Excitation-contraction coupling and extracellular calcium transients in rabbit atrium: reconstruction of basic cellular mechanisms

Reciprocal role of the inward currents ib, Na and i(f) in controlling and stabilizing pacemaker frequency of rabbit sino-atrial node cells

Experiments and computations were done to clarify the role of the various inward currents in generating and modulating pacemaker frequency. Ionic currents in rabbit single isolated sino-atrial (SA) node cells were measured using the nystatin-permeabilized patch-clamp technique. The results were used to refine the Noble-DiFrancesco-Denyer model of spontaneous pacemaker activity of the SA node. This model was then used to show that the pacemaker frequency is relatively insensitive to the magnitude of the sodium-dependent inward background current ib, Na. This is because reducing ib, Na hyperpolarizes the cell and so activates more hyperpolarizing-activated current, i(f), whereas the converse occurs when ib, Na is increased. The result is that i(f) and ib, Na replace one another and so stabilize nodal pacemaker frequency.

Theory of excitation-contraction coupling in cardiac muscle

The consequences of cardiac excitation-contraction coupling by calcium-induced calcium release were studied theoretically, using a series of idealized models solved by analytic and numerical methods. "Common-pool" models, those in which the trigger calcium and released calcium pass through a common cytosolic pool, gave nearly all-or-none regenerative calcium releases (in disagreement with experiment), unless their loop gain was made sufficiently low that it provided little amplification of the calcium entering through the sarcolemma. In the linear (small trigger) limit, it was proven rigorously that no common-pool model can give graded high amplification unless it is operated on the verge of spontaneous oscillation. To circumvent this problem, we considered two types of "local-control" models. In the first type, the local calcium from a sarcolemmal L-type calcium channel directly stimulates a single, immediately opposed SR calcium release channel. This permits high amplification without regeneration, but requires high conductance of the SR channel. This problem is avoided in the second type of local control model, in which one L-type channel triggers a regenerative cluster of several SR channels. Statistical recruitment of clusters results in graded response with high amplification. In either type of local-control model, the voltage dependence of SR calcium release is not exactly the same as that of the macroscopic sarcolemmal calcium current, even though calcium is the only trigger for SR release. This results from the existence of correlations between the stochastic openings of individual sarcolemmal and SR channels. Propagation of regenerative calcium-release waves (under conditions of calcium overload) was analyzed using analytically soluble models in which SR calcium release was treated phenomenalogically. The range of wave velocities observed experimentally is easily explained; however, the observed degree of refractoriness to wave propagation requires either a strong dependence of SR calcium release on the rate of rise of cytosolic calcium or localization of SR release sites to one point in the sarcomere. We conclude that the macroscopic behavior of calcium-induced calcium release depends critically on the spatial relationships among sarcolemmal and SR calcium channels, as well as on their kinetics.

The giant cardiac membrane patch method: stimulation of outward Na(+)-Ca2+ exchange current by MgATP

1. A giant patch method was used to study the stimulatory effect of cytoplasmic MgATP on outward Na(+)-Ca2+ exchange current in inside-out cardiac membrane patches (1-10 G omega seals with 14-24 microns pipette tip diameters) excised from guinea-pig, rabbit and mouse myocytes. 2. To establish the validity of the method with respect to structure, bleb formation was examined with electron microscopy and with confocal fluorescence light microscopy. The blebs, which form as the sarcolemma detaches, excluded intracellular organelles and transverse tubules. The blebbed cells contained normal sarcomeres, sarcoplasmic reticulum, triads and diads. 3. To further establish the validity of the method for ion transport studies, measurements of Na(+)-K+ pump currents and charge movements are described briefly which demonstrate (i) free access to the cytoplasmic membrane side, (ii) MgATP dependence comparable to reconstituted pump (Kd, 94 microns), (iii) fast, rigorous concentration control and (iv) Na(+)-K+ pump densities in the range of whole-cell densities. 4. Stimulation of outward Na(+)-Ca2+ exchange current by MgATP attenuated exchange current decay during step increments of cytoplasmic sodium, shifted the secondary activation of outward exchange current by cytoplasmic calcium to lower free calcium concentrations and, particularly in mouse cardiac sarcolemma, induced cytoplasmic calcium-independent current. 5. Upon removal of MgATP the stimulatory effect usually decayed with a t50 (half-time) of about 3 min. However, the reversal took place much more rapidly (t50, 5-20 s) in patches from individual guinea-pig and rabbit myocyte batches. When decay was rapid, secondary activation by cytoplasmic calcium was shifted to higher free cytoplasmic calcium concentrations (Kd, 10-65 microns-free calcium). 6. With repeated applications of MgATP the rate and magnitude of the stimulatory effect progressively decreased. 7. The Kd for MgATP of the initial rate of stimulation of outward exchange current was 3 mM or greater. When decay was rapid, the steady-state dependence of exchange current on MgATP also had a Kd of 3 mM or greater. 8. Stimulation of Na(+)-Ca2+ exchange current by MgATP occurred in the absence of cytoplasmic calcium with 9 mM-EGTA. 9. The stimulatory effect of 2 mM-MgATP was not inhibited by up to 200 microM of the protein kinase inhibitor 1-(5-isoquinoline sulphonyl)-2-methylpiperazine (H7), or by peptide inhibitors of cyclic AMP-dependent protein kinase, protein kinase C and calcium-calmodulin-dependent protein kinase II.(ABSTRACT TRUNCATED AT 400 WORDS)

