currents (IK, I f) that determine the maximum diastolic potential and the early phase of the spontaneous diastolic depolarization (DD). These early DD events are caused by the prior action potential (AP) and essentially reflect a membrane recovery process. Events following the recovery process that ignite APs have not been recognized and remained a mystery until recently. These critical events are linked to rhythmic intracellular signals initiated by Ca2+ clock (i.e., sarcoplasmic reticulum [SR] cycling Ca2+). Sinoatrial cells, regardless of size, exhibit intense ryanodine receptor (RyR), Na+/Ca2+ exchange (NCX)-1, and SR Ca2+ ATPase-2 immunolabeling and dense submembrane NCX/RyR colocalization; Ca2+ clocks generate spontaneous stochastic but roughly periodic local subsarcolemmal Ca2+ releases (LCR). LCRs generate inward currents via NCX that exponentially accelerate the late DD. The timing and amplitude of LCR/I NCX-coupled events control the timing and amplitude of the nonlinear terminal DD and therefore ultimately control the chronotropic state by determining the timing of the I CaL activation that initiates the next AP. LCR period is precisely controlled by the kinetics of SR Ca2+ cycling, which, in turn, are regulated by 1) the status of protein kinase A-dependent phosphorylation of SR Ca2+ cycling proteins; and 2) membrane ion channels ensuring the Ca2+ homeostasis and therefore the Ca2+ available to Ca2+ clock. Thus, the link between early DD and next AP, missed in earlier studies, is ensured by a precisely physiologically regulated Ca2+ clock within pacemaker cells that integrates multiple Ca2+-dependent functions and rhythmically ignites APs during late DD via LCRs-I NCX coupling.

Chronic blockade of angiotensin II AT1-receptors increased cell-to-cell communication, reduced fibrosis and improved impulse propagation in the failing heart

INTRODUCTION

The influence of chronic administration of losartan on gap junction conductance (gj), conduction velocity and interstitial fibrosis was investigated in the failing heart of 4-month-old cardiomyopathic hamsters (TO-2). After two months of administration of losartan (25 mg/kg/day/po) the number of cell pairs showing very low values of gj (28 nS) was significantly reduced whereas the group of cell pairs with larger values of gj (18-45 nS) was significantly increased. The conduction velocity measured with intracellular microelectrodes in the wall of the left ventricle was enhanced from 38+2.3 cm/s (n=25) (control) to 49+2 cm/s (n=24) (p<0.05) after losartan administration. Moreover, the number of ventricular fibres showing non-propagated action potentials was significantly decreased (p<0.05) by losartan. The % area of interstitial fibrosis measured in the wall of the left ventricle was reduced from 22+1.4% (n=18) to 14+1.3% (n=18) (p<0.05) after losartan administration.

CONCLUSION

Chronic blockade of angiotensin II type 1 receptors increased gj in the failing heart. Moreover, the conduction velocity was enhanced in part by the increase of gj and in part by the decrease of interstitial fibrosis and structural remodelling.

The emergence of a general theory of the initiation and strength of the heartbeat

Sarcoplasmic reticulum (SR) Ca(2+) cycling, that is, the Ca(2+) clock, entrained by externally delivered action potentials has been a major focus in ventricular myocyte research for the past 5 decades. In contrast, the focus of pacemaker cell research has largely been limited to membrane-delimited pacemaker mechanisms (membrane clock) driven by ion channels, as the immediate cause for excitation. Recent robust experimental evidence, based on confocal cell imaging, and supported by numerical modeling suggests a novel concept: the normal rhythmic heart beat is governed by the tight integration of both intracellular Ca(2+) and membrane clocks. In pacemaker cells the intracellular Ca(2+) clock is manifested by spontaneous, rhythmic submembrane local Ca(2+) releases from SR, which are tightly controlled by a high degree of basal and reserve PKA-dependent protein phosphorylation. The Ca(2+) releases rhythmically activate Na(+)/Ca(2+) exchange inward currents that ignite action potentials, whose shape and ion fluxes are tuned by the membrane clock which, in turn, sustains operation of the intracellular Ca(2+) clock. The idea that spontaneous SR Ca(2+) releases initiate and regulate normal automaticity provides the key that reunites pacemaker and ventricular cell research, thus evolving a general theory of the initiation and strength of the heartbeat.

Renin increments the inward calcium current in the failing heart

BACKGROUND

Evidence is available that activation of the renin-angiotensin system is involved in cardiac remodeling. It is unknown whether renin can change the inward calcium current (ICa) in the failing heart. This problem was investigated in the present study.

METHODS

Cardiomyocytes were isolated from the ventricle of 4-month-old cardiomyopathic hamsters and measurements of the L-type ICa were performed using the patch-clamp technique in a whole-cell configuration.

RESULTS

Extracellular renin (128 pmol Ang I/ml per min) plus angiotensinogen (110 pmol angiotensin I generated by renin to exhaustion) incremented the peak ICa density significantly, an effect suppressed by enalapril maleate (10 mol/l) or by losartan (10 mol/l) added to the bath, indicating that the effect of renin plus angiotensinogen was related to the formation of angiotensin I and its conversion to angiotensin II at the surface cell membrane. Renin internalization seems to increment the ICa because intracellular dialysis of renin (128 pmol Ang I/ml per min) plus angiotensinogen (110 pmol angiotensin I generated by renin to exhaustion) also increased the peak ICa density significantly, an effect suppressed by intracellular losartan (10 mol/l) but not by extracellular losartan (10 mol/l).

CONCLUSIONS

Extracellular renin plus angiotensinogen increases the ICa in isolated myocytes from the failing heart of cardiomyopathic hamsters through the formation of angiotensin II and the activation of angiotensin type 1 receptors at the surface cell membrane. A similar increment of ICa was found with intracellular administration of renin plus angiotensinogen. This finding might indicate that renin internalization is involved in control of inward calcium current in the failing heart.

