End Of Depolarization Research Articles

Effect of the anxiolytic fabomotizol on the time parameters of atrial depolarization in the acute phase of myocardial infarction

The aim of the work is to study the effect of the anxiolytic fabomotizole (15 mg/kg, i.v.) on atrial depolarization in the acute phase of myocardial infarction using the method of synchronous multichannel cardioelectrochronotopography. Synchronous multichannel cardioelectrochronotopography was used to study the sequence of depolarization of the atrial epicardium in outbred rats during the acute phase of myocardial infarction caused by ligation of the anterior branch of the left coronary artery with simultaneous administration of the σ1-receptor agonist fabomotizole. Significant differences in the temporal characteristics of atrial depolarization in the acute phase of myocardial infarction were revealed in control animals and animals treated with fabomotizol. In rats against the background of fabomotizol, changes in the time of the onset of epicardial depolarization, the transition of the excitation wave from the right to the left atrium, and the end of depolarization, compared with the initial state, occur by the 5th minute after the ligation of the coronary vessel; in the process of further development of acute myocardial infarction, changes in time parameters are not observed .In animals of the control group, changes in the time parameters of atrial depolarization after coronary vessel ligation develop by 10 minutes after ligation and continue to change as acute myocardial infarction develops. Anxiolytic fabomotizol in the most acute phase of myocardial infarction changes the time parameters of atrial depolarization, preventing the development of atrial fibrillation.

Abstract 1830: New ECG Criteria for Diagnosis of Arrhythmogenic Right Ventricular Dysplasia/Cardiomyopathy

Introduction Desmosomal changes, electrical uncoupling and surviving myocardial bundles embedded in fibrofatty tissue are hallmarks of activation delay (AD) in arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C). AD is pivotal for reentrant mechanisms and thereby ventricular tachycardia (VT). At present, generally accepted Task Force Criteria (TFC) are used for clinical diagnosis of ARVD/C. We propose additional criteria (AC) on AD and VT to improve identification of affected individuals. Methods AD and VT-related electrocardiographic criteria were studied, while off drugs, in 42 patients with proven ARVD/C according to TFC, and 23 controls with idiopathic VT from the RV outflow tract. TFC assessed: epsilon waves, QRS width >110ms in V1–3 in absence of RBBB and negative T-waves in V2 and beyond. The 3 new AC are: prolonged S-wave upstroke in V1–3 (≥55ms from nadir of S to end of depolarization) occurrence of LBBB like VT with axis −30° to −150°and number of different monomorphic VT morphologies (different when Δ axis ≥30°; spontaneous and induced by programmed electrical stimulation (PES)). All ARVD/C patients were screened for mutations in genes encoding desmosomal proteins. Results For comparison of ARVD/C patients (pts) and controls: see table . In ARVD/C pts mean S-wave upstroke duration was 63±20 ms (range 40–140), mean number of different spontaneous VTs 1.8±0.95 (range 0–5) and mean number of spontaneous+induced VTs 2.8±1.08 (range 1–5). All VTs showed LBBB morphology. Plakophilin-2 mutations were identified in 25 (60%) of ARVD/C pts. Parameters measured were not significantly different between mutation carriers and non-carriers. Conclusions The newly proposed criteria are sensitive and specific for ARVD/C and thereby useful for its diagnosis, independent of outcome of DNA analysis. Prolonged S-wave upstroke is the most sensitive ECG indicator of activation delay during sinus rhythm. PES contributes to yield of VT morphologies.

Mitochondrial regulation of the cytosolic Ca2+ concentration and the InsP3-sensitive Ca2+ store in guinea-pig colonic smooth muscle.

