Exchanger Current Research Articles

Autocrine stimulation of cardiac Na(+)-Ca(2+) exchanger currents by endogenous endothelin released by angiotensin II.

The goal of the present study was to evaluate the effects of Ang II on the current produced by the Na(+)-Ca(2+) exchanger (I(NCX)) working in the reverse mode and the possible autocrine role played by the release of endothelin (ET) in these actions. I(NCX) was studied in isolation in cat cardiac myocytes. Angiotensin II (Ang II) (100 nmol/L) increased I(NCX) at potentials higher than 0 mV (at +60 mV: 2.07 +/- 0.22 pA/pF in control versus 2.73 +/- 0.22 pA/pF in Ang II, n=9; P<0.05). The increase in I(NCX) induced by Ang II was prevented by the treatment of the cells with the unspecific blocker of the ET receptors, TAK 044 (1 micromol/L) (at +60 mV: 2.15 +/- 0.27 pA/pF in control versus 2.01+/- 0.26 pA/pF in Ang II, n=5, NS). These results show, for the first time, that the effect of Ang II on I(NCX) is the result of the autocrine actions of ET released by the octapeptide.

Rescue of Contractile Parameters and Myocyte Hypertrophy in Calsequestrin Overexpressing Myocardium by Phospholamban Ablation

Cardiac-specific overexpression of murine cardiac calsequestrin results in depressed cardiac contractile parameters, low Ca(2+)-induced Ca(2+) release from sarcoplasmic reticulum (SR) and cardiac hypertrophy in transgenic mice. To test the hypothesis that inhibition of phospholamban activity may rescue some of these phenotypic alterations, the calsequestrin overexpressing mice were cross-bred with phospholamban-knockout mice. Phospholamban ablation in calsequestrin overexpressing mice led to reversal of the depressed cardiac contractile parameters in Langendorff-perfused hearts or in vivo. This was associated with increases of SR Ca(2+) storage, assessed by caffeine-induced Na(+)-Ca(2+) exchanger currents. The inactivation time of the L-type Ca(2+) current (I(Ca)), which has an inverse correlation with Ca(2+)-induced SR Ca(2+) release, and the relation between the peak current density and half-inactivation time were also normalized, indicating a restoration in the ability of I(Ca) to trigger SR Ca(2+) release. The prolonged action potentials in calsequestrin overexpressing cardiomyocytes also reversed to normal upon phospholamban ablation. Furthermore, ablation of phospholamban restored the expression levels of atrial natriuretic factor and alpha-skeletal actin mRNA as well as ventricular myocyte size. These results indicate that attenuation of phospholamban function may prevent or overcome functional and remodeling defects in hypertrophied hearts.

Bimodal regulation of Na(+)--Ca(2+) exchanger by beta-adrenergic signaling pathway in shark ventricular myocytes.

In shark heart, the Na(+)--Ca(2+) exchanger serves as a major pathway for both Ca(2+) influx and efflux, as there is only rudimentary sarcoplasmic reticulum in these hearts. The modulation of the exchanger by a beta-adrenergic agonist in whole-cell clamped ventricular myocytes was compared with that of the Na(+)--Ca(2+) exchanger blocker KB-R7943. Application of 5 microM isoproterenol and 10 microM KB-R7943 suppressed both the inward and the outward Na(+)--Ca(2+) exchanger current (I(Na--Ca)). The isoproterenol effect was mimicked by 10 microM forskolin. Isoproterenol and forskolin shifted the reversal potential (E(rev)) of I(Na--Ca) by approximately -23 mV and -30 mV, respectively. An equivalent suppression of outward I(Na--Ca) by KB-R7943 to that by isoproterenol produced a significantly smaller shift in E(rev) of about --4 mV. The ratio of inward to outward exchanger currents was also significantly larger in isoproterenol- than in control- and KB-R7943-treated myocytes. Our data suggest that the larger ratio of inward to outward exchanger currents as well as the larger shift in E(rev) with isoproterenol results from the enhanced efficacy of Ca(2+) efflux via the exchanger. The protein kinase A-mediated bimodal regulation of the exchanger in parallel with phosphorylation of the Ca(2+) channel and enhancement of its current may have evolved to satisfy the evolutionary needs for accelerated contraction and relaxation in hearts of animals with vestigial sarcoplasmic Ca(2+) release stores.