Contribution of sarcolemmal sodium-calcium exchange and intracellular calcium release to force development in isolated canine ventricular muscle

The aim of this work was to determine the relationship between peak twitch amplitude and sarcoplasmic reticulum (SR) Ca2+ content during changes of stimulation frequency in isolated canine ventricle, and to estimate the extent to which these changes were dependent upon sarcolemmal Na(+)-Ca2+ exchange. In physiological [Na+]o, increased stimulation frequency in the 0.2-2-Hz range resulted in a positive inotropic effect characterized by an increase in peak twitch amplitude and a decrease in the duration of contraction, measured as changes in isometric force development or unloaded cell shortening in intact muscle and isolated single cells, respectively. Action potentials recorded from single cells indicated that the inotropic effect was associated with a progressive decrease of action potential duration and a marked reduction in average time spent by the cell near the resting potential during the stimulus train. The frequency-dependent increase of peak twitch force was correlated with an increase of Ca2+ uptake into and release from the SR. This was estimated indirectly using the phasic contractile response to rapid (less than 1 s) lowering of perfusate temperature from 37 degrees C to 0-2 degrees C and changes of twitch amplitude resulting from perturbations in the pattern of electrical stimulation. Lowering [Na+]o from 140 to 70 mM resulted in an increase of contractile strength, which was accompanied by a similar increase of apparent SR Ca2+ content, both of which could be abolished by exposure to ryanodine (1 x 10(-8) M), caffeine (3 x 10(-3) M), or nifedipine (2 x 10(-6) M). Increased stimulation frequency in 70 mM [Na+]o resulted in a negative contractile staircase, characterized by a graded decrease of peak isometric force development or unloaded cell shortening. SR Ca2+ content estimated under identical conditions remained unaltered. Rate constants derived from mechanical restitution studies implied that the depressant effect of increased stimulation frequency in 70 mM [Na+]o was not a consequence of a decreased rate of refilling of a releasable pool of Ca2+ within the cell. These results demonstrate that frequency-dependent changes of contractile strength and intracellular Ca2+ loading in 140 mM [Na+]o require the presence of a functional sarcolemmal Na(+)-Ca2+ exchange process. The possibility that the negative staircase in 70 mM [Na+]o is related to inhibition of Ca(2+)-induced release of Ca2+ from the SR by various cellular mechanisms is discussed.

The relationship between contraction and intracellular sodium in rat and guinea-pig ventricular myocytes

1. The contraction, measured optically, and the intracellular Na+ activity (aNai), measured with the Na(+)-sensitive fluorescent dye SBFI, have been recorded simultaneously in rat and guinea-pig ventricular myocytes. 2. In rat and guinea-pig ventricular myocytes at rest, aNai was 7.8 +/- 0.3 mM (n = 4) and 5.1 +/- 0.3 mM (n = 16), respectively. 3. When both rat and guinea-pig ventricular myocytes were stimulated at 1 Hz after a rest there was usually a gradual increase in twitch shortening (referred to as a 'staircase') over several minutes accompanied by an increase in aNai over a similar time course. Twitch shortening increased by 21 +/- 3% (n = 6) and 20 +/- 4% (n = 16) (of steady-state twitch shortening during 1 Hz stimulation) per millimolar rise in aNai in rat and guinea-pig ventricular myocytes, respectively. 4. When rat and guinea-pig ventricular myocytes were exposed to strophanthidin to block the Na(+)-K+ pump, there were increases in twitch shortening and aNai over similar time courses. Twitch shortening increased by 24 +/- 4% (n = 5) and 20 +/- 3% (n = 10) (of control twitch shortening) per millimolar rise in aNai in rat and guinea-pig ventricular myocytes respectively. 5. The inotropic effect of cardiac glycosides, such as strophanthidin, is widely regarded to be principally the result of the rise in aNai. The similarity of the relation between twitch shortening and aNai during the staircase and on application of strophanthidin suggests that the progressive increase in the strength of contraction during the staircase was also linked to the rise in aNai. 6. In guinea-pig, but not rat, ventricular myocytes there was hysteresis in the relation between twitch shortening and aNai on application and wash-off of strophanthidin. This indicates that strophanthidin has another inotropic action in guinea-pig ventricular myocytes. 7. A computer model of excitation-contraction coupling has been developed to simulate the staircase and the action of cardiac glycoside and to account for the relation between contraction and intracellular Na+.

