Evaluation of T- and L-type Ca2+ currents in shark ventricular myocytes

Ionic currents underlying different patterns of electrical activity in working cardiac myocytes of mammals and non-mammalian vertebrates

The orderly contraction of the vertebrate heart is determined by generation and propagation of cardiac action potentials (APs). APs are generated by the integrated activity of time- and voltage-dependent ionic channels which carry inward Na⁺ and Ca²⁺ currents, and outward K⁺ currents. This review compares atrial and ventricular APs and underlying ion currents between different taxa of vertebrates. We have collected literature data and attempted to find common electrophysiological features for two or more vertebrate groups, show differences between taxa and cardiac chambers, and indicate gaps in the existing data. Although electrical excitability of the heart in all vertebrates is based on the same superfamily of channels, there is a vast variability of AP waveforms between atrial and ventricular myocytes, between different species of the same vertebrate class and between endothermic and ectothermic animals. The wide variability of AP shapes is related to species-specific differences in animal size, heart rate, stage of ontogenetic development, excitation-contraction coupling, temperature and oxygen availability. Some of the differences between taxa are related to evolutionary development of genomes, which appear e.g. in the expression of different Na⁺ and K⁺ channel orthologues in cardiomyocytes of vertebrates. There is a wonderful variability of AP shapes and underlying ion currents with which electrical excitability of vertebrate heart can be generated depending on the intrinsic and extrinsic conditions of animal body. This multitude of ionic mechanisms provides excellent material for studying how the function of the vertebrate heart can adapt or acclimate to prevailing physiological and environmental conditions.

Indirect evidence that anoxia exposure and cold acclimation alter transarcolemmal Ca2+ flux in the cardiac pacemaker, right atrium and ventricle of the red-eared slider turtle (Trachemys scripta)

We indirectly assessed if altered transarcolemmal Ca²⁺ flux accompanies the decreased cardiac activity displayed by Trachemys scripta with anoxia exposure and cold acclimation. Turtles were first acclimated to 21 °C or 5 °C and held under normoxic (21N; 5N) or anoxic conditions (21A; 5A). We then compared the response of intrinsic heart rate (f_H) and maximal developed force of spontaneously contracting right atria (F_max,RA), and maximal developed force of isometrically-contracting ventricular strips (F_max,V), to Ni²⁺ (0.1-10 mM), which respectively blocks T-type Ca²⁺ channels, L-type Ca²⁺ channels and the Na⁺-Ca²⁺-exchanger at the low, intermediate and high concentrations employed. Dose-response curves were established in simulated in vivo normoxic (Sim Norm) or simulated in vivo anoxic extracellular conditions (Sim Anx; 21A and 5A preparations). Ni²⁺ decreased intrinsic f_H, F_max,RA and F_max,V of 21N tissues in a concentration-dependent manner, but the responses were blunted in 21A tissues in Sim Norm. Similarly, dose-response curves for F_max,RA and F_max,V of 5N tissues were right-shifted, whereas anoxia exposure at 5 °C did not further alter the responses. The influence of Sim Anx was acclimation temperature-, cardiac chamber- and contractile parameter-dependent. Combined, the findings suggest that: (1) reduced transarcolemmal Ca²⁺ flux in the cardiac pacemaker is a potential mechanism underlying the slowed intrinsic f_H of anoxic turtles at 21 °C, but not 5 °C, (2) a downregulation of transarcolemmal Ca²⁺ flux may aid cardiac anoxia survival at 21 °C and prime the turtle myocardium for winter anoxia and (3) confirm that altered extracellular conditions with anoxia exposure can modify turtle cardiac transarcolemmal Ca²⁺ flux.