Bimodal regulation of Na+-Ca2+ exchanger by beta -adrenergic signaling pathway in shark ventricular myocytes

In shark heart, the Na(+)--Ca(2+) exchanger serves as a major pathway for both Ca(2+) influx and efflux, as there is only rudimentary sarcoplasmic reticulum in these hearts. The modulation of the exchanger by a beta-adrenergic agonist in whole-cell clamped ventricular myocytes was compared with that of the Na(+)--Ca(2+) exchanger blocker KB-R7943. Application of 5 microM isoproterenol and 10 microM KB-R7943 suppressed both the inward and the outward Na(+)--Ca(2+) exchanger current (I(Na--Ca)). The isoproterenol effect was mimicked by 10 microM forskolin. Isoproterenol and forskolin shifted the reversal potential (E(rev)) of I(Na--Ca) by approximately -23 mV and -30 mV, respectively. An equivalent suppression of outward I(Na--Ca) by KB-R7943 to that by isoproterenol produced a significantly smaller shift in E(rev) of about --4 mV. The ratio of inward to outward exchanger currents was also significantly larger in isoproterenol- than in control- and KB-R7943-treated myocytes. Our data suggest that the larger ratio of inward to outward exchanger currents as well as the larger shift in E(rev) with isoproterenol results from the enhanced efficacy of Ca(2+) efflux via the exchanger. The protein kinase A-mediated bimodal regulation of the exchanger in parallel with phosphorylation of the Ca(2+) channel and enhancement of its current may have evolved to satisfy the evolutionary needs for accelerated contraction and relaxation in hearts of animals with vestigial sarcoplasmic Ca(2+) release stores.

Alpha-Adrenoceptor stimulation-mediated negative inotropism and enhanced Na(+)/Ca(2+) exchange in mouse ventricle.

Mechanisms underlying the negative inotropic response to alpha-adrenoceptor stimulation in adult mouse ventricular myocardium were studied. In isolated ventricular tissue, phenylephrine (PE), in the presence of propranolol, decreased contractile force by approximately 40% of basal value. The negative inotropic response was similarly observed under low extracellular Ca(2+) concentration ([Ca(2+)](o)) conditions but was significantly smaller under high-[Ca(2+)](o) conditions and was not observed under low-[Na(+)](o) conditions. The negative inotropic response was not affected by nicardipine, ryanodine, ouabain, or dimethylamiloride (DMA), inhibitors of L-type Ca(2+) channel, Ca(2+) release channel, Na(+)-K(+) pump, or Na(+)/H(+) exchanger, respectively. KB-R7943, an inhibitor of Na(+)/Ca(2+) exchanger, suppressed the negative inotropic response mediated by PE. PE reduced the magnitude of postrest contractions. PE caused a decrease in duration of the late plateau phase of action potential and a slight increase in resting membrane potential; time courses of these effects were similar to that of the negative inotropic effect. In whole cell voltage-clamped myocytes, PE increased the L-type Ca(2+) and Na(+)/Ca(2+) exchanger currents but had no effect on the inwardly rectifying K(+), transient outward K(+), or Na(+)-K(+)-pump currents. These results suggest that the sustained negative inotropic response to alpha-adrenoceptor stimulation of adult mouse ventricular myocardium is mediated by enhancement of Ca(2+) efflux through the Na(+)/Ca(2+) exchanger.

Electrogenic Na(+)/Ca(2+) exchange. A novel amplification step in squid olfactory transduction.

