Ca2+-induced current oscillations in rabbit ventricular myocytes

Modulating cardiac physiology in engineered heart tissue with the bidirectional optogenetic tool BiPOLES

Optogenetic actuators are rapidly advancing tools used to control physiology in excitable cells, such as neurons and cardiomyocytes. In neuroscience, these tools have been used to either excite or inhibit neuronal activity. Cell type-targeted actuators have allowed to study the function of distinct cell populations. Whereas the first described cation channelrhodopsins allowed to excite specific neuronal cell populations, anion channelrhodopsins were used to inhibit neuronal activity. To allow for simultaneous excitation and inhibition, opsin combinations with low spectral overlap were introduced. BiPOLES (Bidirectional Pair of Opsins for Light-induced Excitation and Silencing) is a bidirectional optogenetic tool consisting of the anion channel Guillardia theta anion-conducting channelrhodopsin 2 (GtACR2 with a blue excitation spectrum and the red-shifted cation channel Chrimson. Here, we studied the effects of BiPOLES activation in cardiomyocytes. For this, we knocked in BiPOLES into the adeno-associated virus integration site 1 (AAVS1) locus of human-induced pluripotent stem cells (hiPSC), subjected these to cardiac differentiation, and generated BiPOLES expressing engineered heart tissue (EHT) for physiological characterization. Continuous light application activating either GtACR2 or Chrimson resulted in cardiomyocyte depolarization and thus stopped EHT contractility. In contrast, short light pulses, with red as well as with blue light, triggered action potentials (AP) up to a rate of 240 bpm. In summary, we demonstrate that cation, as well as anion channelrhodopsins, can be used to activate stem cell-derived cardiomyocytes with pulsed photostimulation but also to silence cardiac contractility with prolonged photostimulation.

Simultaneous recordings of action potentials and calcium transients from human induced pluripotent stem cell derived cardiomyocytes

Human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) offer a unique in vitro platform to study cardiac diseases, as they recapitulate many disease phenotypes. The membrane potential (V_m) and intracellular calcium (Ca²⁺) transient (CaT) are usually investigated separately, because incorporating different techniques to acquire both aspects concurrently is challenging. In this study, we recorded V_m and CaT simultaneously to understand the interrelation between these parameters in hiPSC-CMs. For this, we used a conventional patch clamp technique to record V_m, and synchronized this with a Ca²⁺ imaging system to acquire CaT from same hiPSC-CMs. Our results revealed that the CaT at 90% decay (CaT90) was longer than action potential (AP) duration at 90% repolarization (APD90). In addition, there was also a strong positive correlation between the different parameters of CaT and AP. The majority of delayed after depolarizations (DADs) observed in the V_m recording were also characterized by elevations in the intracellular Ca²⁺ level, but in some cases no abnormalities were observed in CaT. However, simultaneous fluctuations in CaT were always observed during early after depolarizations (EADs) in V_m In summary, simultaneous recording of V_m and CaT broadens the understanding of the interrelation between V_m and CaT and could be used to elucidate the mechanisms underlying arrhythmia in cardiac disease condition.

Disordered Myocardial Ca2+ Homeostasis Results in Substructural Alterations That May Promote Occurrence of Malignant Arrhythmias

We aimed to determine the impact of Ca2+-related disorders induced in intact animal hearts on ultrastructure of the cardiomyocytes prior to occurrence of severe arrhythmias. Three types of acute experiments were performed that are known to be accompanied by disturbances in Ca2+ handling. Langedorff-perfused rat or guinea pig hearts subjected to K+-deficient perfusion to induce ventricular fibrillation (VF), burst atrial pacing to induce atrial fibrillation (AF) and open chest pig heart exposed to intramyocardial noradrenaline infusion to induce ventricular tachycardia (VT). Tissue samples for electron microscopic examination were taken during basal condition, prior and during occurrence of malignant arrhythmias. Cardiomyocyte alterations preceding occurrence of arrhythmias consisted of non-uniform sarcomere shortening, disruption of myofilaments and injury of mitochondria that most likely reflected cytosolic Ca2+ disturbances and Ca2+ overload. These disorders were linked with non-uniform pattern of neighboring cardiomyocytes and dissociation of adhesive junctions suggesting defects in cardiac cell-to-cell coupling. Our findings identified heterogeneously distributed high [Ca2+]i-induced subcellular injury of the cardiomyocytes and their junctions as a common feature prior occurrence of VT, VF or AF. In conclusion, there is a link between Ca2+-related disorders in contractility and coupling of the cardiomyocytes pointing out a novel paradigm implicated in development of severe arrhythmias.

