Localization of regions of troponin I important in deactivation of cardiac myofilaments by acidic pH

New Drug Targets and Preclinical Modelling Recommendations for Treating Acute Myocardial Infarction

Acute myocardial infarction (AMI) is the leading cause of morbidity and mortality worldwide and the primary underlying risk factor for heart failure. Despite decades of research and clinical trials, there are no drugs currently available to prevent organ damage from acute ischaemic injuries of the heart. In order to address the increasing global burden of heart failure, drug, gene, and cell-based regeneration technologies are advancing into clinical testing. In this review we highlight the burden of disease associated with AMI and the therapeutic landscape based on market analyses. New studies revealing the role of acid-sensitive cardiac ion channels and other proton-gated ion channels in cardiac ischaemia are providing renewed interest in pre- and post-conditioning agents with novel mechanisms of action that may also have implications for gene- and cell-based therapeutics. Furthermore, we present guidelines that couple new cell technologies and data resources with traditional animal modelling pipelines to help de-risk drug candidates aimed at treating AMI. We propose that improved preclinical pipelines and increased investment in drug target identification for AMI is critical to stem the increasing global health burden of heart failure.

Effects of clinically relevant acute hypercapnic and metabolic acidosis on the cardiovascular system: an experimental porcine study

INTRODUCTION

Hypercapnic acidosis (HCA) that accompanies lung-protective ventilation may be considered permissive (a tolerable side effect), or it may be therapeutic by itself. Cardiovascular effects may contribute to, or limit, the potential therapeutic impact of HCA; therefore, a complex physiological study was performed in healthy pigs to evaluate the systemic and organ-specific circulatory effects of HCA, and to compare them with those of metabolic (eucapnic) acidosis (MAC).

METHODS

In anesthetized, mechanically ventilated and instrumented pigs, HCA was induced by increasing the inspired fraction of CO2 (n = 8) and MAC (n = 8) by the infusion of HCl, to reach an arterial plasma pH of 7.1. In the control group (n = 8), the normal plasma pH was maintained throughout the experiment. Hemodynamic parameters, including regional organ hemodynamics, blood gases, and electrocardiograms, were measured in vivo. Subsequently, isometric contractions and membrane potentials were recorded in vitro in the right ventricular trabeculae.

RESULTS

HCA affected both the pulmonary (increase in mean pulmonary arterial pressure (MPAP) and pulmonary vascular resistance (PVR)) and systemic (increase in mean arterial pressure (MAP), decrease in systemic vascular resistance (SVR)) circulations. Although the renal perfusion remained unaffected by any type of acidosis, HCA increased carotid, portal, and, hence, total liver blood flow. MAC influenced the pulmonary circulation only (increase in MPAP and PVR). Both MAC and HCA reduced the stroke volume, which was compensated for by an increase in heart rate to maintain (MAC), or even increase (HCA), the cardiac output. The right ventricular stroke work per minute was increased by both MAC and HCA; however, the left ventricular stroke work was increased by HCA only. In vitro, the trabeculae from the control pigs and pigs with acidosis showed similar contraction force and action-potential duration (APD). Perfusion with an acidic solution decreased the contraction force, whereas APD was not influenced.

CONCLUSIONS

MAC preferentially affects the pulmonary circulation, whereas HCA affects the pulmonary, systemic, and regional circulations. The cardiac contractile function was reduced, but the cardiac output was maintained (MAC), or even increased (HCA). The increased ventricular stroke work per minute revealed an increased work demand placed by acidosis on the heart.

