Contraction in voltage-clamped, internally perfused single heart cells

Regulation of cardiac calcium signaling by newly identified calcium pump modulators

In cardiomyocytes, the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA2a) is a central component of intracellular Ca2+ regulation. Several heart diseases, including heart failure, are associated with reduced myocardial contraction due to SERCA2a downregulation. Therefore, the need for developing new drugs that could improve SERCA2a function is high. We have recently identified SERCA2a modulators (Compounds 6 and 8) from our screening campaigns and confirmed activation of biochemical SERCA2a ATPase activity and Ca2+ uptake activity. In this study, confocal microscopy and in-cell Ca2+ imaging were used to characterize the effects of these SERCA2a activators on Ca2+ regulation in mouse ventricular myocytes and endoplasmic reticulum (ER) Ca2+ uptake in a HEK293 cell expressing human SERCA2a. Analysis of cytosolic Ca2+ dynamics in cardiomyocytes revealed that both Compounds (6 and 8) increase the action potential-induced Ca2+ transients and sarcoplasmic reticulum (SR) Ca2+ load. While Compound 6 induced a negligible effect on Ca2+ transients invoked by the L-type Ca2+ channel (LTCC) current, Compound 8 increased Ca2+ transients during LTCC activation, suggesting an off-target protein interaction of Compound 8. Analysis of ER Ca2+ transport by human SERCA2a in HEK cells showed that only Compound 6 increased both ER Ca2+ uptake and ER Ca2+ load significantly, whereas Compound 8 had no effect on SERCA2a Ca2+ transport. This study revealed that Compound 6 exhibits promising characteristics that can improve intracellular Ca2+ dynamics by selectively enhancing SERCA2a Ca2+ uptake.

Use of Isolated Adult Myocytes to Evaluate Cardiotoxicity. II. Preparation and Properties*

The preparation and properties of isolated adult cardiac myocytes are reviewed, with the goal being to evaluate their usefulness as a model system for measuring cardiotoxicity. Some important factors in cell isolation methodology which impact on the quality of the preparation are identified, along with criteria for assessing the quality of cells after isolation. By all criteria, myocytes isolated by good procedures appear to largely retain their original properties. Moreover, the distinctive behavior of adult myocytes under metabolic stress endows them with a particular usefulness as monitors of toxicity. Overall, we conclude that the art of adult heart cell isolation and culture is now sufficiently advanced for either freshly isolated cells in suspension or cells in culture to be a useful model system for toxicity studies.

Heterogeneity of transverse-axial tubule system in mouse atria: Remodeling in atrial-specific Na+-Ca2+ exchanger knockout mice

Transverse-axial tubules (TATs) are commonly assumed to be sparse or absent in atrial myocytes from small animals. Atrial myocytes from rats, cats and rabbits lack TATs, which results in a characteristic "V"-shaped Ca release pattern in confocal line-scan recordings due to the delayed rise of Ca in the center of the cell. To examine TAT expression in isolated mouse atrial myocytes, we loaded them with the membrane dye Di-4-ANEPPS to label TATs. We found that >80% of atrial myocytes had identifiable TATs. Atria from male mice had a higher TAT density than female mice, and TAT density correlated with cell width. Using the fluorescent Ca indicator Fluo-4-AM and confocal imaging, we found that wild type (WT) mouse atrial myocytes generate near-synchronous Ca transients, in contrast to the "V"-shaped pattern typically reported in other small animals such as rat. In atrial-specific Na-Ca exchanger (NCX) knockout (KO) mice, which develop sinus node dysfunction and atrial hypertrophy with dilation, we found a substantial loss of atrial TATs in isolated atrial myocytes. There was a greater loss of transverse tubules compared to axial tubules, resulting in a dominance of axial tubules. Consistent with the overall loss of TATs, NCX KO atrial myocytes displayed a "V"-shaped Ca transient with slower and reduced central (CT) Ca release and uptake in comparison to subsarcolemmal (SS) Ca release. We compared chemically detubulated (DT) WT cells to KO, and found similar slowing of CT Ca release and uptake. However, SS Ca transients in the WT DT cells had faster uptake kinetics than KO cells, consistent with the presence of NCX and normal sarcolemmal Ca efflux in the WT DT cells. We conclude that the remodeling of NCX KO atrial myocytes is accompanied by a loss of TATs leading to abnormal Ca release and uptake that could impact atrial contractility and rhythm.

Phenotypic assays for analyses of pluripotent stem cell-derived cardiomyocytes

Stem cell-derived cardiomyocytes (CMs) hold great hopes for myocardium regeneration because of their ability to produce functional cardiac cells in large quantities. They also hold promise in dissecting the molecular principles involved in heart diseases and also in drug development, owing to their ability to model the diseases using patient-specific human pluripotent stem cell (hPSC)-derived CMs. The CM properties essential for the desired applications are frequently evaluated through morphologic and genotypic screenings. Even though these characterizations are necessary, they cannot in principle guarantee the CM functionality and their drug response. The CM functional characteristics can be quantified by phenotype assays, including electrophysiological, optical, and/or mechanical approaches implemented in the past decades, especially when used to investigate responses of the CMs to known stimuli (eg, adrenergic stimulation). Such methods can be used to indirectly determine the electrochemomechanics of the cardiac excitation-contraction coupling, which determines important functional properties of the hPSC-derived CMs, such as their differentiation efficacy, their maturation level, and their functionality. In this work, we aim to systematically review the techniques and methodologies implemented in the phenotype characterization of hPSC-derived CMs. Further, we introduce a novel approach combining atomic force microscopy, fluorescent microscopy, and external electrophysiology through microelectrode arrays. We demonstrate that this novel method can be used to gain unique information on the complex excitation-contraction coupling dynamics of the hPSC-derived CMs.

Measurement of Strain in Cardiac Myocytes at Micrometer Scale Based on Rapid Scanning Confocal Microscopy and Non-Rigid Image Registration

Measurement of cell shortening is an important technique for assessment of physiology and pathophysiology of cardiac myocytes. Many types of heart disease are associated with decreased myocyte shortening, which is commonly caused by structural and functional remodeling. Here, we present a new approach for local measurement of 2-dimensional strain within cells at high spatial resolution. The approach applies non-rigid image registration to quantify local displacements and Cauchy strain in images of cells undergoing contraction. We extensively evaluated the approach using synthetic cell images and image sequences from rapid scanning confocal microscopy of fluorescently labeled isolated myocytes from the left ventricle of normal and diseased canine heart. Application of the approach yielded a comprehensive description of cellular strain including novel measurements of transverse strain and spatial heterogeneity of strain. Quantitative comparison with manual measurements of strain in image sequences indicated reliability of the developed approach. We suggest that the developed approach provides researchers with a novel tool to investigate contractility of cardiac myocytes at subcellular scale. In contrast to previously introduced methods for measuring cell shorting, the developed approach provides comprehensive information on the spatio-temporal distribution of 2-dimensional strain at micrometer scale.

A modified local control model for Ca2+ transients in cardiomyocytes: junctional flux is accompanied by release from adjacent non-junctional RyRs

Excitation-contraction coupling in cardiomyocytes requires Ca(2+) influx through dihydropyridine receptors in the sarcolemma, which gates Ca(2+) release through sarcoplasmic ryanodine receptors (RyRs). Ca(2+) influx, release and diffusion produce a cytosolic Ca(2+) transient. Here, we investigated the relationship between Ca(2+) transients and the spatial arrangement of the sarcolemma including the transverse tubular system (t-system). To accomplish this, we studied isolated ventricular myocytes of rabbit, which exhibit a heterogeneously distributed t-system. We developed protocols for fluorescent labeling and triggered two-dimensional confocal microscopic imaging with high spatiotemporal resolution. From sequences of microscopic images, we measured maximal upstroke velocities and onset times of local Ca(2+) transients together with their distance from the sarcolemma. Analyses indicate that not only sarcolemmal release sites, but also those that are within 1 μm of the sarcolemma actively release Ca(2+). Our data also suggest that release does not occur at sites further than 2.5 μm from the sarcolemma. The experimental data are in agreement with results from a mathematical model of Ca(2+) release and diffusion. Our findings can be explained by a modified local control model, which constrains the region of regenerative activation of non-junctional RyR clusters. We believe that this model will be useful for describing excitation-contraction coupling in cardiac myocytes with a sparse t-system, which includes those from diseased heart tissue as well as atrial myocytes of some species.

Cardiac sodium-calcium exchange and efficient excitation-contraction coupling: implications for heart disease

Cardiovascular disease is a leading cause of death worldwide, with ischemic heart disease alone accounting for >12% of all deaths, more than HIV/AIDS, tuberculosis, lung, and breast cancer combined. Heart disease has been the leading cause of death in the United States for the past 85 years and is a major cause of disability and health-care expenditures. The cardiac conditions most likely to result in death include heart failure and arrhythmias, both a consequence of ischemic coronary disease and myocardial infarction, though chronic hypertension and valvular diseases are also important causes of heart failure. Sodium-calcium exchange (NCX) is the dominant calcium (Ca2+) efflux mechanism in cardiac cells. Using ventricular-specific NCX knockout mice, we have found that NCX is also an essential regulator of cardiac contractility independent of sarcoplasmic reticulum Ca2+ load. During the upstroke of the action potential, sodium (Na+) ions enter the diadic cleft space between the sarcolemma and the sarcoplasmic reticulum. The rise in cleft Na+, in conjunction with depolarization, causes NCX to transiently reverse. Ca2+ entry by this mechanism then "primes" the diadic cleft so that subsequent Ca2+ entry through Ca2+ channels can more efficiently trigger Ca2+ release from the sarcoplasmic reticulum. In NCX knockout mice, this mechanism is inoperative (Na+ current has no effect on the Ca2+ transient), and excitation-contraction coupling relies upon the elevated diadic cleft Ca2+ that arises from the slow extrusion of cytoplasmic Ca2+ by the ATP-dependent sarcolemmal Ca2+ pump. Thus, our data support the conclusion that NCX is an important regulator of cardiac contractility. These findings suggest that manipulation of NCX may be beneficial in the treatment of heart failure.