Olfactory receptor neurons (ORNs) from the squid, Lolliguncula brevis, respond to the odors l-glutamate or dopamine with increases in internal Ca(2+) concentrations ([Ca(2+)](i)). To directly asses the effects of increasing [Ca(2+)](i) in perforated-patched squid ORNs, we applied 10 mM caffeine to release Ca(2+) from internal stores. We observed an inward current response to caffeine. Monovalent cation replacement of Na(+) from the external bath solution completely and selectively inhibited the caffeine-induced response, and ruled out the possibility of a Ca(2+)-dependent nonselective cation current. The strict dependence on internal Ca(2+) and external Na(+) indicated that the inward current was due to an electrogenic Na(+)/Ca(2+) exchanger. Block of the caffeine-induced current by an inhibitor of Na(+)/Ca(2+) exchange (50-100 microM 2',4'-dichlorobenzamil) and reversibility of the exchanger current, further confirmed its presence. We tested whether Na(+)/Ca(2+) exchange contributed to odor responses by applying the aquatic odor l-glutamate in the presence and absence of 2', 4'-dichlorobenzamil. We found that electrogenic Na(+)/Ca(2+) exchange was responsible for approximately 26% of the total current associated with glutamate-induced odor responses. Although Na(+)/Ca(2+) exchangers are known to be present in ORNs from numerous species, this is the first work to demonstrate amplifying contributions of the exchanger current to odor transduction.

Cardiac Unloading Alters Contractility and Calcium Homeostasis in Ventricular Myocytes

M. Ritter, Z. Su, S. Xu, J. Shelby and W. H. Barry. Cardiac Unloading Alters Contractility and Calcium Homeostasis in Ventricular Myocytes. Journal of Molecular and Cellular Cardiology (2000) 32, 577–584. Altered cardiac workload has an important effect on myocyte structure and function. Cardiac hypertrophy resulting from an increase in load has been studied extensively in the past. However, the effects of unloading and atrophy have recently become of more interest since devices for mechanical left ventricular unloading have been introduced into clinical practice for the treatment of patients with terminal heart failure, and a resulting improved cardiac and myocyte contractility have been reported. We used the heterotopic abdominal mouse heart transplant model in order to study the effects of 5 days of unloading on cell size (confocal microscopy), contractility (fractional shortening: video motion), calcium homeostasis ([Ca2+]itransients, SR Ca2+content); and L-type Ca2+and sodium/calcium exchanger currents (whole cell patch clamp technique). We found unloading caused decreased cell volume consistent with atrophy. An increased fractional shortening and [Ca2+]itransient were observed in myocytes from unloaded hearts as compared with controls. Transsarcolemmal ICa,Land INa/Cadensities, and SR Ca2+content were unaltered, as was membrane capacitance. A reduction in cell volume with mainteinance of internal and surface membrane areas, and/or a decrease in concentration of cellular protein Ca2+buffers, may contribute to the increase in the [Ca2+]itransient in this model.

Quantitation of Na/Ca Exchanger Function in Single Ventricular Myocytes

A Na/Ca exchange current can be elicited in voltage clamped single ventricular myocytes by the abrupt removal of extracellular Na+by means of a rapid switcher device. We measured this reverse Na/Ca exchange current in isolated mouse ventricular myocytes from wild-type mice, and from transgenic mice with hearts overexpressing the Na/Ca exchanger. In mouse ventricular myocytes, the current was sensitive to nickel, and was eliminated by removal of intracellular Na+. It was not influenced by 3 mmouabain, and thus not contaminated by Na pump currents. The magnitude of the current reached a plateau within 10–15 min after obtaining a whole cell patch with the pipettes containing EGTA, to buffer [Ca2+]iand in zero extracellular K+concentration to completely inhibit the Na pump, and allow equilibration of pipette Na+with subsarcolemmal [Na+]. The magnitude of the current increased with increases in pipette [Na+]. Comparison of the current magnitudes in wild-type and transgenic myocytes showed a 2.5 and 2.7 fold increase in the current in transgenic myocytes at pipette [Na+] of 10 and 20 mm. The magnitude of this increase in Na/Ca exchanger currents in single transgenic myocytes compares well with the reported 2.5 fold increase in Na+-dependent45Ca2+uptake measured in ventricular sarcolemmal vesicles obtained from transgenic animals. With this approach, we found variation in exchanger current densities in different species, with values for mouse>rat>rabbit>dog>human. This technique should also be useful in quantifying changes in Na/Ca exchanger current density as a consequence of pathologic processes, and exposure to drugs.