Electrical restitution in the endocardium of the intact human right ventricle

OBJECTIVE

To characterise electrical restitution in the intact human heart.

PATIENTS AND METHODS

A series of monophasic action potential electrical restitution curves were constructed from a single right ventricular endocardial site in eight patients (three men) without structural heart disease aged 52-68 (mean 55 years). A combination pacing/monophasic action potential electrode was used to pace and record monophasic action potentials at drive cycle lengths of from 350 ms to 1500 ms. Ventricular extrastimuli were delivered at 20 cycle intervals and decreased from the longest coupling interval attainable without escape beats.

RESULTS

Restitution curves shifted downward and towards the left; steady state action potential duration shifted from the restitution plateau and descended the curve, the amount of shift being linearly related to drive cycle length in two patients in whom the relation could be assessed; the amount of monophasic action potential shortening was a function of the degree of prematurity and that relation was unaffected by drive rate; the magnitude of restitution and the time constant of the restitution curve were not changed significantly by altered drive cycle length.

CONCLUSION

In the intact heart in vivo, electrical restitution (of the monophasic action potential) has similar characteristics to those (of the transmembrane action potential) in cellular preparations in vitro. Thus the alteration of action potential plateau currents by instantaneous rate change or drug effects, which can be directly observed by techniques available to the cellular electrophysiologist, may be indirectly assessed in vivo by characterisation of the effect of these on electrical restitution.

Kinetic models of Na-Ca exchange in ferret red blood cells. Interaction of intracellular Na, extracellular Ca, Cd, and Mn

The kinetic equation that best describes the intracellular Na dependence of Ca influx into ferret red cells is sequential; whether this implies that there is a conformation of the protein that has both Na and Ca ions bound remains to be determined. Cd and Mn substitute very well for Ca on the exchanger in ferret red cells; this suggests that the Ca-binding site does not contain an important thiol and that the one of the Na steps may be rate limiting.

Control of the Na-Ca exchanger in isolated heart cells. II. Beat-dependent activation in normal cells by intracellular calcium

Electrical stimulation of isolated adult rat heart cells in suspension at 4 Hz resulted in a fourfold increase in the rate of sodium influx and efflux across the sarcolemma, with no change in total cell sodium, as measured with 22Na. The magnitude of stimulation-dependent sodium fluxes under these conditions averaged 17 nmol/min/mg protein. The increased rate of efflux was inhibited by tetrodotoxin, verapamil, or dichlorobenzamil and required extracellular calcium. The inhibition by tetrodotoxin was overcome by Bay K 8644. The basal rate of 22Na efflux in cells at rest was inhibited only slightly by dichlorobenzamil. The stimulation-induced efflux was not inhibited by ouabain, but in the presence of ouabain, stimulation increased the rate of accumulation of total sodium by 4 nmol/min/mg. This increase was inhibited by tetrodotoxin or verapamil. A calcium-dependent increase in rate of 22Na influx and efflux could also be induced by KCl addition. This was inhibited by verapamil and dichlorobenzamil but not by tetrodotoxin and was reversed by EGTA, but only after a delay. We conclude the following. 1) The Na-Ca exchanger in cells at rest is no more than 10% activated. 2) The exchanger becomes activated directly or indirectly by calcium that enters the cell through calcium channels during excitation. 3) In this preparation the major part of excitation-induced sodium fluxes are mediated by the Na-Ca exchanger, with only a relatively small direct participation of sodium channels. These channels participate indirectly by promoting calcium channel activation. 4) If all the calcium-dependent sodium fluxes were Na-Ca exchange, then calcium flux through the exchanger per beat would be about sevenfold larger than that through the calcium channels. An undetermined part of the calcium-dependent sodium fluxes, however, could be a direct Na-Na exchange through the activated Na-Ca exchanger.

The slowing of Ca2+ signals by Ca2+ indicators in cardiac muscle

The use of high-affinity fluorescent probes for monitoring intracellular free Ca2+ in cardiac muscle is now widespread. We have investigated the consequences of introducing intracellular buffers with the properties of Fura-2 or Indo-1 on the action potential, Ca2+ transient and contractile activity of the myocardium. Our theoretical results suggest that, at the high intracellular concentrations of these fluorescent probes used on occasion to improve the signal-to-noise ratio of the emitted fluorescence, modulation of action potential profile and attenuation of the amplitudes of the Ca2+ transient and contraction can occur, together with subtle changes in the kinetics of these events.