Local control in cardiac E-C coupling

The development of local control theories in cardiac excitation-contraction coupling solved a major problem in the calcium-induced calcium release (CICR) hypothesis. Local control explained how regeneration, inherent in the CICR mechanism, might be limited spatially to enable graded Ca release (and force production). The key lies in the stochastic recruitment of individual calcium release units (couplons or CRUs) where adjacent CRUs are partially uncoupled by the distance between them. In the CRU, individual groups of sarcoplasmic reticulum calcium release channels (RyRs) are very close to the surface membrane where calcium influx, controlled by membrane depolarization, leads to high local Ca levels that enable a high speed response from RyRs that have a very low probability to opening at resting Ca levels. However, calcium diffusion from an activated CRU results in adjacent CRUs being exposed to much lower levels of Ca and probability of activation. This effectively uncouples the CRUs and limits overall regenerative gain to enable stability without compromising sensitivity. Nevertheless, it is still unclear how the CRU terminates its release of calcium on the physiological timescale, and possible mechanisms (and problems) are briefly reviewed. We suggest that modulation in RyR gating may serve to control average SR Ca levels to regulate other metabolic functions of the sarco(endo)plasmic reticulum beyond regulating contractility. This article is part of a special issue entitled "Local Signaling in Myocytes."

Na+ currents are required for efficient excitation-contraction coupling in rabbit ventricular myocytes: a possible contribution of neuronal Na+ channels

Ca2+ transients were activated in rabbit ventricular cells by a sequence of action potential shaped voltage clamps. After activating a series of control transients, Na+ currents (INa) were inactivated with a ramp from -80 to -40 mV (1.5 s) prior to the action potential clamp. The transients were detected with the calcium indicator Fluo-4 and an epifluorescence system. With zero Na+ in the pipette INa inactivation produced a decline in the SR Ca2+ release flux (measured as the maximum rate of rise of the transient) of 27 ± 4% (n = 9, P < 0.001) and a peak amplitude reduction of 10 ± 3% (n = 9, P < 0.05). With 5 mm Na+ in the pipette the reduction in release flux was greater (34 ± 4%, n = 4, P < 0.05). The ramp effectively inactivates INa without changing ICa, and there was no significant change in the transmembrane Ca2+ flux after the inactivation of INa. We next evoked action potentials under current clamp. TTX at 100 nm, which selectively blocks neuronal isoforms of Na+ channels, produced a decline in SR Ca2+ release flux of 35 ± 3% (n = 6, P < 0.001) and transient amplitude of 12 ± 2% (n = 6, P < 0.05). This effect was similar to the effect of INa inactivation on release flux. We conclude that a TTX-sensitive INa is essential for efficient triggering of SR Ca2+ release. We propose that neuronal Na+ channels residing within couplons activate sufficient reverse Na+-Ca2+ exchanger (NCX) to prime the junctional cleft with Ca2+. The results can be explained if non-linearities in excitation-contraction coupling mechanisms modify the coupling fidelity of ICa, which is known to be low at positive potentials.

Ontogeny of Ca2+-induced Ca2+ release in rabbit ventricular myocytes

It is commonly accepted that L-type Ca(2+) channel-mediated Ca(2+)-induced Ca(2+) release (CICR) is the dominant mode of excitation-contraction (E-C) coupling in the adult mammalian heart and that there is no appreciable CICR in neonates. However, we have observed that cell contraction in the neonatal heart was significantly decreased after sarcoplasmic reticulum (SR) Ca(2+) depletion with caffeine. Therefore, the present study investigated the developmental changes of CICR in rabbit ventricular myocytes at 3, 10, 20, and 56 days of age. We found that the inhibitory effect of the L-type Ca(2+) current (I(Ca)) inhibitor nifedipine (Nif; 15 microM) caused an increasingly larger reduction of Ca(2+) transients on depolarization in older age groups [from approximately 15% in 3-day-old (3d) myocytes to approximately 90% in 56-day-old (56d) myocytes]. The remaining Ca(2+) transient in the presence of Nif in younger age groups was eliminated by the inhibition of Na(+)/Ca(2+) exchanger (NCX) with the subsequent addition of 10 microM KB-R7943 (KB-R). Furthermore, Ca(2+) transients were significantly reduced in magnitude after the depletion of SR Ca(2+) with caffeine in all age groups, although the effect was significantly greater in the older age groups (from approximately 40% in 3d myocytes up to approximately 70% in 56d myocytes). This SR Ca(2+)-sensitive Ca(2+) transient in the earliest developmental stage was insensitive to Nif but was sensitive to the subsequent addition of KB-R, indicating the presence of NCX-mediated CICR that decreased significantly with age (from approximately 37% in 3d myocytes to approximately 0.5% in 56d myocytes). In contrast, the I(Ca)-mediated CICR increased significantly with age (from approximately 10% in 3d myocytes to approximately 70% in 56d myocytes). The CICR gain as estimated by the integral of the CICR Ca(2+) transient divided by the integral of its Ca(2+) transient trigger was smaller when mediated by NCX ( approximately 1.0 for 3d myocytes) than when mediated by I(Ca) ( approximately 3.0 for 56d myocytes). We conclude that the lower-efficiency NCX-mediated CICR is a predominant mode of CICR in the earliest developmental stages that gradually decreases as the more efficient L-type Ca(2+) channel-mediated CICR increases in prominence with ontogeny.

Twenty years of calcium imaging: cell physiology to dye for

The use of fluorescent dyes over the past two decades has led to a revolution in our understanding of calcium signaling. Given the ubiquitous role of Ca(2+) in signal transduction at the most fundamental levels of molecular, cellular, and organismal biology, it has been challenging to understand how the specificity and versatility of Ca(2+) signaling is accomplished. In excitable cells, the coordination of changing Ca(2+) concentrations at global (cellular) and well-defined subcellular spaces through the course of membrane depolarization can now be conceptualized in the context of disease processes such as cardiac arrhythmogenesis. The spatial and temporal dimensions of Ca(2+) signaling are similarly important in non-excitable cells, such as endothelial and epithelial cells, to regulate multiple signaling pathways that participate in organ homeostasis as well as cellular organization and essential secretory processes.

Effects of streptozotocin-induced diabetes on contraction and calcium transport in rat ventricular cardiomyocytes

Cardiovascular diseases are the major cause of morbidity and mortality in diabetic patients. Contractile function of the heart is frequently compromised in the clinical setting and in experimental models of diabetes mellitus (DM). This article investigated the effect of streptozotocin (STZ)-induced type 1 DM on contraction, L-type calcium (Ca2+) current (I(Ca(2+)L)), and on cytosolic calcium concentrations [Ca2+]i in ventricular myocytes of the rat heart. After 4-10 weeks of STZ treatment, blood glucose levels in diabetic animals were significantly (P < 0.05) higher compared to age-matched controls. Diabetic rats have significantly (P < 0.05) reduced body, reduced heart weight, and reduced viability of ventricular myocytes compared to controls. The amplitude of I(Ca(2+)L) and amplitude of contraction were significantly reduced (P < 0.05) at test potentials in the range -10 mV to +20 mV and -30 mV to +40 mV, respectively, in myocytes from diabetic animals compared to age-matched controls. Moreover, there was a significant (P < 0.05) delay in electrically stimulated and caffeine-evoked time to half relaxation of the Ca2+ transient in myocytes from diabetic animals compared to controls. A similar effect was obtained in myocytes treated with a combination of caffeine and nickel chloride (NiCl2). It is concluded that the diabetes-induced voltage-dependent decrease in contraction is associated with reduced Ca2+ channel activities and prolonged diastolic cytosolic Ca2+ compared to age-matched control. Taken together, the results suggest that Ca2+ homeostasis is deranged during DM and this may be expressed at the level of the Na+/Ca2+ exchanger.

Restitution of Ca(2+) release and vulnerability to arrhythmias

New information has recently been obtained along two essentially parallel lines of research: investigations into the fundamental mechanisms of Ca(2+)-induced Ca(2+) release (CICR) in heart cells, and analyses of the factors that control the development of unstable rhythms such as repolarization alternans. These lines of research are starting to converge such that we can begin to understand unstable and potentially arrhythmogenic cardiac dynamics in terms of the underlying mechanisms governing not only membrane depolarization and repolarization but also the complex bidirectional interactions between electrical and Ca(2+) signaling in heart cells. In this brief review, we discuss the progress that has recently been made in understanding the factors that control the beat-to-beat regulation of cardiac Ca(2+) release and attempt to place these results within a larger context. In particular, we discuss factors that may contribute to unstable Ca(2+) release and speculate about how instability in CICR may contribute to the development of arrhythmias under pathological conditions.

L-type Ca2+channel function and expression in neonatal rabbit ventricular myocytes

L-type Ca2+channel-mediated, Ca2+-induced Ca2+release (CICR) is the dominant mode of excitation-contraction (E-C) coupling in the mature mammalian myocardium but is thought to be absent in the fetal and newborn mammalian myocardium. Furthermore, the characteristics and contributors of E-C coupling at the earliest developmental stages are poorly understood. In this study, we measured [3H](+)PN200-110 dihydropyridine binding capacity, functionality and expression of the L-type Ca2+channel, and cytosolic [Ca2+] ([Ca2+]i) at various developmental stages (3, 6, 10, 20, and 56 days old) to characterize ontogenetic changes in E-C coupling. We found that 1) the whole cell L-type Ca2+channel peak current ( ICa) density increased slightly in parallel with cell growth, but the current-voltage relationship, the steady-state activation, and the maximum DHP binding and binding affinity did not exhibit significant developmental changes; 2) sarcoplasmic reticulum Ca2+dependence of inactivation rates of L-type Ca2+channel and peak of ICadensity were only observed after 10 days of age, which temporally coincides with transverse (T)-tubule formation; 3) the relationship between [Ca2+]iand voltage changed from a linear relationship at the earliest developmental stages to a “bell-shaped” relationship at the later developmental stages, presumably corresponding to a switch from reverse-mode Na/Ca exchange-dependent to ICa-dependent E-C coupling; and 4) the expression of two different splice variants of CaV1.2, IVS3A and IVS3B, switched from predominantly IVS3A at the earliest stages to IVS3B at the later developmental stages. Our data suggest that whereas the density of functional dihydropyridine receptors (DHPRs) increases only slightly during ontogeny, the enhancement of functional coupling between DHPR and ryanodine receptor is dramatic between the second and third weeks after birth. Furthermore, we found that the differential expression of splice variants during development temporally correlated with the appearance of ICa-dependent E-C coupling and T-tubule formation.

