Excitation-contraction coupling in heart muscle: Membrane control of development of tension

Cardiac Alternans: From Bedside to Bench and Back

Cardiac alternans arises from dynamical instabilities in the electrical and calcium cycling systems of the heart, and often precedes ventricular arrhythmias and sudden cardiac death. In this review, we integrate clinical observations with theory and experiment to paint a holistic portrait of cardiac alternans: the underlying mechanisms, arrhythmic manifestations and electrocardiographic signatures. We first summarize the cellular and tissue mechanisms of alternans that have been demonstrated both theoretically and experimentally, including 3 voltage-driven and 2 calcium-driven alternans mechanisms. Based on experimental and simulation results, we describe their relevance to mechanisms of arrhythmogenesis under different disease conditions, and their link to electrocardiographic characteristics of alternans observed in patients. Our major conclusion is that alternans is not only a predictor, but also a causal mechanism of potentially lethal ventricular and atrial arrhythmias across the full spectrum of arrhythmia mechanisms that culminate in functional reentry, although less important for anatomic reentry and focal arrhythmias.

Ca2+ signaling of human pluripotent stem cells-derived cardiomyocytes as compared to adult mammalian cardiomyocytes

Human induced pluripotent stem cells derived cardiomyocytes (hiPSC-CMs) have been extensively used for in vitro modeling of human cardiovascular disease, drug screening and pharmacotherapy, but little rigorous studies have been reported on their biophysical or Ca2+ signaling properties. There is also considerable concern as to the level of their maturity and whether they can serve as reliable models for adult human cardiac myocytes. Ultrastructural difference such as lack of t-tubular network, their polygonal shapes, disorganized sarcomeric myofilament, and their rhythmic automaticity, among others, have been cited as evidence for immaturity of hiPSC-CMs. In this review, we will deal with Ca2+ signaling, its regulation, and its stage of maturity as compared to the mammalian adult cardiomyocytes. We shall summarize the data on functional aspects of Ca2+signaling and its parameters that include: L-type calcium channel (Cav1.2), ICa-induced Ca2+release, CICR, and its parameters, cardiac Na/Ca exchanger (NCX1), the ryanodine receptors (RyR2), sarco-reticular Ca2+pump, SERCA2a/PLB, and the contribution of mitochondrial Ca2+ to hiPSC-CMs excitation-contraction (EC)-coupling as compared with adult mammalian cardiomyocytes. The comparative studies suggest that qualitatively hiPSC-CMs have similar Ca2+signaling properties as those of adult cardiomyocytes, but quantitative differences do exist. This review, we hope, will allow the readers to judge for themselves to what extent Ca2+signaling of hiPSC-CMs represents the adult form of this signaling pathway, and whether these cells can be used as good models of human cardiomyocytes.

CRISPR/Cas9 Gene editing of RyR2 in human stem cell-derived cardiomyocytes provides a novel approach in investigating dysfunctional Ca2+ signaling

Type-2 ryanodine receptors (RyR2s) play a pivotal role in cardiac excitation-contraction coupling by releasing Ca²⁺ from sarcoplasmic reticulum (SR) via a Ca²⁺ -induced Ca²⁺ release (CICR) mechanism. Two strategies have been used to study the structure-function characteristics of RyR2 and its disease associated mutations: (1) heterologous cell expression of the recombinant mutant RyR2s, and (2) knock-in mouse models harboring RyR2 point mutations. Here, we establish an alternative approach where Ca²⁺ signaling aberrancy caused by the RyR2 mutation is studied in human cardiomyocytes with robust CICR mechanism. Specifically, we introduce point mutations in wild-type RYR2 of human induced pluripotent stem cells (hiPSCs) by CRISPR/Cas9 gene editing, and then differentiate them into cardiomyocytes. To verify the reliability of this approach, we introduced the same disease-associated RyR2 mutation, F2483I, which was studied by us in hiPSC-derived cardiomyocytes (hiPSC-CMs) from a patient biopsy. The gene-edited F2483I hiPSC-CMs exhibited longer and wandering Ca²⁺ sparks, elevated diastolic Ca²⁺ leaks, and smaller SR Ca²⁺ stores, like those of patient-derived cells. Our CRISPR/Cas9 gene editing approach validated the feasibility of creating myocytes expressing the various RyR2 mutants, making comparative mechanistic analysis and pharmacotherapeutic approaches for RyR2 pathologies possible.

Modeling Electrophysiological Coupling and Fusion between Human Mesenchymal Stem Cells and Cardiomyocytes

Human mesenchymal stem cell (hMSC) delivery has demonstrated promise in preclinical and clinical trials for myocardial infarction therapy; however, broad acceptance is hindered by limited understanding of hMSC-human cardiomyocyte (hCM) interactions. To better understand the electrophysiological consequences of direct heterocellular connections between hMSCs and hCMs, three original mathematical models were developed, representing an experimentally verified triad of hMSC families with distinct functional ion channel currents. The arrhythmogenic risk of such direct electrical interactions in the setting of healthy adult myocardium was predicted by coupling and fusing these hMSC models to the published ten Tusscher midcardial hCM model. Substantial variations in action potential waveform—such as decreased action potential duration (APD) and plateau height—were found when hCMs were coupled to the two hMSC models expressing functional delayed rectifier-like human ether à-go-go K⁺ channel 1 (hEAG1); the effects were exacerbated for fused hMSC-hCM hybrid cells. The third family of hMSCs (Type C), absent of hEAG1 activity, led to smaller single-cell action potential alterations during coupling and fusion, translating to longer tissue-level mean action potential wavelength. In a simulated 2-D monolayer of cardiac tissue, re-entry vulnerability with low (5%) hMSC insertion was approximately eight-fold lower with Type C hMSCs compared to hEAG1-functional hMSCs. A 20% decrease in APD dispersion by Type C hMSCs compared to hEAG1-active hMSCs supports the claim of reduced arrhythmogenic potential of this cell type with low hMSC insertion. However, at moderate (15%) and high (25%) hMSC insertion, the vulnerable window increased independent of hMSC type. In summary, this study provides novel electrophysiological models of hMSCs, predicts possible arrhythmogenic effects of hMSCs when directly coupled to healthy hCMs, and proposes that isolating a subset of hMSCs absent of hEAG1 activity may offer increased safety as a cell delivery cardiotherapy at low levels of hMSC-hCM coupling.

Myocardial infarction—better known as a heart attack—strikes on average every 43 seconds in America. An emerging approach to treat myocardial infarction patients involves the delivery of human mesenchymal stem cells (hMSCs) to the damaged heart. While clinical trials of this therapeutic approach have yet to report adverse effects on heart electrical rhythm, such consequences have been implicated in simpler experimental systems and thus remain a concern. In this study, we utilized mathematical modeling to simulate electrical interactions arising from direct coupling between hMSCs and human heart cells to develop insight into the possible adverse effects of this therapeutic approach on human heart electrical activity, and to assess a novel strategy for reducing some potential risks of this therapy. We developed the first mathematical models of electrical activity of three families of hMSCs based on published experimental data, and integrated these with previously established mathematical models of human heart cell electrical activity. Our computer simulations demonstrated that one particular family of hMSCs minimized the disturbances in cardiac electrical activity both at the single-cell and tissue levels, suggesting that isolating this specific sub-population of hMSCs for myocardial delivery could potentially increase the safety of future hMSC-based heart therapies.