Cytosolic free Ca2+ during operation of sodium-calcium exchange in guinea-pig heart cells

1. Membrane current generated by the Na(+)-Ca2+ exchange mechanism was recorded in single guinea-pig ventricular myocytes using the whole-cell voltage-clamp technique and the intracellular free calcium concentration ([Ca2+]i) was monitored using the fluorescent probe Indo-1, applied intracellularly through a perfused patch pipette. The reversal potential of the exchanger (ENa, Ca) was measured from records of the 2 mM-Ni(2+)-sensitive current and used in an attempt to clamp [Ca2+]i at a level determined by the ionic compositions of the external and pipette solutions. 2. Measurements of ENa, Ca indicated that [Ca2+]i was close to that in the pipette solution when the holding potential was set at the ENa, Ca expected for a 3Na+:1Ca2+ exchanger. The measured value of ENa, Ca was more positive than the theoretical value when the membrane potential was held positive to ENa, Ca and the opposite was true when the holding potential was more negative than the expected ENa, Ca. 3. As Indo-1 diffused into the cell from the whole-cell clamp electrode, the intensities of the fluorescent signals measured at 405 and 480 nm increased with time, with no obvious saturation over a 10-45 min recording period. However, the ratio of these two signals reached a steady level within 5 min after rupture of the patch membrane, when the holding potential was set at the expected ENa, Ca of the exchanger. The intensity ratios measured using pipette solutions containing 600 and 803 nM [Ca2+] were almost equal to the ratios obtained extracellularly from internal solutions of identical compositions, but in experiments using pipette solutions having lower [Ca2+] the intensity ratios measured in myocytes were higher than those obtained extracellularly. 4. If the membrane was depolarized or hyperpolarized, the fluorescence ratio either increased or decreased, respectively. These changes in the fluorescence ratio were virtually blocked by the extracellular application of 2 mM-Ni2+. 5. When the concentration of bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) in the recording pipette was reduced from 30 to 1 mM, an increase in [Ca2+]i was observed during a depolarizing ramp pulse. The Ca2+ influx estimated by integrating the 2 mM-Ni(2+)-sensitive current during the pulse correlated with the increase in [Ca2+]i estimated from Indo-1 using the extracellular calibration curve, but the values of the influx determined directly from Indo-1 fluorescence were always larger than those calculated from the exchanger current.(ABSTRACT TRUNCATED AT 400 WORDS)

The cytosolic calcium transient modulates the action potential of rat ventricular myocytes

1. The modulation of the action potential by the cytosolic Ca2+ (Cai2+) transient was studied in single isolated rat ventricular myocytes loaded with the acetoxymethyl ester form of the Ca(2+)-sensitive fluorescent dye Indo-1. Stimulation following rest and exposure to ryanodine were used to change the amount of Ca2+ released from the sarcoplasmic reticulum and thus the size of the Cai2+ transient. The Cai2+ transient was measured as the change, upon stimulation, in the ratio of Indo-1 fluorescence at 410 nm to that at 490 nm (410/490) and action potentials or membrane currents were recorded using patch-type microelectrodes. 2. When stimulation was initiated following rest, the magnitude of the Cai2+ transient decreased in a beat-dependent manner until a steady state was reached. The negative staircase in the Cai2+ transient was accompanied by a similar beat-dependent decrease in the duration of the action potential, manifested primarily as a gradual loss of the action potential plateau (approximately -45 mV). A slow terminal phase of repolarization of a few millivolts in amplitude was found to parallel the terminal decay of the Cai2+ transient. 3. The terminal portion of phase-plane loops of membrane potential (Vm) vs. Indo-1 ratio from all of the beats of a stimulus train followed a common linear trajectory even though the individual beats differed markedly in the duration and amplitude of the action potential and Cai2+ transient. 4. When the stimulation dependence of the Cai2+ transient was titrated away with submaximal exposure to ryanodine, the stimulation-dependent changes in the action potential plateau and terminal phase of repolarization were also eliminated. The same effect was noted in cells which, fortuitously, did not show a staircase in the Cai2+ transient following a period of rest. 5. When action potentials were triggered immediately following spontaneous release of Ca2+ from the sarcoplasmic reticulum, which results in a small depolarization at the resting potential, phase-plane loops (Vm vs. Indo-1 ratio) of the spontaneous events followed the same linear trajectory as the terminal phase of repolarization in the loops of the stimulated beats. 6. Following repolarization from brief voltage clamp pulses (to minimize time and voltage-dependent currents associated with depolarization), an inward current was observed that rose and fell in phase with the Cai2+ transient. This current was present at -70 mV, near the resting potential, and at -40 mV, a potential relevant to the plateau of the action potential.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of ryanodine and caffeine on Ca-activated current in guinea-pig ventricular myocytes