Angiotensin converting enzyme and the arrhythmogenic action of angiotensin I: cardiac cell membrane as a site of angiotensin I conversion

The influence of angiotensin I (Ang I) on heart excitability and refractoriness was investigated in isolated right ventricular muscle of adult rats as well as in isolated ventricular myocytes. The results indicated that Ang I (10(-8) M) added to the bath solution, decreased the action potential duration from 50.4 +/- 3.6 to 33.9 +/- 3.9 ms (P < 0.05) and reduced significantly the cardiac refractoriness. Consequently, a discharge of spontaneous action potentials was elicited when a second stimulus was applied during the relative refractory period. Moreover, the conduction velocity was reduced from 56.9 +/- 2.9 to 40 +/- 3.2 cm/s (P < 0.05). The question whether the effect of Ang I was related to its conversion to Ang II, was investigated on tissues exposed to enalapril maleate (10(-8) M). Under these conditions, the effect of Ang I was totally suppressed. Similar results were found with losartan (10(-7) M). To investigate if the conversion of Ang I to Ang II occurs at the level of surface cell membrane, measurements of inward calcium current (ICa) were performed in myocytes isolated from the rat ventricle. ICa was measured before and after the administration of Ang I (10(-8) M). The results indicated that Ang I (10(-8) M), added to the bath solution, reduced the peak ICa density by 26.3 +/- 2.6% (P < 0.05), an effect abolished by enalapril maleate (10(-8)M).

CONCLUSION

Evidence is presented for the first time, that Ang I is converted to Ang II at the surface cell membrane in cardiac muscle with consequent generation of cardiac arrhythmias which are elicited by Ang II.

Intracellular and extracellular angiotensin II enhance the L-type calcium current in the failing heart

The influence of intracellular and extracellular angiotensin II (Ang II) on the L-type calcium current of cardiomyocytes isolated from cardiomyopathic hamsters was investigated. The results indicated that Ang II (10(-8) mmol/L), added to the bath, increased the peak inward calcium current (I(Ca)) density by 37+/-3.4% (P<0.05), an effect that depends on the activation of protein kinase C. Intracellular administration of the same dose of Ang II (10(-8) mmol/L) also elicited an increase of peak I(Ca) density but enhanced the rate of I(Ca) inactivation, an effect not seen with extracellular Ang II. Moreover, in control animals, no change in the rate of I(Ca) inactivation was seen with intracellular Ang II. Thapsigargin (1 micromol/L), a potent inhibitor of sarcoplasmic reticulum (SR) ATPase, which depletes the SR, decreased the rate of I(Ca) inactivation elicited by intracellular Ang II, although the cytoplasmic calcium concentration was highly buffered with 10 mmol/L EGTA. These findings might indicate that intracellular Ang II releases calcium from the SR and inactivates I(Ca). The effect of intracellular Ang II on peak I(Ca) was not altered by extracellular losartan (10(-7) mmol/L), supporting the notion that the peptide acted intracellularly. Other studies showed that intracellular Ang I administration (10(-8) mmol/L) enhanced the peak I(Ca) density and the rate of I(Ca) inactivation, an effect that was reduced by intracellular enalaprilat (10(-8) mmol/L). Moreover, intracellular enalaprilat by itself reduced the peak I(Ca) density. These observations might indicate that endogenous Ang II is contributing to I(Ca) modulation in the failing heart.

Further studies on the effect of intracellular angiotensins on heart cell communication: on the role of endogenous angiotensin II

The influence of intracellular angiotensin I (Ang I) and angiotensin II (Ang II) on the process of cell communication was investigated in isolated cell pairs from the failing heart of cardiomyopathic hamsters at 2 and at 6 months of age. Measurements of junctional conductance were performed on weekly coupled ventricular cells (4-5.3 nS) using two separated voltage clamp circuits. The results indicated that at 2 months of age, when no signs of heart failure are detected, the angiotensin converting enzyme (ACE) activity is low and similar to controls (0.26 nmol/mg/min). Here the intracellular dialysis of angiotensin I (10(-8) M) caused a decline of junctional conductance of 33+/-3.6% (n=35) (P<0.05) within 10 min while the administration of the same concentration of Ang I elicited cell uncoupling in cell pairs of 6-month-old cardiomyopathic hamsters in which the ACE activity was enhanced (0.41+/-0.05 nmol/mg/min) (P<0.05). Intracellular administration of angiotensin II in cell pairs of 2-month-old hamsters caused a decline of junctional conductance of only 25+/-4.5% (n=35) (P<0.05) compared to cell uncoupling in 6-month-old cardiomyopathic hamsters. Intracellular losartan(10(-8) M) reduced the effect of intracellular Ang II by 68+/-3.5% (n=28) on 2-month-old hamsters and abolished the effect of the peptide on 6-month-old hamsters. To investigate the influence of endogenous angiotensin II on the regulation of cell coupling, enalapril maleate (10(-8) M) or enalaprilat (10(-9) M) was used. The results indicated that at 2 months of age, no change in cell coupling was elicited by the ACE inhibitor while at 6 months of age, there was an increment of cell coupling of 72+/-6.2% (P<0.05). Similar results were found with intracellular losartan (10(-8) M). These results support the view that endogenous angiotensin II is involved in the regulation of cell communication at an advanced stage of heart failure when the ACE activity is enhanced and the cardiac renin angiotensin system (RAS) is activated.

TNF-alpha rapidly antagonizes the beta-adrenergic responses of the chloride current in guinea-pig ventricular myocytes

The purpose of this study was to test the hypothesis that tumor necrosis factor-alpha (TNF-alpha) rapidly antagonizes the beta-adrenergic responses of the chloride current and to clarify the intracellular mechanisms responsible for the anti-adrenergic action. The whole-cell patch-clamp technique was used to monitor the anti-adrenergic effects of TNF-alpha on the cAMP-dependent chloride current (I(Cl)) recorded from isolated guinea-pig ventricular myocytes. Ramp pulses (+/-120 mV; dv/dt = +/-0.4 V/s) were applied from the holding potential of -40 mV. TNF-alpha rapidly (<15 min) inhibited the isoproterenol (Iso, 0.1 micromol/L)-induced I(Cl) in a concentration-dependent manner (30-1,000 U/ml, IC (50) = 144 U/ml, n=30). The inhibitory action of TNF-alpha was also observed when I(Cl) had been previously stimulated by 1 micromol/L forskolin (n=5). Prior exposure of myocytes to 5 microg/ml pertussis toxin (PTX) hardly affected the anti-adrenergic action of TNF-alpha (n=4). However, when I(Cl) was induced by both 8-bromo-cAMP (100 micromol/L) and isobutylmethylxanthine (0.1 mmol/L), TNF-alpha (1,000 U/ml) failed to decrease I(Cl) amplitude (n=5). Prior exposure of myocytes to 5 mg/ml pertussis toxin (PTX) hardly affected the anti-adrenergic action of TNF-alpha (n=4). Furthermore, despite of the presence of nitro-L-arginine methyl ester (0.1 mmol/L), a nitric oxide synthase (NOS) inhibitor, TNF-alpha reversed the Iso-induced increase in I(Cl) (n=5). These results suggest that TNF-alpha rapidly antagonizes the beta-adrenergic responses of I(Cl) by reducing cAMP concentration. This anti-adrenergic action is mediated by neither the PTX-sensitive G proteins regulatory pathway nor constitutive NOS activation.