Variability in couplon size in rabbit ventricular myocytes

Variation in couplon size is thought to be essential for graded Ca(2+) transients in cardiac myocytes. We examined this variation by investigating spark appearance in rabbit ventricular myocytes at various locations and at potentials from -20 to 0 mV. At 0 mV, sparks appeared at the beginning of the voltage step with a probability of unity. On the other hand, at -20 mV, sparks appeared later during the voltage step with a lower probability. The cumulative spark probabilities at various potentials were fitted with exponential functions of both time and voltage. Spark latency became longer as spark probability decreased at more negative potentials. At -20 mV, the cumulative spark probability and the mean spark latency were not only variable among locations but also inversely related. Under the assumption that a single opening of an L-type Ca(2+) channel triggers a spark, we suggest a simple mathematical explanation for the distribution of spark appearance. The variation in spark probability and latency with location suggests that the couplon size, and hence the number of L-type Ca(2+) channels in a couplon is variable.

Na+/Ca2+ exchange activity in neonatal rabbit ventricular myocytes

Much less is known about the contributions of the Na(+)/Ca(2+) exchanger (NCX) and sarcoplasmic reticulum (SR) Ca(2+) pump to cell relaxation in neonatal compared with adult mammalian ventricular myocytes. Based on both biochemical and molecular studies, there is evidence of a much higher density of NCX at birth that subsequently decreases during the next 2 wk of development. It has been hypothesized, therefore, that NCX plays a relatively more important role for cytosolic Ca(2+) decline in neonates as well as, perhaps, a role in excitation-contraction coupling in reverse mode. We isolated neonatal ventricular myocytes from rabbits in four different age groups: 3, 6, 10, and 20 days of age. Using an amphotericin-perforated patch-clamp technique in fluo-3-loaded myocytes, we measured the caffeine-induced inward NCX current (I(NCX)) and the Ca(2+) transient. We found that the integral of I(NCX), an indicator of SR Ca(2+) content, was greatest in myocytes from younger age groups when normalized by cell surface area and that it decreased with age. The velocity of Ca(2+) extrusion by NCX (V(NCX)) was linear with [Ca(2+)] and did not indicate saturation kinetics until [Ca(2+)] reached 1-3 microM for each age group. There was a significantly greater time delay between the peaks of I(NCX) and the Ca(2+) transient in myocytes from the youngest age groups. This observation could be related to structural differences in the subsarcolemmal microdomains as a function of age.

Differential effects of phosphodiesterase-sensitive and -resistant analogs of cAMP on initiation of contraction in cardiac ventricular myocytes

Amplitudes of cardiac contractions initiated by Ca2+-induced Ca2+ release (CICR) are proportional to the magnitude of Ca2+ current (ICa-L). However, large contractions accompanied by little inward current have been reported in some but not all studies in which cells were dialyzed with different analogs of cAMP. This study compares the effects of different phosphodiesterase (PDE)-resistant and PDE-sensitive analogs of cAMP on CICR, and investigates whether cAMP sensitizes CICR so that small currents induce large contractions. Experiments were conducted in voltage-clamped guinea pig ventricular myocytes at 37 degrees C, with different analogs of cAMP added to patch pipette solutions. With PDE sensitive Tris-cAMP, contraction-voltage relations were bell-shaped and proportional to ICa-L. In contrast, dialysis with PDE-resistant dibutyryl-cAMP resulted in sigmoidal contraction-voltage relations and large responses with little inward current. Similarly, in cells loaded with fura-2, large Ca2+ transients were elicited with little inward current in cells dialyzed with PDE-resistant but not PDE-sensitive cAMP. However, large transients were observed with PDE-sensitive cAMP when PDE was inhibited with 3-isobutyl-1-methylxanthine. When the amplitude of ICa-L was varied by partial block with Cd2+, or by partial inactivation, CICR remained proportional to the amplitude of ICa-L. Thus, cAMP altered the relationship between Ca2+ transients and membrane potential but did not sensitize conventional CICR coupled to ICa-L. Our results show that effects of different analogs of cAMP on contraction depend on the PDE resistance of the analog tested. Furthermore, PDE can play a major role in modulating cardiac contraction by altering the relationship between membrane potential and Ca2+ release.

The voltage-sensitive release mechanism of excitation contraction coupling in rabbit cardiac muscle is explained by calcium-induced calcium release

The putative voltage-sensitive release mechanism (VSRM) was investigated in rabbit cardiac myocytes at 37 degrees C with high resistance microelectrodes to minimize intracellular dialysis. When the holding potential was adjusted from -40 to -60 mV, the putative VSRM was expected to operate alongside CICR. Under these conditions however, we did not observe a plateau at positive potentials of the cell shortening versus voltage relationship. The threshold for cell shortening changed by -10 mV, but this resulted from a similar change of the threshold for activation of inward current. Cell shortening under conditions where the putative VSRM was expected to operate was blocked in a dose dependent way by nifedipine and CdCl2 and blocked completely by NiCl2. "Tail contractions" persisted in the presence of nifedipine and CdCl2 but were blocked completely by NiCl2. Block of early outward current by 4-aminopyridine and 4-acetoamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid (SITS) demonstrated persisting inward current during test depolarizations despite the presence of nifedipine and CdCl2. Inward current did not persist in the presence of NiCl2. A tonic component of cell shortening that was prominent during depolarizations to positive potentials under conditions selective for the putative VSRM was sensitive to rapidly applied changes in superfusate [Na+] and to the outward Na+/Ca2+ exchange current blocking drug KB-R7943. This component of cell shortening was thought to be the result of Na+/Ca2+ exchange-mediated excitation contraction coupling. Cell shortening recorded under conditions selective for the putative VSRM was increased by the enhanced state of phosphorylation induced by isoprenaline (1 microM) and by enhancing sarcoplasmic reticulum Ca2+ content by manipulation of the conditioning steps. Under these conditions, cell shortening at positive test depolarizations was converted from tonic to phasic. We conclude that the putative VSRM is explained by CICR with the Ca2+ "trigger" supplied by unblocked L-type Ca2+ channels and Na+/Ca2+ exchange.

Regulation of junctional and non-junctional sarcoplasmic reticulum calcium release in excitation-contraction coupling in cat atrial myocytes

We have characterized the dependence on membrane potential (V(m)) and calcium current (I(Ca)) of calcium-induced calcium release (CICR) from the junctional-SR (j-SR, in the subsarcolemmal (SS) space) and non-junctional-SR (nj-SR, in the central (CT) region of the cell) of cat atrial myocytes using whole-cell voltage-clamp together with spatially resolved laser-scanning confocal microscopy. Subsarcolemmal and central [Ca(2+)](i) transient amplitudes and I(Ca) had a bell-shaped dependence on V(m), but [Ca(2+)](i) reached a maximum at more negative V(m) (-10 to 0 mV) than I(Ca) (+10 mV). Termination of I(Ca) after a brief depolarization (2.5 to 22.5 ms) immediately interrupted only the SS [Ca(2+)](i) transient, leaving the development of the CT [Ca(2+)](i) transient unaffected. Block of SR function with 20 microM ryanodine and 2 microM thapsigargin, revealed that > 90 % of the control [Ca(2+)](i) transient amplitude was attributable to active SR Ca(2+) release through ryanodine receptors (RyRs). The gain of SR Ca(2+) release was highest in the SS space at negative test potentials and was less pronounced in the CT region. Inhibition of Na(+)-Ca(2+) exchange resulted in prolonged and higher amplitude [Ca(2+)](i) transients, elevated resting [Ca(2+)](i), accelerated propagation of CICR, decreased extrusion of Ca(2+) and an increase in j-SR Ca(2+) load. Increasing the cytosolic Ca(2+) buffer capacity by internal perfusion with 1 mM EGTA limited SR Ca(2+) release to the SS region, indicating that Ca(2+) release from nj-SR is initiated by diffusion of Ca(2+) from the cell periphery and propagating CICR. Junctional-SR Ca(2+) release occurred at discrete sites whose order of activation and amplitude of release varied from beat to beat. In conclusion, during normal excitation-contraction coupling in cat atrial myocytes, only Ca(2+) release from the j-SR is directly activated by Ca(2+) entering via I(Ca). Elevation of SS [Ca(2+)](i) is required to provide the cytosolic Ca(2+) gradient needed to initiate regenerative and propagating CICR from nj-SR.

L-type Ca(2+) currents overlapping threshold Na(+) currents: could they be responsible for the "slip-mode" phenomenon in cardiac myocytes?

Phosphorylation of Na channels has been suggested to increase their Ca permeability. Termed "slip-mode conductance" (SMC), this hypothesis predicts that Ca influx via protein kinase A (PKA)-modified Na channels can induce sarcoplasmic reticulum (SR) Ca release. We tested this hypothesis by determining if SR Ca release is graded with I(Na) in the presence of activated PKA (with Isoproterenol, ISO). V(m), I(m), and [Ca](i) were measured in feline (n=26) and failing human (n=19) ventricular myocytes. Voltage steps from -70 through -40 mV were used to grade I(Na). Na channel antagonists (tetrodotoxin), L-type Ca channel (I(Ca,L)) antagonists (nifedipine, cadmium, verapamil), and agonists (Bay K 8644, FPL 64176) were used to separate SMC from I(Ca,L). In the absence of ISO, I(Na) was associated with SR Ca release in human but not feline myocytes. After ISO, graded I(Na) was associated with small amounts of SR Ca release in feline myocytes and the magnitude of release increased in human myocytes. I(Na)-related SR Ca release was insensitive to tetrodotoxin (n=10) but was blocked by nifedipine (n=10) and cadmium (n=3). SR Ca release was induced over the same voltage range in the absence of ISO with Bay K 8644 and FPL 64176 (n=9). Positive voltage steps (to 0 mV) to fully activate Na channels (SMC) in the presence of ISO and Verapamil only caused SR Ca release when block of I(Ca,L) was incomplete. We conclude that PKA-mediated increases in I(Ca,L) and SR Ca loading can reproduce many of the experimental features of SMC.