1. Action potentials from guinea-pig single ventricular myocytes were interrupted by application of a 300 ms voltage clamp to -40 mV in order to evoke the Ca-activated tail current which is thought to be carried by Na:Ca exchange. Stimulation frequency was 1 Hz and temperature 36 degrees C. 2. The actions of ryanodine (1 microM and 10 microM) and caffeine (1 mM and 10 mM) on Ca-activated tail currents were investigated. 3. Exposure to 10 mM caffeine and ryanodine reduced tail currents associated with very abbreviated (12 ms duration) action potentials and greatly reduced the difference between first and steady-state tail currents at this action potential duration. These observations were interpreted in terms of suppression of Ca release from the sarcoplasmic reticulum (SR) stores. 4. Tail current decay during the voltage clamp is thought to reflect the fall in [Ca]i which accompanies muscle relaxation. Current decay is dependent on Ca extrusion via Na:Ca exchange and on Ca accumulation by the SR stores. Time constants of tail current decay were seen to decrease with increasing action potential duration. This relationship was not affected by 1 mM caffeine or 1 microM ryanodine. Ryanodine at 10 microM and 10 mM caffeine abolished this relationship and increased the time constants of current decay. An increase in the time constant of tail current decay was thought to reflect a reduction in the rate of Ca accumulation by the sarcoplasmic reticulum. 5. The actions of caffeine and ryanodine on the Ca-activated tail currents are consistent with a dose-dependent leakage of Ca from the SR Ca stores. The Ca-activated tail current appears to be a useful tool in the study of Ca homeostasis.

Force production following transient potential changes in voltage-clamped myocardium

Inter-relationships between force, membrane voltage and currents were studied in ferret and guinea-pig papillary muscles using the single sucrose gap technique (37 degrees C). The preparations were held at -90 or -40 mV and depolarized (excited) to 0 mV for 180 ms at 1.0 Hz. At regular intervals the shape of a single clamp pulse (called '1') was varied and its effects were investigated during the same test cycle and in two subsequent test cycles ('2' and '3'). Peak force of contraction 1 (F1) increased with the duration of the test clamp up to 90 ms and was constant thereafter. F1 increased with clamp amplitude (V1) between -30 and 10 mV and decreased at greater amplitudes. This relation was similar to the relation between peak second inward current (I1) and V1. The peak force of contractions 2 and 3 rose with the clamp duration and clamp amplitudes of cycle 1. The relation between F3 and F2 was linear (slope 0.40), except at the lowest and highest F2 values where there was a small deviation. There was an inverse relation between I2 and F2. The results support the idea that increased duration or amplitude of the voltage clamp pulse leads to a greater calcium entry which is manifested in the following potentiated contraction. The relation between F3 and F2 implies that about 40% of calcium recirculates between the contractions. The inverse relationship between F2 and I2 indicates that the second inward current is regulated by release from the sarcoplasmic reticulum via negative feedback.

A model of the single atrial cell: relation between calcium current and calcium release

The hypothesis that calcium release from the sarcoplasmic reticulum in cardiac muscle is induced by rises in free cytosolic calcium (Fabiato 1983, Am. J. Physiol 245) allows the possibility that the release could be at least partly regenerative. There would then be a non-linear relation between calcium current and calcium release. We have investigated this possibility in a single-cell version of the rabbit-atrial model developed by Hilgemann & Noble (1987, Proc. R. Soc. Lond. B 230). The model predicts different voltage ranges of activation for calcium-dependent processes (like the sodium-calcium exchange current, contraction or Fura-2 signals) and the calcium current, in agreement with the experimental results obtained by Earm et al. (1990, Proc. R. Soc. Lond. B 240) on exchange current tails, Cannell et al. (1987, Science, Wash. 238) by using Fura-2 signals, and Fedida et al. (1987, J. Physiol., Lond. 385) and Talo et al. (1988, Biology of isolated adult cardiac myocytes) by using contraction. However, when the Fura-2 concentration is sufficiently high (greater than 200 microM) the activation ranges become very similar as the buffering properties of Fura-2 are sufficient to remove the regenerative effect. It is therefore important to allow for the buffering properties of calcium indicators when investigating the correlation between calcium current and calcium release.

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.

