Reversal potential of the calcium current in bull-frog atrial myocytes

Emerging methods to model cardiac ion channel and myocyte electrophysiology

In the field of cardiac electrophysiology, modeling has played a central role for many decades. However, even though the effort is well-established, it has recently seen a rapid and sustained evolution in the complexity and predictive power of the models being created. In particular, new approaches to modeling have allowed the tracking of parallel and interconnected processes that span from the nanometers and femtoseconds that determine ion channel gating to the centimeters and minutes needed to describe an arrhythmia. The connection between scales has brought unprecedented insight into cardiac arrhythmia mechanisms and drug therapies. This review focuses on the generation of these models from first principles, generation of detailed models to describe ion channel kinetics, algorithms to create and numerically solve kinetic models, and new approaches toward data gathering that parameterize these models. While we focus on application of these models for cardiac arrhythmia, these concepts are widely applicable to model the physiology and pathophysiology of any excitable cell.

Models of the cardiac L-type calcium current: A quantitative review

The L-type calcium current (I CaL ) plays a critical role in cardiac electrophysiology, and models ofI CaL are vital tools to predict arrhythmogenicity of drugs and mutations. Five decades of measuring and modelingI CaL have resulted in several competing theories (encoded in mathematical equations). However, the introduction of new models has not typically been accompanied by a data-driven critical comparison with previous work, so that it is unclear which model is best suited for any particular application. In this review, we describe and compare 73 published mammalianI CaL models and use simulated experiments to show that there is a large variability in their predictions, which is not substantially diminished when grouping by species or other categories. We provide model code for 60 models, list major data sources, and discuss experimental and modeling work that will be required to reduce this huge list of competing theories and ultimately develop a community consensus model ofI CaL . This article is categorized under: Cardiovascular Diseases > Computational Models Cardiovascular Diseases > Molecular and Cellular Physiology.

Ca2+ entry through NaV channels generates submillisecond axonal Ca2+ signaling

Calcium ions (Ca²⁺) are essential for many cellular signaling mechanisms and enter the cytosol mostly through voltage-gated calcium channels. Here, using high-speed Ca²⁺ imaging up to 20 kHz in the rat layer five pyramidal neuron axon we found that activity-dependent intracellular calcium concentration ([Ca²⁺]_i) in the axonal initial segment was only partially dependent on voltage-gated calcium channels. Instead, [Ca²⁺]_i changes were sensitive to the specific voltage-gated sodium (Na_V) channel blocker tetrodotoxin. Consistent with the conjecture that Ca²⁺ enters through the Na_V channel pore, the optically resolved I_Ca in the axon initial segment overlapped with the activation kinetics of Na_V channels and heterologous expression of Na_V1.2 in HEK-293 cells revealed a tetrodotoxin-sensitive [Ca²⁺]_i rise. Finally, computational simulations predicted that axonal [Ca²⁺]_i transients reflect a 0.4% Ca²⁺ conductivity of Na_V channels. The findings indicate that Ca²⁺ permeation through Na_V channels provides a submillisecond rapid entry route in Na_V-enriched domains of mammalian axons.

Nerve cells communicate using tiny electrical impulses called action potentials. Special proteins termed ion channels produce these electric signals by allowing specific charged particles, or ions, to pass in or out of cells across its membrane. When a nerve cell ‘fires’ an action potential, specific ion channels briefly open to let in a surge of positively charged ions which electrify the cell. Action potentials begin in the same place in each nerve cell, at an area called the axon initial segment. The large number of sodium channels at this site kick-start the influx of positively charged sodium ions ensuring that every action potential starts from the same place.

Previous research has shown that, when action potentials begin, the concentration of calcium ions at the axon initial segment also increases, but it was not clear which ion channels were responsible for this entry of calcium. Channels that are selective for calcium ions are the prime candidates for this process. However, research in squid nerve cells gave rise to an unexpected idea by suggesting that sodium channels may not exclusively let in sodium but also allow some calcium ions to pass through. Hanemaaijer, Popovic et al. therefore wanted to test the routes that calcium ions take and see whether the sodium channels in mammalian nerve cells are also permeable to calcium.

Experiments using fluorescent dyes to track the concentration of calcium in rat and human nerve cells showed that calcium ions accumulated at the axon initial segment when action potentials fired. Most of this increase in calcium could be stopped by treating the neurons with a toxin that prevents sodium channels from opening. Electrical manipulations of the cells revealed that, in this context, the calcium ions were effectively behaving like sodium ions. Human kidney cells were then engineered to produce the sodium channel protein. This confirmed that calcium and sodium ions were indeed both passing through the same channel.

