Influence of a period of inactivity on the duration of post-rest action potentials of the mammalian working ventricular myocardium in correlation to the preceding stimulation frequency

Experiments were carried out on the working right ventricular myocardium of adult cats, guinea-pigs and rabbits. Membrane voltage was recorded by the glass microelectrode technique and the preparations were stimulated with frequencies of 5, 1 and 0.2 Hz. After a steady state had been reached, a pause (TP) lasting 10-600 s was interpolated. The influence of TP on the duration (D) of post-rest action potentials (AP) was studied; the effect of the pause was measured at electric zero level (D0) and at further repolarization levels (-20, -40 and -60 mV, given here as D-20, D-40 and D-60). At 1 and 0.2 Hz frequency, the cat myocardium displayed lengthening of the AP proportional to the duration of the pause; at 5 Hz frequency, D0 reacted by lengthening up to TP = 120 s and to further pauses by slight shortening. D-60, at all frequencies, lengthened throughout the whole of the given TP range. The rabbit myocardium, at all the given frequencies, reacted up to TP = 60-120 s by marked shortening of post-rest AP at all repolarization levels; with longer pauses the AP lengthened. At 5 Hz frequency the guinea-pig myocardium reacted similarly to the cat myocardium; at the lower stimulation frequencies, the pause-induced changes in the post-rest AP were less strongly expressed. In all the given types of myocardium, the most pronounced post-rest AP reactions were those at electric zero level (the plateau phase of the AP); towards more negative repolarization values and with lower pre-pause stimulation frequencies they were less strongly expressed.(ABSTRACT TRUNCATED AT 250 WORDS)     

The fertilization potential and associated membrane potential oscillations during the resumption of meiosis in the egg of the ascidian Phallusia mammillata.

The fertilization potential in Phallusia mammillata consisted of an initial rapid depolarization. This initial sperm-triggered depolarization was followed by a phase of membrane depolarization which was of either long or short duration, depending on the eggs. When of long duration, the phase of membrane depolarization was divided into two periods: the first one began with a plateau (Em = +20.2 +/- 1.1 mV; duration = 1.7 +/- 0.14 min) which was followed by a series of membrane potential oscillations (n = 3.1 +/- 0.25) lasting 2.4 +/- 0.2 min. The second period also began as a plateau (Em = approximately 0 mV; duration = 3.40 +/- 0.20 min) which was followed by a series of oscillations (n = 11.5 +/- 0.5) lasting 11.8 +/- 0.6 min, followed by a membrane repolarization. The second series of oscillations often continued rising from the resting potential value. In the eggs displaying a short duration of membrane depolarization, the second period of depolarization was shortened (lasting only 3.5 +/- 0.5 min) since it lacked the second plateau. In addition it displayed a smaller number of oscillations (n = 4.7 +/- 0.6). As a consequence of this shortening, the membrane repolarized sooner. After repolarization, the membrane displayed several potential oscillations that started from the repolarization level. Regardless of the length of the depolarized plateau phases, the total number of membrane oscillations and the time period during which they occurred were constant. Eggs displaying a long depolarization phase had 15.9 +/- 0.6 oscillations in a 19.5 +/- 0.6 min interval, while eggs having a short depolarization phase had 16.0 +/- 0.8 oscillations in a 18.1 +/- 0.3 min interval. The time period during which the potential oscillations occurred corresponded remarkably well with the time of the meiotic divisions: the formation of the first polar body was detected about 80 sec after the end of the first series of oscillations; the second polar body was extruded about 85 sec after the last membrane oscillation occurred.     