Calcium signaling in human stem cell-derived cardiomyocytes: Evidence from normal subjects and CPVT afflicted patients

Derivation of cardiomyocyte cell lines from human fibroblasts (induced pluripotent stem cells, iPSCs) has made it possible not only to investigate the electrophysiological and Ca(2+) signaling properties of these cells, but also to determine the altered electrophysiological and Ca(2+)-signaling profiles of such cells lines derived from patients expressing mutation-inducing pathologies. This approach has the potential of generating in vitro human models of cardiovascular diseases where cellular pathology can be investigated in detail and possibly specific pharmacotherapy developed. Although this approach has been applied to a number of mutations in channel proteins that cause arrhythmias, there are only few detailed reports addressing Ca(2+) signaling pathologies beyond measurements of Ca(2+) transients in intact non-voltage clamped cells. Unfortunately, full understanding of Ca(2+) signaling pathologies remains elusive, not only because of the plethora of Ca(2+) signaling proteins defects that cause arrhythmias and cardiomyopathies, but also because detailed functional properties of Ca(2+) signaling proteins are difficult to obtain. Catecholaminergic polymorphic ventricular tachycardia (CPVT1) is a malignant inherited arrhythmogenic disorder predominantly caused by mutations in the cardiac ryanodine receptor (RyR2). Thus far over 150 mutations in RyR2 have been identified that appear to cause this arrhythmia, a number of which have been expressed and studied in transgenic mice or cell-line models. The development of human iPSC-technology makes it possible to create human heart cell-lines carrying these mutations, making detailed identification of Ca(2+) signaling defects and its specific pharmacotherapy possible. In this review we shall first briefly summarize the essential characteristics of the mammalian cardiac Ca(2+) signaling, then compare them to Ca(2+) signaling phenotypes of human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CM) and to those of rat neonatal cardiomyocytes, and categorize the possible variance in Ca(2+) signaling defects caused by different CPVT-inducing mutations as expressed in hiPSC-CMs.

Ca2+ signaling in human induced pluripotent stem cell-derived cardiomyocytes (iPS-CM) from normal and catecholaminergic polymorphic ventricular tachycardia (CPVT)-afflicted subjects

Derivation of cardiomyocytes from induced pluripotent stem cells (iPS-CMs) allowed us to probe the Ca(2+)-signaling parameters of human iPS-CMs from healthy- and catecholaminergic polymorphic ventricular tachycardia (CPVT1)-afflicted individuals carrying a novel point mutation p.F2483I in ryanodine receptors (RyR2). iPS-CMs were dissociated on day 30-40 of differentiation and patch-clamped within 3-6 days. Calcium currents (ICa) averaged ∼8pA/pF in control and mutant iPS-CMs. ICa-induced Ca(2+)-transients in control and mutant cells had bell-shaped voltage-dependence similar to that of ICa, consistent with Ca(2+)-induced Ca(2+)-release (CICR) mechanism. The ratio of ICa-activated to caffeine-triggered Ca(2+)-transients was ∼0.3 in both cell types. Caffeine-induced Ca(2+)-transients generated significantly smaller Na(+)-Ca(2+) exchanger current (INCX) in mutant cells, reflecting their smaller Ca(2+)-stores. The gain of CICR was voltage-dependent as in adult cardiomyocytes. Adrenergic agonists enhanced ICa, but differentially altered the CICR gain, diastolic Ca(2+), and Ca(2+)-sparks in mutant cells. The mutant cells, when Ca(2+)-overloaded, showed longer and wandering Ca(2+)-sparks that activated adjoining release sites, had larger CICR gain at -30mV yet smaller Ca(2+)-stores. We conclude that control and mutant iPS-CMs express the adult cardiomyocyte Ca(2+)-signaling phenotype. RyR2 F2483I mutant myocytes have aberrant unitary Ca(2+)-signaling, smaller Ca(2+)-stores, higher CICR gains, and sensitized adrenergic regulation, consistent with functionally altered Ca(2+)-release profile of CPVT syndrome.

ASSESMENT OF TOTAL CA2+ HANDLING FOR EXCITATION-CONTRACTION COUPLING IN BEATING LEFT VENTRICLE

We have aimed to assess total Ca 2+ handling in excitation-contraction coupling in a beating left ventricle (LV). Our newly developed integrative analysis method utilizes the internal Ca 2+ recirculation fraction (RF), O2 consumption ( Vo 2) for Ca2+ handling, and O 2 cost of Emax (contractility index) of the LV. We have obtained the O 2 cost of Emax from Vo 2 measured at different contractility levels, and have combined the cost with RF calculated from the beat-constant of the exponential decay component of the postextrasystolic potentiation. Our method calculates the unknown total Ca2+ handling from the RF and the " Ca 2+ handling Vo 2". The calculated total Ca 2+ handling fell between 30 and 110 μmol/kg, depending on contractility and pathological conditions. The present method also enable's reasonable assessment of futile Ca 2+ cycling and of the Ca 2+ reactivity of Emax. Our method seems useful to better understanding of the pathophysiology of total Ca 2+ handling in a beating heart.

Effects of caffeine, verapamil, lithium, and KB-R7943 on mechanics and energetics of rat myocardial bigeminies

We examined the effects of pharmacological alteration of Ca2+sources on mechanical and energetic properties of paired-pulse (“bigeminic”) contractions. The fraction of heat release that is related to pressure development and pressure-independent heat release were measured during isovolumic contractions in arterially perfused rat ventricles. The heat released by regular and bigeminic contractions showed two brief pressure-independent components (H1 and H2) and a pressure-dependent component (H3). We used the ratio of active heat (H′a) to pressure-time integral (PtI) and the ratio of H3 to PtI to estimate the energetic cost of muscle contraction (overall economy) and pressure maintenance (contractile economy), respectively. Neither of these ratios was affected by stimulation pattern. Caffeine (an inhibitor of sarcoplasmic reticulum function) significantly decreased mechanical responses and increased the energetic cost of contraction (Δ = 101 ± 12.6%). Verapamil (an L-type Ca2+channel blocker) decreased pressure maintenance of extrasystolic (Δ = 43.4 ± 3.7%) and postextrasystolic (Δ = 37.5 ± 3.5%) contractions without affecting postextrasystolic potentiation, suggesting that a verapamil-insensitive fraction is responsible for potentiation. The verapamil-insensitive fraction was further studied in the presence of lithium (45 mM) and KB-R7943 (5 μM), inhibitors of the Na+/Ca2+exchanger. Both agents decreased all mechanical responses, including postextrasystolic potentiation (Δ = 67.3 ± 3.3%), without altering overall or contractile economies, suggesting an association of the verapamil-insensitive Ca2+fraction to the sarcolemmal Na+/Ca2+exchanger. The effect of the inhibitors of the Na+/Ca2+exchanger on potentiation suggests an increased participation of extracellular Ca2+(and, thus, a redistribution of the relative participation of the Ca2+pools) during bigeminic contractions in rat myocardium.