Cardiac excitation-contraction coupling: role of membrane potential in regulation of contraction

The steps that couple depolarization of the cardiac cell membrane to initiation of contraction remain controversial. Depolarization triggers a rise in intracellular free Ca(2+) which activates contractile myofilaments. Most of this Ca(2+) is released from the sarcoplasmic reticulum (SR). Two fundamentally different mechanisms have been proposed for SR Ca(2+) release: Ca(2+)-induced Ca(2+) release (CICR) and a voltage-sensitive release mechanism (VSRM). Both mechanisms operate in the same cell and may contribute to contraction. CICR couples the release of SR Ca(2+) closely to the magnitude of the L-type Ca(2+) current. In contrast, the VSRM is graded by membrane potential rather than Ca(2+) current. The electrophysiological and pharmacological characteristics of the VSRM are strikingly different from CICR. Furthermore, the VSRM is strongly modulated by phosphorylation and provides a new regulatory mechanism for cardiac contraction. The VSRM is depressed in heart failure and may play an important role in contractile dysfunction. This review explores the operation and characteristics of the VSRM and CICR and discusses the impact of the VSRM on our understanding of cardiac excitation-contraction coupling.

Regulation of a voltage-sensitive release mechanism by Ca(2+)-calmodulin-dependent kinase in cardiac myocytes

A role for Ca(2+)-calmodulin-dependent kinase (CamK) in regulation of the voltage-sensitive release mechanism (VSRM) was investigated in guinea pig ventricular myocytes. Voltage clamp was used to separate the VSRM from Ca(2+)-induced Ca(2+) release (CICR). VSRM contractions and Ca(2+) transients were absent in cells dialyzed with standard pipette solution but present when 2-5 microM calmodulin was included. Effects of calmodulin were blocked by KN-62 (CamK inhibitor), but not H-89, a protein kinase A (PKA) inhibitor. Ca(2+) current and caffeine contractures were not affected by calmodulin. Transient-voltage relations were bell-shaped without calmodulin, but they were sigmoidal and typical of the VSRM with calmodulin. Contractions with calmodulin exhibited inactivation typical of the VSRM. These contractions were inhibited by rapid application of 200 microM of tetracaine, but not 100 microM of Cd(2+), whereas CICR was inhibited by Cd(2+) but not tetracaine. In undialyzed myocytes (high-resistance microelectrodes), KN-62 or H-89 each reduced amplitudes of VSRM contractions by approximately 50%, but together they decreased VSRM contractions by 93%. Thus VSRM is facilitated by CamK or PKA, and both pathways regulate the VSRM in undialyzed cells.

The mechanisms of sarcoplasmic reticulum Ca2+ release in toad pacemaker cells

The mechanisms of sarcoplasmic reticulum (SR) Ca2+ release in pacemaker cells from the sinus venosus of the cane toad (Bufo marinus) were studied. Single, isolated cells were voltage clamped using a nystatin-perforated patch. Ionic currents and intracellular Ca2+ concentration ([Ca2+]i) were recorded simultaneously. Depolarizations of 300 ms duration from a holding potential of -55 mV produced an inward current which had a bell-shaped relationship with voltage. Inward current first appeared at about -45 mV, reached a maximum of -343 +/- 46 pA at -15 mV and reversed at +45 mV. In contrast the amplitude of the increase in [Ca2+]i caused by depolarization (Ca2+ transient) increased monotonically with the increasing depolarization. At -15 mV the amplitude of the Ca2+ transient was 243 +/- 33 nM and at +45 mV it was 411 +/- 43 nM. The inward current produced by depolarizations to -5 mV was largely eliminated by the L-type Ca2+ channel blocker nifedipine (10 microM) while 37 +/- 7 % of the Ca2+ transient persisted. A significantly larger proportion of the Ca2+ transient (56 +/- 5 %) remained at +85 mV in the presence of nifedipine. The SR Ca2+ pump inhibitor 2, 5-di(tert-butyl)-1,4-hydroquinone (10 microM), which causes depletion of the SR Ca2+, reduced the amplitude of the Ca2+ transient to 34 +/- 1 % of control, irrespective of the voltage. Brief exposure to extracellular Ca2+-free solution abolished the Ca2+ transients caused by depolarization while the caffeine-induced Ca2+ release persisted. Tetrodotoxin (1 microM) had no effect on the amplitude of the depolarization-induced Ca2+ transient, although it reduced the fast component of the inward current. In contrast, Ni2+ (5 mM) abolished the Ca2+ transients at any given voltage. Ni2+ also abolished spontaneous Ca2+ transients. In conclusion, in toad pacemaker cells Ca2+ release from SR contributes approximately 66 % of the Ca2+ involved in the Ca2+ transient and requires extracellular Ca2+ influx to trigger its release. The L-type Ca2+ channels and Na+-Ca2+ exchange are major sources of Ca2+ influx under physiological conditions.

Regulation of contraction and relaxation by membrane potential in cardiac ventricular myocytes

Control of contraction and relaxation by membrane potential was investigated in voltage-clamped guinea pig ventricular myocytes at 37 degrees C. Depolarization initiated phasic contractions, followed by sustained contractions that relaxed with repolarization. Corresponding Ca(2+) transients were observed with fura 2. Sustained responses were ryanodine sensitive and exhibited sigmoidal activation and deactivation relations, with half-maximal voltages near -46 mV, which is characteristic of the voltage-sensitive release mechanism (VSRM) for sarcoplasmic reticulum Ca(2+). Inactivation was not detected. Sustained responses were insensitive to inactivation or block of L-type Ca(2+) current (I(Ca-L)). The voltage dependence of sustained responses was not affected by changes in intracellular or extracellular Na(+) concentration. Furthermore, sustained responses were not inhibited by 2 mM Ni(2+). Thus it is improbable that I(Ca-L) or Na(+)/Ca(2+) exchange generated these sustained responses. However, rapid application of 200 microM tetracaine, which blocks the VSRM, strongly inhibited sustained contractions. Our study indicates that the VSRM includes both a phasic inactivating and a sustained noninactivating component. The sustained component contributes both to initiation and relaxation of contraction.

Voltage-dependent Ca2+ release from the SR of feline ventricular myocytes is explained by Ca2+-induced Ca2+ release

1. Direct voltage-gated (voltage-dependent Ca2+ release, VDCR) and Ca2+ influx-gated (Ca2+-induced Ca2+ release, CICR) sarcoplasmic reticulum (SR) Ca2+ release were studied in feline ventricular myocytes. The voltage-contraction relationship predicted by the VDCR hypothesis is sigmoidal with large contractions at potentials near the Ca2+ equilibrium potential (ECa). The relationship predicted by the CICR hypothesis is bell-shaped with no contraction at ECa. 2. The voltage dependence of contraction was measured in ventricular myocytes at physiological temperature (37 C), resting membrane potential and physiological [K+]. Experiments were performed with cyclic adenosine 3',5'-monophosphate (cAMP) in the pipette or in the presence of the beta-adrenergic agonist isoproterenol (isoprenaline; ISO). 3. The voltage-contraction relationship was bell-shaped in Na+-free solutions (to eliminate the Na+ current and Na+-Ca2+ exchange, NCX) but the relationship was broader than the L-type Ca2+ current (ICa,L)-voltage relationship. 4. Contractions induced with voltage steps from normal resting potentials to -40 mV are thought to represent VDCR rather than CICR. We found that cAMP and ISO shifted the voltage dependence of ICa,L activation to more negative potentials so that ICa,L was always present with steps to -40 mV. ICa,L at -40 mV inactivated when the holding potential was decreased (VŁ = -57.8 +/- 0.49 mV). 5. ISO increased inward current, SR Ca2+ load and contraction in physiological [Na+] and a broad bell-shaped voltage-contraction relationship was observed. Inhibition of reverse-mode NCX, decreasing ICa,L and decreasing SR Ca2+ loading all decreased contractions at strongly positive potentials near ECa. 6. The voltage-contraction relationship in 200 microM cadmium (Cd2+) was bell-shaped, supporting a role of ICa,L rather than VDCR. 7. All results could be accounted for by the CICR hypothesis, and many results exclude the VDCR hypothesis.

Effects of stimulation frequency on calcium transients in noninfarcted myocardium: modulation by chronic captopril treatment

BACKGROUND

Angiotensin-converting enzyme (ACE) inhibition produces beneficial effects in patients and experimental animals after myocardial infarction (MI). However, the mechanisms accounting for these effects are incompletely understood.

METHODS AND RESULTS

We recorded contractile force and intracellular calcium (Ca2+i) transients in papillary muscles from sham-operated rats (n = 8), untreated rats with heart failure after MI (MI; n = 7), and MI rats receiving captopril treatment for 5 weeks (n = 4). All studies were performed 6 weeks after MI or sham surgery. In muscles from sham-operated rats, increasing stimulation frequency from 0.33 to 3.0 Hz was associated with no change in the peak amplitude or the time to the peak of the Ca2+i transients. In contrast, in muscles from MI rats, stimulation at 3.0 Hz caused a marked increase in the amplitude of the Ca2+i transients (170% of baseline), prolongation of the time to the peak of the Ca2+i transient (54 +/- 2 to 84 +/- 8 * ms), and a prominent alternans pattern. Tissue hypoxia did not appear to be responsible for the abnormal response to rapid stimulation in the myocardium from infarcted hearts because bubbling the bath solution with 95% N2/5% CO2 resulted in no change in the amplitude of the Ca2+i transients in muscles from both groups. Muscles from captopril-treated MI rats responded like sham-operated controls, with no change in the amplitude or time course of the Ca2+i transients during rapid stimulation.

CONCLUSION

In myocardium isolated from rats with postinfarction heart failure, increasing stimulation frequency causes marked increases in peak Ca2+i , prolongation of the time course of the Ca2+i transient, and Ca2+i alternans. Despite the increased Ca2+i transients, contractility declined during rapid pacing. We hypothesize that these changes could be explained by a frequency-related decline in intracellular pH and/or a decrease in sarcolemmal Ca2+ extrusion. The frequency-dependent abnormalities of cellular Ca2+ regulation in the infarcted heart are prevented by long-term treatment with an ACE inhibitor.