Sarcoplasmic Reticulum Function in Murine Ventricular Myocytes Overexpressing SR CaATPase

To examine the effects of the overexpression of sarcoplasmic reticulum (SR) CaATPase on function of the SR and Ca2+homeostasis, we measured [Ca2+]itransients (fluo-3), and L-type Ca2+currents (ICa,L), Na/Ca exchanger currents (INa/Ca), and SR Ca2+content with voltage clamp in ventricular myocytes isolated from wild type (WT) mice and transgenic (SRTG) mice. The amplitude of [Ca2+]itransients was insignificantly increased in SRTG myocytes, while the diastolic [Ca2+]itended to be lower. The initial and terminal declines of [Ca2+]itransients were significantly accelerated in SRTG myocytes, implying a functional upregulation of the SR CaATPase. We examined the functional contribution of only the SR CaATPase to the initial and the terminal phase of the decline of [Ca2+]i, by abruptly inhibiting Na/Ca exchange with a rapid switcher device. The rate of [Ca2+] decline mediated by the SR CaATPase was increased by 40% in SRTG compared with WT myocytes. The function of the L-type Ca2+channel was unchanged in SRTG myocytes, while INa/Ca density was slightly (10%) decreased. Measured SR Ca2+content was significantly increased by 29% in SRTG myocytes. Thus, overexpression of SR CaATPase markedly accelerates the decline of [Ca2+]itransients, and induces an increase in SR Ca2+content, with some downregulation of the Na/Ca exchanger.

Influence of prior Na+ pump activity on pump and Na+/Ca2+ exchange currents in mouse ventricular myocytes.

We examined the dependence of peak Na+ pump and Na+/Ca2+ exchanger currents on prior Na+ pump inhibition induced by exposure to zero extracellular K+ in voltage-clamped adult murine ventricular myocytes. Abrupt activation of the Na+ pump by reexposure of myocytes to extracellular K+ with a rapid solution switcher resulted in the development of a transient peak current at approximately 500 ms, followed by a decline over 1-2 min to a steady-state level. The magnitudes of both the peak Na+ pump current (Ip) and the peak outward Na+/Ca2+ exchange current, activated by rapidly reducing extracellular Na+ to zero with the solution switcher, were dependent on previous Na+ pump activity. [Na+] gradients (Na+-binding benzofuran isophthalate fluorescence) between the patch pipette and the bulk cytosol were relatively small and could not account for the large differences between peak and steady-state Ip and reverse Na+/Ca2+ exchanger currents. Our results are consistent with the presence of a subsarcolemmal Na+ concentration gradient, which is similar for the Na+ pump and the Na+/Ca2+ exchanger. These findings also support the hypothesis that the Na+ pump and the Na+/Ca2+ exchanger are colocalized in the sarcolemma.

Regulation of Ca2+ signaling in transgenic mouse cardiac myocytes overexpressing calsequestrin.

To probe the physiological role of calsequestrin in excitation-contraction coupling, transgenic mice overexpressing cardiac calsequestrin were developed. Transgenic mice exhibited 10-fold higher levels of calsequestrin in myocardium and survived into adulthood, but had severe cardiac hypertrophy, with a twofold increase in heart mass and cell size. In whole cell-clamped transgenic myocytes, Ca2+ channel- gated Ca2+ release from the sarcoplasmic reticulum was strongly suppressed, the frequency of occurrence of spontaneous or Ca2+ current-triggered "Ca2+ sparks" was reduced, and the spark perimeter was less defined. In sharp contrast, caffeine-induced Ca2+ transients and the resultant Na+-Ca2+ exchanger currents were increased 10-fold in transgenic myocytes, directly implicating calsequestrin as the source of the contractile-dependent pool of Ca2+. Interestingly, the proteins involved in the Ca2+-release cascade (ryanodine receptor, junctin, and triadin) were downregulated, whereas Ca2+-uptake proteins (Ca2+-ATPase and phospholamban) were unchanged or slightly increased. The parallel increase in the pool of releasable Ca2+ with overexpression of calsequestrin and subsequent impairment of physiological Ca2+ release mechanism show for the first time that calsequestrin is both a storage and a regulatory protein in the cardiac muscle Ca2+-signaling cascade. Cardiac hypertrophy in these mice may provide a novel model to investigate the molecular determinants of heart failure.