Assessment of systolic and diastolic ventricular properties via pressure-volume analysis: a guide for clinical, translational, and basic researchers

Assessment of left ventricular systolic and diastolic pump properties is fundamental to advancing the understanding of cardiovascular pathophysiology and therapeutics, especially for heart failure. The use of end-systolic and end-diastolic pressure-volume relationships derived from measurements of instantaneous left ventricular pressure-volume loops emerged in the 1970s as a comprehensive approach for this purpose. As invasive and noninvasive techniques for measuring ventricular volume improved over the past decades, these relations have become commonly used by basic, translational, and clinical researchers. This review summarizes 1) the basic concepts underlying pressure-volume analysis of ventricular and myocardial systolic and diastolic properties, 2) deviations from ideal conditions typically encountered in real-life applications, 3) how these relationships are appropriately analyzed, including statistical analyses, and 4) the most common problems encountered by investigators and the appropriate remedies. The goal is to provide practical information and simple guidelines for accurate application and interpretation of pressure-volume data as they pertain to characterization of ventricular and myocardial properties in health and disease.

Load independence of temperature-dependent Ca2+ recirculation fraction in canine heart

Intramyocardial Ca(2+) recirculation fraction (RF) critically determines the economy of excitation-contraction coupling. RF is obtainable from the exponential decay of the postextrasystolic potentiation of left ventricular (LV) contractility. We have shown that RF remains unchanged despite increasing LV volume (LVV) at normothermia, but decreases with increasing temperature at a constant LVV. However, it remains unknown whether the temperature-dependent RF was not due to the simultaneously changed peak LV pressure (LVP) at a constant LVV. We hypothesized that this temperature-dependent RF would be independent of the simultaneous change in LVP. We used nine excised, cross-circulated canine hearts and allowed their LVs to contract isovolumically. During stable regular beats at 500 msec intervals, we inserted an extrasystolic beat at 360 msec interval followed by the postextrasystolic beats (PESs) at 500 msec intervals. We equalized the temperature-dependent peak LVPs of the regular beats at 36 degrees C and 38 degrees C to the peak LVP level of the stable regular beat at 33 degrees C by adjusting LVV. We fitted the same equation: nEmax = a.exp[-(i - 1)/tau(e)] + b.exp[-(i - 1)/tau(s)]cos[pi(i - 1)] + 1, used before to the normalized Emax (maximum elastance) values of PESi (i = 1-6) relative to the regular beat Emax. RF given by exp(-1/tau(e)) decreased by 19% to 38 degrees C from 33 degrees C. The temperature coefficient (Q(10)) of 1/RF was significantly greater than 1.3. The present results indicated a similar temperature dependence of RF and its Q(10) to those we observed previously without equalizing peak LVP. Thus, the temperature-dependent RF is independent of ventricular loading conditions.

Learning about Cardiac Calcium Signaling from Genetic Engineering

Genetic engineering has already provided critical data on the Ca-induced Ca(2+) release (CICR) hypothesis issues and promises even greater future insights. The two approaches employed thus far are (1) the construction of transgenic animal models with deletion or overexpression of Ca(2+) signaling proteins, and (2) direct structure-function studies of these proteins in artificial systems. In our laboratory both approaches have provided some insight into molecular modulation of CICR and the pathophysiology arising from the deletion or overactivity of these proteins. Probing the cytoplasmic segments of the carboxyl c-terminal tail of Ca(2+) channel, we identified two calcium sensing and calmodulin binding domains (LA and K) that have been implicated in Ca(2+)-induced inactivation of Ca(2+) channels. Introducing these peptides into atrial myocytes, where a large fraction of Ca(2+) release sites are unassociated with the dihydropyridine receptors (DHPRs) (no t-tubules), suggests that LA, but not K motif, increases the sensitivity of RyRs to Ca(2+), is responsible for the higher frequency of Ca(2+) sparks in the peripheral sites, and provides for the voltage dependence of CICR. Genetic overexpression or deletion of the primary proteins of the Ca(2+) signaling cascade also provides supportive evidence for the Ca(2+) current (I(Ca))-gated CICR mechanism, generates some novel and unexpected cardiac phenotypes in transgenic mice, and suggests that Ca(2+) signaling defects can trigger compensatory molecular mechanisms that underlie the observed cardiac phenotype and pathophysiology.

Tonic component of myocardial contraction

Calcium transients and contractions of cardiac myocytes consist of phasic component, relaxing spontaneously independently of membrane voltage and of the tonic component (TC) relaxing only upon repolarization. Experimental data reviewed in this article suggest that most Ca(2+) activating TC is released from sarcoplasmic reticulum (SR) via the ryanodine receptors (RyRs). Most likely these RyRs are activated by sustained Ca(2+) influx. However, its route may differ depending on species and state of the cells. It seems that in rat RyRs responsible for TC are activated by the sustained Ca(2+) current. In guinea-pig the blockers of Ca(2+) current or reverse mode Na(+)/Ca(2+) exchange do not inhibit TC, so these routes seem unlikely. In myocytes of the failing human hearts TC is activated mostly via the reverse mode Na(+)/Ca(2+) exchange and contribution of SR is negligible. The mechanism of TC in the normal human cardiomyocytes has not been investigated. Thus, despite investigation of TC for half a century many problems concerning the mechanism of its activation and maintenance as well as its physiological meaning remain unsolved.

Alternans decay of postextrasystolic potentiation in human left ventricle

An organ-level assessment of the total Ca2+ handled in the excitation-contraction coupling in a beating heart has been accomplished in canine left ventricles (LVs). This approach combines the intramyocardial Ca2+ recirculation fraction (RF) with the cardiac O2 consumption for the excitation-contraction coupling. The RF has conventionally been obtained from the exponential decay of the postextrasystolic (PES) potentiation of myocardial contractility. However, in canine LVs, the PES contractility in terms of Emax (end-systolic pressure-volume ratio) has been shown to decay generally in alternans under both physiological and pathological conditions. Nevertheless, the RF can be obtained from the exponential decay component in the PES Emax alternans decay. We expected that the same Ca2+ assessment could be applied to the human heart. As the first step, we investigated whether the PES Emax would decay in alternans or exponentially in patient LVs. We retrospectively analyzed 13 patient cases that had stable regular beats unexpectedly interrupted by a spontaneous extrasystole followed by a PES compensatory pause during their diagnostic examination. These patients had either mitral regurgitation, old myocardial infarction, or dilated cardiomyopathy. Their LV Emax decayed consistently in alternans within the first several PES beats. These Emax alternans decays resemble those reported in canine LVs. This finding suggests for the first time the applicability of the same organ-level RF assessment method developed for canine hearts to human hearts.

Arterial and Left Ventricular Pressures Illude Transient Alternans of Contractility during Postextrasystolic Potentiation

We have previously found that the postextrasystolic (PES) potentiation (PESP) of the left ventricular (LV) contractility (Emax) decays typically in transient alternans even in the normally ejecting canine heart. This contradicted the general expectation that arterial pressure (AP) and LV pressure (LVP) usually decay exponentially during PESP. We hypothesized this contradiction to be due to the different cardiodynamic behaviors of AP and LVP from LV Emax during PESP. We tested this hypothesis by measuring AP, LVP, LV volume, Emax, effective arterial elastance (Ea) as an index of afterload, and pulse pressure (PP) during PESP in eight anesthetized open-chest dogs by using the conductance catheter system. We changed Ea by changing the total peripheral resistance (TPR) with methoxamine hydrochloride (iv) and repeated the measurements. Although the Emax alternans patterns during PESP were comparable between the normal and high afterloads, LVP and PP were slightly potentiated and alternated under the normal afterload, whereas LVP and PP were obviously potentiated and alternated under the high afterload. We also simulated the effects of Ea/Emax on the transient alternans of AP and LVP on a computer. Despite the same alternans pattern of Emax, a higher Ea/Emax, which is typical in heart failure, caused a larger PP alternans, whereas a lower Ea/Emax, which is typical in normal hearts, almost eliminated it. These results suggest that a transient alternans of LV contractility during PESP could be overlooked when AP and LVP are monitored in in situ normal hearts.