Role of Na-Ca exchange in the action potential changes caused by drive in cardiac myocytes exposed to different Ca2+ loads

The role of Na-Ca exchange in the membrane potential changes caused by repetitive activity ("drive") was studied in guinea pig single ventricular myocytes exposed to different [Ca2+]o. The following results were obtained. (i) In 5.4 mM [Ca2+]o, the action potentials (APs) gradually shortened during drive, and the outward current during a train of depolarizing voltage clamp steps gradually increased. (ii) The APs shortened more and were followed by a decaying voltage tail during drive in the presence of 5 mM caffeine; the outward current became larger and there was an inward tail current on repolarization during a train of depolarizing steps. (iii) These effects outlasted drive so that immediately after a train of APs, currents were already bigger and, after a train of steps, APs were already shorter. (iv) In 0.54 mM [Ca2+]o, the above effects were much smaller. (v) In high [Ca2+]o APs were shorter and outward currents larger than in low [Ca2+]o. (vi) In 10.8 mM [Ca2+]o, both outward and inward currents during long steps were exaggerated by prior drive, even with steps (+80 and +120 mV) at which there was no apparent inward current identifiable as ICa. (vii) In 0.54 mM [Ca2+]o, the time-dependent outward current was small and prior drive slightly increased it. (viii) During long steps, caffeine markedly increased outward and inward tail currents, and these effects were greatly decreased by low [Ca2+]o. (ix) After drive in the presence of caffeine, Ni2+ decreased the outward and inward tail currents. It is concluded that in the presence of high [Ca2+]o drive activates outward and inward Na-Ca exchange currents. During drive, the outward current participates in the plateau shortening and the inward tail current in the voltage tail after the action potential.Key words: ventricular myocytes, repetitive activity, outward and inward Na-Ca exchange currents, caffeine, nickel.

Effect of the microtubule polymerizing agent taxol on contraction, Ca2+ transient and L-type Ca2+ current in rat ventricular myocytes

1. Microtubules form part of the cytoskeleton. Their role in adult ventricular myocytes is not well understood although microtubule proliferation has previously been linked with reduced contractile function. 2. We investigated the effect of the anti-tumour drug taxol, a known microtubule polymerizing agent, on Ca2+ handling in adult rat ventricular myocytes. 3. Treatment of cells with taxol caused proliferation of microtubules. 4. In taxol-treated cells there was a reduction in the amplitude of contraction, no significant effect on the amplitude of L-type Ca2+ current, but a significant reduction in the amplitude of the Ca2+ transient. 5. Caffeine was used to release Ca2+ from the sarcoplasmic reticulum (SR). There was a significant reduction in the ratio of electrically stimulated : caffeine-induced Ca2+ transients in taxol-treated cells. This observation is consistent with the hypothesis that taxol reduces fractional SR Ca2+ release. 6. We suggest that the negative inotropic effect of taxol may, at least in part, be the result of reduced release of Ca2+ from the SR. Microtubules may be important regulators of Ca2+ handling in the heart.

Role of cAMP-dependent protein kinase A in activation of a voltage-sensitive release mechanism for cardiac contraction in guinea-pig myocytes

1. Ionic currents and unloaded cell shortening were recorded from guinea-pig ventricular myocytes with single electrode voltage clamp techniques and video edge detection at 37 C. Patch pipettes (1-3 MOmega) were used to provide intracellular dialysis with pipette solutions. 2. Na+ currents were blocked with 200 microM lidocaine. Contractions initiated by the voltage-sensitive release mechanism (VSRM) and Ca2+-induced Ca2+ release (CICR) in response to L-type Ca2+ current (ICa,L) were separated with voltage clamp protocols. 3. Without 8-bromo cyclic adenosine 3',5'-monophosphate (8-Br-cAMP) in the pipette, small VSRM-induced contractions occurred transiently in only 13% of myocytes. In contrast, large ICa,L-induced contractions were demonstrable in 100% of cells. 4. Addition of 10 or 50 microM 8-Br-cAMP to the pipette increased the percentage of cells exhibiting VSRM contractions to 68 and 93%, respectively. With 50 microM 8-Br-cAMP, contractions initiated by the VSRM and ICa,L were not significantly different in amplitude. 5. 8-Br-cAMP-supported VSRM contractions had characteristics of the VSRM shown previously in undialysed myocytes. Cd2+ (100 microM) blocked ICa,L and ICa,L contractions but not VSRM contractions. 8-Br-cAMP-supported contractions exhibited steady-state inactivation with parameters characteristic of the VSRM, as well as sigmoidal contraction-voltage relations. 6. Without 8-Br-cAMP in the pipette, contraction-voltage relations determined with steps from a post-conditioning potential (Vpc) of either -40 or -65 mV were bell shaped, with a threshold near -35 mV. With 50 microM 8-Br-cAMP in the pipette, contraction-voltage relations from a Vpc of -65 mV were sigmoidal and the threshold shifted to near -55 mV. Contraction-voltage relations remained bell shaped in the presence of 8-Br-cAMP when the Vpc was -40 mV. 7. H-89, which inhibits cAMP-dependent protein kinase A (PKA), significantly reduced the amplitudes of VSRM contractions by approximately 84% with 50 microM 8-Br-cAMP in the pipette. H-89 also significantly reduced the amplitudes of peak ICa, L and ICa,L contractions, although to a lesser extent. 8. We conclude that intracellular dialysis with patch pipettes disrupts the adenylyl cyclase-PKA phosphorylation cascade, and that the VSRM requires intracellular phosphorylation to be available for activation. Intracellular dialysis with solutions that do not maintain phosphorylation levels inhibits a major mechanism in cardiac excitation- contraction coupling.

Na-Ca exchange and the trigger for sarcoplasmic reticulum Ca release: studies in adult rabbit ventricular myocytes

The importance of Na-Ca exchange as a trigger for sarcoplasmic reticulum (SR) Ca release remains controversial. Therefore, we measured whole-cell Ca currents (ICa), Na-Ca exchange currents (INaCa), cellular contractions, and intracellular Ca transients in adult rabbit cardiac myocytes. We found that changing pipette Na concentration markedly affected the relationship between cell shortening (or Ca transients) and voltage, but did not affect the Ca current-voltage relationship. We then inhibited Na-Ca exchange and varied SR content (by changing the number of conditioning pulses before each test pulse). Regardless of SR Ca content, the relationship between contraction and voltage was bell-shaped in the absence of Na-Ca exchange. Next, we rapidly and completely blocked ICa by applying nifedipine to cells. Cellular shortening was variably reduced in the presence of nifedipine. The component of shortening blocked by nifedipine had a bell-shaped relationship with voltage, whereas the "nifedipine-insensitive" component of contraction increased with voltage. With the SR disabled (ryanodine and thapsigargin pretreatment), ICa could initiate late-peaking contractions that were approximately 70% of control amplitude. In contrast, nifedipine-insensitive contractions could not be elicited in the presence of ryanodine and thapsigargin. Finally, we recorded reverse Na-Ca exchange currents that were activated by membrane depolarization. The estimated sarcolemmal Ca flux occurring by Na-Ca exchange (during voltage clamp steps to +30 mV) was approximately 10-fold less than that occurring by ICa. Therefore, Na-Ca exchange alone is unlikely to raise cytosolic Ca concentration enough to directly activate the myofilaments. We conclude that reverse Na-Ca exchange can trigger SR Ca release. Because of the sigmoidal relationship between the open probability of the SR Ca release channel and pCa, the effects of ICa and INaCa may not sum in a linear fashion. Rather, the two triggers may act synergistically in the modulation of SR release.

The control of Ca release from the cardiac sarcoplasmic reticulum: regulation versus autoregulation

This review discusses the mechanism and regulation of Ca release from the cardiac sarcoplasmic reticulum. Ca is released through the Ca release channel or ryanodine receptor (RyR) by the process of calcium-induced Ca release (CICR). The trigger for this release is the L-type Ca current with a small contribution from Ca entry on the Na-Ca exchange. Recent work has shown that CICR is controlled at the level of small, local domains consisting of one or a small number of L-type Ca channels and associated RyRs. Ca efflux from the s.r. in one such unit is seen as a 'spark' and the properties of these sparks produce controlled Ca release from the s.r. A major factor controlling the amount of Ca released from the s.r. and therefore the magnitude of the systolic Ca transient is its Ca content. The Ca content depends on both the properties of the s.r. and the cytoplasmic Ca concentration. Changes of s.r. Ca content and the Ca released affect the sarcolemmal Ca and Na-Ca exchange currents and this acts to control cell Ca loading and the s.r. Ca content. The opening probability of the RyR can be regulated by various physiological mediators as well as pharmacological compounds. However, it is shown that, due to compensatory changes of s.r. Ca, modifiers of the RyR only produce transient effects on systolic Ca. We conclude that, although the RyR can be regulated, of much greater importance to the control of Ca efflux from the s.r. are effects due to changes of s.r. Ca content.

Contribution of a voltage-sensitive calcium release mechanism to contraction in cardiac ventricular myocytes

The contribution of a voltage-sensitive release mechanism (VSRM) for sarcoplasmic reticulum (SR) Ca2+ to contraction was investigated in voltage-clamped ventricular myocytes at 37 degrees C. Na+ current was blocked with lidocaine. The VSRM exhibited steady-state inactivation (half-inactivation voltage: -47.6 mV; slope factor: 4.37 mV). When the VSRM was inactivated, contraction-voltage relationships were proportional to L-type Ca2+ current (ICa-L). When the VSRM was available, the relationship was sigmoidal, with contractions independent of voltage positive to -20 mV. VSRM and ICa-L contractions could be separated by activation-inactivation properties. VSRM contractions were extremely sensitive to ryanodine, thapsigargin, and conditioning protocols to reduce SR Ca2+ load. ICa-L contractions were less sensitive. When both VSRM and ICa-L were available, sigmoidal contraction-voltage relationships became bell-shaped with protocols to reduce SR Ca2+ load. Myocytes demonstrated restitution of contraction that was slower than restitution of ICa-L. Restitution was a property of the VSRM. Thus activation and recovery of the VSRM are important in coupling cardiac contraction to membrane potential, SR Ca2+ load, and activation interval.