A Conservation Principle and its Effect on the Formulation of Na–Ca Exchanger Current in Cardiac Cells

In this paper we show the presence of a latent conservation principle in the formulation of ionic currents in cardiac cells and examine its effect on a formulation of the sodium–calcium exchange current appearing in the Noble model of the sinoatrial node cell in the mammalian heart [see Nobleet al. (1989) or Winslowet al. (1991)]. Our objective is to show that this formulation, if not corrected, will result in a serious instability in this cardiac cell model. In particular, under certain initial conditions, the solutions of the model equations will blow-up in finite time. We also propose a correction to the model equation of the sodium–calcium exchange current, and we show that the modified model agrees favorably with the original model. These phenomena also occur in the other cardiac cell models, such as those modeling the Purkinje fiber, the atrial cell and the ventricular cell. The changes proposed in this paper can be applied directly to these models as well.

Na(+)-Ca2+ exchange currents in cortical neurons: concomitant forward and reverse operation and effect of glutamate.

Na(+)-Ca2+ exchanger-associated membrane currents were studied in cultured murine neocortical neurons, using whole-cell recording combined with intracellular perfusion. A net inward current specifically associated with forward (Na+(o)-Ca2+(i)) exchange was evoked at -40 mV by switching external 140 mM Li+ to 140 mM Na+. The voltage dependence of this current was consistent with that predicted for 3Na+:1Ca2+ exchange. As expected, the current depended on internal Ca2+, and could be blocked by intracellular application of the exchanger inhibitory peptide, XIP. Raising internal Na+ from 3 to 20 mM or switching the external solution from 140 mM Li+ to 30 mM Na+ activated outward currents, consistent with reverse (Na+(i)-Ca2+(o)) exchange. An external Ca2(+)-sensitive current was also identified as associated with reverse Na(+)-Ca2+ exchange based on its internal Na+ dependence and sensitivity to XIP. Combined application of external Na+ and Ca2+ in the absence of internal Na+ triggered a 3.3-fold larger inward current than the current activated in the presence of 3 mM internal Na+, raising the intriguing possibility that Na(+)-Ca2+ exchangers might concurrently operate in both the forward and the reverse direction, perhaps in different subcellular locations. With this idea in mind, we examined the effect of excitotoxic glutamate receptor activation on exchanger operation. After 3-5 min of exposure to 100-200 microM glutamate, the forward exchanger current was significantly increased even when external Na+ was reduced to 100 mM, and the external Ca2(+)-activated reverse exchanger current was eliminated.

Regulation of cardiac sodium-calcium exchanger by beta-adrenergic agonists.

Na+-Ca2+ exchanger and Ca2+ channel are two major sarcolemmal Ca2+-transporting proteins of cardiac myocytes. Although the Ca2+ channel is effectively regulated by protein kinase A-dependent phosphorylation, no enzymatic regulation of the exchanger protein has been identified as yet. Here we report that in frog ventricular myocytes, isoproterenol down-regulates the Na+-Ca2+ exchanger, independent of intracellular Ca2+ and membrane potential, by activation of the beta-receptor/adenylate-cyclase/cAMP-dependent cascade, resulting in suppression of transmembrane Ca2+ transport via the exchanger and providing for the well-documented contracture-suppressant effect of the hormone on frog heart. The beta-blocker propranolol blocks the isoproterenol effect, whereas forskolin, cAMP, and theophylline mimic it. In the frog heart where contractile Ca2+ is transported primarily by the Na+-Ca2+ exchanger, the beta-agonists' simultaneous enhancement of Ca2+ current, ICa, and suppression of Na+-Ca2+ exchanger current, INa-Ca would enable the myocyte to develop force rapidly at the onset of depolarization (enhancement of ICa) and to decrease Ca2+ influx (suppression of INa-Ca) later in the action potential. This unique adrenergically induced shift in the Ca2+ influx pathways may have evolved in response to paucity of the sarcoplasmic reticulum Ca2+-ATPase/phospholamban complex and absence of significant intracellular Ca2+ release pools in the frog heart.