Postextrasystolic contractility normally decays in alternans in canine in situ heart

We have reported that the postextrasystolic (PES) potentiation of left ventricular (LV) contractility usually decays in alternans at heart rates above 80-100 beats/min in the canine excised, cross-circulated heart. We examined whether the PES contractility would also decay in alternans even in the canine in situ heart presumably more physiological than the excised heart. In anesthetized, ventilated, and open-chest mongrel dogs, we measured LV pressure and volume with a micromanometer and a conductance catheter cannulated into the LV and obtained LV end-systolic maximum elastance (E(max)) as the reasonably load-independent contractility index. We inserted an extrasystole followed by a compensatory pause into steady-state regular beats at heart rates above 90 beats/min and analyzed the PES decay pattern of E(max). We found that E(max) potentiated in the first PES beat decayed in alternans within 5-6 PES beats. This indicates that PES contractility also decays in alternans in the normal canine in situ heart.

Exponential fitting of postextrasystolic potentiation may underestimate the cardiac Ca2+ recirculation fraction: a theoretical analysis

The recirculation fraction of intramyocardial Ca(2+) (RF) has conventionally been obtained from the monotonic decay of postextrasystolic potentiation (PESP). The used assumption is that the decay is exponential. However, we have found that PESP usually decays in alternans even at spontaneous heart rates (>100 beats/min) in excised, cross-circulated canine heart preparations under normal coronary perfusion and normothermia. We have already devised a means of extracting the exponential decay component for RF calculation by subtracting the oscillatory component from the alternans PESP decay by a curve-fitting method. Using mathematics, we assessed the possible error in estimated RF when an exponential curve was naively fit to the alternans PESP decay. We obtained results showing that the exponential assumption may considerably underestimate RF even when the alternans is trivial with the oscillatory component of only 10% of the exponential component.

Temperature-dependent postextrasystolic potentiation and Ca(2+) recirculation fraction in canine hearts

We have found that cardiac temperature proportionally changes O(2) cost of contractility, defined as O(2) consumption for myocardial total Ca(2+) handling normalized to contractility in terms of the end-systolic pressure-volume ratio (maximal elastance, E(max)), in the canine left ventricle (temperature sensitivity, Q(10) = 2). We have separately found that a decrease in the recirculation fraction (RF) of Ca(2+) within myocardial cells underlies an increased O(2) cost of E(max) in stunned hearts. We therefore hypothesized that a similar change in RF would underlie the Q(10) of O(2) cost of E(max). We tested this hypothesis by analyzing RF calculated from an exponential decay component of the transiently alternating postextrasystolic potentiation in the canine left ventricle. RF decreased from 0.7 to 0.5 as cardiac temperature increased from 33 to 38 degrees C with Q(10) of 0.5, reciprocal to that of O(2) cost of E(max). We conclude that Q(10) of ATP-consuming reactions involved in Ca(2+) handling and E(max) response to it could reasonably account for the reciprocal Q(10) of RF and O(2) cost of E(max).

New calculation of internal Ca(2+) recirculation fraction from alternans decay of postextrasystolic potentiation

In our previous studies, we calculated the internal Ca(2+) recirculation fraction (RF) after obtaining the beat decay constant (tau(e)) of the monoexponential component in the postextrasystolic potentiation (PESP) of the alternans decay by curve fitting. However, this method sometimes suffers from the sensitive variation of tau(e) with small noises in the measured contractilities of the 5th and 6th postextrasystolic (PES) beats in the tail of the exponential component. We now succeeded in preventing this problem by a new method to calculate RF without obtaining tau(e). The equation for the calculation in the new method expresses an alternans decay of PESP as a recurrence formula of PESP. It can calculate RF directly from the contractilities of the 1st through the 4th PES beats without any fitting procedure. To evaluate the reliability of the new method, we calculated RF from the alternans decay of PESP of the left ventricle (LV) of the canine excised cross-circulated heart preparation by both the original fitting and the new method. Although there was no significant difference in the mean value of the obtained RF between these two methods, the variance of RF was smaller with the new method than with the original method. Thus the new method proved useful and more reliable than the original fitting method.

Coupling interval from slow to tachycardiac pacing decides sustained alternans pattern

We discovered that the coupling beat interval from a slow to a tachycardiac pacing period considerably affected the pattern of the beat-to-beat alternation of the tachycardia-induced sustained contractile alternans. We analyzed the relationship between the coupling interval and the pattern and amplitude of the alternans in the isovolumic left ventricle of canine blood-perfused hearts. The alternans pattern and amplitude varied transiently over the first 30-50 beats and became gradually stable over the first minute in all 12 hearts. We discovered that stable alternans, even under the same tachycardiac pacing, had three different strong-weak beat patterns depending on the coupling interval. A relatively short coupling interval produced a representative sustained alternans of the strong and weak beats. A relatively long coupling interval produced a similar sustained alternans but in a reversed order of even- and odd-numbered beats counted from the coupling interval. However, sustained alternans disappeared after 1-3 specific coupling intervals. We conclude that ventricular pacing rate does not solely determine the pattern and amplitude of sustained contractile alternans induced by tachycardia.

Total Ca2+ handling for E-C coupling in the whole heart: An integrative analysis

We assessed total Ca2+ handling (transport, flux) in excitation-contraction (E-C) coupling in a beating left ventricle (LV). We developed a new integrative analysis method that utilizes the internal Ca2+ recirculation fraction (RF), O2 consumption ([Formula: see text]o2) for Ca2+ handling, and O2 cost of Emax (contractility index) of the LV. We obtained the RF from the beat constant of the exponential decay component of the postextrasystolic potentiation, and the O2 cost of Emax from [Formula: see text]o2measured at different Emax. Our equation calculated the unknown total Ca2+ handling, futile Ca2+ cycling, and Ca2+ reactivity of Emax from the RF and Ca2+ handling [Formula: see text]o2. The calculated total Ca2+ handling fell between 30 and 110 µmol/kg, depending on Emax and pathological conditions. Our method also allowed an assessment of futile Ca2+ cycling and Ca2+ reactivity of Emax in a beating LV. These data are not available using conventional methods. Our method can be used to better understand the pathophysiology of total Ca2+ handling in a beating heart.Key words: excitation-contraction coupling, myocardial Ca2+, contractility, cardiac O2 consumption.

2,3-Butanedione monoxime suppresses primarily total calcium handling in canine heart

Whether 2,3-butanedione monoxime (BDM, < or = 5mmol/l) suppresses primarily crossbridge cycling or total Ca(2+) handling in the blood-perfused whole heart remains controversial. Although BDM seems to suppress primarily total Ca(2+) handling in canine hearts, more evidence is lacking. We therefore analyzed the cardiac mechanoenergetics, namely, E(max) (contractility), PVA (total mechanical energy), and O(2) consumption of canine BDM-treated hearts by our recently developed integrative method to assess myocardial total Ca(2+) handling. This method additionally required the internal Ca(2+) recirculation fraction. We obtained this from the beat constant of the exponential decay component of the postextrasystolic potentiation. Our analysis indicated significant decreases in both internal Ca(2+) recirculation fraction and total Ca(2+) handling in the BDM-treated heart, but virtually no change in the reactivity of E(max) to total Ca(2+) handling. This result corroborates the view that BDM suppresses primarily total Ca(2+) handling rather than crossbridge cycling in the canine blood-perfused heart.