1. Mitochondrial regulation of the cytosolic Ca2+ concentration ([Ca2+]c) in guinea-pig single colonic myocytes has been examined, using whole-cell recording, flash photolysis of caged InsP3 and microfluorimetry. 2. Depolarization increased [Ca2+]c and triggered contraction. Resting [Ca2+]c was virtually restored some 4 s after the end of depolarization, a time when the muscle had shortened to 50 % of its fully relaxed length. The muscle then slowly relaxed (t = 17 s). 3. The decline in the Ca2+ transient was monophasic but often undershot or overshot resting levels, depending on resting [Ca2+]c. The extent of the overshoot or undershoot increased with increasing peak [Ca2+]c. 4. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP; 5 microM), which dissipates the mitochondrial proton electrochemical gradient and therefore prevents mitochondrial Ca2+ accumulation, slowed Ca2+ removal at high ( > 300 nM) but not at lower [Ca2+]c and abolished [Ca2+]c overshoots. Oligomycin B (5 microM), which prevents mitchondrial ATP production, affected neither the rate of decline nor the magnitude of the overshoot. 5. During depolarization, the global rhod-2 signal (which represents the mitochondrial matrix Ca2+ concentration, [Ca2+]m) rose slowly in a CCCP-sensitive manner during and for about 3 s after depolarization had ended. [Ca2+]m then slowly decreased over tens of seconds. 6. Inhibition of sarcoplasmic reticulum Ca2+ uptake with thapsigargin (100 nM) reduced the undershoot and increased the overshoot. 7. Flash photolysis of caged InsP3 (20 microM) evoked reproducible increases in [Ca2+]c. CCCP (5 microM) reduced the magnitude of the [Ca2+]c transients evoked by flash photolysis of caged InsP3. Oligomycin B (5 microM) did not reduce the inhibition of the InsP3-induced Ca2+ transient by CCCP thus minimizing the possibility that CCCP lowered ATP levels by reversing the mitochondrial ATP synthase and so reducing SR Ca2+ refilling. 8. While CCCP reduced the magnitude of the InsP3-evoked Ca2+ signal, the internal Ca2+ store content, as assessed by the magnitude of ionomycin-evoked Ca2+ release, did not decrease significantly. 9. [Ca2+]c decline in smooth muscle, following depolarization, may involve mitochondrial Ca2+ uptake. Following InsP3-evoked Ca2+ release, mitochondrial uptake of Ca2+ may regulate the local [Ca2+]c near the InsP3 receptor so maintaining the sensitivity of the InsP3 receptor to release Ca2+ from the SR.

Age-associated changes of cytoplasmic calcium homeostasis in cerebellar granule neurons in situ: investigation on thin cerebellar slices.

Mechanisms of cytoplasmic calcium homeostasis were investigated in adult and old CBA mice. The cytoplasmic calcium concentration ([Ca2+]i) was measured on fura-2/AM loaded granule neurons in acutely isolated cerebellar slices. The resting [Ca2+]i was significantly higher in senile cerebellar granule neurons, being on average 60 +/- 15 nM (n = 163) in adult and 107 +/- 12 nM (n = 129) in old neurons. The depolarization-induced [Ca2+]i transients were markedly altered in old neurons as compared with adult ones: their amplitude was smaller by about five times, the rate of rise was prolonged about two times, and the complete recovery to the resting level after the end of depolarization was about five times longer. The amplitude of calcium release from caffeine/Ca(2+)-sensitive endoplasmic reticulum calcium stores also become significantly smaller in old neurons (the amplitudes of [Ca2+]i transients evoked by 30 mM caffeine were 75 +/- 27 nM (n = 29) in adult and 25 +/- 10 nM (n = 23) in old neurons). We conclude that neuronal aging is associated with prominent changes in the mechanisms responsible for [Ca2+]i regulation. These changes presumably include lowering of voltage-gated plasmalemmal Ca2+ influx and slowing down of Ca2+ extrusion from the cytoplasm.

Calcium homeostasis in aged neurones

Mechanisms of cytoplasmic calcium homeostasis were investigated in peripheral and central neurones isolated from neonatal, adult and old Wistar rats and in granule neurones in acutely prepared cerebellar slices of adult and old CBA mice. The cytoplasmic calcium concentration ([Ca 2+] i) was measured by either indo-1- or fura-2-based microfluorimetry. The resting [Ca 2+]. was significantly higher in senile neurones. The depolarization-induced [Ca 2+] i transients were markedly altered in old neurones when compared with adult ones: the age-associated changes in stimulus-evoked [Ca 2+] i signalling comprised of (i) significant decrease of the amplitudes of [Ca 2+ i transients; (ii) prolongation of the rising phase and (iii) prominent deceleration of the recovery of the [Ca 2+]elevation towards the resting level after the end of depolarization. The amplitudes of calcium release from caffeine/Ca 2+-sensitive endoplasmic reticulum calcium stores became significantly smaller in old central neurones, whereas they remained unaffected in peripheral neurones. Based on our observations we can conclude that ageing of the nervous system is associated with significant changes in mechanisms of [Ca 2+] i homeostasis in individual neurones. These changes lead to a stable increase in the resting [Ca 2+] i and to a substantial prolongation of stimulus-evoked [Ca 2+] i signals. We could suggest also that the ability of the old neurones to handle Ca 2+ loads is diminished, which may determine higher vulnerability of aged neurones to excess of calcium ions.