Ionic mechanisms of arrhythmogenesis

The understanding of ionic mechanisms underlying cardiac rhythm disturbances (arrhythmias) is an issue of significance in the medical science community. Several advances in molecular, cellular, and optical techniques in the past few decades have substantially increased our knowledge of ionic mechanisms that are thought to underlie arrhythmias. The application of these techniques in the study of ion channel biophysics and regulatory properties has provided a wealth of information, with some important therapeutic implications for dealing with the disease. In this review, we briefly consider the cellular and tissue manifestations of a number of cardiac rhythm disturbances, while focusing on our current understanding of the ionic current mechanisms that have been implicated in such rhythm disturbances.

The effect of high [K(+)]o on spontaneous Ca(2+) waves in freshly isolated interstitial cells of Cajal from the rabbit urethra

Interstitial cells of Cajal (ICC) act as putative pacemaker cells in the rabbit urethra. Pacemaker activity in ICC results from spontaneous global Ca²⁺ waves that can be increased in frequency by raising external [K⁺]. The purpose of this study was to elucidate the mechanism of this response. Intracellular [Ca²⁺] was measured in fluo‐4‐loaded smooth muscle cells (SMCs) and ICC using a Nipkow spinning disk confocal microscope. Increasing [K⁺]_o to 60 mmol/L caused an increase in [Ca²⁺]_i accompanied by contraction in SMCs. Raising [K⁺]_o did not cause contraction in ICC, but the frequency of firing of spontaneous calcium waves increased. Reducing [Ca²⁺]_o to 0 mmol/L abolished the response in both cell types. Nifedipine of 1 μmol/L blocked the response of SMC to high [K⁺]_o, but did not affect the increase in firing in ICC. This latter effect was blocked by 30 μmol/L NiCl₂ but not by the T‐type Ca²⁺ channel blocker mibefradil (300 nmol/L). However, inhibition of Ca²⁺ influx via reverse‐mode sodium/calcium exchange (NCX) using either 1 μmol/L SEA0400 or 5 μmol/L KB‐R7943 did block the effect of high [K⁺]_o on ICC. These data suggest that high K⁺ solution increases the frequency of calcium waves in ICC by increasing Ca²⁺ influx through reverse‐mode NCX.

Pacemaker activity in ICC results from spontaneous global Ca²⁺ waves that can be increased in frequency by raising external [K⁺]. The experiments described support the hypothesis that high K⁺ solution increases the frequency of calcium waves in ICC by increasing Ca²⁺ influx through reverse‐mode NCX.

Optical imaging of voltage and calcium in cardiac cells & tissues

Cardiac optical mapping has proven to be a powerful technology for studying cardiovascular function and disease. The development and scientific impact of this methodology are well-documented. Because of its relevance in cardiac research, this imaging technology advances at a rapid pace. Here, we review technological and scientific developments during the past several years and look toward the future. First, we explore key components of a modern optical mapping set-up, focusing on: (1) new camera technologies; (2) powerful light-emitting-diodes (from ultraviolet to red) for illumination; (3) improved optical filter technology; (4) new synthetic and optogenetic fluorescent probes; (5) optical mapping with motion and contraction; (6) new multiparametric optical mapping techniques; and (7) photon scattering effects in thick tissue preparations. We then look at recent optical mapping studies in single cells, cardiomyocyte monolayers, atria, and whole hearts. Finally, we briefly look into the possible future roles of optical mapping in the development of regenerative cardiac research, cardiac cell therapies, and molecular genetic advances.

Enhancement of Asynchronous and Train-Evoked Exocytosis in Bovine Adrenal Chromaffin Cells Infected With a Replication Deficient Adenovirus