Elucidation of isoform-dependent pH sensitivity of troponin i by NMR spectroscopy

Myocardial ischemia is characterized by reduced blood flow to cardiomyocytes, which can lead to acidosis. Acidosis decreases the calcium sensitivity and contractile efficiency of cardiac muscle. By contrast, skeletal and neonatal muscles are much less sensitive to changes in pH. The pH sensitivity of cardiac muscle can be reduced by replacing cardiac troponin I with its skeletal or neonatal counterparts. The isoform-specific response of troponin I is dictated by a single histidine, which is replaced by an alanine in cardiac troponin I. The decreased pH sensitivity may stem from the protonation of this histidine at low pH, which would promote the formation of electrostatic interactions with negatively charged residues on troponin C. In this study, we measured acid dissociation constants of glutamate residues on troponin C and of histidine on skeletal troponin I (His-130). The results indicate that Glu-19 comes in close contact with an ionizable group that has a pK(a) of ∼6.7 when it is in complex with skeletal troponin I but not when it is bound to cardiac troponin I. The pK(a) of Glu-19 is decreased when troponin C is bound to skeletal troponin I and the pK(a) of His-130 is shifted upward. These results strongly suggest that these residues form an electrostatic interaction. Furthermore, we found that skeletal troponin I bound to troponin C tighter at pH 6.1 than at pH 7.5. The data presented here provide insights into the molecular mechanism for the pH sensitivity of different muscle types.

Pathogenic peptide deviations support a model of adaptive evolution of chordate cardiac performance by troponin mutations

In cardiac muscle, the troponin (cTn) complex is a key regulator of myofilament calcium sensitivity because it serves as a molecular switch required for translating myocyte calcium fluxes into sarcomeric contraction and relaxation. Studies of several species suggest that ectotherm chordates have myofilaments with heightened calcium responsiveness. However, genetic polymorphisms in cTn that cause increased myofilament sensitivity to activating calcium in mammals result in cardiac disease including arrhythmias, diastolic dysfunction, and increased susceptibility to sudden cardiac death. We hypothesized that specific residue modifications in the regulatory arm of troponin I (TnI) were critical in mediating the observed decrease in myofilament calcium sensitivity within the mammalian taxa. We performed large-scale phylogenetic analysis, atomic resolution molecular dynamics simulations and modeling, and computational alanine scanning. This study provides evidence that a His to Ala substitution within mammalian cardiac TnI (cTnI) reduced the thermodynamic potential at the interface between cTnI and cardiac TnC (cTnC) in the calcium-saturated state by disrupting a strong intermolecular electrostatic interaction. This key residue modification reduced myofilament calcium sensitivity by making cTnI molecularly untethered from cTnC. To meet the requirements for refined mammalian adult cardiac performance, we propose that compensatory evolutionary pressures favored mutations that enhanced the relaxation properties of cTn by decreasing its sensitivity to activating calcium.

Differential effects of a green tea-derived polyphenol (-)-epigallocatechin-3-gallate on the acidosis-induced decrease in the Ca(2+) sensitivity of cardiac and skeletal muscle

(-)-Epigallocatechin-3-gallate (EGCg), a green tea-derived polyphenol, has received much attention as a protective agent against cardiovascular diseases. In this study, we determined its effects on the acidosis-induced change in the Ca(2+) sensitivity of myofilaments in myofibrils prepared from porcine ventricular myocardium and chicken pectoral muscle. EGCg (0.1 mM) significantly inhibited the decrease caused by lowering the pH from 7.0 to 6.0 in the Ca(2+) sensitivity of myofibrillar ATPase activity in cardiac muscle, but not in skeletal muscle. Studies on recombinant mouse cardiac troponin C (cTnC) and chicken fast skeletal troponin C (sTnC) using circular dichroism and intrinsic and extrinsic fluorescence spectroscopy showed that EGCg bound to cTnC with a dissociation constant of approximately 3-4 muM, but did not bind to sTnC. By presumably binding to the cTnC C-lobe, EGCg decreased Ca(2+) binding to cTnC and overcame the depressant effect of protons on the Ca(2+) sensitivity of the cardiac contractile response. To demonstrate isoform-specific effects of the action of EGCg, the pH sensitivity of the Ca(2+) response was examined in cardiac myofibrils in which endogenous cTnC was replaced with exogenous sTnC or cTnC and in skeletal myofibrils in which the endogenous sTn complex was replaced with whole cardiac Tn complex (cTn). The results suggest that the binding of EGCg to the cardiac isoform-specific TnC or Tn complex alters the effect of pH on myofilament Ca(2+) sensitivity in striated muscle.