Enhanced inhibition of L-type calcium currents by troglitazone in streptozotocin-induced diabetic rat cardiac ventricular myocytes

1. Troglitazone, an insulin-sensitizing agent shown to improve cardiac function in both experimental animals and patients with diabetes, inhibits voltage-dependent L-type Ca(2+) currents (I(Ca,L)) in cardiac myocytes, which may underlie its cardioprotective effects. However, inhibition by troglitazone of I(Ca,L) in diabetic cardiac myocytes has not been characterized. 2. Using whole-cell voltage-clamp techniques, I(Ca,L) was measured in ventricular myocytes isolated from 4-6 weeks streptozotocin (STZ)-induced diabetic rats and age-matched control rats. 3. Under control conditions with CsCl internal solution, diabetic myocytes did not differ from control myocytes in membrane capacitance, current density or voltage-dependent properties of I(Ca,L). 4. Troglitazone decreased amplitude of I(Ca,L) in both control and diabetic myocytes in a concentration-dependent manner. This inhibition was more potent in diabetic than in control myocytes; half-maximum inhibitory concentrations of troglitazone measured at a holding potential of -50 mV were 4.3 and 9.5 micromol l(-1), respectively. 5. Troglitazone at 5 micromol l(-1) did not significantly influence the voltage dependency of steady-state inactivation or the inactivation time course of I(Ca,L) in either control or diabetic myocytes. 6. Since troglitazone inhibits I(Ca,L) more effectively in STZ-induced diabetic ventricular myocytes, this agent may prevent cardiac dysfunction in diabetes.

Regulation of K(ATP) channels by P(2Y) purinoceptors coupled to PIP(2) metabolism in guinea pig ventricular cells

We used patch-clamp techniques to elucidate the regulatory mechanisms of ATP-sensitive K(+) (K(ATP)) channels by stimulation of P(2) purinoceptors in guinea pig ventricular myocytes. Extracellular ATP at 0.1 mM transiently inhibited by 90.5 +/- 5.0% the whole cell K(ATP) channel current evoked by a reduction in intracellular ATP concentration to 0.5 mM and exposure to 30 microM pinacidil. ADP and AMP (both 1 mM) also decreased the current by 42.8 +/- 9.3% and 9.4 +/- 4.8%, respectively, but adenosine did not, even at 10 mM. ATP-induced channel inhibition was hardly observed in the presence of 0.2 mM suramin, 0.2 mM guanosine 5'-O-(2-thiodiphosphate), or 0.1 mM compound 48/80, whereas it was not influenced by the presence of 0.1 microM staurosporine or 10 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid in the pipette. In the presence of 10 microM wortmannin or the absence of ATP in the cytosol, the ATP-induced channel inhibition was irreversible. Phosphatidylinositol 4,5-bisphosphate (PIP(2)) at 0.1 mM in the outside-out patch pipette prevented ATP-induced channel inhibition. The half-maximal internal ATP concentrations for inhibition of channel activity determined in inside-out membrane patches were 13.8 microM in the presence and 1.12 mM in the absence of 0.1 mM ATP at the external side. It is concluded that activity of K(ATP) channels is modulated by extracellular ATP by a mechanism involving P(2Y) purinoceptors coupled to GTP-binding proteins associated with reduction of the sarcolemmal PIP(2) concentration via stimulation of phospholipase C.

Electrophysiological analysis of the negative chronotropic effect of endothelin-1 in rabbit sinoatrial node cells

1. Electrophysiological effects of endothelin-1 (ET-1) were studied in rabbit sinoatrial node (SAN) using conventional microelectrode and whole-cell voltage and current recordings. 2. In rabbit SAN, RT-PCR detected ET(A) endothelin receptor mRNA. ET-1 (100 nM) increased the cycle length of action potentials (APs) from 305 +/- 15 to 388 +/- 25 ms; this effect was antagonised by the ET(A) receptor-selective antagonist BQ-123 (1 microM). ET-1 increased AP duration (APD50) by 22%, depolarised the maximum diastolic potential (MDP) from -59 +/- 1 to -53 +/- 2 mV, shifted the take-off potential by +5 mV and decreased the pacemaker potential (PMP) slope by 15%. Under exactly the same experimental conditions, ET-1 caused a positive chronotropic effect in guinea-pig SAN with a decrease of 13% in APD50, a shift of -4 mV in the take-off potential and an increase of 8% in the PMP slope. 3. Rabbit SAN exhibited two major cell types, distinguished both by their appearances and by their electrophysiological responses to ET-1. Whereas the spontaneous pacing rate and the PMP slope were similarly decreased by ET-1 (10 nM) in both cell types, ET-1 depolarised MDP from -67 +/- 1 to -62 +/- 4 mV in spindle-shaped cells but hyperpolarised it from -73 +/- 1 to -81 +/- 3 mV in rod-shaped cells. ET-1 decreased APD50 by 8 and 52% and shifted the take-off potential by +5 and -9 mV in spindle- and rod-shaped cells, respectively. 4. ET-1 decreased the high-threshold calcium current (I(CaL)) by about 50% in both cell types, without affecting its voltage dependence, and decreased the delayed rectifier K+ current (I(K)) with significant shifts (of +4.7 and +14.0 mV in spindle- and rod-shaped cells, respectively) in its voltage dependence. It was exclusively in rod-shaped cells that ET-1 activated a sizeable amount of time-independent inward-rectifying current. 5. The hyperpolarisation-activated current (I(f)), observed exclusively in spindle-shaped cells, was significantly increased by ET-1 at membrane potentials between -74.7 and -84.7 mV whereas it was significantly decreased at more negative potentials. ET-1 significantly decreased the slope of the current-voltage (I-V) relation of the I(f) tail without changing its half-maximum voltage. 6. The overall negative chronotropic influence of ET-1 on the whole rabbit SAN is interpreted as resulting from the integration of its different actions on spindle- and rod-shaped SAN cells through electrotonic interaction.