Bovine adrenal chromaffin cells share many characteristics with neurons and are often used as a simple model system to study ion channels and neurotransmitter release. We infected bovine adrenal chromaffin cells with a replication deficient adenovirus that induces expression of the common reporters β-galactosidase and Green Fluorescent Protein via a bicistronic sequence. In perforated-patch recordings performed 48-h postinfection, peak calcium currents were reduced 32%, primarily due to loss of ω-conotoxin-GVIA-sensitive current. In contrast, sodium currents were increased 17%. Exocytosis, detected as an increase in membrane capacitance immediately after a single step depolarization, was reduced in proportion to the decrease in calcium influx. However, capacitance continued to increase for seconds after the depolarization. The amplitude of this poststimulus drift, or asynchronous exocytosis, was approximately three times that which occurred in a small fraction of control cells. Exocytosis evoked by repetitive stimulation with a train of brief depolarizations was increased 50%. Intracellular calcium levels measured during and after stimulation were lower, not higher, in adenovirus-infected cells. Electroporated cells showed reduced calcium currents but no enhancement of exocytosis. Cells infected with UV-irradiated virus showed reduced calcium currents and enhancement of exocytosis, but the changes were smaller than those caused by intact virus. Our results are consistent with the idea that adenovirus capsid and adenoviral DNA contribute to a Ca2+influx- and [Ca2+]i-independent enhancement of exocytosis in bovine chromaffin cells.

Rab3A negatively regulates activity-dependent modulation of exocytosis in bovine adrenal chromaffin cells

Members of the Rab family of monomeric GTPases have been implicated in vesicle trafficking, and Rab3A, located on synaptic vesicles in neurones and secretory vesicles in neuroendocrine cells, is likely to be involved in vesicle fusion leading to neurotransmitter release. A hydrolysis-deficient mutant of Rab3A, Rab3AQ81L, has been shown to potently inhibit hormone release. Here we show that the inhibition of hormone release by Rab3AQ81L is activity-dependent. Bovine adrenal chromaffin cells were induced to express Rab3AQ81L and green fluorescent protein by adenoviral gene transfer of a bicistronic construct. Fluorescent cells were stimulated with single depolarizations and trains of depolarizing pulses in whole cell perforated patch clamp recordings, and exocytosis was detected with cell capacitance measurements and carbon fibre amperometry. When single depolarizations were used to evoke exocytosis, cells expressing Rab3AQ81L showed a 50% reduction in response amplitude. When trains of brief depolarizations (10 or 40 ms) were used to evoke exocytosis, responses rapidly declined to zero in cells expressing Rab3AQ81L. Wild-type Rab3A had effects similar to Rab3AQ81L, causing significant inhibition of exocytosis only during repetitive stimulation. Expression of Rab5A did not alter exocytosis evoked by single depolarizations or repetitive stimulation. Applying a long duration depolarization in the middle of a stimulus train revealed that exocytotic efficacy (capacitance increase per amount of calcium influx) was not decreased in Rab3AQ81L-expressing cells. Instead, the activity-dependent increase in exocytotic efficacy observed in control cells did not occur in Rab3AQ81L-expressing cells. Our results suggest that Rab3A in the GTP bound conformation prevents activity-dependent facilitation.

Effects of Na+/Ca2+ exchange induced by SR Ca2+ release on action potentials and afterdepolarizations in guinea pig ventricular myocytes

In cardiac cells, evoked Ca2+ releases or spontaneous Ca2+ waves activate the inward Na+/Ca2+ exchange current (INaCa), which may modulate membrane excitability and arrhythmogenesis. In this study, we examined changes in membrane potential due to INaCa elicited by sarcoplasmic reticulum (SR) Ca2+ release in guinea pig ventricular myocytes using whole cell current clamp, fluorescence, and confocal microscopy. Inhibition of INaCa by Na+-free, Li+-containing Tyrode solution reversibly abbreviated the action potential duration at 90% repolarization (APD90) by 50% and caused SR Ca2+ overload. APD90 was similarly abbreviated in myocytes exposed to the Na+/Ca2+ exchange inhibitor KB-R7943 (5 microM) or after inhibition of SR Ca2+ release with ryanodine (20 microM). In the absence of extracellular Na+, spontaneous SR Ca2+ releases caused minimal changes in resting membrane potential. After the myocytes were returned to Na+-containing solution, the potentiated intracellular Ca2+ concentration ([Ca2+]i) transients dramatically prolonged APD90 and [Ca2+]i oscillations caused delayed and early afterdepolarizations (DADs and EADs). Laser-flash photolysis of caged Ca2+ mimicked the effects of spontaneous [Ca2+]i oscillations, confirming that APD prolongation, DADs, and EADs could be ascribed to intracellular Ca2+ release. These results suggest that Na+/Ca2+ exchange is a major physiological determinant of APD and that INaCa activation by spontaneous SR Ca2+ release/oscillations, depending on the timing, can account for both DADs and EADs during SR Ca2+ overload.