These results shed new light on the relationship between calcium ions and sodium channels within the mammalian nervous system and that this interplay occurs at the axon initial segment of the cell. Genetic mutations that ‘nudge’ sodium channels towards favoring calcium entry are also found in patients with autism spectrum disorders, and so this new finding may contribute to our understanding of these conditions.

Piezo Is Essential for Amiloride-Sensitive Stretch-Activated Mechanotransduction in Larval Drosophila Dorsal Bipolar Dendritic Sensory Neurons

Stretch-activated afferent neurons, such as those of mammalian muscle spindles, are essential for proprioception and motor co-ordination, but the underlying mechanisms of mechanotransduction are poorly understood. The dorsal bipolar dendritic (dbd) sensory neurons are putative stretch receptors in the Drosophila larval body wall. We have developed an in vivo protocol to obtain receptor potential recordings from intact dbd neurons in response to stretch. Receptor potential changes in dbd neurons in response to stretch showed a complex, dynamic profile with similar characteristics to those previously observed for mammalian muscle spindles. These profiles were reproduced by a general in silico model of stretch-activated neurons. This in silico model predicts an essential role for a mechanosensory cation channel (MSC) in all aspects of receptor potential generation. Using pharmacological and genetic techniques, we identified the mechanosensory channel, DmPiezo, in this functional role in dbd neurons, with TRPA1 playing a subsidiary role. We also show that rat muscle spindles exhibit a ruthenium red-sensitive current, but found no expression evidence to suggest that this corresponds to Piezo activity. In summary, we show that the dbd neuron is a stretch receptor and demonstrate that this neuron is a tractable model for investigating mechanisms of mechanotransduction.

An excess-calcium-binding-site model predicts neurotransmitter release at the neuromuscular junction

Despite decades of intense experimental studies, we still lack a detailed understanding of synaptic function. Fortunately, using computational approaches, we can obtain important new insights into the inner workings of these important neural systems. Here, we report the development of a spatially realistic computational model of an entire frog active zone in which we constrained model parameters with experimental data, and then used Monte Carlo simulation methods to predict the Ca(2+)-binding stoichiometry and dynamics that underlie neurotransmitter release. Our model reveals that 20-40 independent Ca(2+)-binding sites on synaptic vesicles, only a fraction of which need to bind Ca(2+) to trigger fusion, are sufficient to predict physiological release. Our excess-Ca(2+)-binding-site model has many functional advantages, agrees with recent data on synaptotagmin copy number, and is the first (to our knowledge) to link detailed physiological observations with the molecular machinery of Ca(2+)-triggered exocytosis. In addition, our model provides detailed microscopic insight into the underlying Ca(2+) dynamics during synapse activation.

Cell and tissue responses to electric shocks

AIM

Existing models of myocardial membrane kinetics have not been able to reproduce the experimentally-observed negative bias in the asymmetry of transmembrane potential changes (DeltaV(m)) induced by strong electric shocks. The goals of this study are (1) to demonstrate that this negative bias could be reproduced by the addition, to the membrane model, of electroporation and an outward current, I(a), part of the K(+) flow through the L-type Ca(2+)-channel, and (2) to determine how such modifications in the membrane model affect shock-induced break excitation in a 2D preparation.

METHODS AND RESULTS

We conducted simulations of shocks in bidomain fibres and sheets with membrane dynamics represented by the Luo-Rudy dynamic model (LRd'2000), to which electroporation (LRd + EP model) and the outward current, I(a), activated upon strong shock-induced depolarization (aLRd model) was added. Assuming I(a) is a part of K(+) flow through the L-type Ca(2+)-channel enabled us to reproduce both the experimentally observed rectangularly-shaped positive DeltaV(m) and the value of near 2 of the negative-to-positive DeltaV(m) ratio. In the sheet, I(a) not only contributed to the negative bias in DeltaV(m) asymmetry at sites polarized by physical and virtual electrodes, but also restricted positive DeltaV(m). Electroporation, in its turn, was responsible for the decrease in cathode-break excitation threshold in the aLRd sheet, compared with the other two cases, as well as for the occurrence of the excitation after the shock-end rather than during the shock.