Ca 2+ -Induced Current Oscillations in Rabbit Ventricular Myocytes

Transient currents are activated by spontaneous Ca2+ oscillations in rabbit ventricular myocytes. We investigated the ionic basis for these transient currents under conditions in which K+ currents would be expected to be blocked. Holding cells under voltage clamp at positive potentials leads to a rise in intracellular Ca2+ via reversal of the Na+-Ca2+ exchanger and subsequently to the initiation of spontaneous Ca2+ transients, presumably from a Ca2+-overloaded sarcoplasmic reticulum. The current transients associated with these Ca2+ transients reversed at about +10 to +15 mV under conditions of approximately symmetrical Cl-. In the absence of Cl-, this current was inward at all potentials examined over the range from -88 to +72 mV, consistent with a Na+-Ca2+ exchanger current. In the absence of Na+, the repetitive spontaneous Ca2+ transients could be initiated by a brief train of depolarizations to activate the inward Ca2+ current. Under such conditions, the current was found to reverse at -3 mV when the equilibrium potential of Cl- (ECl) was -2 mV, and the reversal potential shifted to -32 mV when internal Cl- was lowered, to make ECl -33 mV. Thus, in the absence of Na+, it appears that the current is exclusively a Ca2+-activated Cl- current. There is no evidence to indicate the presence of a Ca2+-activated cationic conductance. Further, our results demonstrate that the Ca2+-activated Cl- conductance can carry inward current at potentials more negative to ECl in rabbit ventricular myocytes and is therefore likely to contribute to the arrhythmogenic delayed afterdepolarizations that occur in Ca2+-overloaded cells.

Species differences in the activity of the Na(+)‐Ca2+ exchanger in mammalian cardiac myocytes.

1. Species differences in the activity of the exchanger were evaluated in isolated myocytes from rat, guinea-pig, hamster ventricles and human atria. Fluorescence measurements using fura-2 were carried out in conjunction with the whole-cell patch-clamp technique for simultaneous recording of membrane currents and intracellular Ca2+ concentration. 2. Ca2+ release from sarcoplasmic reticulum (SR) induced either by rapid application of caffeine or by Ca2+ current elicited inward Na(+)-Ca2+ exchange currents (INa-Ca). The magnitude of INa-Ca was largest in hamster, smallest in rat, with guinea-pig and human myocytes having intermediate values. The ratio of caffeine-induced exchanger current densities, normalized with respect to the peak Ca2+ release, was 4:2:1.5:1 for hamster > guinea-pig > or = human > or = rat myocytes. 3. The rates of Ca2+ removal in the presence of caffeine, which reflect primarily the Ca2+ extruding activity of the Na(+)-Ca2+ exchanger, followed the same order of hamster > guinea-pig > or = human > or = rat. 4. The kinetics of INa-Ca vs. Ca2+ transients were different among species. In rat myocytes, the kinetics of the INa-Ca and the Ca2+ transients were similar, with INa-Ca linearly proportional to intracellular Ca2+ concentration ([Ca2+]i). In hamster myocytes, the time course of INa-Ca tracked only the declining phase of the Ca2+ transient with INa-Ca having faster kinetics during the Ca2+ release. These findings suggest that the Ca2+ concentrations in the vicinity of the exchanger were significantly higher than those of the cytosol during Ca2+ release in hamster myocytes. 5. We concluded that there are significant species differences in the exchanger activity of cardiac myocytes, arising from differences in exchanger densities, their modulation and/or their spatial distribution with respect to the ryanodine receptors of cardiac myocytes.

Extracellular osmotic pressure modulates sodium‐calcium exchange in isolated guinea‐pig ventricular myocytes.

1. The sensitivity of the cardiac Na(+)-Ca2+ exchange current to changes in osmotic pressure was investigated in guinea-pig ventricular myocytes, using the whole-cell patch-clamp technique. 2. A hyposmotic challenge applied by removal of sucrose from the standard bathing solution reduced exchanger current, measured as the Ni(2+)-sensitive component of whole-cell transsarcolemmal current. These changes were fully reversible. 3. No response of whole-cell current to hyposmosis was observed when Ca2+ was removed from the bathing solution by chelation with 1 mM EGTA. 4. Inclusion of 25 microM exchanger inhibitory peptide (XIP) in the pipette solution caused a marked reduction in the Ni(2+)-sensitive component of membrane current, but the percentage change in Ni(2+)-sensitive membrane slope conductance evoked by hyposmosis was the same as when XIP was omitted from the pipette solution. 5. Exposure of cells to hyperosmotic solutions produced variable responses. In a majority of cells, solutions 30% hyperosmotic compared with control evoked a persistent increase in exchanger current, whereas for solutions 50% hyperosmotic, a larger but transient increase in current was observed. 6. Over a wide range of osmolalities (50-130% of isosmotic) the changes in Ni(2+)-sensitive membrane slope conductance were linearly related to the changes in extracellular osmotic pressure. 7. We propose that one consequence of exposing ventricular myocytes to anisosmotic solutions is modulation of Na(+)-Ca2+ exchange current.