Presence of a calcium-activated chloride current in mouse ventricular myocytes

The properties of several components of outward K(+) currents, including the pharmacological and kinetics profiles as well as the respective molecular correlates, have been identified in mouse cardiac myocytes. Surprisingly little is known with regard to the Ca(2+)-activated ionic currents. We studied the Ca(2+)-activated transient outward currents in mouse ventricular myocytes. We have identified a 4-aminopyridine (4-AP)- and tetraethyl ammonium-resistant transient outward current that is Ca(2+) dependent. The current is carried by Cl(-) and is critically dependent on Ca(2+) influx via voltage-gated Ca(2+) channels and the sarcoplasmic reticulum Ca(2+) store. The current can be blocked by the anion transport blockers niflumic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. Single channel recordings reveal small conductance channels (approximately 1 pS in 140 mM Cl(-)) that can be blocked by anion transport blockers. Ensemble-averaged current faithfully mirrors the transient kinetics observed at the whole level. Niflumic acid (in the presence of 4-AP) leads to prolongation of the early repolarization. Thus this current may contribute to early repolarization of action potentials in mouse ventricular myocytes.

Simultaneous maps of optical action potentials and calcium transients in guinea-pig hearts: mechanisms underlying concordant alternans

1. The mechanisms underlying electro-mechanical alternans caused by faster heart rates were investigated in perfused guinea-pig hearts stained with RH237 and Rhod-2 AM to simultaneously map optical action potentials (APs) and intracellular free Ca2+ (Ca2+i). 2. Fluorescence images of the heart were focused on two 16 x 16 photodiode arrays to map Ca2+i (emission wavelength (lamdda;em) = 585 +/- 20 nm) and APs (lamdda;em > 715 nm) from 252 sites. Spatial resolution was 0.8 mm x 0.8 mm per diode and temporal resolution 4000 frames s-1. 3. The mean time-to-peak for APs and [Ca2+]i was spatially homogeneous (8.8 +/- 0.5 and 25.6 +/- 5.0 ms, respectively; n = 6). The durations of APs (APDs) and Ca2+i transients were shorter at the apex and progressively longer towards the base, indicating a gradient of ventricular relaxation. 4. Restitution kinetics revealed increasingly longer delays between AP and Ca2+i upstrokes (9.5 +/- 0.4 to 11.3 +/- 0.4 ms) with increasingly shorter S1-S2 intervals, particularly at the base, despite nearly normal peak [Ca2+]i. 5. Alternans of APs and Ca2+i transients were induced by a decrease++ in cycle length (CL), if the shorter CL captured at the pacing site and was shorter than refractory periods (RPs) near the base, creating heterogeneities of conduction velocity. 6. Rate-induced alternans in normoxic hearts were concordant (long APD with large [Ca2+]i) across the epicardium, with a magnitude (difference between odd-even signals) that varied with the local RP. Alternans were initiated by gradients of RP, producing alternans of conduction that terminated spontaneously without progressing to fibrillation.

Neuronal Ca2+ -activated Cl- channels--homing in on an elusive channel species

Ca2+ -activated Cl- channels control electrical excitability in various peripheral and central populations of neurons. Ca2+ influx through voltage-gated or ligand-operated channels, as well as Ca2+ release from intracellular stores, have been shown to induce substantial Cl- conductances that determine the response to synaptic input, spike rate, and the receptor current of various kinds of neurons. In some neurons, Ca2+ -activated Cl- channels are localized in the dendritic membrane, and their contribution to signal processing depends on the local Cl- equilibrium potential which may differ considerably from those at the membranes of somata and axons. In olfactory sensory neurons, the channels are expressed in ciliary processes of dendritic endings where they serve to amplify the odor-induced receptor current. Recent biophysical studies of signal transduction in olfactory sensory neurons have yielded some insight into the functional properties of Ca2+ -activated Cl- channels expressed in the chemosensory membrane of these cells. Ion selectivity, channel conductance, and Ca2+ sensitivity have been investigated, and the role of the channels in the generation of receptor currents is well understood. However, further investigation of neuronal Ca2+ -activated Cl- channels will require information about the molecular structure of the channel protein, the regulation of channel activity by cellular signaling pathways, as well as the distribution of channels in different compartments of the neuron. To understand the physiological role of these channels it is also important to know the Cl- equilibrium potential in cells or in distinct cell compartments that express Ca2+ -activated Cl- channels. The state of knowledge about most of these aspects is considerably more advanced in non-neuronal cells, in particular in epithelia and smooth muscle. This review, therefore, collects results both from neuronal and from non-neuronal cells with the intent of facilitating research into Ca2+ -activated Cl- channels and their physiological functions in neurons.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