CONCLUSIONS

The incorporation of electroporation and I(a) in a membrane model ensures match between simulation results and experimental data. The use of the aLRd model results in a lower threshold for shock-induced break excitation.

Asymmetry in membrane responses to electric shocks: insights from bidomain simulations

Models of myocardial membrane dynamics have not been able to reproduce the experimentally observed negative bias in the asymmetry of transmembrane potential changes (DeltaVm) induced by strong electric shocks delivered during the action potential plateau. The goal of this study is to determine what membrane model modifications can bridge this gap between simulation and experiment. We conducted simulations of shocks in bidomain fibers and sheets with membrane dynamics represented by the LRd'2000 model. We found that in the fiber, the negative bias in DeltaVm asymmetry could not be reproduced by addition of electroporation only, but by further addition of hypothetical outward current, Ia, activated upon strong shock-induced depolarization. Furthermore, the experimentally observed rectangularly shaped positive DeltaVm, negative-to-positive DeltaVm ratio (asymmetry ratio) = approximately 2, electroporation occurring at the anode only, and the increase in positive DeltaVm caused by L-type Ca2+-channel blockade were reproduced in the strand only if Ia was assumed to be a part of K+ flow through the L-type Ca2+-channel. In the sheet, Ia not only contributed to the negative bias in DeltaVm asymmetry at sites polarized by physical and virtual electrodes, but also restricted positive DeltaVm. Inclusion of Ia and electroporation is thus the bridge between experiment and simulation.

Dynamical description of sinoatrial node pacemaking: improved mathematical model for primary pacemaker cell

We developed an improved mathematical model for a single primary pacemaker cell of the rabbit sinoatrial node. Original features of our model include 1) incorporation of the sustained inward current (I(st)) recently identified in primary pacemaker cells, 2) reformulation of voltage- and Ca(2+)-dependent inactivation of the L-type Ca(2+) channel current (I(Ca,L)), 3) new expressions for activation kinetics of the rapidly activating delayed rectifier K(+) channel current (I(Kr)), and 4) incorporation of the subsarcolemmal space as a diffusion barrier for Ca(2+). We compared the simulated dynamics of our model with those of previous models, as well as with experimental data, and examined whether the models could accurately simulate the effects of modulating sarcolemmal ionic currents or intracellular Ca(2+) dynamics on pacemaker activity. Our model represents significant improvements over the previous models, because it can 1) simulate whole cell voltage-clamp data for I(Ca,L), I(Kr), and I(st); 2) reproduce the waveshapes of spontaneous action potentials and ionic currents during action potential clamp recordings; and 3) mimic the effects of channel blockers or Ca(2+) buffers on pacemaker activity more accurately than the previous models.

Cholinergic modulation of the basal L-type calcium current in ferret right ventricular myocytes

The effects of the cholinergic muscarinic agonist carbachol (CCh) on the basal L-type calcium current, I(Ca,L), in ferret right ventricular (RV) myocytes were studied using whole cell patch clamp. CCh produced two major effects : (i) in all myocytes, extracellular application of CCh inhibited I(Ca,L) in a reversible concentration-dependent manner; and (ii) in many (but not all) myocytes, upon washout CCh produced a significant transient stimulation of I(Ca,L) ('rebound stimulation'). Inhibitory effects could be observed at 1 x 10(-10) M CCh. The mean steady-state inhibitory concentration-response relationship was shallow and could be described with a single Hill equation (maximum inhibition = 34.5 %, IC50 = 4 x 10(-8) M, Hill coefficient n = 0.60). Steady-state inhibition (1 or 10 microM CCh) had no significant effect on I(Ca,L) selectivity or macroscopic (i) activation characteristics, (ii) inactivation kinetics, (iii) steady-state inactivation or (iv) kinetics of recovery from inactivation. Maximal inhibition of nitric oxide synthase (NOS) activity (preincubation of myocytes in 1 mM L-NMMA (N(G)-monomethyl-L-arginine) + 1 mM L-NNA (N(G)-nitro-L-arginine) for 2-3 h plus inclusion of 1 mM L-NMMA + 1 mM L-NNA in the patch pipette solution) produced no significant attenuation of the CCh-mediated inhibition of I(Ca,L). Protocols involving (i) the nitric oxide (NO) scavenger PTIO (2-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide; 200 microM), (ii) imposition of a 'cGMP clamp' (100 microM 8-Bromo-cGMP), and (iii) inhibition of soluble guanylyl cyclase (ODQ (1H-[1,2,4,]oxadiazolo(4,3,-a)quinoxalin-1-one), 50 microM) all failed to attenuate CCh-mediated inhibition of I(ca,L). While CCh consistently inhibited basal I(Ca,L) in all RV myocytes studied, not all myocytes displayed rebound stimulation upon CCh washout. However, there was no difference between CCh-mediated inhibition of I(Ca,L) between these two RV myocyte types, and in myocytes displaying rebound stimulation neither ODQ nor 8-Bromo-cGMP (8-Br-cGMP) altered the effect. We conclude that NO production, activation of soluble guanylyl cyclase, or changes in intracellular cGMP levels are not obligatorily involved in muscarinic-mediated modulation of basal I(Ca,L) in ferret RV myocytes.