Enhanced Na(+)-Ca2+ exchange in the infarcted heart. Implications for excitation-contraction coupling

Cellular Ca2+ regulation is abnormal in diseased hearts. We designed this study to assess the role of the Na(+)-Ca2+ exchanger in excitation-contraction coupling in surviving myocardium of the infarcted heart. We measured cellular contractions and whole-cell currents in single left ventricular myocytes isolated from the hearts of rabbits with healed myocardial infarction (MI). Eight weeks after MI, rabbits had left ventricular dysfunction without overt heart failure. Myocytes isolated from regions adjacent to the infarcted zone were significantly longer than cells from control hearts. At low stimulation rates (0.5 Hz), the amplitude of field-stimulated contractions was increased (11.6 +/- 0.5% versus 10.2 +/- 0.6% resting cell length), whereas the time to peak shortening and action potential duration were prolonged in the MI cells. When stimulation frequency was increased to 2.0 Hz, cellular shortening did not change or decreased in myocytes from infarcted hearts, whereas control cells had a positive shortening-interval relationship. Cells from infarcted hearts had a significantly decreased (31%) L-type Ca2+ current (ICa) density but no change in the current-voltage relationship or the kinetics of ICa inactivation. Maximal Na(+)-Ca2+ exchange current density was significantly increased (32%) in the cells from infarcted hearts. Sarcoplasmic reticulum (SR) Ca2+ content during a stable train of contractions, as estimated from caffeine-induced inward currents, was slightly increased (P = NS) in the MI myocytes. To determine whether Na(+)-Ca2+ exchange influenced SR Ca2+ content, cells were clamped at potentials between -70 and +90 mV for 400 ms. The amplitude of the contraction during a subsequent clamp step to +10 mV was then measured as an index of SR loading that occurred during the preceding clamp step. Steps to positive potentials produced greater augmentation of the subsequent contraction in MI than in control myocytes. In myocytes from the infarcted heart, increased activity of the Na(+)-Ca2+ exchanger may promote Ca2+ entry or decrease Ca2+ extrusion. This relative augmentation of inward Ca2+ flux by the exchanger may enhance SR Ca2+ loading and thus support contractility that would otherwise be impaired as a result of decreased Ca2+ current. However, Ca2+ influx by the exchanger may contribute to the prolongation of contractions in myocytes from infarcted hearts.

Low efficiency of Ca2+ entry through the Na(+)-Ca2+ exchanger as trigger for Ca2+ release from the sarcoplasmic reticulum. A comparison between L-type Ca2+ current and reverse-mode Na(+)-Ca2+ exchange

It has been proposed that Ca2+ entry through the Na(+)-Ca2+ exchanger can contribute significantly to the trigger for Ca2+ release from the sarcoplasmic reticulum (SR). We have compared the characteristics of Ca2+ release triggered by reverse-mode Na(+)-Ca2+ exchange and by L-type Ca2+ current (ICaL) during depolarizing steps in single guinea pig ventricular myocytes (whole-cell voltage clamp, fluo 3 and fura-red as [Ca2+]i indicators, 36 +/- 1 degrees C, K(+)-based pipette solution with 20 mmol/L [Na+]). Conditioning pulses to +60 mV ensured comparable Ca2+ loading of the SR. In the presence of ICaL, [Ca2+]i transients typically have an early and rapid rising phase reflecting Ca2+ release, which has a bell-shaped voltage dependence with a peak at +10 mV. With Ca2+ entry through Na(+)-Ca2+ exchange only (20 mumol/L nisoldipine), Ca2+ release flux from the SR is decreased and directly related to the amplitude of the depolarizing step. Ca2+ release is preceded by a significant delay (81 +/- 21 ms at +20 mV, 24 +/- 4 ms at +70 mV) related to Ca2+ entry through the exchanger. Triggered release interrupts Ca2+ entry, as evidenced by reversal of the exchanger current. At potentials positive to +40 mV, Ca2+ influx through Na(+)-Ca2+ exchange, calculated from the outward exchange current, reaches magnitudes comparable to ICaL, but Ca2+ release due to reverse-mode Na(+)-Ca2+ exchange still has a significant delay. We calculated trigger efficiency as the ratio between the maximal rate of Ca2+ release and the Ca2+ influx preceding this release; efficiency of reverse-mode Na(+)-Ca2+ exchange is approximately four times less than that of ICaL. With both ICaL and reverse-mode Na(+)-Ca2+ exchange present, Ca2+ release is triggered by ICaL, and a contribution of reverse-mode Na(+)-Ca2+ exchange to the trigger could not be detected at potentials below +60 mV. These characteristics of reverse-mode Na(+)-Ca2+ exchange predict that its role as a trigger for Ca2+ release during the action potential is likely to be negligible.

Dietary and physiological studies to investigate the relationship between calcium and magnesium signalling in the mammalian myocardium

This study employs both dietary and physiological studies to investigate the relationship between calcium (Ca2+) and magnesium (Mg2+) signalling in the mammalian myocardium. Rats maintained on a low Mg2+ diet (LMD; 39 mg Kg-1 Mg2+ in food) consumed less food and grew more slowly than control rats fed on a control Mg2+ diet (CMD; 500 mg Kg-1 Mg2+ in food). The Mg2+ contents of the heart and plasma were 85 +/- 3% and 34 +/- 6.5%, respectively relative to the control group. In contrast, Ca2+ contents in the heart and plasma were 177 +/- 5% and 95 +/- 3%. The levels of potassium (K+) was raised in the plasma (129 +/- 16%) and slightly decreased in the heart (88 +/- 6%) compared to CMD. Similarly, sodium (Na+) contents were slightly higher in the heart and lowered in the plasma of low Mg2+ diet rats compared to control Mg2+ diet rat. Perfusion of the isolated Langendorff's rat heart with a physiological salt solution containing low concentrations (0-0.6 mM) of extracellular magnesium [Mg2+]o resulted in a small transient increase in the amplitude of contraction compared to control [Mg2+]o (1.2 mM). In contrast, elevated [Mg2+]o (2-7.2 mM) caused a marked and progressive decrease in contractile force compared to control. In isolated ventricular myocytes the L-type Ca2+ current (ICa,L) was significantly (p < 0.001) attenuated in cells dialysed with 7.1 mM Mg2+ compared to cells dialysed with 2.9 microM Mg2+. The results indicate that hypomagnesemia is associated with decreased levels of Mg2+ and elevated levels of Ca2+ in the heart and moreover, internal Mg2+ is able to modulate the Ca2+ current through the L-type Ca2+ channel which in turn may be involved with the regulation of contractile force in the heart.

Ca2+ influx during the cardiac action potential in guinea pig ventricular myocytes

The relative contributions of L-type Ca2+ current (ICa) and Na+/Ca2+ exchange to Ca2+ influx during the cardiac action potential (AP) are unknown. In this study, we have used an AP recorded under physiological conditions as the command voltage applied to voltage-clamped ventricular myocytes. ICa (measured as nifedipine-sensitive membrane current) had a complex multiphasic time course during the AP. Peak ICa was typically 4 pA/pF, after which it rapidly declined (to about 60% of peak) during the rising phase of the cell-wide Ca2+ transient before increasing to a second, more sustained component. The initial decline in ICa was sensitive to the amount of Ca2+ released by the sarcoplasmic reticulum (SR), and conditions that reduce the amplitude of the Ca2+ transient (such as rest or brief application of caffeine) increased net Ca2+ influx via ICa. Dissection of the Na+/Ca2+ exchange current at the start of the AP suggested that Ca2+ influx via Na+/Ca2+ exchange is less than 30% of that due to ICa. From these data, we suggest that ICa is the primary source of Ca2+ that triggers SR Ca2+ release, even at the highly depolarized membrane potentials associated with the AP. However, Ca2+ influx via Na+/Ca2+ exchange is not negligible and may activate some Ca2+ release from the SR, especially when ICa is reduced. We propose that SR Ca2+ release inhibits ICa within the same beat, thereby providing a negative feedback mechanism that may serve to limit Ca2+ influx as well as to regulate the amount of Ca2+ stored within the SR.

Effect on the indo-1 transient of applying Ca2+ channel blocker for a single beat in voltage-clamped guinea-pig cardiac myocytes

1. We used rapid solution changes to investigate the mechanisms which trigger Ca2+ release from the sarcoplasmic reticulum (SR) in guinea-pig ventricular myocytes. We patch-clamped myocytes at 36 degrees C and used indo-1 to monitor intracellular Ca2+. Before each test pulse, we established a standard level of SR Ca2+ load by applying a train of conditioning pulses. 2. We switched rapidly to 32 microM nifedipine (an L-type Ca2+ current (ICa,L) blocker) 8 s before a test pulse, and just after applying nifedipine we applied a ramp depolarization to pre-block Ca2+ channels. We found that ICa,L elicited by the following test pulse was inhibited almost completely (98-99% inhibition). 3. The indo-1 transient elicited by an 800 ms depolarizing pulse showed a rapid initial rise which was inhibited by ryanodine-thapsigargin. This indicated that the rapid rise was due to Ca2+ release from the SR, and therefore provides an index of SR Ca2+ release. 4. In cells dialysed internally with 10 mM Na(+)-containing solution, nifedipine application before a +10 mV test pulse blocked 62% of the rapid initial phase of the indo-1 transient. Calibration curves of indo-1 for intracellular Ca2+ (using a KD of indo-1 for Ca2+ of either 250 or 850 nM, the reported range) indicated that between 67 and 76% of the Ca2+i transient was inhibited by nifedipine. Thus, in cells dialysed with 10 mM Na+ and depolarized to +10 mV, and in the absence of ICa,L, this suggests that another trigger mechanism for SR release is able to trigger between 33 and 24% of the Ca2+i transient. 5. For a given dialysing Na+ concentration, the fraction of indo-1 transient which was inhibited by nifedipine decreased as test potential became more positive. In cells dialysed with 10 mM Na+ and pulsed to +110 mV, 24% of the rapid phase of the indo-1 transient was inhibited by nifedipine (equivalent to between 27 and 37% of the Ca2+i transient). 6. For a given test potential, the fraction of the indo-1 transient which was inhibited by nifedipine decreased as dialysing Na+ concentration increased. In cells dialysed with Na(+)-free solution and pulsed to +10 mV, 84% of the indo-1 transient was inhibited by nifedipine (equivalent to between 88 and 91% of the Ca2+i transient). In contrast, in cells dialysed with 20 mM Na+ and pulsed to +10 mV, 41% of the indo-1 transient was inhibited by nifedipine (equivalent to between 47 and 57% of the Ca2+i transient). 7. Dialysing cells with different Na+ concentrations could lead to a different SR Ca2+ content. We therefore manipulated the conditioning train before each test pulse to change the extent of SR loading. For each dialysing Na+ concentration, we found no change in the degree to which nifedipine blocked the indo-1 transient when SR content was either increased or decreased. 8. The results support the idea that both ICa, L and a second mechanism are able to trigger SR release and the resulting Ca2+i transient. When ICa, L was blocked with nifedipine, the fraction of Ca2+i transient which remained increased with more positive test potential and higher internal Na+. This is consistent with the hypothesis that the second SR trigger mechanism is Ca2+ entry via reverse Na(+)-Ca2+ exchange, elicited by a step change in membrane potential.