Interaction of inhomogeneities of repolarization with anisotropic propagation in dog atria. A mechanism for both preventing and initiating reentry

Having found the regional differences in right atrial action potentials shown in an accompanying article, we tested two seemingly paradoxical hypotheses: 1) The spatial pattern of repolarization provides a protective mechanism against reentry, and 2) repolarization inhomogeneities interact with anisotropic discontinuous propagation to produce reentry. Measurement of multidimensional refractory periods demonstrated an anisotropic distribution within large bundles with the longest refractory periods in the medial upper crista terminalis (sinus node area), a distribution similar to that of action potential durations. Also, discontinuities of repolarization were found at muscle bundle junctions. Early premature impulses originating in the sinus node area propagated throughout the right atrial preparations without conduction disturbances or reentry. Conversely, early premature impulses that originated at sites distal to the sinus node area resulted in localized conduction block at multiple sites, which frequently produced complex conduction changes and reentry. The critical nature of the site of origin of a premature impulse in initiating reentry was related to locations where the steepest repolarization gradients occurred: within anisotropic bundles in the direction of highest axial resistance (across fibers) and at muscle bundle junctions that represented localized discontinuities of axial resistance. The multiple conduction abnormalities at localized sites interacted to produce different types of reentry at a larger size scale (25 mm2 to several cm2). In each case, neither repolarization inhomogeneities (leading circle concept) nor anisotropic discontinuous propagation was the only "mechanism" involved. That is, reentry at a macroscopic size scale occurred as a result of a combined repolarization-anisotropic discontinuous propagation mechanism.

Changes in cytosolic calcium monitored by inward currents during action potentials in guinea-pig ventricular cells

Action potentials were recorded from single cells isolated from guinea-pig ventricular muscle. Contraction was measured with an optical technique. Tail currents thought to be activated by cytosolic calcium were recorded when action potentials were interrupted by application of a voltage-clamp. A family of tail currents was recorded by interrupting the action potential at various times after the upstroke. The envelope of tail current amplitudes was taken as an index of changes in cytosolic calcium. Consistent with this interpretation, tail currents were negligible following intracellular loading with the calcium chelator BAPTA to suppress calcium transients. The cytosolic calcium transient estimated from the envelope of tails reached a peak approximately 50 ms after the upstroke of the action potential, and fell close to diastolic levels before repolarization was complete; 10 mM caffeine delayed the time to peak contraction, and caused a prolongation of the cytosolic calcium transient estimated from the envelope of tail currents. Caffeine also induced the appearance of a distinct late plateau phase of the action potential. Intracellular BAPTA suppressed the late plateau, contraction and tail currents in cells exposed to caffeine. Exposure to caffeine increased the time constant for decay of tail currents (from approximately 25 to 70 ms). When action potentials were greatly abbreviated by interruption with a voltage-clamp, a progressive decline occurred in the subsequent three contractions and tail currents. There was a progressive reversal of these effects over four responses when the full action potential duration was restored. None of these effects was observed in cells exposed to caffeine. Calcium-activated tail currents appear to be a useful qualitative index of changes in cytosolic calcium. The observations are consistent with the suggestion that cytosolic calcium is reduced during the plateau by a combination of calcium extrusion through Na-Ca exchange and calcium uptake into caffeine-sensitive stores. It also appears that reduction of stores loading during abbreviated action potentials reduces subsequent contraction in cells not exposed to caffeine.

The control of calcium influx by cytoplasmic calcium in mammalian heart muscle

The role of Ca2+ in the initiation and maintenance of contraction has been extensively studies. Many of these studies have focused on how Ca2+ influx and efflux affect cytoplasmic Ca2+ (Cai) and, therefore, contraction in cardiac muscle. However, it has recently become apparent that Cai itself may play a major role in the control of Ca2+ influx and efflux from cardiac muscle. Here we review current ideas on the mechanisms underlying Ca2+ homeostasis in cardiac muscle, with specific attention to how Cai may control Ca2+ influx, both under normal and pathological conditions.

Free calcium in rat papillary muscle at contraction assessed with Ca-selective microelectrodes

Direct measurements of free intracellular calcium (Ca)i are needed for an understanding of the regulation of contractility. An on-line measurement of (Ca)i with Ca-selective microelectrodes in intact muscle strips provides a suitable means of investigating this problem, although considerable methodologic difficulties exist. Measurements of (Ca)i concentrations during muscle contraction have been carried out by different methods such as Ca-binding techniques and aequorin luminescence, but remain unsatisfying, since they were not performed on intact muscle strips. The authors' measurements were carried out with Ca-selective microelectrodes on rat papillary muscles (stretched to optimal length in a perfusion bath of 1.54 mL at 30 degrees C). The impalement of electrodes was considered adequate when the heights of Ca-electrode potential and membrane potential remained constant for more than twenty minutes. For provoking contractile responses, the authors replaced the normal Tyrode solution by a caffeine-containing contracture solution (content in mM: 0 NaCl [choline], 4 CaCl2, 30 KCl, 25 caffeine, 1.05 MgCl2). Ca-selective microelectrodes were calibrated before and after each measurement and only those impalements were taken as adequate that showed identical calibration curves before and after the experiment. They measured the (Ca)i at 20%, 50%, and 80% of maximal contractile force and obtained (Ca)i concentrations of 1.1 +/- 0.3 microM (at 20%), 3.6 +/- 1.2 microM (at 50%), and 11.8 +/- 0.27 microM (at 80%) (n = 6, +/- SEM). These results represent the fist on-line measurements of the myocardial (Ca)i concentrations with Ca++-selective microelectrodes in intact muscle strips during various degrees of contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium-calcium exchange during the action potential in guinea-pig ventricular cells