Action potentials of the rabbit, guinea pig, dog and albino rat working ventricular myocardium under interpolated extrasystole conditions during postnatal ontogenesis

Experiments were carried out on the working myocardium of the right heart ventricle of newborn and adult rabbits, guinea-pigs, dogs and albino rats. In the dog, the guinea-pig and the rabbit, after ten action potentials (AP) elicited with 1 Hz frequency we always interpolated an extrasystole at an interval (TE) of 100-900 ms. In albino rats we used a basic frequency of 2 Hz and a TE of 30-370 ms from the last regular AP. Using glass microelectrodes, we recorded the extrasystolic AP (EAP) and the next subsequent AP (2AP). The results were evaluated by constructing graphs of the correlations of the duration of the plateau phase (D0) to TE and of the duration of repolarization to -60 mV level (D60) to the TE. In the myocardium of newborn rabbits, guinea-pigs and dogs, with short TE both D0 and D60 of the EAP are shorter than in the steady state (SS), while for the 2AP the same parameters are influenced only a little. As the TE lengthens, the EAP gradually acquire a length corresponding more to the SS. With TE longer than half the duration of the cycle in the steady state the EAP return to normal, while the 2AP become shorter. The effect of extrasystole on the rat EAP and 2AP diminished with advancing age. In the myocardium of adult rabbits and adult guinea-pigs, and slightly in the myocardium of adult dogs and newborn rats, we observed that the duration of the EAP, with certain TE, was greater than in the steady state.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of ajmaline on action potential and ionic currents in rat ventricular myocytes

The effect of ajmaline on action potential (AP) and ionic current components has been investigated in right ventricular myocytes of rat at room temperature using the whole cell patch clamp technique. Ajmaline decreased the upstroke velocity ((dV/dt)max) of AP and the AP amplitude, increased the AP duration measured at 50 and 90% repolarization, and reversibly inhibited most components of membrane ionic current in a concentration-dependent manner. The following values of IC50 and of the Hill coefficient (nH) resulted from approximation of the measured data by the Hill formula: for fast sodium current (INa) IC50=27.8+/-1.14 micromol/l and nH=1.27+/-0.25 at holding potential -75 mV, IC50=47.2+/-1.16 micromol/l and nH=1.16+/-0.21 at holding potential -120 mV; for L-type calcium current (ICa-L) IC50=70.8+/-0.09 micromol/l and n(H)=0.99+/-0.09; for transient outward potassium current (Ito) IC50=25.9+/-2.91 micromol/l and nH=1.07+/-0.15; for ATP-sensitive potassium current (IK(ATP)) IC50=13.3+/-1.1 micromol/l and nH=1.16+/-0.15. The current measured at the end of 300 ms depolarizing impulse was composed of an ajmaline-insensitive component and a component inhibited with IC50=61.0+/-1.1 micromol/l and nH=0.91+/-0.08. At hyperpolarizing voltages, ajmaline at high concentration of 300 micromol/l reduced the inward moiety of time-independent potassium current (IK1) by 36%. The results indicate that the inhibition of INa causes both the decreased rate of rise of depolarizing phase and the lowered amplitude of AP. The inhibition of Ito is responsible for the ajmaline-induced AP prolongation.     

Action potential characterization in intact mouse heart: steady-state cycle length dependence and electrical restitution

Transgenic mice have been increasingly utilized to investigate the molecular mechanisms of cardiac arrhythmias, yet the rate dependence of the murine action potential duration and the electrical restitution curve (ERC) remain undefined. In the present study, 21 isolated, Langendorff-perfused, and atrioventricular node-ablated mouse hearts were studied. Left ventricular and left atrial action potentials were recorded using a validated miniaturized monophasic action potential probe. Murine action potentials (AP) were measured at 30, 50, 70, and 90% repolarization (APD(30)-APD(90)) during steady-state pacing and varied coupling intervals to determine ERCs. Murine APD showed rate adaptation as well as restitution properties. The ERC time course differed dramatically between early and late repolarization: APD(30) shortened with increasing S1-S2 intervals, whereas APD(90) was prolonged. When fitted with a monoexponential function, APD(30) reached plateau values significantly faster than APD(90) (tau = 29 +/- 2 vs. 78 +/- 6 ms, P < 0.01, n = 12). The slope of early APD(90) restitution was significantly <1 (0.16 +/- 0.02). Atrial myocardium had shorter final repolarization and significantly faster ERCs that were shifted leftward compared with ventricular myocardium. Recovery kinetics of intracellular Ca(2+) transients recorded from isolated ventricular myocytes at 37 degrees C (tau = 93 +/- 4 ms, n = 18) resembled the APD(90) ERC kinetics. We conclude that mouse myocardium shows AP cycle length dependence and electrical restitution properties that are surprisingly similar to those of larger mammals and humans.     