The Fura-2 transient can show two types of voltage dependence at 36 degrees C in ventricular myocytes isolated from the rat heart

We used the whole-cell patch-clamp method to investigate the voltage dependence of the L-type Ca current (ICa,L) and intracellular Ca (Cai) transient in ventricular myocytes isolated from the rat heart. Intracellular Ca was monitored using Fura-2 and the experiments were carried out at 36 degrees C. We measured ICa,L by using a caesium-based internal dialysis solution to eliminate interfering K currents. The voltage dependence of peak ICa,L amplitude was bell-shaped: ICa,L was maximal at +10 mV and declined at more positive potentials. When ICa,L was integrated over the first 25 ms to estimate the magnitude of Ca entry, this had a very similar voltage dependence to peak ICa,L. In all cells, phasic Fura-2 transients were abolished by 5 microM ryanodine (a blocker of the sarcoplasmic reticulum, SR) showing that the Fura-2 transient provided an index of the magnitude of SR Ca release. For experiments measuring the Cai transient, we used a K-based internal dialysis solution to preserve normal excitation-contraction coupling. In 30-40% of cells, we found that the Fura-2 transient had a bell-shaped voltage dependence. This suggests that, in these cells, the primary trigger mechanism for Ca-induced Ca-release might have been Ca entry via ICa,L. In the remaining 60-70% of cells, the voltage dependence of the Fura-2 transient was not bell-shaped. The Fura-2 transient reached a maximum with a pulse to +10 mV, and the amplitude of the transient did not decline significantly at more positive potentials to this. In cells with a non-bell-shaped voltage dependence of the Fura-2 transient, pulses to potentials as far positive as +140 mV elicited phasic Fura-2 transients. Since this potential exceeded the Nernst potential for Ca, it was unlikely there was any trigger Ca entry via ICa,L at this potential. This would suggest that, in these cells, another trigger for SR Ca release (in addition to ICa,L) might be present. We conclude that rat ventricular myocytes, produced using a standard isolation technique and under standard recording conditions, can show either a bell-shaped or a sigmoidal voltage dependence of the Fura-2 transient.

Effect on the fura-2 transient of rapidly blocking the Ca2+ channel in electrically stimulated rabbit heart cells

1. We used a rapid solution switcher technique to investigate mechanisms that might trigger intracellular Ca2+ release in rabbit ventricular myocytes. The study was carried out at 36 degrees C, intracellular Ca2+ (Ca2+i) was monitored with fura-2, and myocytes were electrically stimulated. 2. In patch-clamped cells, using the switcher to apply 20 microM nifedipine (an L-type Ca2+ current (ICa,L) blocker) 4 s before a depolarization to +10 mV reduced the amplitude of ICa,L to 10.25 +/- 2.25% of control (mean +/- S.E.M., n = 7 cells). 3. In externally stimulated cells, a rapid switch to 20 microM nifedipine 4 s before a stimulus reduced the amplitude of the fura-2 transient to 64.01 +/- 2.09% of control (mean +/- S.E.M., n = 19 cells). Using an in vivo calibration curve for fura-2, this was equivalent to a reduction in the Ca2+ transient to 50% during nifedipine application. Since an identical nifedipine switch reduced ICa,L to 10.25%, it would seem that blocking a large fraction of ICa,L inhibited only half the Ca2+ transient. 4. The Na(+)-Ca2+ exchanger is inhibited by 5 mM nickel. Switching to 20 microM nifedipine +5 mM nickel 4 s before a stimulus abolished the fura-2 transient completely, consistent with the hypothesis that Ca2+ entry via reverse Na(+)-Ca2+ exchange might trigger a fraction of the fura-2 transient that remained during nifedipine. 5. After the Na(+)-K+ pump was inhibited by strophanthidin to increase intracellular Na+ (Na+i), a switch to 20 microM nifedipine became progressively less effective in reducing the fura-2 transient. This suggests that as Na+i rose, other mechanisms (perhaps reverse Na(+)-Ca2+ exchange) appeared able to substitute for ICa,L in triggering the Ca2+ transient. 6. In cells depleted of Nai+ to inhibit the triggering of sarcoplasmic reticulum (SR) Ca2+ release by reverse Na(+)-Ca2+ exchange, a nifedipine switch reduced the fura-2 transient to 10.9 +/- 4.19% (mean +/- S.E.M., n = 7; equivalent to 6.5% of the Ca2+ transient). 7. A switch to Na(+)-free (Li+) solution 100 ms before an electrical stimulus caused an increase in the fura-2 transient of 12.2 +/- 1.5% (mean +/- S.E.M., n = 7; equivalent to a 22% increase in the Ca2+ transient). 8. The results confirm that ICa,L is an important trigger for SR Ca2+ release and the resulting Ca2+ transient. However, since 50% of the Ca2+ transient remained when ICa,L was largely inhibited, it would seem likely that other SR trigger mechanisms might exist in addition. These data are consistent with the idea that Ca2+ entry via reverse Na(+)-Ca2+ exchange during the upstroke of the normal cardiac action potential might trigger a fraction of SR Ca2+ release and the resulting Ca2+ transient.

Evidence that reverse Na-Ca exchange can trigger SR calcium release

Several results suggest that the Na-Ca exchange can function as a trigger promoting SR Ca release and ensuing contractions. First, if the Ca current was the sole trigger for contraction we would expect the relationship between triggered contractions and voltage to be similar to the relationship between Ca current and contraction. When Na is present in the pipette this is not observed. Between -40 and +10 mV the relationships between contractions and voltage and current and voltage are similar. At potentials positive to 10 mV the Ca current declines as expected but contractions either decline much more slowly or continue to increase depending upon the concentration of intracellular Na. In addition, we have observed that contractions can be activated when Ca current is largely or completely blocked. Since these contractions are sensitive to the presence of ryanodine and thapsigargin they appear to be triggered by Na-Ca exchange. Also, contractions that are activated in the presence of nifedipine are sensitive to the Na-Ca exchange inhibitor XIP. Finally, rapid removal of extracellular Na apparently stimulates enough reverse exchange triggering of SR Ca release without affecting the SR content. It is clear that the shape of the shortening voltage relationship depends upon the concentration of dialyzing Na. This is likely to occur for two reasons. Either the shape of the shortening voltage relationship depends upon the extent to which Na-Ca exchange contributes a trigger for SR Ca release or alternatively the shape of the shortening voltage relationship depends upon SR Ca content. The latter is known to depend upon the Na concentration. In addition it is now established that the gain of SR Ca release is influenced by SR content. However, we studied triggered contractions in the absence of a Na gradient when the only available trigger is the Ca current. We measured triggered contractions over a range of voltages between -30 and +60 mV. Between each measurement we reestablished the Na gradient and activated a series of conditioning pulses to standardize the SR Ca content. Just before a test pulse we removed extracellular Na and activated either 3 or 6 pulses to produce two different SR Ca loads (in the absence of a Na gradient entering Ca cannot be extruded and therefore changes the SR Ca content). Regardless of the number of prepulses in the absence of a Na gradient the shortening voltage relationship was similar and bell shaped. From this we conclude that the shape of the relationship between shortening and voltage does not depend upon SR Ca content. Therefore, we conclude that the asymmetry in the shortening voltage relationship that depends upon intracellular Na is due to a contribution of reverse Na-Ca exchange. It is too early to say what the physiological significance (if any) of triggering by reverse exchange actually is. However, it does seem likely that it might provide a powerful inotropic mechanism. For example intracellular Na might be expected to change with heart rate and to be elevated at higher heart rates. Presumably this increased intracellular Na would tend to favor triggering by reverse exchange and would therefore enhance contractility at a time when it would be most required.

Action potential duration modulates calcium influx, Na(+)-Ca2+ exchange, and intracellular calcium release in rat ventricular myocytes

The experimental work summarized in this paper and described in more detail in our previous publication demonstrates a very important functional role for Na(+)-Ca2+ exchange in intracellular Ca2+ homeostasis in ventricular myocytes from rat hearts. Ca2+ homeostasis in mammalian cardiac myocytes can be considered to be the result of four interactive processes: (i) Ca2+ influx through L-type Ca2+ channels, (ii) Ca2+ release from the SR and its subsequent re-uptake, (iii) intracellular Ca/+ buffering, and (iv) Ca2+ extrusion across the sarcolemma. Our results demonstrate a number of interesting features of these processes. (1) When the action potential voltage-clamp technique is used to identify the size and time-course of Ca2+ fluxes during the action potential, both the peak current and the associated influx of Ca2+ are relatively large as was previously demonstrated by Isenberg and his colleagues. (2) Nevertheless, this source of Ca2+ is unable, by itself, to produce a significant twitch, which is consistent with previous data from rat ventricle. (3) This Ca2+ influx, however, does represent the trigger for SR Ca2+ release. (4) The Na(+)-Ca2+ exchanger on the SR is able, on average, to extrude all the Ca2+ which enters through L-type Ca2+ channels, although it provides relatively little Ca2+, i.e., during the course of the normal action potential there is no significant reverse Na(+)-Ca2+ exchange activity, at least under our experimental conditions. Our results also suggest that although the L-type Ca2+ current cannot by itself trigger and control contraction its amplitude, frequency, and time-course can alter the rate and the extent of Ca2+ release from the SR. Recently, detailed mathematical formulations and a direct demonstration of some of these phenomena have been published. Stern and Stern and Lakatta predicted more than three years ago that the concentration and the time-course of change in concentration of Ca2+ very near the release sites of the SR may be critical determinants of the overall release process. Within the past year Wier and his colleagues and also Lederer et al. have combined electrophysiological measurements with recordings of localized intracellular Ca2+ (made using a confocal microscope) and have shown that rapid, and relatively large, but very localized changes in intracellular Ca2+ due to Ca2+ influx through L-type Ca2+ channels are responsible for triggering, and to some extent, controlling the release of Ca2+ from the SR. However, it has also been shown that this release depends importantly on the loading or priming state of the SR. Perhaps not surprisingly, the massive release of Ca2+ from the SR can, itself, alter the pattern of subsequent SR release events (cf. Ref. 46) and the time-course of Ca2+ influx through the L-type Ca2+ channels. Thus, although our relatively crude measurements have clearly demonstrated the relationship between L-type Ca2+ channel activity and Na+-Ca2+ exchanger function during a normal cardiac action potential in rat ventricle, they fall far short of any delineation of the functional roles of either of these processes in overall Ca2+ homeostasis. This additional information can, in principle, be obtained from studies in which cellular microanatomy can be visualized dynamically in conjunction with localized changes in intracellular Ca2+ as well as Ca2+ of L-type Ca2+ channels, SR release, and cell shortening.