Reappraisal of the role of sodium ions in excitation-contraction coupling in frog twitch muscle.

Tetanic and twitch tension were recorded on isolated frog twitch fibres under experimental conditions modifying the influx of sodium ions. In current clamp conditions replacing Li+ for Na+ did not modify the electrical activity but drastically decreased the plateau of tetanic tension. In voltage clamp conditions replacing Li+ for Na+ did not modify the inward currents but induced a marked decrease of the plateau of the tetanic tension for depolarizations between the activation threshold and the reversal potential of sodium current. Under veratridine treatment, during tetanic depolarization, a slow inward sodium (or lithium) current developed. This induced a parallel increase of the tetanic tension which was much more pronounced in sodium than in lithium containing solution. The twitch tension obtained during short depolarization was increased by greater than 100% during veratridine treatment with a sizeable decrease (40%) of the delay between the end of depolarization and the beginning of tension. All these results could be reproduced in calcium-free solution. Our data confirm that the entry of sodium ions (and to a lesser extent of lithium ions) is able to modulate the release of calcium from the sarcoplasmic reticulum (SR). We discuss these results in terms of a model where sodium ions entering the compartment between the tubular membrane and the SR junctional membrane carry counter charges through the SR K+ channels and help to maintain the SR Ca2+ release. This could occur in particular during a physiological tetanic contraction where the junctional compartment is probably filled with Na+ ions and depleted of K+ ions.

Patterns of synaptic input to identified flight motoneurons in the locust

1. We examined the pattern of synaptic input to identified flight motoneurons of the locust,Locusta migratoria. The preparation we used was one in which rhythmic motor activity resembling flight activity could be generated following removal of all sensory input from wing receptors. Intracellular recordings were made from the main neuropil processes of all the 17 different types of fast motoneurons in the meso- and metathoracic ganglia which innervate the main flight muscles. Motoneurons were identified anatomically by intracellular staining with Lucifer yellow following recording. 2. The pattern of synaptic input was similar in all elevator motoneurons (4 different types in each of the two ganglia). Three distinct components were observed in the profile of synaptic activity in these motoneurons. The first was a relatively slow depolarization beginning immediately after the end of a depolarization in depressor motoneurons, the second was a rapid depolarization which began at a variable interval after the beginning of the initial slow depolarization, and the third was a rapid repolarization occurring in-phase with depressor depolarizations. The similarity of synaptic input to elevator motoneurons prevented the use of physiological criteria for the identification of individual elevator motoneurons. 3. Unlike the elevators, the depressor motoneurons do not form a homogeneous group with regard to the pattern of centrally derived synaptic input. Different patterns of input were observed in different depressor motoneurons, including differences in input to homologous motoneurons in the meso- and metathoracic ganglia. The most obvious difference between the profiles of synaptic input to elevator and depressor motoneurons was that the depolarizations in the latter consisted of only a single component, the duration of which increased relatively slowly with increases in cycle time. Knowledge of the recording site within a ganglion and the pattern of synaptic input allowed depressor motoneurons to be identified individually. 4. Simultaneous recordings from an elevator and a depressor motoneuron within the same ganglion showed that the synaptic input to the two groups of motoneurons is not symmetrically timed. Each depolarization in depressor motoneurons began immediately following the end of a rapid depolarization in elevators, but the beginning of the rapid depolarizations in elevators was delayed following the end of each depressor depolarization by an amount depending on cycle time. A single depressor depolarization never occurred in the absence of a preceding rapid elevator depolarization. We conclude that the basic unit of activity in the central oscillator is an elevator followed by a depressor depolarization sequence. 5. Simultaneous recordings from homologous motoneurons in the meso- and metathoracic ganglia of deafferented preparations revealed that the depolarizations in elevator motoneurons in the two ganglia were in-phase. In depressors, however, the depolarizations in hindwing motoneurons led those in homologous forewing motoneurons by 5 to 15 ms indicating that central processes are partly, but not completely, responsible for establishing the relative timing of fore and hind wing movements in normal flight. This basic difference in the intersegmental driving of elevator and depressor motoneurons in deafferented preparations was confirmed in electromyographic recordings from homologous fore- and hindwing muscles.