新生大鼠离体脊髓薄片侧角中间外侧核细胞的电生理特性

在新生大鼠离体脊髓薄片的中间外侧核作细胞内记录,研究细胞膜的静态与动态电生理特性。细胞的静息电位(RP)变动于-46—-70mV,膜的输入阻抗为108.3±67.9MΩ(X±SD,下同),时间常数9.9±5.6ms,膜电容138.6±124.2pF。用去极化电流进行细胞内刺激时,大部份细胞(85.4％)能产生高频率连续发放,其余细胞(15.6％)仅产生初始单个发放。胞内直接刺激引起的动作电位(AP)幅度为63.4±9.0mV,时程2.4±0.6ms,阈电位水平在RP基础上去极18.7±6.2mV。大部份细胞的锋电位后存在明显的超极化后电位,其幅度为5.1±2.7mV、持续90±31.8ms。刺激背根可在记录细胞引起EPSP或顺向AP,少数细胞尚出现IPSP。而刺激腹根则可引起逆向AP。     

Effects of Cl- deficiency on the membrane potential in mouse pancreatic beta-cells

The membrane potential of mouse pancreatic beta-cells was measured with microelectrodes. In the resting cell (3 mM D-glucose), the membrane potential was -63 +/- 3 mV (mean +/- S.E. for four experiments). In the presence of 3 mM D-glucose, total Cl- substitution by isethionate induced a depolarization by 3-4 mV, and readmission of Cl- induced a hyperpolarization by 3-5 mV. At 10 mM glucose, reduction of Cl- to 12 mM by substituting isethionate for Cl- reversibly shifted the repolarization potential by 6-9 mV in the positive direction and stimulated the burst activity during the initial 2-3 min by increasing the fraction of plateau phase. This was followed by a gradual inhibition of electrical activity, including decrease in fraction of plateau phase and slow wave amplitude. Total substitution of Cl- by isethionate or methyl sulphate reversibly shifted the repolarization potential by 3-4 mV in the positive direction and rapidly inhibited the electrical burst pattern without any initial stimulation. Glucose-induced (10 mM) insulin release (15 min) and 45Ca2+ uptake (3 min) were strongly inhibited by reducing the Cl- concentration to 10 mM (isethionate as substitute) and were further inhibited by further reduction of the Cl- concentration. It is suggested that beta-cells are equipped with on electrogenic Cl- flux, which can affect the burst pattern of electrical activity. The inhibitory effects of Cl- substitution may be explained by an influence of Cl- on the voltage-controlled Ca2+ channels.     

Alteration of the transmembrane K+ gradient during development of delayed rectifier in isolated rat pulmonary arterial cells

The properties of the tail current associated with the delayed rectifier K+ current (IK) in isolated rat pulmonary artery smooth muscle cells were examined using the whole cell patch clamp technique. The tail currents observed upon repolarization to -60 mV after brief (e.g., 20 ms) or small (i.e. to potentials negative of 0 mV) depolarizations were outwardly directed, as expected given the calculated K+ reversal potential of -83 mV. The tail currents seen upon repolarization after longer (e.g., 500 ms) and larger (e.g., to +60 mV) depolarizations tended to be inwardly directed. Depolarizations of intermediate strength and/or duration were followed by biphasic tail currents, which were inwardly directed immediately upon repolarization, but changed direction and became outwardly directed before deactivation was complete. When cells were depolarized to +60 mV for 500 ms both IK and the subsequent inward tail current at -60 mV were similarly blocked by phencyclidine. Both IK and the inward tail current were also blocked by 4-aminopyridine. Application of progressively more depolarized 30 s preconditioning potentials inactivated IK, and reduced the inward tail current amplitude with a similar potential dependency. These results indicated that the inward tail current was mediated by IK. The reversal potential of the tail current became progressively more positive with longer depolarizations to +60 mV, shifting from -76.1 +/- 2.2 mV (n = 10) after a 20-ms step to -57.7 +/- 3.5 mV (n = 9) after a 500-ms step. Similar effects occurred when extracellular K+ and Na+ were replaced by choline. When extracellular K+ was raised to 50 mM, the tail current was always inwardly directed at -60 mV, but showed little change in amplitude as the duration of depolarization was increased. These observations are best explained if the dependencies of tail current direction and kinetics upon the duration of the preceding depolarization result from an accumulation of K+ at the external face of the membrane, possibly in membrane invaginations. A mathematical model which simulates the reversal potential shift and the biphasic kinetics of the tail current on this basis is presented.     