Intracellular calcium level required for calpain activation in a single myocardial cell

We have hypothesized that calpain mediates myocardial injury induced by Ca(2+)overload. However, in vitro study demonstrated that the calcium requirement for calpain activation is around 10 microm, which is difficult to reach without the cell collapsing. Furthermore, because calpastatin is abundant in the myocardial cell, calpain may not be activated in physiological conditions. To elucidate whether calpain is activated by the calcium concentration reachable in myocardial living cells, we measured the calpain activity and the calcium concentration simultaneously in isolated guinea-pig cardiomyocytes. t-Butoxycarbonyl-Leu-Met-7-amino-4chlorimethylcoumarin (Boc-Leu-Met-CMAC), a fluorescent substrate of calpain, and/or fura red, a calcium indicator, were loaded into isolated cardiomyocytes together, and their fluorescence were measured separately. Intracellular Ca overload was induced by changing the superfusate from normal Tyrode solution to a sodium-free one. After changing the solution, fluorescence intensity of fura red and Boc-Leu-Met-CMAC did not change for a while, then fluorescence intensity of fura red began to rise. This was followed by the fluorescence intensity of Boc-Leu-Met-CMAC starting to rise 160+/-45 s after [Ca(2+)](i)increase. The relative fluorescence intensity of fura red increased to 1.37+/-0.32 folds of the control at the point that calpain became active. The calcium concentration at this point was estimated as 451 n m. These results indicate that calpain is activated by the slight rise of Ca concentration in intact cardiomyocytes.

Morphological and membrane characteristics of spider and spindle cells isolated from rabbit sinus node

This study reports the comparative quantitative, morphological, and electrophysiological properties of two pacemaker cell types, spider and spindle-shaped cells, isolated from the rabbit sinoatrial node. Isolated nodal cells were studied with perforated and ruptured patch whole cell recording techniques. The basic spontaneous cycle length of the spider cells was 381 +/- 12 ms, and the basic spontaneous cycle length of the spindle cells was 456 +/- 17 ms (n = 12, P < 0.05). The spider cells had a more positive maximum diastolic potential (-54 +/- 1 mV) compared with the spindle cells (-68 +/- 1mV, P < 0.05). The overshoot and action potential amplitudes were also smaller in the spider cells. The hyperpolarization-activated inward (I(f)) current density, measured from their tail currents, was 15 +/- 1.3 pA/pF for the spider cells and 9 +/- 0.7 pA/pF for the spindle cells (P < 0.01). I(f) current activation voltage was more positive in the spider cells than the spindle cells. Isoproterenol (1 microM) decreased the spontaneous cycle length of the spider cells by 28 +/- 3% and the spindle cells by 20 +/- 1.5% (P < 0.05). Acetylcholine (0.5 microM) hyperpolarized the membrane potential of the spider cells to -86 +/- 0.7 mV and the spindle cells to -76 +/- 0.8 mV (P < 0.05). In summary, there are at least two distinct pacemaker cell types in the sinus node with different electrophysiological characteristics.

State-dependent modification of ATP-sensitive K+ channels by phosphatidylinositol 4,5-bisphosphate

With inside-out patch recordings in ventricular myocytes from the hearts of guinea pigs, we studied ATP-sensitive K+ (K(ATP)) channels activated by phosphatidylinositol 4,5-bisphosphate (PIP2) with respect to sensitivity to ATP when in either a rundown state (RS) or a non-rundown state (NRS). Rundown of K(ATP) channels was induced by exposure either to ATP-free solution or to ATP-free solution containing 19 microM Ca2+. Exposure of membrane patches to 10 microM PIP2 reactivated channels with both types of rundown. The reactivation by PIP2 did not require ATP in the bath. The IC50 of channels recovered from RS and before the rundown was 37.1 and 31.1 microM, respectively. PIP2 irreversibly increased the mean current when the channel was in the NRS. This was associated with a shift of IC50 to 250.6 microM after PIP2 exposure. PIP2 activates NRS K(ATP) channels by decreasing their sensitivity to ATP, whereas PIP2 reactivates RS-K(ATP) channels independently of ATP without changing ATP sensitivity.

On the mechanism of ADP-induced alteration of sulphonylurea sensitivity in cardiac ATP-sensitive K(+) channels

1. To study the mechanism of regulation of sulphonylurea sensitivity in ATP-sensitive K(+) (K(ATP)) channels, we used the inside-out patch clamp technique in guinea-pig ventricular myocytes. 2. In the absence of nucleotides, the half maximal concentration of tolbutamide inhibition of K(ATP) channels (IC(50)) was 0.4 mM, and it decreased to 0.1 mM when 0.1 mM ATP was added. 3. Increasing the ADP concentration from 0 to 0.1 and 0.3 mM in the absence of ATP shifted the IC(50) from 0.4 to 5.3 and 11.4 mM, respectively. Increasing the ADP concentration further to 1 and 3 mM conversely reduced the IC(50) to 9.5 and 4.4 mM, respectively. 4. In the absence of Mg(2+) and ADP, the IC(50) was calculated to 16.6 mM which was found to be less, 12.3, 5.1 and 2.5 mM, respectively, when the ADP concentration was increased to 0.1, 0.3 and 1 mM. 5. The IC(50)s for tolbutamide obtained at various concentrations of ADP in the presence of Mg(2+) were best fitted by equations reflecting a model that assumed two binding sites for ADP; one is a high affinity site that reduces the sensitivity to the sulphonylurea, while the other is a low affinity site that increases such sensitivity. Dissociation constants calculated for ADP to sites 1 and 2 were 2.6 microM and 46.7 mM, respectively. In the absence of Mg(2+), data were fitted by equations corresponding to a single site model (site 2); the dissociation constant for ADP was 25.0 mM. 6. It is concluded that ADP modifies tolbutamide sensitivity by binding to two sites. The high affinity site is strongly Mg(2+)-dependent, whereas the low affinity site is Mg(2+)-independent.

Na+-Ca2+ exchange current from rabbit isolated atrioventricular nodal and ventricular myocytes compared using action potential and ramp waveforms

We measured and compared Na-Ca exchanger current (INa-Ca) from rabbit isolated ventricular and atrioventricular (AV) nodal myocytes, using action potential (AP) and ramp voltage commands. Whole cell patch-clamp recordings were made at 35-37 degrees C; INa-Ca was measured as 5 mM nickel (Ni)- sensitive current with major interfering voltage and calcium-activated currents blocked. In ventricular cells a 2-s descending ramp elicited INa-Ca showing outward rectification and a reversal potential (Erev) of -13.1 +/- 1. 2 mV (n = 12; mean +/- SEM). With a ventricular AP as the voltage command, the profile of INa-Ca followed the applied waveform closely. The current-voltage relation during AP repolarization was almost linear and showed an Erev of -38.3 +/- 5.3 mV (n = 6). As INa-Ca depended on the applied voltage waveform, comparisons between the two cell types utilized the same command waveform (a series of AV nodal APs). In ventricular myocytes this elicited INa-Ca that reversed near -38 mV and was inwardly directed during the pacemaker potential. This command was also applied to AV node cells; mean INa-Ca density at all voltages encompassed by the AP (-70 to +30 mV) did not differ significantly from that in ventricular myocytes (P > 0.05, ANOVA). This finding was confirmed using brief (250 ms) voltage ramp protocols (P > 0.1 ANOVA). These data represent the first direct measurements of AV nodal INa-Ca and suggest that the exchanger may be functionally expressed to similar levels in the two cell types. They may also suggest a possible role for INa-Ca during the pacemaker potential in AV node as inward INa-Ca was observed over the pacemaker potential range even with bulk internal Ca buffered to a low level.