Molecular identification of a TTX-sensitive Ca(2+) current

The TTX-sensitive Ca(2+) current [I(Ca(TTX))] observed in cardiac myocytes under Na(+)-free conditions was investigated using patch-clamp and Ca(2+)-imaging methods. Cs(+) and Ca(2+) were found to contribute to I(Ca(TTX)), but TEA(+) and N-methyl-D-glucamine (NMDG(+)) did not. HEK-293 cells transfected with cardiac Na(+) channels exhibited a current that resembled I(Ca(TTX)) in cardiac myocytes with regard to voltage dependence, inactivation kinetics, and ion selectivity, suggesting that the cardiac Na(+) channel itself gives rise to I(Ca(TTX)). Furthermore, repeated activation of I(Ca(TTX)) led to a 60% increase in intracellular Ca(2+) concentration, confirming Ca(2+) entry through this current. Ba(2+) permeation of I(Ca(TTX)), reported by others, did not occur in rat myocytes or in HEK-293 cells expressing cardiac Na(+) channels under our experimental conditions. The report of block of I(Ca(TTX)) in guinea pig heart by mibefradil (10 microM) was supported in transfected HEK-293 cells, but Na(+) current was also blocked (half-block at 0.45 microM). We conclude that I(Ca(TTX)) reflects current through cardiac Na(+) channels in Na(+)-free (or "null") conditions. We suggest that the current be renamed I(Na(null)) to more accurately reflect the molecular identity of the channel and the conditions needed for its activation. The relationship between I(Na(null)) and Ca(2+) flux through slip-mode conductance of cardiac Na(+) channels is discussed in the context of ion channel biophysics and "permeation plasticity."

Ca2+ influx via the L-type Ca2+ channel during tail current and above current reversal potential in ferret ventricular myocytes

1. Current through L-type Ca2+ channels (ICa) was measured electrophysiologically at the same time as Ca2+ influx was measured by trapping entering Ca2+ with a high concentration of indo-1 (> 1 mM) in ferret ventricular myocytes. 2. Na+-free conditions prevented Na+-Ca2+ exchange and K+ currents were blocked by Cs+ and TEA. Thapsigargin (5 microM) prevented Ca2+ uptake and release by the sarcoplasmic reticulum. ICa was pre-activated by brief pulses to +120 mV (the equilibrium potential for Ca2+, ECa), followed by steps to different membrane potentials (Em, -80 to +100 mV), in some cases in the presence of the Ca2+ channel agonist FPL-64176. 3. Integrated ICa ( 82 ICa) was linearly related to the change in the concentration of Ca2+ bound to indo-1, which was assessed by the fluorescence difference signal DeltaFd (Fd = F500 - F400). This created an internal calibration of DeltaFd as a measure of Ca2+ influx. 4. The DeltaFd/ 82 ICadt relationship was virtually unchanged at all measurable inward ICa (at Em from -80 to +50 mV). This indicates that the fractional current carried by Ca2+ and channel selectivity are unchanged over this Em range, and also that the selectivity for Ca2+ is very high. 5. Ca2+ influx was readily detected by DeltaFd beyond the ICa reversal potential (+65 to +100 mV) and was not abolished until Em was +120 mV (i.e. ECa). This is explained by the fact that inward Ca2+ flux at the ICa reversal potential is exactly balanced by outward Cs+ current through the Ca2+ channels and can be described by classic Goldman flux analysis with a Ca2+/Cs+ selectivity of the order of 5000. 6. This result also emphasizes that net Ca2+ influx via Ca2+ channels occurs over a voltage range where the net channel current is outward.