New potential uses for cardiac troponins

This review briefly synthesizes the molecular biology of troponin, which is currently the best biochemical marker for the detection of cardiac injury and, thus, acute myocardial infarction as well. Potential new uses for the marker based on these insights, with a specific interest in cardiac troponin fragments that potentially could be linked to distinct clinical conditions, are described. Some of the clinical problems clinicians are faced with including how to use the markers in renal failure and the difficulties associated with the heterogeneity of current troponin assays are also discussed. Finally, we present the possibility of specific cardiac troponin fragments resulting from modification or degradation, associated with distinct pathological processes, as new potential uses for this biomarker.

Myofilament calcium sensitivity does not affect cross-bridge activation-relaxation kinetics

We employed single myofibril techniques to test whether the presence of slow skeletal troponin-I (ssTnI) is sufficient to induce increased myofilament calcium sensitivity (EC50) and whether modulation of EC50 affects the dynamics of force development. Studies were performed using rabbit psoas myofibrils activated by rapid solution switch and in which Tn was partially replaced for either recombinant cardiac Tn(cTn) or Tn composed of recombinant cTn-T (cTnT) and cTn-C (cTnC), and recombinant ssTnI (ssTnI-chimera Tn). Tn exchange was performed in rigor solution (0.5 mg/ml Tn; 20°C; 2 h) and confirmed by SDS-PAGE. cTnI exchange induced a decrease in EC50; ssTnI-chimera Tn exchange induced a further decrease in EC50 (in μM: endogenous Tn, 1.35 ± 0.08; cTnI, 1.04 ± 0.13; ssTnI-chimera Tn, 0.47 ± 0.03). EC50 was also decreased by application of 100 μM bepridil (control: 2.04 ± 0.03 μM; bepridil 1.35 ± 0.03 μM). Maximum tension was not different between any groups. Despite marked alterations in EC50, none of the dynamic activation-relaxation parameters were affected under any condition. Our results show that 1) incorporation of ssTnI into the fast skeletal sarcomere is sufficient to induce increased myofilament Ca2+ sensitivity, and 2) the dynamics of actin-myosin interaction do not correlate with EC50. This result suggests that intrinsic cross-bridge cycling rate is not altered by the dynamics of thin-filament activation.

Acute heart failure: inotropic agents and their clinical uses

Inotropic agents are indispensable for the improvement of cardiac contractile dysfunction in acute or decompensated heart failure. Clinically available agents, including sympathomimetic amines (dopamine, dobutamine, noradrenaline) and selective phosphodiesterase-3 inhibitors (amrinone, milrinone, olprinone and enoximone) act via cAMP/protein kinase A (PKA)-mediated facilitation of intracellular Ca2+ mobilisation. Phosphodiesterase-3 inhibitors also have a vasodilatory action, which plays a role in improving haemodynamic parameters in certain patients, and are termed inodilators. The available inotropic agents suffer from risks of Ca2+ overload leading to arrhythmias, myocardial cell injury and ultimately, cell death. In addition, they are energetically disadvantageous because of an increase in activation energy and cellular metabolism. Furthermore, they lose their effectiveness under pathophysiological conditions, such as acidosis, stunned myocardium and heart failure. Pimobendan and levosimendan (that act by a combination of an increase in Ca2+ sensitivity and phosphodiesterase-3 inhibition) appear to be more beneficial among existing agents. Novel Ca2+ sensitisers that are under basic research warrant clinical trials to replace available inotropic agents.