1. Slow inward tail currents attributable to electrogenic sodium-calcium exchange can be recorded by imposing hyperpolarizing voltage clamp pulses during the normal action potential of isolated guinea-pig ventricular cells. The hyperpolarizations return the membrane to the resting potential (between -65 and -88 m V) allowing an inward current to be recorded. This current usually has peak amplitude when repolarization is imposed during the first 50 ms after the action potential upstroke, but becomes negligible once the final phase of repolarization is reached. The envelope of peak current tail amplitudes strongly resembles that of the intracellular calcium transient recorded in other studies. 2. Repetitive stimulation producing normal action potentials at a frequency of 2 Hz progressively augments the tail current recorded immediately after the stimulus train. Conversely, if each action potential is prematurely terminated at 0.1 Hz, repetitive stimulation produces a tail current much smaller than the control value. The control amplitude of inward current is only maintained if interrupted action potentials are separated by at least one full 'repriming' action potential. These effects mimic those on cell contraction (Arlock & Wohlfart, 1986) and suggest that progressive changes in tail current are controlled by variations in the amplitude and time course of the intracellular calcium transient. 3. When intracellular calcium is buffered sufficiently to abolish contraction, the tail current is abolished. Substitution of calcium with strontium greatly reduces the tail current. 4. The inward tail current can also be recorded at more positive membrane potentials using standard voltage clamp pulse protocols. In this way it was found that temperature has a large effect on the tail current, which can change from net inward at 22 degrees C to net outward at 37 degrees C. The largest inward currents are usually recorded at about 30 degrees C. It is shown that this effect is attributable predominantly to the temperature sensitivity of activation of the delayed potassium current, iK, whose decay can then mask the slow tail current at high temperatures. 5. Studies of the relationship between the tail current and the membrane calcium current, iCa, have been performed using a method of drug application which is capable of perturbing iCa in a very rapid and highly reversible manner. Partial block of iCa with cadmium does not initially alter the size of the associated inward current tail. When iCa is increased by applying isoprenaline, the percentage augmentation of the associated tail current is much greater but occurs more slowly.(ABSTRACT TRUNCATED AT 400 WORDS)

Use-dependent reduction and facilitation of Ca2+ current in guinea-pig myocytes

1. Action potentials, calcium currents (iCa) and cell contraction have been recorded from single guinea-pig myocytes during periods of stimulation from rest. Voltage clamp was carried out using a single microelectrode. Cell contraction was measured optically. All experiments were performed at 18-22 degrees C. 2. An inverse relationship was observed between cell contraction and action potential duration or iCa. Mixed trains of action potentials and voltage clamp pulses preserved this relationship. Long voltage clamp pulses induced negative 'staircases' of iCa and positive 'staircases' of cell contraction. A facilitation of iCa was observed during repetitive stimulation with clamp pulses of 100 ms duration or less and was accompanied by a decrease in cell contraction. 3. The voltage dependence of inward current staircases was found to depend on Ca2+ entry rather than membrane voltage for long voltage clamp pulses and was not affected by 30 mM-TEA or 50 microM-TTX. Current reduction was greatest at 0 mV (P less than 0.05) when iCa was largest. Changes in cell contraction during pulse trains showed a similar voltage dependence. The time constant of current staircases was only mildly voltage dependent. 4. Interference with normal cellular mechanisms for Ca2+ uptake and release by strontium, 1-5 mM-caffeine and 1 microM-ryanodine increased current staircases and could abolish iCa facilitation with short clamp pulses. 5. Variations in the level of Ca2+-dependent inactivation of iCa can explain many features of the changes in iCa during stimulation after rest. Long clamp pulses (or action potentials) may increase cell Ca2+ loading and inhibit iCa. Short clamp pulses reduce available Ca2+ for cell contraction and this may reflect a lowered myoplasmic Ca2+ level which allows facilitation of iCa.