Nonglutamate pore residues in ion selection and conduction in voltage-gated Ca2+ channels

High-affinity, intrapore binding of Ca(2+) over competing ions is the essential feature in the ion selectivity mechanism of voltage-gated Ca(2+) channels. At the same time, several million Ca(2+) ions can travel each second through the pore of a single open Ca(2+) channel. How such high Ca(2+) flux is achieved in the face of tight Ca(2+) binding is a current area of inquiry, particularly from a structural point of view. The ion selectivity locus comprises four glutamate residues within the channel's pore. These glutamates make unequal contributions to Ca(2+) binding, underscoring a role for neighboring residues in pore function. By comparing two Ca(2+) channels (the L-type alpha(1C), and the non-L-type alpha(1A)) that differ in their pore properties but only differ at a single amino acid position near the selectivity locus, we have identified the amino-terminal neighbor of the glutamate residue in motif III as a determinant of pore function. This position is more important in the function of alpha(1C) channels than in alpha(1A) channels. For a systematic series of mutations at this pore position in alpha(1C), both unitary Ba(2+) conductance and Cd(2+) block of Ba(2+) current varied with residue volume. Pore mutations designed to make alpha(1C) more like alpha(1A) and vice versa revealed that relative selectivity for Ba(2+) over K(+) depended almost solely on pore sequence and not channel type. Analysis of thermodynamic mutant cycles indicates that the motif III neighbor normally interacts in a cooperative fashion with the locus, molding the functional behavior of the pore.

Ca2+ flux through promiscuous cardiac Na+ channels: slip-mode conductance

The tetrodotoxin-sensitive sodium ion (Na+) channel is opened by cellular depolarization and favors the passage of Na+ over other ions. Activation of the beta-adrenergic receptor or protein kinase A in rat heart cells transformed this Na+ channel into one that is promiscuous with respect to ion selectivity, permitting calcium ions (Ca2+) to permeate as readily as Na+. Similarly, nanomolar concentrations of cardiotonic steroids such as ouabain and digoxin switched the ion selectivity of the Na+ channel to this state of promiscuous permeability called slip-mode conductance. Slip-mode conductance of the Na+ channel can contribute significantly to local and global cardiac Ca2+ signaling and may be a general signaling mechanism in excitable cells.

Comparison of sarcolemmal calcium channel current in rabbit and rat ventricular myocytes

1. Fundamental properties of Ca2+ channel currents in rat and rabbit ventricular myocytes were measured using whole cell voltage clamp. 2. In rat, as compared with rabbit myocytes, Ca2+ channel current (ICa) was half-activated at about 10 mV more negative potential, decayed slower, was half-inactivated (in steady state) at about 5 mV more positive potential, and recovered faster from inactivation. 3. These features result in a larger steady-state window current in rat, and also suggest that under comparable voltage clamp conditions, including action potential (AP) clamp, more Ca2+ influx would be expected in rat myocytes. 4. Ca2+ channel current carried by Na+ and Cs+ in the absence of divalent ions (Ins) also activated at more negative potential and decayed more slowly in rat. 5. The reversal potential for Ins was 6 mV more positive in rabbit, consistent with a larger permeability ratio (PNa/PCs) in rabbit than in rat. ICa also reversed at slightly more positive potentials in rabbit (such that PCa/PCs might also be higher). 6. Ca2+ influx was calculated by integration of ICa evoked by voltage clamp pulses (either square pulses or pulses based on recorded rabbit or rat APs). For a given clamp waveform, the Ca2+ influx was up to 25% greater in rat, as predicted from the fundamental properties of ICa and Ins. 7. However, the longer duration of the AP in rabbit myocytes compensated for the difference in influx, such that the integrated Ca2+ influx via ICa in response to the species-appropriate waveform was about twice as large as that seen in rat.