Characterization of two distinct depolarization-activated K+ currents in isolated adult rat ventricular myocytes

Depolarization-activated outward K+ currents in isolated adult rat ventricular myocytes were characterized using the whole-cell variation of the patch-clamp recording technique. During brief depolarizations to potentials positive to -40 mV, Ca(2+)-independent outward K+ currents in these cells rise to a transient peak, followed by a slower decay to an apparent plateau. The analyses completed here reveal that the observed outward current waveforms result from the activation of two kinetically distinct voltage-dependent K+ currents: one that activates and inactivates rapidly, and one that activates and inactivates slowly, on membrane depolarization. These currents are referred to here as Ito (transient outward) and IK (delayed rectifier), respectively, because their properties are similar (although not identical) to these K+ current types in other cells. Although the voltage dependences of Ito and IK activation are similar, Ito activates approximately 10-fold and inactivates approximately 30-fold more rapidly than IK at all test potentials. In the composite current waveforms measured during brief depolarizations, therefore, the peak current predominantly reflects Ito, whereas IK is the primary determinant of the plateau. There are also marked differences in the voltage dependences of steady-state inactivation of these two K+ currents: IK undergoes steady-state inactivation at all potentials positive to -120 mV, and is 50% inactivated at -69 mV; Ito, in contrast, is insensitive to steady-state inactivation at membrane potentials negative to -50 mV. In addition, Ito recovers from steady-state inactivation faster than IK: at -90 mV, for example, approximately 70% recovery from the inactivation produced at -20 mV is observed within 20 ms for Ito; IK recovers approximately 25-fold more slowly. The pharmacological properties of Ito and IK are also distinct: 4-aminopyridine preferentially attenuates Ito, and tetraethylammonium suppresses predominantly IK. The voltage- and time-dependent properties of these currents are interpreted here in terms of a model in which Ito underlies the initial, rapid repolarization phase of the action potential (AP), and IK is responsible for the slower phase of AP repolarization back to the resting membrane potential, in adult rat ventricular myocytes.     

Hypothermic effects on action potential and force production of hedgehog and guinea pig papillary muscles

Action potentials and isometric force were recorded in papillary muscles from guinea pigs and summer hedgehogs at different temperatures between 37 and 0 degrees C. The action potential of the hedgehog was of a lower amplitude (mean 83 +/- 6 mV) than that of the guinea pig (mean 110 +/- 5 mV). The action potential duration at 50% repolarization was 22 +/- 2 msec in the hedgehog as compared to 105 +/- 11 msec in the guinea pig. Moreover, there was no distinct plateau phase of the hedgehog action potential. Lowering temperature prolonged the action potential duration in the two preparations by about the same percentage. However, the guinea pig preparation became progressively less excitable below 20 degrees C. Lowered temperature produced a positive inotropic effect in the guinea pig, whereas this effect was very slight in the hedgehog heart. Postextrasystolic potentiation was seen in the guinea pig but not in the hedgehog preparation. It is suggested that this difference between the preparations may be due to a greater relative amount of activator calcium in the hedgehog heart. The difference in cold tolerance between the preparations may reflect a difference in chemical composition of the sarcolemma.     