Characteristics of atrial re-entry and meander computed from a model of a rabbit single atrial cell

A two-dimensional excitable medium model of atrial tissue has been constructed by incorporating the Earm-Hilgemann-Noble excitation equations for membrane voltage-dependent ionic currents and pump exchanger currents, time-dependent changes in intracellular and extracellular ionic concentrations, and storage and release of Ca 2+, for an atrial cell in an homogeneous partial differential equation. A diffusion coefficient of 1.25 cm 2sec −1gives the conduction velocity of a solitary planar wave as 0.6 m sec −1. A spiral wave with a period of 78 msec develops from a broken wavefront, and initially rotates around a circular core of 2 mm diameter. After 1.5 sec of rotation, a biperiodic meander, with 6 epicycles/cycle, develops. This simple meander pattern is reflected in a period-6 modulation of the intervals between successive wavefronts.

A mathematical model of a rabbit sinoatrial node cell.

A mathematical model for the electrophysiological responses of a rabbit sinoatrial node cell that is based on whole cell recordings from enzymatically isolated single pacemaker cells at 37 degrees C has been developed. The ion channels, Na(+)-K+ and Ca2+ pumps, and Na(+)-Ca2+ exchanger in the surface membrane (sarcolemma) are described using equations for these known currents in mammalian pacemaker cells. The extracellular environment is treated as a diffusion-limited space, and the myoplasm contains Ca(2+)-binding proteins (calmodulin and troponin). Original features of this model include 1) new equations for the hyperpolarization-activated inward current, 2) assessment of the role of the transient-type Ca2+ current during pacemaker depolarization, 3) inclusion of an Na+ current based on recent experimental data, and 4) demonstration of the possible influence of pump and exchanger currents and background currents on the pacemaker rate. This model provides acceptable fits to voltage-clamp and action potential data and can be used to seek biophysically based explanations of the electrophysiological activity in the rabbit sinoatrial node cell.

Cholinergic and Adrenergic Modulation of Ion Channels in a Rabbit Sinoatrial Node Model

We have modified our rabbit SA node (SAN) model [1] to show the effects of the cholinergic and adrenergic modulation of certain ion channels. Our study centers on the direct and indirect effects of bath applications of acetylcholine (ACh) in the presence and absence of isoprenaline (ISO). The direct effect is mediated by an ACh-sensitive K+ membrane channel while the indirect effect is mediated by the second messenger cyclic adenosine monophosphate (cAMP) which modulates three specific membrane currents If, IcaL and Ik. Our model [l] has two major components: a sarcolemmal model and a lumped fluid compartment model. The sarcolemma is characterized by a Hodgkin-Huxley (HH)-type membrane model containing the ionic currents, membrane pump and exchanger currents that are known to be present in the rabbit SAN cell. The description of the cAMP change is added to the fluid compartment of the SAN model. Cytosolic cAMP concentration changes are defined by (1) a cAMP production term which is modulated by concentrations of neurotransmitters (ISO and ACH) and (2) a degradation term which represents removal of channel phosphorylation by phosphatases and cleavage of cAMP by phosphosodiesterase. Intracellular cAMP levels indirectly modulate IcaL, Ik and If. The direct effect of ACh upon the muscarinic G-protein mediated K+ channel (Ik,ACh) is also included in our model. The inhibitory effects of bath applications of ACh on the cycle length of the spontaneous activity are observed along with the excitatory effects of ISO upon the cycle length The phase sensitivity of the SAN cell is studied in response to both transiently- and periodically-applied ACh stimuli.