A dynamic model of the cardiac ventricular action potential. I. Simulations of ionic currents and concentration changes

A mathematical model of the cardiac ventricular action potential is presented. In our previous work, the membrane Na+ current and K+ currents were formulated. The present article focuses on processes that regulate intracellular Ca2+ and depend on its concentration. The model presented here for the mammalian ventricular action potential is based mostly on the guinea pig ventricular cell. However, it provides the framework for modeling other types of ventricular cells with appropriate modifications made to account for species differences. The following processes are formulated: Ca2+ current through the L-type channel (ICa), the Na(+)-Ca2+ exchanger, Ca2+ release and uptake by the sarcoplasmic reticulum (SR), buffering of Ca2+ in the SR and in the myoplasm, a Ca2+ pump in the sarcolemma, the Na(+)-K+ pump, and a nonspecific Ca(2+)-activated membrane current. Activation of ICa is an order of magnitude faster than in previous models. Inactivation of ICa depends on both the membrane voltage and [Ca2+]i. SR is divided into two subcompartments, a network SR (NSR) and a junctional SR (JSR). Functionally, Ca2+ enters the NSR and translocates to the JSR following a monoexponential function. Release of Ca2+ occurs at JSR and can be triggered by two different mechanisms, Ca(2+)-induced Ca2+ release and spontaneous release. The model provides the basis for the study of arrhythmogenic activity of the single myocyte including afterdepolarizations and triggered activity. It can simulate cellular responses under different degrees of Ca2+ overload. Such simulations are presented in our accompanying article in this issue of Circulation Research.

Ionic selectivity of the mechano-electrical transduction channels in the hair cells of the frog sacculus

In the isolated saccular macula of Rana esculenta extracellular hair cell receptor currents evoked by mechanical stimulation of the otolithic membrane were recorded under transepithelial voltage clamp conditions. The ionic selectivity of the mechano-electrical transduction channels of the hair cells was determined by examining the effects of different concentrations of Ca2+ and K+ in the apical solution on the transepithelial voltage at which the extracellular receptor current was zero (Vrev). Changing the concentration of Ca2+ from 0.26 mM to 0.026 and to 2.6 mM at a constant K+ concentration caused changes in Vrev of -15 +/- 7 mV (mean +/- SD; n = 9) and 20 +/- 6 mV (n = 13), respectively. The relative ionic permeabilities of the transduction channels were estimated from a modified Goldman, Hodgkin and Katz equation, assuming that 80% of the transepithelial resistance is located in the apical membranes of the hair cells. The permeability of the transduction channels for Ca2+ was found to be two orders of magnitude larger than that for K+. The measured effects on Vrev of changing the concentration of K+ at constant ionic strength and at different constant Ca2+ concentrations were well predicted by the same equation. These results indicate that the transduction channels of the frog saccular hair cells are highly selective to Ca2+.

Citrate alters Ca channel gating and selectivity in rabbit ventricular myocytes

Addition of 10 mM citrate at constant free extracellular Ca concentration [( Ca]o; 2 mM) reduced contraction in rabbit ventricular muscle and isolated myocytes. We have recently shown that extracellular citrate decreases contraction and Ca current (ICa) in cardiac muscle by a direct effect on Ca channels rather than by Ca buffering per se [D. M. Bers, L. V. Hryshko, S. M. Harrison, and D. Dawson. Am. J. Physiol. 260 (Cell Physiol. 29): C900-C909, 1991]. Citrate rapidly depressed peak ICa and shifted both the peak ICa and the apparent reversal potential (Erev) to more negative potentials. When the impermeant cations, tetraethylammonium or N-methylglucamine were used instead of intracellular Cs, the citrate-induced shift in Erev was reduced or eliminated but depression of ICa was still observed. Thus citrate appears to alter the selectivity (PCa/PCs) of the Ca channel and reduce ICa. We also studied the effects of citrate on Na current through the Ca channel, observed when the divalent cation concentration is submicromolar. This current, termed INS for nonspecific, also exhibited leftward shifts in peak INS and smaller changes in Erev in the presence of citrate. However, neither peak INS nor single-channel conductance were affected by citrate. Thus the reduced PCa/PCs is due primarily to alteration of Ca permeation rather than monovalent cation permeation. Activation and inactivation curves for both ICa and INS were shifted toward more negative potentials by citrate. The shifts in gating and peak current to more negative membrane potentials would be consistent with a surface charge effect. The much larger shift in Erev for ICa (than for INS) is consistent with a reduction in Ca selectivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and localization of two ion-binding sites within the pore of cardiac L-type calcium channels