Postextrasystolic contractile decay always contains exponential and alternans components in canine heart

In isolated, blood-perfused canine hearts, postextrasystolic potentiation (PESP) decays monotonically after a noncompensatory pause following a spontaneous extrasystole (ES). The monotonic PESP decay yields myocardial internal Ca(2+) recirculation fraction (RF). We have found that after a compensatory pause (CP), PESP decays in alternans, consisting of an exponential and a sinusoidal decay component. We have proposed that this exponential component also yields RF. In the present study, we examined the reliability of this alternative method by widely changing the ES coupling interval (ESI), CP, and heart rate in the canine excised, cross-circulated left ventricle. We found that all PESP decays consisted of the sum of an exponential and a sinusoidal decay component of variable magnitudes whether a CP existed or not. Their decay constants as well as the calculated RF were independent of the ESI and CP. This confirmed the utility of our alternative RF determination method regardless of the ESI, CP, and heart rate. Direct experimental evidence of Ca(2+) dynamics supportive of this alternative method, however, remains to be obtained.

Energy-wasteful total Ca(2+) handling underlies increased O(2) cost of contractility in canine stunned heart

Postischemic myocardial stunning halved left ventricular contractility [end-systolic maximum elastance (E(max))] and doubled the O(2) cost of E(max) in excised cross-circulated canine heart. We hypothesized that this increased O(2) cost derived from energy-wasteful myocardial Ca(2+) handling consisting of a decreased internal Ca(2+) recirculation, some futile Ca(2+) cycling, and a depressed Ca(2+) reactivity of E(max). We first calculated the internal Ca(2+) recirculation fraction (RF) from the exponential decay component of postextrasystolic potentiation. Stunning significantly accelerated the decay and decreased RF from 0.63 to 0. 43 on average. We then combined the decreased RF with the halved E(max) and its doubled O(2) cost and analyzed total Ca(2+) handling using our recently developed integrative method. We found a decreased total Ca(2+) transport and a considerable shift of the relation between futile Ca(2+) cycling and Ca(2+) reactivity in an energy-wasteful direction in the stunned heart. These changes in total Ca(2+) handling reasonably account for the doubled O(2) cost of E(max) in stunning, supporting the hypothesis.

Myocardial mechanical restitution and potentiation partly underlie alternans decay of postextrasystolic potentiation: simulation

We have reported that the postextrasystolic potentiation (PESP) decays in alternans or monotonically, respectively, depending on whether the first postextrasystolic beat interval has a compensatory pause or not, in the canine left ventricle. To get better mechanistic insight into the alternans PESP decay, we hypothesized that the myocardial mechanical restitution and potentiation could partly account for both types of PESP decay. To test this hypothesis, we simulated PESP decay on a computer using a documented equation combining myocardial mechanical restitution and potentiation. We changed the first postextrasystolic beat interval after a fixed extrasystolic beat interval without changing regular and other postextrasystolic beat intervals. The simulated PESP decayed in alternans or monotonically as a function only of the first postextrasystolic beat interval. Thus, the myocardial mechanical restitution and potentiation could partly account for both alternans and monotonic decay of PESP. We conclude that myocardial mechanical restitution and potentiation may partly underlie the initial two alternating beats, the first beat being the most potentiated and the second beat being the most depressed, of alternans PESP decay in the canine heart.

Influence of transgenic overexpression of phospholamban on postextrasystolic potentiation

Twelve mice with PLB overexpression (PLBOE), and 11 isogenic FVB/N wild-type (WT) controls, were anesthetized and instrumented with a 1.4 F Millar catheter in the LV and a 1 F pacemaker in the right atrium. At a cycle length of 200 ms and a fixed extrastimulus of 120 ms, extrastimuli with increasing intervals (PESI) up to 1000 ms were introduced, and the peak rates of LV isovolumic contraction (+/- dP/dtmax) were normalized and fit to monoexponential equations. In a subset of animals, the protocols were repeated after ryanodine (4 ng/g) was given to deplete SR Ca2+ stores. The time constant and the plateau of the exponential curve fits were significantly greater in PLBOE than WT (107.8 +/- 7.0 v 75.2 +/- 5.5 ms and 1.39 +/- 0.03 v 1.08 +/- 0.02, both P < 0.05). At 200, 600 and 1000 ms, the normalized dP/dt was significantly greater in PLBOE than WT. After ryanodine, normalized dP/dt was significantly decreased in PLBOE, but unchanged in WT. The protein levels of the sodium-calcium exchanger normalized to calsequestrin were increased 3.7 +/- 0.3-fold in PLBOE compared to controls. In conclusion, the phospholamban level is a critical determinant of postextrasystolic potentiation in this transgenic model, and is differentially impaired by ryanodine at long diastolic intervals in PLBOE v controls. These differences may be due in part to changes in the protein level and resultant activity of the sodium calcium exchanger.

Total Ca handling in canine mild Ca overload failing heart

We analyzed total Ca handling of the left ventricle (LV) in the mildly failing heart preparation induced by a temporary intracoronary Ca overloading intervention in eight excised and cross-circulated canine hearts. This Ca intervention consisted of interruption of coronary blood perfusion by Ca-free oxygenated Tyrode perfusion for 10 min followed by high-Ca (16mmol/l) oxygenated Tyrode perfusion for 5 min. This intervention decreased the LV contractility index, Emax (end-systolic maximum elastance), by 40% after restoration of the blood cross-circulation. We expected a Ca overload or paradox failing heart resembling the postischemic stunned heart and being characterized by an increased O2 cost of Emax. However, LV O2 consumption under mechanically unloading conditions decreased by 30% from control without increasing the O2 cost of Emax. To obtain a mechanistic view of this failing heart, we investigated cardiac total Ca handling by our integrative analysis method. In this method, we obtained the internal Ca recirculation fraction (RF) from the decay beat constant of the postextrasystolic potentiation following each sporadic spontaneous extrasystole in these failing LVs. We combined the RF with the decreased Emax and the unchanged O2 cost of Emax in our recently developed formula of total Ca handling. We found that these failing LVs had a slightly but significantly increased RF accompanied by either a slightly increased futile Ca cycling or a slightly decreased Ca reactivity of Emax, or both. Any of these three possible changes can account for the unchanged O2 cost of Emax. This result indicates that the present mildly failing heart has not yet fallen into a typical Ca overload or paradox by the temporary Ca overloading intervention.