Muscarinic regulation of Ca2+ signaling in mammalian atrial and ventricular myocardium

The differential regulation of the contractility of mammalian atrial and ventricular myocardium upon activation of muscarinic receptors can be ascribed, for the most part, to alterations in intracellular Ca2+ transients. However, alterations in myofibrillar sensitivity to Ca2+ ions also contribute to such regulation. In atrial muscle, the following actions are all associated with the corresponding alterations in the amplitude of Ca2+ transients in the same direction as those in the strength of the contractile force: (1) the direct inhibitory action on the basal force of contraction; (2) the increase (recovery) in force that is induced during the prolonged stimulation of muscarinic receptors; and (3) the rebound increase in force induced by washout of muscarinic receptor agonists. In addition, for a given decrease in force induced by muscarinic receptor stimulation in atrial muscle, the amplitude of Ca2+ transients is decreased to a smaller extent than the decrease in amplitude induced by reduction of extracellular Ca2+ concentration ([Ca2+]o), an indication that muscarinic receptor stimulation might increase myofibrillar sensitivity to Ca2+ ions simultaneously with the reduction in the amplitude of Ca2+ transients during induction of the direct inhibitory action. In mammalian ventricular myocardium, the direct inhibitory action of muscarinic receptor stimulation exhibits a wide range of species-dependent variation. A pronounced direct inhibitory action is induced in ferret papillary muscle, which is also associated with a definite increase in myofibrillar sensitivity to Ca2+ ions. By contrast, in the ventricular myocardium of other species including the rabbit and the dog, muscarinic receptor stimulation scarcely affects the baseline Ca2+ transients and the force, but it results in a pronounced decrease in Ca2+ transients and force when applied in the presence of beta-adrenoceptor stimulation, a phenomenon known as 'accentuated antagonism' or the 'indirect inhibitory action' of muscarinic receptor stimulation in mammalian ventricular myocardium. During induction of the indirect inhibitory action in mammalian ventricular myocardium, muscarinic receptor stimulation reverses all the effects induced by beta-adrenoceptor stimulation, including the increase in Ca2+ transients, the positive inotropic and lusitropic effects, and the decrease in myofibrillar sensitivity to Ca2+ ions. The relationship between the amplitude of Ca2+ transients and force is unaffected during induction of the indirect inhibitory action in rabbit and dog ventricular myocardium. The direct and indirect inhibitory actions of muscarinic receptor stimulation on Ca2+ transients have clearly different dependences on frequency: the former is more pronounced at a higher rate of stimulation, while the latter is more pronounced at a lower rate. The more complex interaction of muscarinic receptor and beta-adrenoceptor stimulation in mammalian atrial muscle and ferret ventricular muscle might be explained by the contribution of both the direct and the indirect regulatory mechanisms to the interaction.

Effect of tedisamil on cell communication, impulse propagation, and excitability of the failing heart

In the present work, the effect of tedisamil on gap junctional conductance (gj) and conduction velocity was investigated in the failing heart of cardiomyopathic hamsters (TO-2 strain). It was found that tedisamil (10(-7) M) increased gj by 53.8+/-1% (n = 23) in cell pairs isolated from 2 months old cardiomyopathic hamsters. The effect of tedisamil was suppressed by intracellular dialysis of an inhibitor of protein kinase A and also by adenosine indicating that the drug increases gj through the activation of adenylcyclase. Tedisamil also increased the conduction velocity and cardiac refractoriness of ventricular muscle from young cardiomyopathic hamsters. At an advanced stage of the disease, however, when the beta-adrenoceptor, adenylcyclase signaling system is impaired, tedisamil was unable to increase gj. The present results indicate that the antiarrhythmic action of tedisamil is in part related to an increase in junctional conductance and conduction velocity.

Electrophysiological effects of amrinone on the automaticity and membrane current system of the rabbit sinoatrial node cells

To elucidate the physiological role of phosphodiesterase (PDE) in cardiac pacemaker cells, we studied the electrophysiological effects of amrinone, an inhibitor of PDE type III, on the spontaneous action potential (AP) and membrane currents, using small preparations (0.2 x 0.2 x 0.1 mm) of rabbit sinoatrial (SA) node cells. Amrinone (0.1-1.0 mM) progressively increased the AP amplitude, maximal rate of depolarization, and spontaneous firing frequency, shortened the AP duration, and made the threshold potential more negative. In voltage-clamp experiments using double microelectrode techniques, 0.1 mM amrinone increased the Ca2+ current (I(Ca)) obtained on step depolarization from -40 to -10 mV by 25.86% +/-4.6% (P < 0.05, n = 6), the delayed rectifier K+ current (I(K)) tail obtained on repolarization from 10 to -60 mV by 22.8%+/-4.7% (P < 0.05, n = 6), and the hyperpolarization-activated inward current (Ih) at -90 mV by 19.5%+/-7.3% (P < 0.05, n = 6), respectively. Amrinone did not affect the slope factors of either the inactivation curve for I(Ca) (finfinity curve) or the activation curve for the delayed rectifier I(K) (pinfinity curve). These results suggest that this PDE III inhibitor exerts a positive chronotropic action by enhancing the availability and the conductance of all the tested membrane currents in rabbit SA node cells.

Atrial natriuretic factor reduces cell coupling in the failing heart, an effect mediated by cyclic GMP

The influence of the atrial natriuretic factor (ANF) on heart-cell communication was investigated in cell pairs isolated from the ventricle of cardiomyopathic hamsters (BIO TO-2; 11 months old), and the results were compared with controls (F1B) of same age. The results indicated that ANF (10(-8) M) added to the bath caused a decline in junctional conductance (gj) of 48 +/- 2% (n = 15) within 90 s. The effect of ANF was suppressed by HS-142-1, a specific antagonist of guanylyl cyclase ANF receptor. Moreover, the decline in gj elicited by ANF was related to the synthesis of cyclic guanosine monophosphate (cGMP). Indeed, dibutyryl-cGMP (10(-4) M) decreased gj by 80 +/- 3.5% (n = 15) within 90 s, and zaprinast, a selective inhibitor of cGMP phosphodiesterase, enhanced the effect of ANF on gj. The possible relationship between ischemia, ANF release, and impairment of cell coupling is discussed.