The effects of acidosis and bicarbonate on action potential repolarization in canine cardiac Purkinje fibers

Studies were performed on canine cardiac Purkinje fibers to evaluate the effects of acidosis and bicarbonate (HCO3) on action potential repolarization. Extracellular pH (pHe) was reduced from 7.4 to 6.8 by increasing carbon dioxide (CO2) concentration from 4 to 15% in a HCO3-buffered solution or by NaOH titration in a Hepes-buffered solution. Both types of acidosis produced a slowing of the rate of terminal repolarization (i.e., period of repolarization starting at about -60 mV and ending at the maximum diastolic potential) with an attendant increase in action potential duration of 10--20 ms. This was accompanied by a reduction in the maximum diastolic potential of 2--8 mV. In contrast, if the same pH change was made by keeping CO2 concentration constant and lowering extracellular HCO3 from 23.7 to 6.0 mM, in addition to the slowing of terminal repolarization, the plateau was markedly prolonged resulting in an additional 50- to 80-ms increase in action potential duration. If pHe was held constant at 7.4 and HCO3 reduced from 23.7 mM to 0 (Hepes-buffered solution), the changes in repolarization were nearly identical to those seen in 6.0 mM HCO3 except that terminal repolarization was unchanged. This response was unaltered by doubling the concentration of Hepes. Reducing HCO3 to 12.0 mM produced changes in repolarization of about one-half the magnitude of those in 6.0 mM HCO3. These findings suggest that in Purkinje fibers, HCO3 either acts as a current that slows repolarization or modulates the ionic currents responsible for repolarization.     

Calcium-sensitive action potential of long duration in the fertilized egg of the ctenophore Mnemiopsis leidyi

The membrane properties of fertilized eggs of the ctenophore Mnemiopsis leidyi were studied using standard microelectrode techniques. The resting potential was approximately -80 mV, and was dependent on the extracellular K concentration. Depolarizing current injections elicited an action potential with an initial peak amplitude of +20 to +40 mV (duration about 5 sec) and a long lasting (duration 3 to 10 min) plateau phase. The depolarizing phase and the plateau phase appeared to have different ionic mechanisms. The entire action potential could be prevented by removal of extracellular Ca, but only the amplitude of the depolarizing phase, not the plateau phase, was dependent on the extracellular Ca concentration. The plateau phase was not observed in the absence of Ca, but in the presence of Ca its duration was dependent on the external Ca concentration. The data suggest that the plateau phase is activated as a consequence of Ca influx during the initial depolarizing phase. Removal of external Na resulted in only minor changes in the waveform of repolarization. The action potential was resistant to low concentrations of Mn and Cd in the presence of Ca. The role of this action potential in ctenophore development is not known, but in its waveform and duration it resembles the sperm-gated potentials that have been seen in eggs of other phyla. These experiments show ctenophore embryos to be excitable at very early stages, and suggest their utility in the study of the differentiation of cellular electrical properties.     

Electromechanical relationships of rabbit papillary muscle under interpolated extrasystole conditions and after a pause

The aim of the study was to describe and attempt to explain certain specific features of electromechanical coupling in the rabbit myocardium. Electromechanical correlations in the papillary muscles of the right ventricle of adult rabbit hearts were studied by a programmed stimulation technique. The duration of action potentials (AP) was measured in the plateau phase (Do, ms) and at -80 mV level (D80, ms), together with the intensity of the corresponding isometric contractions (MG, arbitrary units). After twenty AP of 1 Hz frequency, we interpolated an extrasystole with a variable interval (TE = 100-900 ms) and measured D0 and MG of the premature AP and the first AP of the subsequent cycle. When a steady state at 1 Hz frequency had been reached, we interpolated pauses (Tp) of 5 to 600 s and read D80 and MG of the first to the tenth contraction after the pause. The extrasystole D0 attained the maximum at TE = 260 ms and then fell abruptly. The MG of extrasystoles with a longer TE grew from the lowest value (0.3), attained 1 at TE = 700 ms and then remained stable. The first contraction after extrasystole displayed distinct postextrasystolic potentiation (MG = 2), while D0 was unwontedly short. Prolongation of TE was accompanied by an increase in its D0 value and by a steep drop in MG (to as little as 0.2). D80 of the first AP evoked after the pause fell proportionally to log Tp and then, from Tp = 60 s, gradually rose. The MG value of the first contraction after the pause fell proportionally to log Tp. The AP recovered much more rapidly than contractility from the effect of the pause. In the discussion, an attempt is made to explain the found correlations on the basis of differences in the behaviour of the calcium current channel system in the rabbit myocardium and a commentary on electromechanical correlations is based on the hypothesis that the free sarcoplasmic calcium level determines both membrane electrogenesis and the inotropic state.     