A quantitative analysis of cardiac myocyte relaxation: a simulation study

The determinants of relaxation in cardiac muscle are poorly understood, yet compromised relaxation accompanies various pathologies and impaired pump function. In this study, we develop a model of active contraction to elucidate the relative importance of the [Ca2+]i transient magnitude, the unbinding of Ca2+ from troponin C (TnC), and the length-dependence of tension and Ca2+ sensitivity on relaxation. Using the framework proposed by one of our researchers, we extensively reviewed experimental literature, to quantitatively characterize the binding of Ca2+ to TnC, the kinetics of tropomyosin, the availability of binding sites, and the kinetics of crossbridge binding after perturbations in sarcomere length. Model parameters were determined from multiple experimental results and modalities (skinned and intact preparations) and model results were validated against data from length step, caged Ca2+, isometric twitches, and the half-time to relaxation with increasing sarcomere length experiments. A factorial analysis found that the [Ca2+]i transient and the unbinding of Ca2+ from TnC were the primary determinants of relaxation, with a fivefold greater effect than that of length-dependent maximum tension and twice the effect of tension-dependent binding of Ca2+ to TnC and length-dependent Ca2+ sensitivity. The affects of the [Ca2+]i transient and the unbinding rate of Ca2+ from TnC were tightly coupled with the effect of increasing either factor, depending on the reference [Ca2+]i transient and unbinding rate.

A dynamic model of excitation-contraction coupling during acidosis in cardiac ventricular myocytes

Acidosis in cardiac myocytes is a major factor in the reduced inotropy that occurs in the ischemic heart. During acidosis, diastolic calcium concentration and the amplitude of the calcium transient increase, while the strength of contraction decreases. This has been attributed to the inhibition by protons of calcium uptake and release by the sarcoplasmic reticulum, to a rise of intracellular sodium caused by activation of sodium-hydrogen exchange, decreased calcium binding affinity to Troponin-C, and direct effects on the contractile machinery. The relative contributions and concerted action of these effects are, however, difficult to establish experimentally. We have developed a mathematical model to examine altered calcium-handling mechanisms during acidosis. Each of the alterations was incorporated into a dynamical model of pH regulation and excitation-contraction coupling to predict the time courses of key ionic species during acidosis, in particular intracellular pH, sodium and the calcium transient, and contraction. This modeling study suggests that the most significant effects are elevated sodium, inhibition of sodium-calcium exchange, and the direct interaction of protons with the contractile machinery; and shows how the experimental data on these contributions can be reconciled to understand the overall effects of acidosis in the beating heart.

Specific enhancement of sarcomeric response to Ca2+protects murine myocardium against ischemia-reperfusion dysfunction

Alteration in myofilament response to Ca2+is a major mechanism for depressed cardiac function after ischemia-reperfusion (I/R) dysfunction. We tested the hypothesis that hearts with increased myofilament response to Ca2+are less susceptible to I/R. In one approach, we studied transgenic (TG) mice with a constitutive increase in myofilament Ca2+sensitivity in which the adult form of cardiac troponin I (cTnI) is stoichiometrically replaced with the embryonic/neonatal isoform, slow skeletal TnI (ssTnI). We also studied mouse hearts with EMD-57033, which acts specifically to enhance myofilament response to Ca2+. We subjected isolated, perfused hearts to an I/R protocol consisting of 25 min of no-flow ischemia followed by 30 min of reperfusion. After I/R, developed pressure and rates of pressure change were significantly depressed and end-diastolic pressure was significantly elevated in nontransgenic (NTG) control hearts. These changes were significantly blunted in TG hearts and in NTG hearts perfused with EMD-57033 during reperfusion, with function returning to nearly baseline levels. Ca2+- and cross bridge-dependent activation, protein breakdown, and phosphorylation in detergent-extracted fiber bundles were also investigated. After I/R NTG fiber bundles exhibited a significant depression of cross bridge-dependent activation and Ca2+-activated tension and length dependence of activation that were not evident in TG preparations. Only NTG hearts demonstrated a significant increase in cTnI phosphorylation. Our results support the hypothesis that specific increases in myofilament Ca2+sensitivity are able to diminish the effect of I/R on cardiac function.