Voltage dependence of sodium-calcium exchange current in guinea-pig atrial myocytes determined by means of an inhibitor

1. Spontaneous transient inward currents (Iti) caused by cyclic release of Ca2+ ions from the sarcoplasmic reticulum were studied in cultured atrial myocytes from hearts of adult guinea-pigs. K+ channel currents were blocked by replacing K+ on both sides of the membrane by Cs+; the L-type Ca2+ current was inhibited by D600. 2. The voltage dependence of peak Iti and the background current displayed distinct outward-going rectification. The I-V curves for both currents approach each other at strongly positive membrane potentials but do not intersect. 3. 3'-4'Dichlorobenzamil (DCB) causes a concentration-dependent inhibition of peak Iti and a shift of the holding current (at -60 to -40 mV) in the inward direction. Inhibition of Iti is half-maximal at a concentration of 30 microM. 4. DCB reduces the outward-rectifying component of both peak Iti and the background current. The I-V curves of the control and DCB-inhibited currents intersect at ca. +10 mV (peak Iti) and negative to -75 mV (background current), suggesting the reversal potential of the DCB-inhibited current to be shifted by ca. 85 mV in the positive direction if Cai2+ rises following Ca2+ release. 5. The voltage dependence of the DCB-inhibited currents is highly compatible with the concept of Na+-Ca2+ exchange being the charge-carrying mechanism of the outward-rectifying background current. Ca2+ release from the SR alters the I-V curve of this current according to the shift of the thermodynamic driving force.

Transient inward current in guinea-pig atrial myocytes reflects a change of sodium-calcium exchange current

1. Enzymatically isolated, cultured myocytes from hearts of adult guinea-pigs were voltage clamped with a whole-cell patch-clamp technique. The pipette-filling solution for internal dialysis contained 65 mM-citrate and 50 microM-EGTA as Ca2+-chelating agents and 20 mM-Na+. Potassium channel currents were blocked by replacing this ion on both sides of the membrane by Cs+. 2. In the above conditions myocytes develop spontaneous transient inward currents (Iti) at constant negative membrane holding potentials. At a given membrane potential Iti can be recorded with constant amplitude and frequency for periods of up to ca. 40 min. A membrane current with similar properties can be evoked by superfusion of the cell with caffeine-containing (5-10 mM) solution. 3. Depolarization results in a reduction of Iti amplitude and a prolongation of its duration. After a step change of the membrane potential to ca. -10 mV or a less-negative level only one inward current change is observed. Thereafter the membrane current remains inward with regard to the instantaneous current at this membrane potential. Complete relaxation of Iti then is only observed after repolarization to a more-negative membrane potential. 4. The current change caused by sarcoplasmic Ca2+ release is inward in a range of membrane potentials between -90 and +75 mV. A reversal of Iti was never detected. 5. Both the instantaneous current-voltage (I-V) relation and voltage dependence of peak Iti display distinct outward rectification. Both I-V relations can be described by a formalism suggested for a membrane current caused by electrogenic Na+-Ca2+ exchange (INa, Ca) assuming a 3:1 stoichiometry and a single energy barrier in the electric field of the membrane. 6. An increase of the time integral of Iti at the holding potential is observed after depolarizations to positive membrane potentials, where the outward-rectifying current component is prominent. This supports the view that the outward current represents INa, Ca in the 'reverse mode', carrying Ca2+ ions into the cell. 7. After prolonged cell dialysis a run-down of Iti is observed. Since strong depolarizations in this condition still can cause inward currents upon repolarization, run-down is likely to reflect an impairment of sarcoplasmic reticulum function rather than an effect of cell dialysis on the exchanger. 8. We conclude that under the present conditions a membrane current is measured, which to a large extent determines the 'passive' I-V curve of the myocytes. This current is modified by a rise in Ca2+(i) following sarcoplasmic Ca2+ release.(ABSTRACT TRUNCATED AT 400 WORDS)

Experimental and theoretical work on excitation and excitation-contraction coupling in the heart

A combination of experimental and theoretical work has been used to investigate the movements of calcium during cardiac excitation. In addition to calcium entry through several types of calcium channel, calcium efflux occurs to balance the entry during each cycle of activity. Measurements of net membrane calcium movements have been made with the right time resolution by Don Hilgemann in Los Angeles by investigating fast extracellular calcium transients. This work shows that, in mammalian cardiac cells, net calcium exit occurs quite early during repolarization and is nearly complete by the time the resting potential is re-established. These results correlate very well indeed with measurements made in the Oxford laboratory of calcium-activated inward current in single cardiac myocytes. Both approaches are consistent with the view that calcium efflux occurs largely through the sodium-calcium exchange process. Modelling of this process in equations developed recently with Dario DiFrancesco, Susan Noble and Don Hilgemann succeeds in reproducing both the ionic current changes and the fast extracellular calcium transients.