Excitation-Contraction Coupling in the Goldfish (Carassius auratus) Intact Heart

Cardiac physiology of fish models is an emerging field given the ease of genome editing and the development of transgenic models. Several studies have described the cardiac properties of zebrafish (Denio rerio). The goldfish (Carassius auratus) belongs to the same family as the zebrafish and has emerged as an alternative model with which to study cardiac function. Here, we propose to acutely study electrophysiological and systolic Ca²⁺ signaling in intact goldfish hearts. We assessed the Ca²⁺ dynamics and the electrophysiological cardiac function of goldfish, zebrafish, and mice models, using pulsed local field fluorescence microscopy, intracellular microelectrodes, and flash photolysis in perfused hearts. We observed goldfish ventricular action potentials (APs) and Ca²⁺ transients to be significantly longer when compared to the zebrafish. The action potential half duration at 50% (APD₅₀) of goldfish was 370.38 ± 8.8 ms long, and in the zebrafish they were observed to be only 83.9 ± 9.4 ms. Additionally, the half duration of the Ca²⁺ transients was also longer for goldfish (402.1 ± 4.4 ms) compared to the zebrafish (99.1 ± 2.7 ms). Also, blocking of the L-type Ca²⁺ channels with nifedipine revealed this current has a major role in defining the amplitude and the duration of goldfish Ca²⁺ transients. Interestingly, nifedipine flash photolysis experiments in the intact heart identified whether or not the decrease in the amplitude of Ca²⁺ transients was due to shorter APs. Moreover, an increase in temperature and heart rate had a strong shortening effect on the AP and Ca²⁺ transients of goldfish hearts. Furthermore, ryanodine (Ry) and thapsigargin (Tg) significantly reduced the amplitude of the Ca²⁺ transients, induced a prolongation in the APs, and altogether exhibited the degree to which the Ca²⁺ release from the sarcoplasmic reticulum contributed to the Ca²⁺ transients. We conclude that the electrophysiological properties and Ca²⁺ signaling in intact goldfish hearts strongly resembles the endocardial layer of larger mammals.

Expression of calcium channel transcripts in the zebrafish heart: dominance of T-type channels

Calcium channels are necessary for cardiac excitation-contraction (E-C) coupling, but Ca²⁺ channel composition of fish hearts is still largely unknown. To this end, we determined transcript expression of Ca²⁺ channels in the heart of zebrafish (Danio rerio), a popular model species. Altogether, 18 Ca²⁺ channel α-subunit genes were expressed in both atrium and ventricle. Transcripts for 7 L-type (Ca_v1.1a, Ca_v1.1b, Ca_v1.2, Ca_v1.3a, Ca_v1.3b, Ca_v1.4a, Ca_v1.4b), 5 T-type (Ca_v3.1, Ca_v3.2a, Ca_v3.2b, Ca_v3.3a, Ca_v3.3b) and 6 P/Q-, N- and R-type (Ca_v2.1a, Ca_v2.1b, Ca_v2.2a, Ca_v2.2b, Ca_v2.3a, Ca_v2.3b) Ca²⁺ channels were expressed. In the ventricle, T-type channels formed 54.9%, L-type channels 41.1% and P/Q-, N- and R-type channels 4.0% of the Ca²⁺ channel transcripts. In the atrium, the relative expression of T-type and L-type Ca²⁺ channel transcripts was 64.1% and 33.8%, respectively (others accounted for 2.1%). Thus, at the transcript level, T-type Ca²⁺ channels are prevalent in zebrafish atrium and ventricle. At the functional level, peak densities of ventricular T-type (I_CaT) and L-type (I_CaL) Ca²⁺ current were 6.3±0.8 and 7.7±0.8 pA pF^-1, respectively. I_CaT mediated a sizeable sarcolemmal Ca²⁺ influx into ventricular myocytes: the increment in total cellular Ca²⁺ content via I_CaT was 41.2±7.3 µmol l^-1, which was 31.7% of the combined Ca²⁺ influx (129 µmol l^-1) via I_CaT and I_CaL (88.5±20.5 µmol l^-1). The diversity of expressed Ca²⁺ channel genes in zebrafish heart is high, but dominated by the members of the T-type subfamily. The large ventricular I_CaT is likely to play a significant role in E-C coupling.

Characterization of the functional and anatomical differences in the atrial and ventricular myocardium from three species of elasmobranch fishes: smooth dogfish (Mustelus canis), sandbar shark (Carcharhinus plumbeus), and clearnose skate (Raja eglanteria)