Expression of Slow Skeletal Troponin I in Adult Mouse Heart Helps to Maintain the Left Ventricular Systolic Function During Respiratory Hypercapnia

Compared with the adult, neonatal heart muscle is less sensitive to deactivation by acidic pH. We hypothesized that expression of slow skeletal troponin I (ssTnI), the embryonic isoform, in adult heart would help maintain left ventricular (LV) systolic function during respiratory hypercapnia. We assessed LV function by transthoracic 2D-targeted M-mode and pulsed Doppler echocardiography in transgenic (TG) mice in which cardiac TnI was replaced with ssTnI and in nontransgenic (NTG) littermates. Anesthetized mice were ventilated with either 100% oxygen or 35% CO 2 balanced with oxygen. Arterial blood pH with 35% CO 2 decreased to the same levels in both groups of animals. In the absence of propranolol, the LV fractional shortening was higher in TG compared with NTG mice throughout most of the experimental protocol. LV diastolic function was impaired in TG compared with NTG mice both at 100% oxygen and 35% CO 2 because E-to-A wave ratio of mitral flow was significantly lower, and E-wave deceleration time and LV isovolumic relaxation time were longer in TG compared with NTG mice. When compensatory mechanisms that occur through stimulation of β-adrenergic receptors during hypercapnia were blocked by continuous perfusion with propranolol, we found that NTG mice died within 3 to 4 minutes after switching to 35% CO 2 , whereas TG mice survived. Our experiments demonstrate the first evidence that specific replacement of cardiac TnI with ssTnI has a protective effect on the LV systolic function during hypercapnic acidosis in situ.

Structural based insights into the role of troponin in cardiac muscle pathophysiology

Troponin is a molecular switch, directly regulating the Ca2+-dependent activation of myofilament in striated muscle contraction. Cardiac troponin is subject to covalent and noncovalent modifications; phosphorylation modulates myofilament physiology, mutations are linked to familial hypertrophic cardiomyopathy, intracellular acidification causes myocardial infarction, and cardiotonic drugs modify myofilament response to Ca2+. The structure of troponin provides insights into the mechanism of this molecular switch and an understanding of the effects of protein modification under pathophysiological conditions. Although the structure of troponin C has been solved in various Ca2+-bound states for some time, structural information on troponin I and troponin T has only emerged recently. This review summarizes recent advances on the structure of complexes of troponin subunits with the aim of assessing how these proteins interact with each other to execute its role as a molecular switch and how covalent and noncovalent modifications affect the structure of troponin and the switch mechanism. We focus on pinpointing the specific amino acid residues involved in phosphorylation and mutation and the pH sensitive regions in the structure of troponin. We also present recent structural work that have identified the docking sites of several cardiotonic drugs on cardiac troponin C and discuss their relevance in the direction of troponin based drug design in the therapy of heart disease.

The therapeutic potential of novel cardiotonic agents

During the course of treatment of heart failure patients, cardiotonic agents are inevitable for improvement of myocardial dysfunction. Clinically available agents, such as beta-adrenoceptor agonists and selective phosphodiesterase 3 inhibitors, act mainly via cyclic AMP/protein kinase A-mediated facilitation of Ca(2+) mobilisation (upstream mechanism). These agents are associated with the risk of Ca(2+) overload leading to arrhythmias, myocardial cell injury and premature cell death. In addition, they are energetically disadvantageous because of an increase in activation energy and metabolic effects. Cardiac glycosides act also via an upstream mechanism and readily elicit Ca(2+) overload with a narrow safety margin. No currently available agents act primarily via an increase in the myofilament sensitivity to Ca(2+) ions (central and/or downstream mechanisms). Novel Ca(2+) sensitisers under basic research may deserve clinical trials to examine the therapeutic potential to replace currently employed agents in acute and chronic heart failure patients. Molecular mechanisms of action of Ca(2+) sensitisers are divergent. In addition, they show a wide range of discrete pharmacological profiles due to additional actions associated with individual compounds. Therefore, the outcome of clinical trials has to be explained carefully based on these mechanisms of actions.