Differential expression of gap junction proteins in the canine sinus node

Electrical coupling of pacemaker cells at gap junctions appears to play an important role in sinus node function. Although the major cardiac gap junction protein, connexin43 (Cx43), is expressed abundantly in atrial and ventricular muscle, its expression in the sinus node has been a subject of controversy. The objectives of the present study were to determine whether Cx43 is expressed by sinus node myocytes, to characterize the spectrum of connexin expression phenotypes in sinus node pacemaker cells, and to define the spatial distribution of different connexin phenotypes in the intact sinus node. To fulfill these objectives, we performed high-resolution immunohistochemical analysis of disaggregated adult canine sinus node preparations. Using enhanced tissue preservation and antigen retrieval techniques, we also performed immunohistochemical studies on sections of intact canine sinus node tissue. Analysis of disaggregated sinus node preparations revealed three populations of pacemaker cells distinguished on the basis of connexin immunohistochemical phenotype: approximately 55% of cells expressed only connexin40 (Cx40); 30% to 35% of cells expressed Cx43, connexin45 (Cx45), and Cx40; and the remaining cells had no detectable connexin expression. In immunostained sections of intact sinus node, Cx43- and Cx45-positive cells were limited in their distribution and were observed in discrete bundles that appeared to abut atrial myocytes. In contrast, Cx40 immunoreactive signal was widely distributed in the sinus node region. These results indicate that subsets of pacemaker cells express distinct connexin phenotypes. Differential expression of connexins could create regions within the sinus node with different conduction properties, thereby contributing to the nonuniform conduction properties seen in this tissue.

Influence of alpha-adrenergic-receptor activation on junctional conductance in heart cells: interaction with beta-adrenergic adrenergic agonists

The influence of phenylephrine (10(-6) M) on the regulation of junctional conductance (gj) was investigated in heart-cell pairs isolated from the ventricles of adult rats. The results indicated that phenylephrine reduced gj by 45% (SEM, +/- 3.4; n = 20; p < 0.05) within 2 min of it's administration to the bath. The effect of phenylephrine was dose dependent and was abolished by prazosin (10(-6) M). Moreover, the activation of protein kinase C seems essential for the effect of phenylephrine on gj, because previous inhibition of protein kinase C reduced the effect of the drug. Norepinephrine (10(-6) M) or epinephrine (10(-6) M) increased gj by 56% (SEM, +/- 5.3; p < 0.05; n = 14) and 43.6% (SEM, +/- 4.1; n = 12; p < 0.05), respectively, and their effects were larger in the alpha 1-adrenergic receptor was blocked with prazosin. The results indicate that alpha-adrenergic activation reduces gj and interacts with the influence of beta-adrenergic stimulation on junctional conductance.

Electrophysiological properties of morphologically distinct cells isolated from the rabbit atrioventricular node

1. Experiments were conducted using the whole-cell patch clamp technique to determine the electrophysiological properties and ionic currents of ovoid and rod-shaped single isolated calcium-tolerant rabbit atrioventricular (AV) nodal cells. 2. Action potential morphologies observed in these cells were similar to those obtained previously from intracellular recordings of intact atrioventricular nodal preparations: ovoid cells had N- or NH-like action potential configurations (see below), whereas rod-shaped cells had AN-like configurations. 3. Action potential restitution in AV nodal cells was characterized by a progressive increase in overshoot potential, maximal upstroke velocity (Vmax) and action potential duration, as well as a decrease in latency from stimulus to Vmax. In rod-shaped cells, premature stimuli could induce regenerative membrane responses before full action potential repolarization, whereas ovoid cells showed only post-repolarization refractoriness. In ovoid cells stimulated at the low stimulus intensities there was no shortening of the action potential duration and the most premature action potentials were often prolonged. 4. The quasi-steady-state current-voltage relationship of ovoid cells was significantly steeper, at both depolarized and hyperpolarized potentials, than that of either the rod-shaped AV nodal cells or atrial cells. The rod-shaped AV nodal cells and the atrial cells had similar current-voltage (I-V) relationships in the positive potential range, but the I-V curves crossed over at potentials of about-90 mV. 5. A hyperpolarization-activated inward current (I(f)) was apparent in the range between -60 and -90 mV in 95% of the ovoid cells (n = 75), whereas in 88% of rod-shaped cells (n = 16) I(f) was activated at more negative potentials. The magnitude of I(f) in ovoid cells, measured at -100 mV, was approximately 25 times that in rod-shaped cells. 6. A rapid inward current (INa) greater than 1 nA was found in all rod-shaped cells (n = 16) but in only 30% of ovoid cells (n = 75). A transient outward current (I(to)) was found in 93% of rod-shaped cells (n = 14) and in 42% of ovoid cells (n = 54). The combination of I(to) and INa was found in 93% of rod-shaped cells but in only 24% of ovoid cells. 7. These results suggest that there are at least two populations of isolated AV nodal cells with distinct action potentials and ionic current profiles that may contribute to the complex electrophysiological properties observed in the intact AV node.

Renin-angiotensin system and cell communication in the failing heart

The influence of heart failure on the process of cell communication was investigated in cell pairs isolated from the ventricle of cardiomyopathic hamsters (11 months old) and the results compared with age-matched normal hamsters. The gap junctional conductance (gj) was measured with two voltage-clamp amplifiers. The results showed two major populations of cell pairs with respect to gj values: one with very low values (0.8 to 2.5 nS) and the other with higher values (7 to 35 nS). In normal hamsters, the most frequent gj values were in the range of 40 to 100 nS. Angiotensin II (Ang 11, 1 microg/mL) caused cell uncoupling in myopathic myocytes with low gj but reduced gj by 53 +/- 6.6 percent (+/- SE) in cell pairs with higher gj values (7 to 35 nS). The effect of Ang II on gj of myopathic cell pairs was suppressed by losartan (10(-7) mol/L). In cardiomyopathic cell pairs with low gj (0.8 to 2.5 nS), enalapril (1 microg/mL) caused an appreciable increase in gj (219 +/- 20.3 percent), whereas in cell pairs with higher gj (7 to 35 nS), the gj increment was smaller (80 +/- 10.8 percent) but still larger than that seen in controls (33 +/- 5.4 percent). Intracellular dialysis of Ang I (10(-8) mol/L) abolished cell communication in myopathic cell pairs with low gj (0.8 to 2.5 nS) and reduced gj by 66 +/- 1.7 percent in the other pairs (7 to 35 nS). The effect of Ang I on gj was greatly reduced by enalaprilat (10(-9) mol/L) added to the cytosol. Dialysis of Ang II (10(-8) mol/L) into the myopathic cell reduced gj by 48 +/- 4.2 percent, an effect abolished by losartan (10(-8) mol/L). The results indicate that the decline in gj seen in the ventricle of cardiomyopathic hamsters is in part due to activation of the cardiac renin-angiotensin system.