Calcium transients which accompany the activation of sodium current in rat ventricular myocytes at 37 degrees C: a trigger role for reverse Na-Ca exchange activated by membrane potential?

We investigated the role of the fast sodium current (INa) in triggering Ca release from the sarcoplasmic reticulum (SR), using adult rat left ventricular myocytes, loaded with Fura-2 to measure intracellular Ca (Cai), which were whole-cell patch-clamped at 35-37 degrees C. Before each test pulse, a series of 400-ms conditioning pulses to +10 mV were applied to establish a constant level of SR Ca load. Pulses were applied every 15 s. A test pulse from -80 mV to -50 mV elicited a rapid INa and a phasic Cai transient. When the solution perfusing a myocyte was rapidly switched for 15 s before a test pulse to one containing the L-type Ca channel blocker nifedipine (20 microM), the test pulse still activated INa and a phasic Cai transient, the amplitude of which was not significantly different from control (P > 0.05; t-test). When a rapid switch to 20 microM nifedipine plus 30 microM tetrodotoxin (TTX) was made 15 s before a test pulse, both INa and the Cai transient were completely abolished (n = 6). When a switch was made to Na-free (Li) solution, which contained 20 microM nifedipine to block L-type Ca current, ICa,L, there was no significant difference in the Cai transient amplitude from that of control (P > 0.05; n = 6). Brief depolarising test pulses (-80 mV to +20 mV, 10 ms duration) to simulate membrane potential escape also elicited a Cai transient which attained 90.0% (+/-2.8%; n = 7) of the Cai transient activated by a conditioning pulse to +10 mV. The Cai transient with a brief pulse was not significantly affected by application of 20 microM nifedipine (P > 0.05), but adding TTX with nifedipine reduced the Cai transient amplitude to 76.9% (+/-6.8%; P < 0.02; n = 8). In four cells, the Cai transient remaining in the presence of nifedipine plus TTX was abolished by adding 5 mM Ni. These data are consistent with voltage escape during activation of INa leading to a trigger Ca entry via a mechanism other than L-type Ca channels or subsarcolemmal Na accumulation with reverse Na-Ca exchange. The block by Ni of the Cai transient suggests that a brief membrane potential escape might directly activate reverse mode Na-Ca exchange to trigger SR release, and this mechanism would seem to account largely for the Cai transient which accompanies INa in rat myocytes, under these experimental recording conditions.

Effects of action potential duration on excitation-contraction coupling in rat ventricular myocytes. Action potential voltage-clamp measurements

Although each of the fundamental processes involved in excitation-contraction coupling in mammalian heart has been identified, many quantitative details remain unclear. The initial goal of our experiments was to measure both the transmembrane Ca2+ current, which triggers contraction, and the Ca2+ extrusion due to Na(+)-Ca2+ exchange in a single ventricular myocyte. An action potential waveform was used as the command for the voltage-clamp circuit, and the membrane potential, membrane current, [Ca2+]i, and contraction (unloaded cell shortening) were monitored simultaneously. Ca(2+)-dependent membrane current during an action potential consists of two components: (1) Ca2+ influx through L-type Ca2+ channels (ICa-L) during the plateau of the action potential and (2) a slow inward tail current that develops during repolarization negative to approximately -25 mV and continues during diastole. This slow inward tail current can be abolished completely by replacement of extracellular Na+ with Li+, suggesting that it is due to electrogenic Na(+)-Ca2+ exchange. In agreement with this, the net charge movement corresponding to the inward component of the Ca(2+)-dependent current (ICa-L) was approximately twice that during the slow inward tail current, a finding that is predicted by a scheme in which the Ca2+ that enters during ICa is extruded during diastole by a 3 Na(+)-1 Ca2+ electrogenic exchanger. Action potential duration is known to be a significant inotropic variable, but the quantitative relation between changes in Ca2+ current, action potential duration, and developed tension has not been described in a single myocyte. We used the action potential voltage-clamp technique on ventricular myocytes loaded with indo 1 or rhod 2, both Ca2+ indicators, to study the relation between action potential duration, ICa-L, and cell shortening (inotropic effect). A rapid change from a "short" to a "long" action potential command waveform resulted in an immediate decrease in peak ICa-L and a marked slowing of its decline (inactivation). Prolongation of the action potential also resulted in slowly developing increases in the magnitude of Ca2+ transients (145 +/- 2%) and unloaded cell shortening (4.0 +/- 0.4 to 7.6 +/- 0.4 microns). The time-dependent nature of these effects suggests that a change in Ca2+ content (loading) of the sarcoplasmic reticulum is responsible. Measurement of [Ca2+]i by use of rhod 2 showed that changes in the rate of rise of the [Ca2+]i transient (which in rat ventricle is due to the rate of Ca2+ release from the sarcoplasmic reticulum) were closely correlated with changes in the magnitude and the time course of ICa-L.(ABSTRACT TRUNCATED AT 400 WORDS)

Fractional SR Ca release is regulated by trigger Ca and SR Ca content in cardiac myocytes

The release of sarcoplasmic reticulum (SR) Ca in cardiac muscle during excitation-contraction coupling is known to be graded by the amount of activating Ca outside the SR (i.e., Ca-induced Ca release). However, little is known about how intra-SR Ca affects the release process. In this study we assessed how the fractional SR Ca release as described by Bassani et al. [Am. J. Physiol. 265 (Cell Physiol. 34): C533-C540, 1993] is affected by alteration of trigger Ca and of SR Ca content. Experiments were done with isolated ferret ventricular myocytes using indo 1 to measure Ca concentration, perforated patch to measure Ca current (ICa), caffeine application to release SR Ca, and thapsigargin to completely block SR Ca uptake. For what we consider a Normal SR Ca load and trigger Ca [action potential at 0.5 Hz with 2 mM extracellular Ca concentration ([Ca]o)], 35 +/- 3% of the SR Ca content was released at a twitch. Changing trigger Ca by altering [Ca]o (to 0.5 and 8 mM) at a test twitch changed this fractional SR Ca release to 10 +/- 2 and 59 +/- 6%, with the same SR Ca load (and peak ICa changed in a parallel manner in separate voltage-clamp experiments). Three different levels of SR Ca load were studied (Low, Normal, and High; by action potential stimulation at different frequencies from 0.05 to 0.8 Hz) using the same standard test trigger Ca (2 mM). Surprisingly, the High-load condition only increased SR Ca content by approximately 4% but appeared to be very close to the limiting SR Ca capacity.(ABSTRACT TRUNCATED AT 250 WORDS)

Contractions in guinea-pig ventricular myocytes triggered by a calcium-release mechanism separate from Na+ and L-currents

1. Unloaded cell shortening and membrane currents were examined in isolated guinea-pig ventricular myocytes at 37 degrees C using video edge detection and single-electrode voltage clamp. 2. Inward Na+ currents were eliminated by lidocaine, tetrodotoxin, replacement of extracellular Na+ with choline chloride or sucrose, or by voltage inactivation of Na+ channels. In the absence of Na+ current, the threshold for contraction was approximately -50 or -55 mV. 3. Verapamil (5 microM) and nifedipine (2 microM) failed to inhibit contractions at negative membrane potentials when positive conditioning pulses were used to maintain intracellular Ca2+ stores via Na(+)-Ca2+ exchange. In contrast, 200 microM Ni2+ inhibited these contractions. 4. Contractions were abolished when the extracellular solution was nominally Ca2+ free. However, contractions were restored by as little as 50 microM extracellular Ca2+. 5. Ryanodine (30 nM) completely abolished contractions initiated by depolarizing steps from -65 to -40 mV, but had minimal effects on contractions initiated by depolarizing steps from -40 to +5 mV. Subtraction of contraction-voltage relations determined in the presence of ryanodine from control relations revealed a ryanodine-sensitive component of contraction. This component activated at -55 mV and reached a plateau near -25 mV. 6. The amplitudes of contractions initiated by depolarizing steps from -40 mV were directly proportional to the magnitude of Ca2+ current (ICa). In contrast, contractions initiated by steps from either -55 or -65 mV were not proportional to ICa. These contractions appeared at potentials negative to the threshold for L-type Ca2+ current, increased to a plateau at more positive potentials and did not decrease at potentials at which ICa decreased. 7. Subtraction of the contraction-voltage relationship determined from a membrane potential of -40 mV from that at -55 mV revealed a component of contraction with a negative activation threshold whose amplitude was not proportional to inward current. The shape of this relationship was virtually identical to that of the ryanodine-sensitive component of contraction. 8. This study identifies a component of contraction associated with Ca2+ release from sarcoplasmic reticulum (SR) which can be separated from other mechanisms of contraction on the basis of membrane potential. Our observations suggest that this voltage-dependent release mechanism is a true trigger mechanism which activates a portion of cardiac contraction which is attributable to SR Ca2+ release.

The effects of mechanical loading and changes of length on single guinea-pig ventricular myocytes

1. The effects of mechanical loading and changes of length on the contraction of single guinea-pig ventricular myocytes has been investigated. 2. Cell shortening was monitored during isotonic contractions (in which the cell shortened freely) and after attaching carbon fibres of known compliance to the ends of the cell, so that the cell contracted auxotonically (the cell both shortened and developed force). 3. Mechanically loading the cells decreased the amount of shortening during a contraction and abbreviated the contraction. There were, however, no consistent changes in the action potential or the [Ca2+]i transient (measured with the fluorescent dye fura-2). 4. Increasing stimulation rate increased the size of the contraction and the [Ca2+]i transient in both isotonic and auxotonic conditions. The increase in the size of the contraction induced by an increase in stimulation rate was greater in auxotonic conditions but the increase in the size of the [Ca2+]i transient was not. 5. When cells were stretched, there was a step increase in the size of the contraction and a prolongation of its time course. However, neither the size nor the time course of the accompanying [Ca2+]i transient was significantly altered by this intervention. 6. When a stretch was maintained, a further, slow increase in the size of the contraction occurred during the following 3-11 min, in about half the cells studied. The probability of this slow response occurring was increased if the initial degree of activation of the cell was decreased. 7. These data suggest that the mechanisms underlying the responses to mechanical loading and changes of length are the same in both multicellular and single cell preparations of cardiac muscle.