Muscarinic regulation of Ca2+ signaling in mammalian atrial and ventricular myocardium

The differential regulation of the contractility of mammalian atrial and ventricular myocardium upon activation of muscarinic receptors can be ascribed, for the most part, to alterations in intracellular Ca2+ transients. However, alterations in myofibrillar sensitivity to Ca2+ ions also contribute to such regulation. In atrial muscle, the following actions are all associated with the corresponding alterations in the amplitude of Ca2+ transients in the same direction as those in the strength of the contractile force: (1) the direct inhibitory action on the basal force of contraction; (2) the increase (recovery) in force that is induced during the prolonged stimulation of muscarinic receptors; and (3) the rebound increase in force induced by washout of muscarinic receptor agonists. In addition, for a given decrease in force induced by muscarinic receptor stimulation in atrial muscle, the amplitude of Ca2+ transients is decreased to a smaller extent than the decrease in amplitude induced by reduction of extracellular Ca2+ concentration ([Ca2+]o), an indication that muscarinic receptor stimulation might increase myofibrillar sensitivity to Ca2+ ions simultaneously with the reduction in the amplitude of Ca2+ transients during induction of the direct inhibitory action. In mammalian ventricular myocardium, the direct inhibitory action of muscarinic receptor stimulation exhibits a wide range of species-dependent variation. A pronounced direct inhibitory action is induced in ferret papillary muscle, which is also associated with a definite increase in myofibrillar sensitivity to Ca2+ ions. By contrast, in the ventricular myocardium of other species including the rabbit and the dog, muscarinic receptor stimulation scarcely affects the baseline Ca2+ transients and the force, but it results in a pronounced decrease in Ca2+ transients and force when applied in the presence of beta-adrenoceptor stimulation, a phenomenon known as 'accentuated antagonism' or the 'indirect inhibitory action' of muscarinic receptor stimulation in mammalian ventricular myocardium. During induction of the indirect inhibitory action in mammalian ventricular myocardium, muscarinic receptor stimulation reverses all the effects induced by beta-adrenoceptor stimulation, including the increase in Ca2+ transients, the positive inotropic and lusitropic effects, and the decrease in myofibrillar sensitivity to Ca2+ ions. The relationship between the amplitude of Ca2+ transients and force is unaffected during induction of the indirect inhibitory action in rabbit and dog ventricular myocardium. The direct and indirect inhibitory actions of muscarinic receptor stimulation on Ca2+ transients have clearly different dependences on frequency: the former is more pronounced at a higher rate of stimulation, while the latter is more pronounced at a lower rate. The more complex interaction of muscarinic receptor and beta-adrenoceptor stimulation in mammalian atrial muscle and ferret ventricular muscle might be explained by the contribution of both the direct and the indirect regulatory mechanisms to the interaction.

The role of calcium in the response of cardiac muscle to stretch

This review focuses on the complex interactions between two major regulators of cardiac function; Ca2+ and stretch. Initial consideration is given to the effect of stretch on myocardial contractility and details the rapid and slow increases in contractility. These are shown to be related to two diverse changes in Ca2+ handling (enhanced myofilament Ca2+ sensitivity and increased intracellular Ca2+ transient, respectively). Interaction between stretch and Ca2+ is also demonstrated with respect to the rhythm of cardiac contraction. Stretch has been shown to alter action potential configuration, generate stretch-activated arrhythmias, and increase the rate of beating of the sino-atrial node. A variety of Ca(2+)-dependent mechanisms including attenuation of Ca2+ extrusion via Na+/Ca2+ exchange, Ca2+ entry through stretch-activated channels (SACs) and mobilisation of intracellular Ca2+ stores have been proposed to account for the effect of stretch on rhythm. Finally, the interaction between stretch and Ca2+ in the secretion of natriuretic peptides and onset of hypertrophy is discussed. Evidence is presented that Ca2+ (entering through L-type Ca2+ channels or SACs, or released from sarcoplasmic reticular stores) influences secretion of both atrial and B-type natriuretic peptide; there is data to support both positive and negative modulation by Ca2+. Ca2+ also appears to be important in the pathway that leads to expression of precursors of hypertrophic protein synthesis. In conclusion, two of the major regulators of cardiac muscle function, Ca2+ and stretch, interact to produce effects on the heart; in general these effects appear to be additive.

A new integrative method to quantify total Ca2+ handling and futile Ca2+ cycling in failing hearts

Ca2+ handling in excitation-contraction coupling requires considerable O2 consumption (VO2) in cardiac contraction. We have developed an integrative method to quantify total Ca2+ handling in normal hearts. However, its direct application to failing hearts, where futile Ca2+ cycling via the Ca2+-leaky sarcoplasmic reticulum (SR) required an increased Ca2+ handling VO2, was not legitimate. To quantify total Ca2+ handling even in such failing hearts, we combined futile Ca2+ cycling with Ca2+ handling VO2 and the internal Ca2+ recirculation fraction via the SR. We applied this method to the canine heart mechanoenergetics before and after intracoronary ryanodine at nanomolar concentrations. We found that total Ca2+ handling per beat was halved after the ryanodine treatment from approximately 60 micromol/kg left ventricle before ryanodine. We also found that futile Ca2+ cycling via the SR increased to >1 cycle/beat after ryanodine from presumably zero before ryanodine. These results support the applicability of the present method to the failing hearts with futile Ca2+ cycling via the SR.

A novel molecular determinant for cAMP-dependent regulation of the frog heart Na+-Ca2+ exchanger

Na+-Ca2+ exchanger is one of the major sarcolemmal Ca2+ transporters of cardiac myocytes. In frog ventricular myocytes the exchanger is regulated by isoproterenol via a beta-adrenoreceptor/adenylate-cyclase/cAMPdependent signaling pathway providing a molecular mechanism for the relaxant effect of the hormone. Here, we report on the presence of a novel exon of 27-base pair insertion, which generates a nucleotide binding motif (P-loop) in the frog cardiac Na+-Ca2+ exchanger. To examine the functional role of this motif, we constructed a full-length frog heart Na+-Ca2+ exchanger cDNA (fNCX1a) containing this exon. The functional expression of fNCX1a in oocytes showed characteristic voltage dependence, divalent (Ni2+, Cd2+) inhibition, and sensitivity to cAMP in a manner similar to that of native exchanger in frog myocytes. In oocytes expressing the dog heart NCX1 or the frog mutant (DeltafNCX1a) lacking the 9-amino acid exon, cAMP failed to regulate Na+-dependent Ca2+ uptake. We suggest that this motif is responsible for the observed cAMP-dependent functional differences between the frog and the mammalian hearts.

Effects of Ca2+ and epinephrine on Ca2+ recirculation fraction and total Ca2+ handling in canine left ventricles

We investigated the effects of intracoronary Ca2+ and epinephrine on the intracellular Ca2+ recirculation fraction (RF) and total Ca2+ handling in the left ventricle (LV) of the excised cross-circulated canine heart preparation. We analyzed LV postextrasystolic potentiation (PESP) following a spontaneous extrasystole that occurred sporadically under constant atrial pacing. All PESPs decayed in alternans and none decayed monotonically. We extracted an exponential decay component from the alternans PESP, determined its beat constant (taue), and calculated RF = exp(-1/taue). Increased intracoronary Ca2+ slightly increased taue and RF, but epinephrine did not change them, although both agents enhanced LV contractility 2-3 times. Neither Ca2+ nor epinephrine affected the sinusoidal decay of the alternans PESP. These results indicate that RF via the sarcoplasmic reticulum was slightly augmented by Ca2+, but not by epinephrine. We combined these RF data with LV Ca2+ handling O2 consumption data and obtained 40-110 micromol/kg as the total amount of Ca2+ handled in one cardiac cycle in the control and enhanced contractile states. These results indicate that this new LV-level approach seems to better the understanding of the Ca2+ mass dynamics responsible for the mechanoenergetics enhanced by inotropic interventions.