Action potentials of the heart atria and ventricles of Rana temporaria frogs and Cyprinus carpio carp]

Using microelectrode technique, studies have been made on electrophysiological indices (amplitude of AP, amplitude of the plateau, latent period of AP, duration of maximal depolarization, duration of repolarization at different levels) of cells of isolated atrium and ventricles of the carp during both the spontaneous activity and electrical stimulation. The obtained amplitude-temporal parameters were compared to those of the heart in the frog R. temporaria. It was found that the amplitude of AP, the amplitude of the plateau and the duration of the latent period of AP in both the atrium and ventricles of the carp significantly (p less than 0.01) differ from the corresponding indices of the frog. On the contrary, the duration of maximal depolarization and repolarization in cells from homologous parts of the heart is very close in the species investigated.     

The ionic basis of cardiac activity in the bivalve mollusc Perna perna

The ionic basis of cardiac activity and aspects of excitation-contraction (E-C) coupling were investigated in the isolated heart of the bivalve mollusc Perna perna, using the sucrose-gap technique. The role of the principal ions was established employing artificial seawater, in which specific ion concentrations were modified, and ion channel blockers. The mean membrane resting potential (MP) and the action potential (AP) were -33+/-0.7 mV (n=89) and 13+/-0.3 mV (n=71), respectively. The MP potential was primarily dependent on K(+) ions. Three types of cardiac APs were identified: fast, slow and spike-plateau potentials. Cardiac activity was maintained in Na(+)- or Ca(2+)-free salines but ceased when either Cd(2+) or EDTA was added to these salines. Other Ca(2+) channel blockers reduced the amplitude and increased duration of the cardiac APs. Tetrodotoxin (TTX) and procaine did not alter the AP. The data showed that the depolarizing phase of the AP was dependent on Ca(2+) influx while the plateau phase, when present, resulted from Na(+) influx that was modulated by Ca(2+). The mechanical responses were more sensitive to changes in extracellular Ca(2+) concentration than were the electrical responses.     

Properties of aconitine-modified sodium channels in single cells of mouse ventricular myocardium

Effects of the plant alkaloid Aconitine on the kinetics of sodium channels were studied in enzymatically isolated single cells of the mouse ventricular myocardium. Aconitine (1 mumol/l) induced a prolongation of the 90% repolarization of action potentials from 52.4 +/- 3.7 ms to 217.0 +/- 12.5 ms. Delayed terminal repolarization and oscillatory afterpotentials preceded spontaneous activity with high frequencies. Peak sodium currents were diminished from 28.0 +/- 9.0 to 14.0 +/- 6.0 nA. The reversal potential of the sodium current was shifted from 16.0 +/- 11.0 to -8.0 +/- 6.0 mV (52.5 mmol/l extracellular sodium concentration) suggesting a decreased selectivity of the Aconitine-modified Na channels. The m-affinity-curves were shifted 31 mV towards more negative potentials at a constant slope. The h affinity-curves were shifted in the same direction by 13 mV. The slope parameter of the h affinity-voltage relationship was enlarged from 9.1 +/- 2.2 mV to 15.6 +/- 4.4 mV. Shifts in m affinity and h affinity resulted in an increased "window". The alkaloid modified channels inactivated extremely slowly at potentials negative to -40 mV, but showed a fast and complete inactivation at potentials positive to -40 mV.     