Characteristics of the delayed rectifier K current compared in myocytes isolated from the atrioventricular node and ventricle of the rabbit heart

The delayed rectifier potassium current (IK) is known to be important in action potential repolarisation and may contribute to the diastolic pacemaker depolarisation in pacemaker cells from the heart. In this study, using whole-cell patch clamp, we investigated the characteristics of IK in morphologically normal cells from the atrioventricular node (AVN) and ventricle of the rabbit heart. Cells were held at -40 mV and 5 microM external nifedipine was used to block L-type calcium current (ICa,L). Significant IK was observed with pulses to potentials more positive than -30 mV. The steady-state activation curve in both cell types showed maximal activation at between + 10 and + 20 mV. Half-maximal activation of IK occurred at -4.9 and -4.1 mV with slope factors of 8.3 and 12.4 mV in ventricular and AVN cells, respectively. Using pulses of increasing duration, significant IK tails after repolarisation from + 40 mV were observed with pulses of 20 ms and increased with pulses up to 100-120 ms in both cell types. Pulses of longer duration did not activate further IK and this suggested that only the rapid component of IK, called IKr, was present in either cell type. Moreover, IK tails after pulses to all potentials were blocked completely by E-4031, a selective blocker of IKr. The reversal potential of IK varied with the concentration of external K. Superfusion of AVN cells with medium containing 4, 15 and 40 mM [K+]o resulted in reversal potentials of -81, -56 and -32 mV, respectively, which are close to values predicted if the IK channel were highly selective for K. The time constants for deactivation of IK in ventricle and AVN on return to -40 mV after a 500-ms activating pulse to + 60 mV were 480 ms and 230 ms, respectively. The faster deactivation of IK in AVN cells was a distinguishing feature and suggests that there may be differences in the IKr channel protein between ventricular and AVN cells.

Impaired regulation of cell communication by beta-adrenergic receptor activation in the failing heart

We investigated the influence of beta-adrenergic receptor activation on the control of gap junctional conductance (gj) in the heart of cardiomyopathic hamsters (11 months old). We measured gj in isolated ventricular cell pairs using two voltage-clamp circuits. Administration of isoproterenol (10(-6) mol/L) to the bath had no effect on gj in myopathic cell pairs but increased gj by 45 +/- 3% (+/- SE) in normal hamsters. Moreover, forskolin (10(-7) mol/L), an activator of adenyl cyclase, did not change gj in myopathic cells but enhanced gj by 23 +/- 2.8% in controls. Similar results were obtained with isobutylmethylxanthine (10(-6) mol/L), a phosphodiesterase inhibitor. Dibutyryl-cAMP (10(-6) mol/L), however, increased gj of cardiomyopathic cell pairs by 58 +/- 2.1% within 2 minutes and enhanced gj in controls by 50 +/- 3.6%. The effect of dibutyryl-cAMP on gj of myopathic cells was suppressed by intracellular dialysis of an inhibitor of protein kinase A. These observations indicate that the regulation of gj by the beta-adrenergic receptor-G protein-adenyl cyclase signaling system is greatly impaired in the failing heart but the ability of cAMP to increase gj is still preserved.

The actions of nickel on membrane currents activated by hyperpolarisation in single cells from the rabbit atrioventricular node

1. The atrioventricular node (AVN) is vital for cardiac function as it normally provides the only conduction route for the cardiac impulse from atria to ventricles and can act as a pacemaker for the ventricles if the sinoatrial node (SAN) fails. We have shown previously that whilst 80-90% of AVN myocytes do not possess If (we have termed these type 1 cells), a small proportion (10-20%) of AVN cells (type 2) do exhibit If. 2. The present study describes the effects of the divalent cation nickel (Ni) on membrane currents activated by hyperpolarising voltage clamps from -40/mV in type 1 and type 2 cells at 35 degrees C, using the whole cell patch clamp technique. In type 2 cells 5 mM Ni enhanced the amplitude of If. At -120 mV the mean Ni-activated If was -1.85 +/- 0.28 pA/pF (mean +/- SEM; n = 5). Ni significantly enhanced If at -70 mV and at all potentials negative to this (p < 0.05 at -70, -80, -90 and -110 mV; 0.05 < p < 0.1 at -100 mV; p < 0.005 at -120 mV). 3. In type 1 cells, which exhibit a small time-independent inward current on hyperpolarisation there was no activation of If by Ni (p > 0.1 at all potentials between -40 mV and -120 mV). 4. In type 1 cells 5 mM Ni significantly reduced the time-independent inward current activated by a hyperpolarising pulse to -120 mV (p < 0.02) and had a smaller effect at -110 and -100 mV (0.05 < p < 0.1 at these potentials). With pulses to less negative potentials there was no significant alteration of the time-independent current. 5. An additional observation was that the fast sodium current activated on repolarisation of the membrane potential to -40 mV after a hyperpolarising voltage clamp appeared to be blocked by Ni. However, this apparent blockade reflected a positive shift in the activation threshold for INa, since a repolarising step to -30 mV could still elicit INa. 6. Ni is known to block sarcolemmal Na/Ca exchange in cardiac cells, and one possible mechanism for the enhancement of If by Ni in type 2 cells is increased intracellular Ca via Na/Ca exchange blockade increasing If. The reduction in end pulse current in type 1 cells is also consistent with Na/Ca exchange current blockade. A second possibility of the enhanced If in type 2 cells with Ni is a positive shift of the activation curve for If in the presence of an increased concentration of external divalent cations.