Sarcoplasmic reticulum Ca2+ content, L-type Ca2+ current and the Ca2+ transient in rat myocytes during beta-adrenergic stimulation

1. The effect of beta-adrenergic stimulation on the relationship between the intracellular Ca2+ transient and the amplitude of the L-type Ca2+ current (ICa) has been investigated in ventricular myocytes isolated from rat hearts. Intracellular [Ca2+] was monitored using fura-2 during field stimulation and while membrane potential was controlled using voltage clamp techniques. 2. The increase in the amplitude, and the rate of decline, of the Ca2+ transient produced by isoprenaline (1.0 mumol l-1) was not significantly different in myocytes generating action potentials and in those voltage clamped with pulses of constant duration and amplitude. 3. Under control conditions, the current-voltage (I-V) relationship for ICa was bell shaped. The amplitude of the Ca2+ transient also showed a bell-shaped voltage dependence. In the presence of isoprenaline, the amplitude of both ICa and the Ca2+ transient was greater at all test potentials and the I-V relationship maintained its bell-shaped voltage dependence. However, the size of the Ca2+ transient was no longer graded with changes in the amplitude of ICa: a small ICa could now elicit a maximal Ca2+ transient. 4. Rapid application of caffeine (10 mmol l-1) was used to elicit Ca2+ release from the sarcoplasmic reticulum (SR). Isoprenaline increased the integral of the subsequent rise in cytoplasmic [Ca2+] to 175 +/- 13% of control. 5. Abbreviation of conditioning pulse duration in the presence of isoprenaline was used to reduce the amplitude of the Ca2+ transient to control levels. Under these conditions, the amplitude of the Ca2+ transient was again graded with the amplitude of ICa in the same way as under control conditions. 6. Nifedipine (2 mumol l-1) was also used to decrease Ca2+ transient amplitude in the presence of isoprenaline. In the presence of isoprenaline and nifedipine, the amplitude of the Ca2+ transient again showed a bell-shaped voltage dependence. 7. The SR Ca(2+)-ATPase inhibitor thapsigargin (2.5 mumol l-1) reduced the effect of isoprenaline on the amplitude of the Ca2+ transient. In the presence of thapsigargin, the size of the Ca2+ transient increased as ICa increased in response to isoprenaline. 8. These data suggest that the increase in the amplitude of the Ca2+ transient produced by beta-adrenergic stimulation in cardiac muscle is due to an increase in the gain of the SR Ca2+ release process, due principally to an increase in the Ca2+ content of the SR.

Calcium equally increases the internal calcium recirculation fraction before and after beta-blockade in canine left ventricles

We studied whether intracoronary Ca administration after beta-blockade would increase the internal Ca recirculation fraction (RF) analogously to the Ca administration before beta-blockade. This was performed in excised cross-circulated canine hearts. We analyzed the exponential decay component of the postextrasystolic potentiation (PESP) following a spontaneous extrasystole. All the PESPs decayed in alternans with atrial pacing at a constant rate. We obtained the time constant (tau(e)) of the monoexponential decay component of the alternans PESP. An increment of intracoronary Ca by 1.5 mmol/l enhanced the left ventricular contractility index Emax (end-systolic maximum elastance) by 2.5 times before and after beta-blockade with propranolol. The intracoronary Ca after beta-blockade slightly but significantly increased tau(e), and hence increased RF calculated from tau(e) by RF = exp(-1/tau(e)). This was analogous to the slightly increased tau(e) and RF with Ca before beta-blockade. We speculate that the myocardial cyclic AMP-dependent phosphorylation level would not significantly alter the effect of intracoronarily administered Ca on myocardial Ca handling, in terms of tau(e) and RF.

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.

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.

The effects of sevoflurane on contractile and electrophysiologic properties in isolated guinea pig papillary muscles

We examined, in guinea pig papillary muscles, whether the negative inotropic effect of sevoflurane is due to the depression of the influx of extracellular Ca2+ or to inhibition of the availability of intracellularly stored Ca2+. Sevoflurane decreased action potential duration and contractile force in a concentration-dependent fashion in normally polarized guinea pig papillary muscles. Sevoflurane produced a depression of contractile force with different rates or patterns of stimulation in the rested state and at low stimulation frequencies. In a potentiated state, sevoflurane did not depress contractile forces. Although sevoflurane decreased action potential duration and contractile force in a concentration-dependent fashion in normal Tyrode's solution, in high K+ Tyrode's solution, it caused a depression of contractile force without a shortening of action potential duration. Sevoflurane also depressed contractile force in normal and high K+ Tyrode's solution with ryanodine 1 microM. Our results suggest that in myocardial contractile force the negative inotropic effect of sevoflurane might be caused by depression of transsarcolemmal Ca2+ influx, accompanied by shortening of the action potential duration.

Mechanical restitution and recirculation fraction in cardiac myocytes and left ventricular muscle of adult rats

Unloaded cell shortening was measured in electrically stimulated myocytes from adult rat hearts to compare the contractile response to stimulation with that in isometrically contracting left ventricular papillary muscles under similar experimental conditions, but preloaded to produce maximum twitch tension. Mechanical restitution in cells followed a biexponential function with time constants of 0.19 +/- 0.03 s and 36.4 +/- 10.2 s (7 cells from 5 hearts, n = 7/5). The time constants for papillary muscles were 0.58 +/- 0.05 s and 14.6 +/- 1.0 s (n = 6/6). In myocytes, maximum post-rest potentiation occurred after 30 to 60 s of rest. The potentiation after 60 s of rest was 2.48 +/- 0.31 times the steadystate in cells and 2.63 +/- 0.16 in papillary muscles. Recirculation fraction of C2+ as calculated from the decay of post-rest potentiation was 0.84 +/- 0.04 in single cells and 0.59 +/- 0.02 in papillary muscles (p < 0.005). Caffeine (3mM) abolished post-rest potentiation in both types of preparations. The numerical values for the time constants of mechanical restitution, potentiation factor and recirculation fraction in papillary muscles did not depend on preload. It is concluded that interval-dependent changes of contractility are preserved in single cardiac cells but the kinetics of decay of potentiation appear to have changed quantitatively.

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)

Excitation and contraction in atrial and ventricular myocardium of the guinea-pig

Several parameters of excitation-contraction coupling were compared in two types of muscle, using thin strips from the left atria and papillary muscles from the right ventricles of guinea-pigs. (1) The duration of the action potential and twitch is much longer in ventricular than in atrial muscle. (2) Mechanical restitution can usually be described by a monoexponential function in ventricular and biexponential function in atrial muscle. (3) Post-extrasystolic potentiation, when related to the steady state force, is greater in ventricular muscle. (4) When priming with paired-pulse stimulation, mechanical restitution can be studied after the short interval and after the long interval. In atrial muscle, mechanical restitution is very similar after the short and long intervals but in ventricular muscle they are different in size. (5) Ryanodine (10(-6) M) can decrease the steady state force to about 10% of control in atrial but only to about 35% in ventricular muscle. Ryanodine (10(-8) M) causes the slow phase of restitution in atrial muscle to disappear but in ventricular muscle only increases the rate of mechanical restitution. (6) Ca-antagonists (Cd2+ 0.2 mM) can decrease the steady state force to zero in atrial and ventricular muscle. Ca-antagonists, in low concentrations (Cd2+ 0.01 mM), mainly affected the fast phase of mechanical restitution. (7) The recirculation fraction of calcium was about 0.64 in atrial and 0.27 in ventricular muscle. The findings are discussed in the light of known ultrastructural differences between atrial and ventricular myocardium.