Block of sodium current in myelinated nerve fibre with enkephalins

The effects of leu- and met-enkephalin were investigated on the node of Ranvier of isolated nerve fibres of frog under current and voltage clamp conditions. When added to the external solution, enkephalins (1–5 mM) caused a slight decrease in peak Na + and steady state K + currents. The action potential was not significantly affected. When added to the internal medium (by diffusion from the two cut ends of the fibre), enkephalins (<5 mM) drastically reduced the peak Na + current without significantly affecting the steady state K + current. The block of Na + current was greatly accelerated and enhanced by repetitive depolarizations. The sodium current slowly recovered after the end of pulsing. The extent of the block and rate of accumulation increased with increasing magnitude and frequency of the depolarizing pulses. Internal applications of enkephalins induced a blockade of the action potential when the fibre was stimulated at frequencies above 0.1 Hz. The results suggest that during depolarizations, enkephalin molecules plug the inner end of Na + channels or immobilize the channel gates leaving the channels in a closed configuration, and remain in or near the pores for a long time after the end of depolarizations. Possible physiological significance and molecular mode of action of enkephalins on myelinated nerve fibres are discussed.

A voltage-sensitive persistent calcium conductance in neuronal somata of Helix.

1. An intracellular voltage clamp in conjunction with a patch pipette utilizing feed-back to monitor local current from the soma membrane were used to analyse transient and stationary currents in bursting pacemaker neurones in Helix pomatia and H. levantina. 2. A weak, net inward current flows during small (less than or equal 20 mV) depolarizations. This current exhibits slow activation kinetics, persistence during prolonged depolarization, and slow turning off at end of depolarization. Consequently, the steady-state current-voltage curve exhibits a region of negative resistance from about -55 to -35 mV. 3. The slow inward current and the negative resistance characteristic are rapidly and completely abolished by substitution of Co2+ or La3+ for Ca2+ and are partially blocked by the Ca-blocking drug D-600. Substitution of Tris or glucose for Na+ significantly reduces the inward current only after 15-20 min exposure, recovery being equally slow. 4. The inward current and the negative resistance characteristic of the I-V curve are greatly enhanced by Ba2+ substitution for Ca2+. This is ascribed in part to Ba2+ carrying current through the slow inward current channels and in part to a suppression of the late K+ current by Ba2+. 5. The inward current is also present in many non-bursting neurones but fails to appear as a net inward current due to short circuiting by a leakage current or by the delayed potassium current. In these cells the slow inward current contributes to inward going rectification. Replacement of Ca2+ with Ba2+ enhances the current so as to produce a net inward current during small depolarizations in these neurones. 6. It is concluded that the slow inward current is carried primarily by Ca2+ in the soma membrane of bursting pace-maker neurones and a number of non-bursting cells examined in the parietal ganglion of Helix. 7. The sensitivity to small depolarizations and persistence during prolonged depolarization suggests two roles for the Ca system in the generation of slow pace-maker oscillations. In this model the Ca system contributes to the slow depolarization which constitutes the onset of the pace-maker wave, and also contributes to the increment in [Ca] in which activates the Ca-sensitive K+ conductance responsible for repolarization. The inhibition of spontaneous bursting by Ca-blocking agents supports this model.

Usefulness of vectorcardiography combined with his bundle recordings and cardiac pacing in evaluation of the preexcitation (Wolff-Parkinson-White) syndrome

Vectorcardiograms (Frank system) were recorded simultaneously with His bundle electrograms during atrial stimulation in 3 patients with intermittent preexcitation Wolff-Parkinson-White (WPW) syndrome. In 2 cases with classic WPW, type A, loops were obtained during (1) exclusive conduction through the normal pathway, (2) coexisting accessory bundle and normal pathway conduction, and (3) exclusive accessory pathway conduction. Combination beats (resulting from ventricular depolarization through both pathways) showed an initial delay associated with anteriorly placed QRS loops and maximal vectors. When exclusive accessory bundle conduction occurred, a rightward and anteriorly oriented terminal delay was added to the initial slowing. This terminal delay was not ascribed to right bundle branch block but to the specific pattern of activation of an impulse apparently propagating from the posterosuperior wall of the left ventricle. In fact, these vectorcardiograms were similar to those recorded from patients with ectopic impulses originating at the posterosuperior left ventricular wall. When exclusive conduction through the normal pathway was associated with right bundle branch block, the prolonged St-V interval coexisted with a loop showing only terminal conduction disturbances. The loops seen during exclusive accessory bundle conduction in the patient with WPW, type B, had a leftward, posterior and superior orientation. The spatial delay was diffuse, although more marked toward the end of depolarization. This terminal (leftward) conduction defect was not attributed to right bundle branch block. The sequence of ventricular activation (as recorded at the body surface) was similar to that occurring during right ventricular apical pacing. Resemblance between vectorcardiograms showing exclusive accessory bundle conduction and those produced by direct stimulation of specific ventricular sites suggests, but certainly does not prove, that the electrical impulse is propagated from equivalent sites.