We assessed the functional properties in atrial and ventricular myocardium (using isolated cardiac strips) of smooth dogfish (Mustelus canis), clearnose skate (Raja eglanteria), and sandbar shark (Carcharhinus plumbeus) by blocking Ca²⁺ release from the sarcoplasmic reticulum (SR) with ryanodine and thapsigargin and measuring the resultant changes in contraction-relaxation parameters and the force-frequency relationship at 20 °C and 30 °C. We also examined ultrastructural differences with electron microscopy. In tissues from smooth dogfish, net force (per cross-sectional area) and measures of the speeds of contraction and relaxation were all higher in atrial than ventricular myocardium at both temperatures. Atrial-ventricular differences were evident in the other two species primarily in measures of the rates of contraction and relaxation. Ryanodine-thapsigargin treatment reduced net force and its maximum positive first derivative (i.e., contractility), and increased time to 50 % relaxation in atrial tissue from smooth dogfish at 30 °C. It also increased times to peak force and half relaxation in clearnose skate atrial and ventricular tissue at both temperatures, but only in atrial tissue from sandbar shark at 30 °C; indicating that SR involvement in excitation-contraction (EC) coupling is species- and temperature-specific in elasmobranch fishes, as it is in teleost fishes. Atrial and ventricular myocardium from all three species displayed a negative force-frequency relationship, but there was no evidence that SR involvement in EC coupling was influenced by heart rate. SR was evident in electron micrographs, generally located in proximity to mitochondria and intercalated discs, and to a lesser extent between the myofibrils; with mitochondria being more numerous in ventricular than atrial myocardium in all three species.

Ca(2+)-Currents in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes Effects of Two Different Culture Conditions

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) provide a unique opportunity to study human heart physiology and pharmacology and repair injured hearts. The suitability of hiPSC-CM critically depends on how closely they share physiological properties of human adult cardiomyocytes (CM). Here we investigated whether a 3D engineered heart tissue (EHT) culture format favors maturation and addressed the L-type Ca²⁺-current (I_Ca,L) as a readout. The results were compared with hiPSC-CM cultured in conventional monolayer (ML) and to our previous data from human adult atrial and ventricular CM obtained when identical patch-clamp protocols were used. HiPSC-CM were two- to three-fold smaller than adult CM, independently of culture format [capacitance ML 45 ± 1 pF (n = 289), EHT 45 ± 1 pF (n = 460), atrial CM 87 ± 3 pF (n = 196), ventricular CM 126 ± 8 pF (n = 50)]. Only 88% of ML cells showed I_Ca, but all EHT. Basal I_Ca density was 10 ± 1 pA/pF (n = 207) for ML and 12 ± 1 pA/pF (n = 361) for EHT and was larger than in adult CM [7 ± 1 pA/pF (p < 0.05, n = 196) for atrial CM and 6 ± 1 pA/pF (p < 0.05, n = 47) for ventricular CM]. However, ML and EHT showed robust T-type Ca²⁺-currents (I_Ca,T). While (−)-Bay K 8644, that activates I_Ca,L directly, increased I_Ca,Lto the same extent in ML and EHT, β₁- and β₂-adrenoceptor effects were marginal in ML, but of same size as (−)-Bay K 8644 in EHT. The opposite was true for serotonin receptors. Sensitivity to β₁ and β₂-adrenoceptor stimulation was the same in EHT as in adult CM (−logEC₅₀: 5.9 and 6.1 for norepinephrine (NE) and epinephrine (Epi), respectively), but very low concentrations of Rp-8-Br-cAMPS were sufficient to suppress effects (−logEC₅₀: 5.3 and 5.3 respectively for NE and Epi). Taken together, hiPSC-CM express I_Ca,L at the same density as human adult CM, but, in contrast, possess robust I_Ca,T. Increased effects of catecholamines in EHT suggest more efficient maturation.

Temperature-dependence of L-type Ca(2+) current in ventricular cardiomyocytes of the Alaska blackfish (Dallia pectoralis)

To lend insight into the overwintering strategy of the Alaska blackfish (Dallia pectoralis), we acclimated fish to 15 or 5 °C and then utilized whole-cell patch clamp to characterize the effects of thermal acclimation and acute temperature change on the density and kinetics of ventricular L-type Ca(2+) current (I Ca). Peak I Ca density at 5 °C (-1.1 ± 0.1 pA pF(-1)) was 1/8th that at 15 °C (-8.8 ± 0.6 pA pF(-1)). However, alterations of the Ca(2+)- and voltage-dependent inactivation properties of L-type Ca(2+) channels partially compensated against the decrease. The time constant tau (τ) for the kinetics of inactivation of I Ca was ~4.5 times greater at 5 °C than at 15 °C, and the voltage for half-maximal inactivation was shifted from -23.3 ± 1.0 mV at 15 °C to -19.8 ± 1.2 mV at 5 °C. These modifications increase the open probability of the channel and culminate in an approximate doubling of the L-type Ca(2+) window current, which contributes to approximately 15% of the maximal Ca(2+) conductance at 5 °C. Consequently, the charge density of I Ca (Q Ca) and the total Ca(2+) transferred through the L-type Ca(2+) channels (Δ[Ca(2+)]) were not as severely reduced at 5 °C as compared to peak I Ca density. In combination, the results suggest that while the Alaska blackfish substantially down-regulates I Ca with acclimation to low temperature, there is sufficient compensation in the kinetics of the L-type Ca(2+) channel to support the level of cardiac performance required for the fish to remain active throughout the winter.