Effect of physiological heart rate changes on left ventricular dimensions and mitral blood flow velocities in the normal fetus

M-mode echo recordings of the left ventricle (LV) and inflow LV Doppler velocimetry were performed in nine normal fetuses at a gestational age of 36-39 weeks. In each fetus approximately 80 consecutive cardiac cycles were digitized. The duration of each cardiac cycle (T) and the corresponding end-diastolic (EDD), end-systolic (ESD) dimensions of LV or the peak velocity of early (E) and late atrial (A) mitral flow parameters was calculated. The role of sonographic parameters on current (Tn) and preceding (Tn-1) cardiac cycles was assessed using linear regression. Significant dependency of ventricular EDD and transmitral A peak velocity upon Tn was demonstrated. We speculate that atrial systole has an important role to play in the beat-to-beat regulation of fetal stroke volume.

Signaling of Ca2+ release and contraction in cardiac myocytes

Cross signaling between Ca2+ channel and ryanodine receptor was explored in whole cell clamped rat ventricular myocyte under conditions where global myoplasmic Ca2+ concentrations were strongly buffered by dialyzing the myocytes with high concentrations of Fura 2 and EGTA. Ca2+ channel and ryanodine receptor were respectively activated by a depolarizing pulse to -10 mV and rapid (< 50 ms) application of 5 mM caffeine. Temporal analysis of kinetics of inactivation of Ca2+ channel with respect to the time of application of caffeine pulse provided experimental evidence that signalling between the ryanodine and Ca2+ channel is mediated exclusively through the Ca2+ microdomains surrounding the DHP/ryanodine receptor complex independent of global myoplasmic Ca2+ concentrations.

Contractile failure in early myocardial ischemia: models and mechanisms

In early myocardial ischemia we find a number of salient electrical and ionic alterations. This article reviews action potential shortening, K accumulation, and contractile failure. Enhanced K efflux during early myocardial ischemia has been attributed to a number of mechanisms, including: the inhibition of active K uptake, osmotic changes, efflux of K ions linked to anion extrusion, cation exchange, altered cellular energy levels, in particular, the opening of ATP-dependent K channels, the involvement of other ion channels, a H/K-ion exchanger, and a catecholamine-dependent pathway. The different mechanisms are discussed. Action potential shortening was described as a salient characteristic of myocardial ischemia in 1954 by Trautwein and Dudel, and was attributed to enhanced outward current. Recently it has been shown by several authors that ATP-dependent potassium channels play a key role in this context. Contractile failure in early myocardial ischemia has been explained by shortening of the action potential duration, reduced cytoplasmic free calcium levels, intracellular acidification, and a rise in inorganic phosphate and Mg. In summary, it is concluded that ATP-dependent K channels may be involved in each of these three phenomena.

Effects of lisinopril on cardiac contractility and ionic currents

1. The effects of lisinopril, an angiotensin-converting enzyme inhibitor, were studied on cardiac contractile force, action potential characteristics and membrane ionic currents. 2. In guinea-pig atria, lisinopril (0.001-1 microM) exerted a negative inotropic effect which was accompanied by a shortening of the time to peak tension and time for total contraction. However, it did not modify atrial rate or the characteristics of the ventricular action potentials recorded either in normally polarized or in depolarized papillary muscles. 3. In isolated guinea-pig ventricular myocytes, lisinopril had no effect on the inward L-type Ca2+ (ICa,L), the inward rectifier (IK1) or the delayed rectifier K+ currents (IK), but abolished the stimulation-dependent facilitation of the ICa,L. Furthermore, it did not alter a cloned human cardiac K+ current (hKv1.5) expressed in a mouse L cell line (Ltk-). 4. The absence of negative inotropic effects in patients with congestive heart failure can be explained by the potent arterial vasodilator action of lisinopril which reduced left ventricular afterload overriding the expected direct cardiodepressant effects of the drug.

Influences of stimulation frequency and temperature on interval-force relationships in guinea-pig papillary muscles

Relationships between contractile force and the preceding and pre-preceding stimulation intervals were studied in papillary muscles by interposing variable test intervals during steady-state pacing. The strength of test contractions increased exponentially to a maximum as the preceding (test) interval was lengthened. Contractility decreased as an exponential function of pre-preceding interval. At 37 degrees C, the half times for these processes were unaffected by increasing the steady-state frequency from 1 to 3 Hz. At 27 degrees C, the force increase with preceding interval was accelerated and the decay with pre-preceding interval was retarded as the stimulation frequency was increased from 0.33 to 2 Hz. The time-courses of force increase and decay were similar to each other during stimulation at an optimum frequency characteristic for the temperature. Cooling from 37 to 27 degrees C prolonged the half times for force increase and decay by factors of 4.5 and 3 respectively. The slope of the linear relationship between the force of the contraction pre-preceded by the test interval and the immediately subsequent contraction (recirculation fraction) was also halved. These results suggest that high stimulation frequency and low temperature uncouples cellular processes underlying the interval dependence of cardiac contractility. The temperature sensitivities are consistent with these processes being enzymatic. The reduced recirculation fraction provides a mechanism for the lowered threshold frequency for sustained mechanical alternans at 27 degrees C.

Effects of lisinopril on electromechanical properties and membrane currents in guinea-pig cardiac preparations

1. The effects of the angiotensin-converting enzyme inhibitor, lisinopril, were studied in guinea-pig atria and papillary muscles and in single isolated ventricular cells. 2. In isolated right atria, lisinopril (0.001-10 microM) decreased the amplitude and rate of the spontaneous contractions. In electrically driven left atria this negative inotropic effect was accompanied by a shortening of the time to peak tension and time for total contraction. 3. Lisinopril did not modify the electrophysiological characteristics of the ventricular action potentials recorded in papillary muscles perfused with normal Tyrode solution or elicited by isoprenaline in papillary muscles perfused with 27 mM K Tyrode solution. 4. In single ventricular cells, lisinopril (10 microM) had no effect on the inward L-type Ca2+ (ICa,L), the inward rectifier (IK1) or the delayed rectifier K+ currents (IK). However, it abolished the stimulation-dependent facilitation of the L-type Ca2+ current. 6. These results indicate that the negative inotropic effect of lisinopril cannot be explained by a decrease in Ca2+ entry through L-type channels and suggest that lisinopril may possibly act at an intracellular site to reduce contractile force.

The energetics of shortening amphibian cardiac muscle

An isolated amphibian cardiac muscle preparation, toad ventricular strip, was used to examine the energetics of shortening. Simultaneous measurements of force and length changes and the associated heat production were made. Both the isometric heat/stress and the enthalpy (heat+work)/load relationships were similar to those previously reported in mammalian cardiac muscle. The activation metabolism was higher in this preparation and, like its mammalian counterpart, was length dependent. The heat production measured in an isometric contraction was approximately 50% higher than that observed at the same stress level in rodent mammalian cardiac muscle. This did not affect the maximum isotonic mechanical efficiency (work divided by enthalpy) of the preparation which, at an afterload of 20% of the maximum stress was 18.1 +/- 1.7% (n = 8). There was no evidence for a shortening heat component in this preparation during isotonic contractions. It appears therefore that the energetics of shortening amphibian cardiac muscle closely resemble the energetics of mammalian cardiac tissue.