Influence of intracellular renin on heart cell communication

The influence of intracellular renin and angiotensinogen on the control of cell-to-cell communication in heart muscle was investigated in cell pairs isolated from adult rat ventricle. Junctional conductance was measured with two separated voltage-clamp circuits. Intracellular dialysis of renin (0.2 pmol/L) caused a decrease in junctional conductance of 29 +/- 3.8% (+/- SEM, P < .05) in 7 minutes. The effect of renin on junctional conductance seems to be mainly due to the synthesis of Ang II because enalaprilat (10(-9) mol/L) dialyzed into the cell caused an appreciable reduction in the effect of renin. The intracellular administration of renin (0.2 pmol/L) plus angiotensinogen (0.4 pmol/L) produced a faster and stronger fall in junctional conductance (84.3 +/- 1.35%, P < .05), and the effect was greatly reduced by enalaprilat. The effects of both renin and angiotensinogen on junctional conductance were not related to a fall in surface cell membrane resistance or a change in series resistance. The effect of renin on junctional conductance was blocked by intracellular administration of a renin inhibitor (S 2864). Moreover, renin dialyzed into just one cell of the pair induced rectification of the junctional membrane, which was prevented by enalaprilat. The results support the view that an intracrine renin-angiotensin system in the heart regulates intercellular communication.

Rate-dependent prolongation of action potential duration in single ventricular myocytes obtained from hearts of rats with streptozotocin-induced chronic diabetes sustained for 30-32 weeks

We examined the characteristics of the action potentials of single ventricular myocytes obtained from the hearts of rats with chronically-induced diabetes. Male Wistar rats were made diabetic by injecting streptozotocin (65 mg/kg) and 30-32 weeks later the hearts were excised and used for an electrophysiological study. Action potentials were recorded from isolated right ventricular myocytes by an electrode fabricated for patch clamp in the whole-cell recording configuration. The action potential durations (APDs) of steady state chronic diabetic rat myocytes were longer than those of age-matched normal rat myocytes at all levels of repolarization (APD25, APD50, APD75, and APD90). As the stimulation frequency was increased (0.2-2 Hz), the APDs were lengthened in both diabetic and normal rats, and the difference of APDs between the groups was greater when the stimulation frequency was higher. When we examined alterations of APDs under conditions of train stimulation (2Hz, 20 stimuli), (1) the APDs in both groups were prolonged, and (2) the degree of prolongation of APD was significantly greater and the rate of APD prolongation was significantly faster in myocytes from the diabetic rats. The prolongation of APD in these heart cells is probably secondary to alteration of the transient outward current Ito, and sheds light on repolarization abnormality in cases of diabetic cardiomyopathy.

The hyperpolarisation-activated current, I(f), is not required for pacemaking in single cells from the rabbit atrioventricular node

The atrioventricular node (AVN) is vital for cardiac function. One of its properties is that it can act as a pacemaker for the ventricles if the sinoatrial node fails. This study investigates the role of the hyperpolarisation-activated inward current (I(f)) in generating pacemaker activity in morphologically normal single cells isolated from the rabbit AVN. Whole-cell patch-clamp recordings show that 80%-90% of AVN myocytes do not possess I(f), but nevertheless generate spontaneous action potentials with normal pacemaker depolarisations before each action potential upstroke. We have termed this type of cell "type 1". A small proportion (10%-20%) of spontaneously active AVN cells (type 2) do exhibit I(f). A 100 nM solution of isoprenaline increased the action potential rate of type 1 cells by 31%. In these cells isoprenaline did not activate any I(f) whereas in type 2 cells it clearly increased the amplitude of I(f). Manganese at 2 mM also increased the amplitude of I(f) in type 2 cells, but did not reveal I(f) in type 1 cells. We conclude that, whilst I(f) may play a role in modulating pacemaker activity in type 2 cells, in the majority of AVN cells (type 1) pacemaker depolarisation normally occurs in the complete absence of I(f). Furthermore, the inability of both isoprenaline and Mn to reveal I(f) in type 1 cells suggests that I(f) channels may be absent in these cells.

Effects of gap junction conductance on dynamics of sinoatrial node cells: two-cell and large-scale network models

A computational model of single rabbit sinoatrial (SA) node cells has been revised to fit data on regional variation of rabbit SA node cell oscillation properties. The revised model simulates differences in oscillation frequency, maximum diastolic potential, overshoot potential, and peak upstroke velocity observed in cells from different regions of the node. Dynamic properties of electrically coupled cells, each with different intrinsic oscillation frequency, are studied as a function of coupling conductance. Simulation results demonstrate at least four distinct regimes of behavior as coupling conductance is varied: a) independent oscillation (Gc < 1 pS); b) complex oscillation (1 < or = Gc < 220 pS); c) frequency, but not waveform entrainment (Gc > or = 220 pS); and d) frequency and waveform entrainment (Gc > or = 50 nS). The conductance of single cardiac myocyte gap junction channels is about 50 pS. These simulations therefore show that very few gap junction channels between each cell are required for frequency entrainment. Analyses of large-scale SA node network models implemented on the Connection Machine CM-200 supercomputer indicate that frequency entrainment of large networks is also supported by a small number of gap junction channels between neighboring cells.

Intracellular calcium transients recorded with Fura-2 in spontaneously active myocytes isolated from the atrioventricular node of the rabbit heart

We have used the fluorescent Ca indicator Fura-2 to assess the changes in intracellular calcium (Cai) in single spontaneously active myocytes isolated from the rabbit atrioventricular node (AVN). Simultaneous recordings of membrane potential and the Fura-2 ratio signal (which reflects Cai) showed that a transient rise of Cai occurred with each spontaneous action potential (AP). The AP upstroke preceded the rise in Cai and repolarization of the AP occurred faster than the decline of Cai. The level of Cai remained raised and progressively declined towards a baseline diastolic level during the subsequent pacemaker depolarization. The Fura-2 (Cai) transient in spontaneously active AVN cells had a time-to-peak of 49.2 +/- 5.4 ms (mean +/- s.e.m.; n = 7) and declined with a single exponential time course (time constant = 139.8 +/- 23.9 ms; n = 7). Application of 10 microM ryanodine completely and irreversibly abolished the Cai transient, identifying the sarcoplasmic reticulum (SR) as the major source of releasable Ca. Both removal of external Ca and block of L-type Ca channels (with 2 microM nifedipine) also abolished Cai transients, suggesting that Ca entry via L-type Ca-channels is involved in triggering the SR Ca release underlying the Cai transient. Removal of external Na (in the presence of 20 microM nifedipine to block L-type Ca channels) caused a reversible increase in Cai, showing that Na/Ca exchange is present in AVN cells and that it is involved in Cai regulation. Spontaneous Cai transients were abolished by 1 microM acetylcholine, and this was associated with a hyperpolarization of membrane potential and cessation of action potentials.(ABSTRACT TRUNCATED AT 250 WORDS)