ELECTRO-MECHANICAL COUPLING IN CRAYFISH MUSCLE FIBERS EXAMINED BY THE VOLTAGE CLAMP METHOD

1. Electro-mechanical coupling in the crayfish short muscle fibers was investigated by the voltage clamp method using intracellular microelectrodes.2. The change in membrane potential was uniformly distributed when depolarization was more negative than -20mV and longer than 100 msec.3. The mechanical threshold potential was -50 to -58mV. The curve relating the tension to the membrane potential was nearly linear in the range between -45 and -20mV for every duration of depolarization. The slope of the curve was 0.3 of the tension at 0mV for 10mV change in potential. The tension-membrane potential curve obtained in the potassium contracture experiments was quite similar to that in the voltage clamp experiments.4. The rising phase of isometric contraction, except for the initial phase within 0.1 sec, was explained by two exponential components. The time constants were 0.4-0.6 sec and 0.08-0.12 sec at 20-25°C, and they were independent of the membrane potential.5. The relaxation phase after the end of depolarization was also explained by three exponential components. The time constants were 0.4-0.65 sec, 0.10-0.12 sec, and 0.05-0.08 sec at 20-27°C, and they were independent of the level and duration of the clamped membrane potential.6. The mechanical threshold potential was unaffected by the change in the speed of depolarization. The maximum rate of tension rise did not decrease until the speed of depolarization was reduced to the critical value of 40-80 mV/sec.

Tension fall after contraction of bullfrog atrial muscle examined with the voltage clamp technique.

1) Tension fall after termination of a depolarization was studied on the bullfrog atrial muscle under voltage clamp by the double glycerol-gap technique.2) The tension fall was composed of three different phases:(a) initial retardation, (b) principal rapid relaxation, and (c) final slow relaxation.3) The initial and second phases appeared exponential with time after repolarization. At 17°C, the time constant of the former was 0.07-0.08sec and that of the latter, 0.3-0.4sec for 25 to 80mV depolarizations.4) The tension and the rate of tension fall during the second principal phase appeared almost proportional to the maximal tension at the end of depolarization, and strongly depended on the membrane potential levels before and after repolarization.5) The principal tension fall during relaxation at different potential levels was also exponential, and the time constant increased in a hyperbolic manner with depolarization.6) The relationship between the logarithm of late tension around 1 sec after repolarization and the membrane potential level during relaxation was linear for the depolarizing side. For the hyperpolarizing side the tension, becoming independent of the potential, appeared as a mere function of time.7) These results are discussed in connection with possibilities that the initial retardation of tension fall relates to probable Ca influx immediately after repolarization and the second principal phase of relaxation, to the Na-Ca exchange mechanism.

The effects of metal ions and caffeine on electro-mechanical coupling in crayfish muscle fibers.

1. The effects of divalent metal ions on electro-mechanical coupling were examined in crayfish muscle fibers, using the voltage clamp method. Metal ions were added by substitution for Na. The relation between tension and membrane potential was directly figured by leading tension and potential to the Y and X axes of the oscilloscope and by depolarizing the membrane very slowly with the linearly rising pulse.2. Replacement of NaCl with equimolar choline chloride shifted the tension-membrane potential relation toward a more negative potential.3. Divalent metal ions elevated the mechanical threshold potential and decreased the tension at the given membrane potential. They not only shifted the curve relating tension and membrane potential along the potential axis but also decreased the slope of the curve. The order of the inhibitory action was Cd>Co≥Mn>Ni>Ca≥Mg>Sr>Ba. Their inhibitory effects appeared fairly rapidly after addition of metal ions and disappeared, fully reversibly, after their removal.4. The time courses of tension rise after the start of depolarization and its fall after the end of depolarization were explained as the sum of two or three exponential components. The time constants were not changed by metal ions.5. Caffeine lowered the mechanical threshold potential and shifted the tension-membrane potential relation toward a more negative potential.6. It is suggested that the inhibitory effect of metal ions on tension results from their stabilizing action on the Ca-permeability of the internal membrane system.