Electrical excitability of the heart in a Chondrostei fish, the Siberian sturgeon (Acipenser baerii)

Sturgeon (family Acipenseridae) are regarded as living fossils due to their ancient origin and exceptionally slow evolution. To extend our knowledge of fish cardiac excitability to a Chondrostei fish, we examined electrophysiological phenotype of the Siberian sturgeon ( Acipenser baerii) heart with recordings of epicardial ECG, intracellular action potentials (APs), and sarcolemmal ion currents. Epicardial ECG of A. baerii had the typical waveform of the vertebrate ECG with Q-T interval (average duration of ventricular AP) of 650 ± 30 ms and an intrinsic heart rate of 45.5 ± 5 beats min−1 at 20°C. Similar to other fish species, atrial AP was shorter in duration (402 ± 33 ms) than ventricular AP (585 ± 40) ( P < 0.05) at 20°C. Densities of atrial and ventricular Na+ currents were similar (−47.6 ± 4.5 and −53.2 ± 5.1 pA/pF, respectively) and close to the typical values of teleost hearts. Two major K+ currents, the inward rectifier K+ current ( IK1), and the delayed rectifier K+ current ( IKr) were found under basal conditions in sturgeon cardiomyocytes. The atrial IKr (3.3 ± 0.2 pA/pF) was about twice as large as the ventricular IKr (1.3 ± 0.4 pA/pF) ( P < 0.05) conforming to the typical pattern of teleost cardiac IKr. Divergent from other fishes, the ventricular IK1 was remarkably small (−2.5 ± 0.07 pA/pF) and not different from that of the atrial myocytes (−1.9 ± 0.06 pA/pF) ( P > 0.05). Two ligand-gated K+ currents were also found: ACh-activated inward rectifier ( IKACh) was present only in atrial cells, while ATP-sensitive K+ current ( IKATP) was activated by a mitochondrial blocker, CCCP, in both atrial and ventricular cells. The most striking difference to other fishes appeared in Ca2+ currents ( ICa). In atrial myocytes, ICa was predominated by nickel-sensitive and nifedipine-resistant T-type ICa, while ventricular myocytes had mainly nifedipine-sensitive and nickel-resistant L-type ICa. ICaT/ ICaL ratio of the sturgeon atrial myocytes (2.42) is the highest value ever measured for a vertebrate species. In ventricular myocytes, ICaT/ ICaL ratio was 0.09. With the exception of the large atrial ICaT and small ventricular IK1, electrical excitability of A. baerii heart is similar to that of teleost hearts.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

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.

Voltage-dependent calcium channels in ventricular cells of rainbow trout: effect of temperature changes in vitro

Voltage-dependent calcium channels (VDCC) in ventricular myocytes from rainbow trout (Oncorhynchus mykiss) were investigated in vitro using the perforated patch-clamp technique, which maintains the integrity of the intracellular milieu. First, we characterized the current using barium as the charge carrier and established the doses of various pharmacological agents to use these agents in additional studies. Second, we examined the current at several physiological temperatures to determine temperature dependency. The calcium currents at 10 degrees C (acclimation temperature) were identified as L-type calcium currents based on their kinetic behavior and response to various calcium channel agonists and antagonists. Myocytes were chilled (4 degrees C) and warmed (18 and 22 degrees C), and the response of VDCC to varying temperatures was observed. There was no significant dependency of the current amplitude and kinetics on temperature. Amplitude decreased 25-36% at 4 degrees C (Q(10) approximately 1.89) and increased 18% at 18 degrees C (Q(10) approximately 1.23) in control, Bay K8644 (Bay K)-, and forskolin-enhanced currents. The inactivation rates (tau(i)) did not demonstrate a temperature sensitivity for the VDCC (Q(10) 1.23-1. 92); Bay K treatment, however, increased temperature sensitivity of tau(i) between 10 and 18 degrees C (Q(10) 3.98). The low Q(10) values for VDCC are consistent with a minimal temperature sensitivity of trout myocytes between 4 and 22 degrees C. This low-temperature dependency may provide an important role for sarcolemmal calcium channels in adaptation to varying environmental temperatures in trout.