L-type Ca channels from porcine cardiac sarcolemma were incorporated into planar lipid bilayers. We characterized interactions of permeant and blocking ions with the channel's pore by (a) studying the current-voltage relationships for Ca2+ and Na+ when equal concentrations of the ions were present in both internal and external solutions, (b) testing the dose-dependent block of Ba2+ currents through the channels by internally applied cadmium, and (c) examining the dose and voltage dependence of the block of Na+ currents through the channels by internally and externally applied Ca2+. We found that the I-V relationship for Na+ appears symmetrical through the origin when equal concentrations of Na+ are present on both sides of the channel (gamma = 90 pS in 200 mM NaCl). The conductance for outward Ca2+ currents with 100 mM Ca2+ on both sides of the channel is approximately 8 pS, a value identical to that observed for inward currents when 100 mM Ca2+ was present outside only. This provides evidence that ions pass through the channel equally well regardless of the direction of net flux. In addition, we find that internal Cd2+ is as effective as external Cd2+ in blocking Ba2+ currents through the channels, again suggesting identical interactions of ions with each end of the pore. Finally, we find that micromolar Ca2+, either in the internal or in the external solution, blocks Na+ currents through the channels. The affinity for internally applied Ca2+ appears the same as that for externally applied Ca2+. The voltage dependence of the Ca(2+)-block suggests that the sites to which Ca2+ binds are located approximately 15% and approximately 85% of the electric field into the pore. Taken together, these data provide direct experimental evidence for the existence of at least two ion binding sites with high affinity for Ca2+, and support the idea that the sites are symmetrically located within the electric field across L-type Ca channels.

Electrical effects of okadaic acid extracted from black sponge on rabbit sinus node

1. Effects of okadaic acid on electrical responses and spontaneous activity in the dominant pacemaker cells of rabbit sinus node were investigated by use of microelectrode techniques. 2. Okadaic acid (10(-5) M to 4 x 10(-5) M) caused a shortening of cycle length of spontaneous firing (SPCL) accompanied by increases in both maximum upstroke velocity at phase 0 (Vmax) and amplitude of action potential. 3. All of the effects of okadaic acid were relatively well preserved in a low-Ca2+ medium (0.12 mM). Okadaic acid restored the spontaneous activity of sinus node pacemaker cells even in a Ca2(+)-deficient medium. 4. The effects of okadaic acid were markedly inhibited or abolished in a low Na+ medium (24 mM or 70 mM) and in the presence of a slow channel blocking agent, verapamil (10(-6) M). 5. In voltage-clamp experiments using a two-microelectrode technique, okadaic acid (10(-5) M) caused an increase in the slow inward current without affecting the outward current. At a higher concentration (4 x 10(-5) M), the drug increased the outward current. 6. These results indicate that okadaic acid causes an increase in spontaneous activity of sinus node pacemaker cells mediated by an enhancement of slow inward current (Isi) through verapamil-sensitive Ca2+ channels.

Effects of magnesium on inactivation of the voltage-gated calcium current in cardiac myocytes

The effects of changes in intracellular and extracellular free ionized [Mg2+] on inactivation of ICa and IBa in isolated ventricular myocytes of the frog were investigated using the whole-cell configuration of the patch-clamp technique. Intracellular [Mg2+] was varied by internal perfusion with solutions having different calculated free [Mg2+]. Increasing [Mg2+]i from 0.3 mM to 3.0 mM caused a 16% reduction in peak ICa amplitude and a 36% reduction in peak IBa amplitude, shifted the current-voltage relationship and the inactivation curve approximately 10 mV to the left, decreased relief from inactivation, and caused a dramatic increase in the rate of inactivation of IBa. The shifts in the current-voltage and inactivation curves were attributed to screening of internal surface charge by Mg2+. The increased rate of inactivation of IBa was due to an increase in both the steady-state level of inactivation as well as an increase in the rate of inactivation, as measured by two-pulse inactivation protocols. Increasing external [Mg2+] decreased IBa amplitude and shifted the current-voltage and inactivation curves to the right, but, in contrast to the effect of internal Mg2+, had little effect on the inactivation kinetics or the steady-state inactivation of IBa at potentials positive to 0 mV. These observations suggest that the Ca channel can be blocked quite rapidly by external Mg2+, whereas the block by [Mg2+]i is time and voltage dependent. We propose that inactivation of Ca channels can occur by both calcium-dependent and purely voltage-dependent mechanisms, and that a component of voltage-dependent inactivation can be modulated by changes in cytoplasmic Mg2+.