Transmembrane electrical characteristics of cultured human skeletal muscle cells

Skeletal muscle explants from normal subjects were established from biopsy material on collagen. Cellular outgrowth appeared within 3-4 days, and fusion of myoblasts was observed in 5-10 days. Multinucleated myotubes were impaled under high optical magnification, at 37 degrees C, with conventional glass microelectrodes. The mean resting potential was -44.4 mV +/- 2.4 (n = 399); -33 +/- 2.3 mV at 9 days (n = 10) vs -48 +/- 2.5 mV (n = 15) at 27 days. The average input resistance (Rin) was 9.7 M omega (n = 83). Action potentials could be elicited by electrical stimulation and had a mean amplitude of 55.9 +/- 2.1 mV with a mean maximum rate of rise (Vmax) of 72.1 +/- 7.5 V/s. The mean overshoot was 13.9 +/- 2.3 mV, and the action potential duration determined at 50% of repolarization (APD50) was 8.0 msec (n = 7). The resting membrane potential showed a depolarization of 23 mV/decade for extracellular potassium ion concentration ([K]o) between 4.5-100 mM. Thus, we have established the normal resting potential and maximum rate of rise of the action potential for human myotubes in culture. We have shown that the values for these are less than those previously reported in cultured avian and rodent cells. In addition, we have shown that the response in our system of the resting potential to change in extracellular potassium concentration is blunted compared to studies using isolated muscle, suggesting an increase in ratio of sodium to potassium permeability. Cultured human muscle cells depolarized in the presence of ouabain.     

Slow inward Ca current in frog heart: theoretical evidence against a voltage-dependent inactivation

The validity of a Hodgkin-Huxley type voltage-dependent inactivation of slow inward Ca current (Isi) was tested in frog heart using a computer simulation. The time course of Isi was calculated during the development of a frog atrial action potential (AP). With a time constant of inactivation (tauf) of 55 ms at a membrane potential (Em) of -15 mV, the variation of Isi was biphasic: after a transient increase followed by a decrease to zero, Isi partially "reactivated" (at the beginning of the AP repolarization phase) and then fully deactivated. The "reactivation" phase of Isi developed whether tauf was an increasing, decreasing, U-shaped, or bell-shaped function of Em. The addition of an independent and slower process responsible for the recovery from inactivation only partly suppressed the "reactivation" phase. However, until now there was no experimental evidence supporting such a biphasic variation of Isi during AP repolarization. Thus our results indicate that the Hodgkin-Huxley type model of the voltage-dependence of Isi-inactivation process may not correctly represent the actual behavior of frog cardiac muscle.     

Inward current generated by Na-Ca exchange during the action potential in single atrial cells of the rabbit.

To investigate the underlying ionic mechanism of the late plateau phase of the action potential in rabbit atrium the whole-cell patch-clamp technique with intracellular perfusion was used. We recorded the inward current during repolarizations following a brief 2 ms depolarizing pulse to +40 mV from a holding potential of between -70 and -80 mV. The development of this current coincides with the onset of the late plateau phase of the action potential. Peak activation of the current occurs about 10 ms from the beginning of the depolarizing pulse, and it decays spontaneously with a slow timecourse. Its voltage dependency from -40 mV to +40 mV shows very steep activation (-40 to -20 mV) and shows almost the same maximum magnitude between -10 mV and +40 mV. This behaviour is quite different from that of the calcium current. The inward current and the late plateau phase of the action potential were both abolished by the application of 5 mM EGTA, 1 microM ryanodine and by reducing the Na+ gradient. The fully activated current-voltage relation of the inward current was plotted as the difference current before and after treatment with Ryanodine, Diltiazem, 20 mM Na+ inside or 30% Na+ outside and shows an exponential voltage dependence with the largest magnitude of the current occurring at negative potentials. The current-voltage (I-V) curve was well fitted by the Na-Ca exchange equation, i = A exp (-(1 - r)EF/RT). The results suggest that the inward current contributes to the generation of the late plateau phase of the rabbit atrial action potential, and is activated by intracellular calcium released from the sarcoplasmic reticulum. Sarcoplasmic reticulum calcium release appears to be triggered both by the membrane voltage and by the calcium current. It is concluded that the inward current is generated by Na-Ca exchange.