A novel anionic conductance affects action potential duration in isolated rat ventricular myocytes

Effects of extracellular anions were studied in electrophysiological experiments on freshly isolated rat ventricular myocytes. Under current-clamp, action potential duration (APD) was prolonged by reducing the extracellular Cl(-) concentration and shortened by replacement of extracellular Cl(-) with I(-). Under voltage-clamp, membrane potential steps or ramps evoked an anionic background current (I(AB)) carried by either Cl(-), Br(-), I(-) or NO(3)(-). Activation of I(AB) was Ca(2+)- and cyclic AMP-independent, and was unaffected by cell shrinkage. I(AB) was insensitive to stilbene and fenamate anion transport blockers at concentrations that inhibit Ca(2+)-, cyclic AMP- and swelling-activated Cl(-) currents in ventricular cells of other mammals. These results suggest that I(AB) may be carried by a novel class of Cl(-) channel. Correlation of anion substitution experiments on membrane current and action potentials revealed that I(AB) could play a major role in controlling rat ventricular APD. These findings have important implications for those studying cardiac Cl(-) channels as potential targets for novel antiarrythmic agents.

Ca2+-activated non-selective cation current in rabbit ventricular myocytes

Oscillatory currents (OCs) were studied in isolated rabbit ventricular myocytes with whole cell mode voltage clamp using Na+-free intracellular and extracellular solutions under conditions where K+ currents were anticipated to be eliminated or minimized. All OCs were dependent on release of Ca2+ from the sarcoplasmic reticulum (SR) because they were associated with intracellular Ca2+ ([Ca2+]i) transients, and were suppressed by high concentrations of BAPTA (20 mmol l-1) or pretreatment with the SR antagonist agents ryanodine (10 micromol l-1) or thapsigargin (1 micromol l-1). The reversal potential (Vrev) for OCs shifted with changes in the calculated Vrev for Cl- (ECl) but was between ECl and the calculated Vrev for elemental monovalent cations (ECat), indicating that more than one Ca2+-activated current contributed to OCs. Addition of the Ca2+-activated Cl- current (ICl(Ca)) antagonist, niflumic acid, shifted the OC Vrev to ECat, suggesting that ICl(Ca) and a Ca2+-activated non-selective cation current (ICAN) contributed to the observed OCs. A reduced niflumic acid-insensitive Ca2+-activated OC persisted following marked symmetrical reduction of Cl- in the intracellular and extracellular solutions. Subsequent removal of all extracellular monovalent cations, by N-methyl-D-glucamine (NMDG) substitution, eliminated OCs and the inward holding current suggesting that ICAN and ICl(Ca) accounted for all or most of the Ca2+-activated OC in the absence of Na+. The OC Vrev was equal to ECl in the absence of monovalent elemental cations. Under these conditions niflumic acid eliminated all OCs. Macroscopic OC is partially due to ICAN in rabbit ventricular myocytes.

Calcium extrusion during aftercontractions in cardiac myocytes: the role of the sodium-calcium exchanger in the generation of the transient inward current

Spontaneous release of calcium from the sarcoplasmic reticulum leads to delayed afterdepolarizations which may represent an arrhythmogenic mechanism in the intact heart. The current underlying delayed afterdepolarizations is the transient inward current, but how this is triggered by a spontaneous rise in cytoplasmic calcium concentration is a matter of debate. We have investigated this by rapid application of caffeine to isolated guinea-pig cardiac myocytes, before and after drive train-induced aftercontractions. Mean (+/- s.e.m.) sarcoplasmic reticulum content reduced from 85 +/- 11 micromol/l accessible cell volume to 53 +/- 9 micromol/l accessible cell volume (n=11) during the course of the aftercontraction. The charge movement expected to result from extrusion of this calcium via the sodium-calcium exchanger was 70.1 +/- 5.4 pC, compared with charge measured during the transient inward current of 70.1 +/- 10.8 pC in the same cells (P=0.9969). Rapid inhibition of the sodium-calcium exchanger, by replacement of the superfusate with a sodium and calcium free solution between the end of the drive train and the aftercontraction, completely abolished the transient inward current (from 90.4 +/- 10.2 pA inward current to 23.8 +/- 14.9 pA outward current, P<0.001). We conclude that the transient inward current in this species is explained entirely by sodium-calcium exchange current without the need to invoke other calcium-activated conductances.