Studies of the sodium-calcium exchanger in bull-frog atrial myocytes

1. Experimental measurements and computer simulations have been used in attempts to identify an exchanger current (Iex) generated by an electrogenic Na+-Ca2+ exchanger in single cells from bull-frog atrium. 2. Voltage clamp measurements of an inward 'slow tail' current observed upon repolarization after depolarizing clamp pulses that elicit net inward Ca2+ currents (ICa) (see Campbell, Giles & Shibata, 1988c), show that these slow tails have a cationic dependence different from ICa. Slow tails are large and prominent in normal [Na+]o solutions containing either Ca2+ or Sr2+, but they are markedly reduced or absent in Ba2+, Ca2+-free, and Na+-free solutions. 3. Kinetic measurements on the slow tails show that they are not generated by deactivation of ICa, and suggest that they may be due to activation of Iex at negative potentials (-70 to -100 mV). 4. Computer simulations of the influx, buffering, and extrusion of Ca2+ provide further indirect evidence that the slow tails correspond to Iex. In addition, these calculations give insights into one plausible mechanism of Ca2+ homeostasis in frog atrium. When the Na+-Ca2+ exchanger formalism of Mullins (1979, 1981), as modified by DiFrancesco & Nobel (1985), is combined with equations for intracellular Ca2+ buffering by myoplasmic proteins (cf. Robertson, Johnson & Potter, 1981), slow inward tails are produced which are qualitatively similar to those recorded experimentally. 5. Comparisons of the size and time course of ICa with those of Iex suggest that Iex does not generate a physiologically significant current (or membrane potential change) during the plateau of the action potential. However, at potentials near the resting potential the inward current due to Iex may be significant. 6. Our theoretical results suggest that in the intact single atrial cell myoplasmic Ca2+-binding proteins (e.g. calmodulin and troponin) could be physiologically important modulators of the amplitude, polarity and kinetics of Iex. Hence, the specificity, capacity and kinetics of intracellular Ca2+ binding are essential components of any quantitative treatment of Iex in excitable tissue.

Ion transfer characteristics of the calcium current in bull-frog atrial myocytes

1. Voltage clamp studies on single cells from bull-frog atrium have been carried out to study the ion transfer characteristics of the calcium current, ICa. In agreement with the preliminary results of Hume & Giles (1983), a TTX-resistant, 'second transient inward current' was recorded consistently. Its average peak size at 0 mV in 2.5 mM [Ca2+]o Ringer solution was approximately -200 pA, and it was blocked by Cd2+ and La3+ but not by tetrodotoxin (TTX, 3 x 10(-6) M). 2. The peak size of this current increases by approximately 4 times when [Ca2+]o is raised from 1.25 to 7.5 mM, indicating that Ca2+ is a major charge carrier. 3. A well-defined reversal potential, Erev, for ICa can be recorded in normal Ringer solution and also when Ba2+ or Sr2+ serve as the charge carriers. When [Ca2+]o is changed the shifts in Erev follow the predictions of a Nernstian Ca2+ electrode. However, all Erev values are well below those predicted from the thermodynamic Nernstian ECa values (see Campbell, Giles, Hume, Noble & Shibata, 1988a). 4. The Ca2+ current exhibits voltage-dependent inactivation, whether the direction of net current flow is inward or outward; however, the rate of inactivation is affected by the species of cation carrying the current. Inactivation is reduced substantially in Ba2+ Ringer solution. 5. Magnesium (5 mM) is not a significant carrier or blocker of ICa in normal [Ca2+]o Ringer solution; however, 5 mM [Mg2+]o can block the current carried by either Sr2+ or Ba2+. In the absence of Mg2+, equimolar substitutions of Sr2+ or Ba2+ for Ca2+ result in larger currents than those carried by Ca2+ in the normal Ringer solution. 6. Sodium appears not to be a significant charge carrier in the presence of normal [Ca2+]o. However, after free [Ca2+]o has been reduced to extremely low levels (less than 10(-6) M) Na+ can carry a significant fraction of 'ICa'. Thus, it appears that the high selectivity of ICa for Ca2+ ions depends upon the presence of Ca2+. 7. 'Slow tails' are frequently recorded after repolarizing clamp steps back to the holding potential. These 'slow tails' are prominent in normal [Na+]o, [Ca2+]o and [Sr2+]o Ringer solution; however, they are markedly reduced in [Ba2+]o, in Na+-free and Ca2+-free Ringer solutions. Experimental and theoretical work suggests these slow tails may be generated by an electrogenic Na+-Ca2+ exchanger (see Campbell, Giles, Robinson & Shibata, 1988b).(ABSTRACT TRUNCATED AT 400 WORDS)