Calcium-dependent modulation of the plateau phase of action potential in isolated ventricular cells of rabbit heart

[Ca2+]i-dependent modulation of the action potential has been studied in Fura-2 dialysed ventricular myocytes of the rabbit using the whole-cell current-clamp method. Fifteen consecutive action potentials (AP1-AP15) and [Ca2+]i transients were elicited at a frequency of 0.2 Hz. A single, brief application of caffeine (during AP9) first enhanced and thereafter attenuated the [Ca2+]i transients accompanying AP9 and AP10-AP12, respectively. This approach provided direct comparison between time courses of action potentials: during the initial steady state (e.g. AP8) and when Ca2+ release from the sarcoplasmic reticulum was increased by caffeine (AP9) or decreased by depletion (AP10). The increase in [Ca2+]i facilitated repolarization and decreased action potential duration. However, action potentials at reduced Ca2+ release (AP10) had longer duration than during steady state. The caffeine-induced changes in L-type Ca2+ current (ICa,L), during voltage-clamp conditions partially explained the effects of caffeine on action potentials. When ICa,L was blocked by 500 micromol L-1 Cd2+, enhanced [Ca2+]i transients revealed an extra current component which was outward at +10 mV and inward at the resting membrane potential (most probably the transient inward current). In the presence of Cd2+, however, AP8 and AP10 had identical time courses, suggesting that ICa,L alone was responsible for the lengthening of AP10. Alterations in the transmembrane Na+ gradient resulted in changes of the steady state action potential durations (AP8) consistently with the expected modulation of the Na+-Ca2+ exchange current. However, the contribution of this current to the [Ca2+]i-dependent behaviour of action potential plateau could not be demonstrated.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

INaCa and ICl(Ca) contribute to isoproterenol-induced delayed after depolarizations in midmyocardial cells

The contributions of electrogenic sodium/calcium exchange current (INaCa), calcium-activated chloride conductance [ICl(Ca)], and calcium-activated nonselective cation conductance to delayed afterdepolarizations (DAD) were examined. Nonselective cation channels were absent in canine M cells, since inhibition of INaCa and ICl(Ca) eliminated all calcium-activated currents without abolishing cell shortening. After the cells were treated with isoproterenol and ouabain to increase calcium loading, INaCa was 168 +/- 30 x 10(-3) pC/pF and ICl(Ca) was 114 +/- 24 x 10(-3) pC/pF. Transient overlapping inward and outward currents were evoked positive to the chloride reversal potential (ECl). Outward current was chloride sensitive, and inward current was blocked by replacement of external sodium with lithium. When ECl was -50 mV, triggered activity occurred in normal external sodium and persisted after inhibition of INaCa. Steps to -80 mV revealed oscillating inward currents in normal sodium and chloride, which persisted after inhibition of INaCa. When ECl was equal to -113 mV, ICl(Ca) opposed INaCa at the resting potential. DAD occurred in normal sodium, and inhibition of outward ICl(Ca) provoked triggered activity. We conclude that INaCa represents approximately 60% of the total calcium-activated current at resting potentials but that both INaCa and ICl(Ca) work in concert to cause DAD in calcium-overloaded cells.

Do beta 2-adrenergic receptors modulate Ca2+ in adult rat ventricular myocytes?

We examined the role of beta 2-adrenergic receptors (ARs) in modulating calcium homeostasis in rat ventricular myocytes. Zinterol (10 microM), an agonist with a 25-fold greater affinity for beta 2-ARs over beta 1-ARs, modestly enhanced L-type calcium current (ICa) magnitude by approximately 30% and modestly accelerated the rate of Ca2+ concentration ([Ca2+]) decline (approximately 35%) but had little effect on the magnitude of the [Ca2+] transient (a nonsignificant 6% increase). However, 1 microM of the highly selective beta 1-AR antagonist CGP-20712A completely blocked the ICa increase induced by 10 microM zinterol. Pretreatment of cells with pertussis toxin (PTX) did not alter ICa enhancement by 10 microM zinterol, although it did abolish the ability of acetylcholine to block the forskolin-induced enhancement of ICa. Zinterol (10 microM) approximately doubled adenosine 3',5'-cyclic monophosphate (cAMP) accumulation, although one-half of this increase was blocked by CGP-20712A. In contrast, 1 microM of the nonselective beta-agonist isoproterenol increased cAMP production 15-fold. Thus we found no evidence that activation of beta 2-ARs modulates calcium homeostasis in rat ventricular myocytes, even after treatment with PTX.