The Special Structure and Function of Troponin I in Regulation of Cardiac Contraction and Relaxation

In this chapter I review evidence for a pivotal role of the sarcomeric thin filament protein, troponin I, in cardiac muscle activation and its modulation by covalent modifications, sarcomere length, and intracellular pH. This evidence demonstrates that the cardiac variant of troponin I (cTnI), which is the only isoform expressed in the adult myocardium, has unique structure and function that are specialized for extrinsic and intrinsic control of cardiac contraction and relaxation.

Single mutation (A162H) in human cardiac troponin I corrects acid pH sensitivity of Ca2+-regulated actomyosin S1 ATPase

In contrast to skeletal muscle, the efficiency of the contractile apparatus of cardiac tissue has long been known to be severely compromised by acid pH as in the ischemia of myocardial infarction and other cardiac myopathies. Recent reports (Westfall, M. V., and Metzger, J. M. (2001) News Physiol. Sci. 16, 278-281; Li, G., Martin, A. F., and Solaro, R. J. (2001) J. Mol. Cell. Cardiol. 33, 1309-1320) have indicated that the reduced Ca(2+) sensitivity of cardiac contractility at low pH (<or=pH 6.5) is attributable to structural difference(s) in the cardiac and skeletal inhibitory components (TnIs) of their troponins. Here, using a reconstituted Ca(2+)-regulated human cardiac troponin-tropomyosin actomyosin S1 ATPase assay, we report that a single TnI mutation, A162H, restores Ca(2+) sensitivity at pH 6.5 to that at pH 7.0. Levels of inhibition (pCa 7.0), activation (pCa 4.0), and cooperativity of ATPase activity were minimally affected. Two other mutations (Q155R and E164V) also previously suggested by us (Pearlstone, J. R., Sykes, B. D., and Smillie, L. B. (1997) Biochemistry 36, 7601-7606) and involving charged residues showed no such effects. With fast skeletal muscle troponin, a single TnI H130A mutation reduced Ca(2+) sensitivity at pH 6.5 to levels approaching the cardiac system at pH 6.5. These observations provide structural insight into long-standing physiological and clinical phenomena and are of potential relevance to therapeutic treatments of heart disease by gene transfer, stem cell, and cell transplantation approaches.

Mechanisms of action of novel cardiotonic agents

Regulation of myocardial contractility by cardiotonic agents is achieved by an increase in intracellular Ca2+ mobilization (upstream mechanism), an increase in Ca2+ binding affinity to troponin C (central mechanism), or facilitation of the process subsequent to Ca2+ binding to troponin C (downstream mechanism). cAMP mediates the regulation induced by Ca2+ mobilizers such as beta-adrenoceptor agonists and selective phosphodiesterase III inhibitors acting through the upstream mechanism. These agents act likewise on the central mechanism to decrease Ca2+ sensitivity of troponin C in association with the cAMP-mediated phosphorylation of troponin I. In addition to such a well-known action of cAMP, recent experimental findings have revealed that Ca2+ sensitizers, such as levosimendan, OR-1896, and UD-CG 212 Cl, require the cAMP-mediated signaling for induction of Ca2+ sensitizing effect. These agents shift the [Ca2+] -force relationship to the left, but their positive inotropic effect (PIE) is inhibited by carbachol, which suppresses selectively the cAMP-mediated PIE. These findings imply that cAMP may play a crucial role in increasing the myofilament Ca2+ sensitivity by cross-talk with the action of individual cardiotonic agents. No clinically available cardiotonic agents act primarily via Ca2+ sensitization, but the PIE of pimobendan and levosimendan is partly mediated by an increase in myofilament Ca2+ sensitivity. Evidence is accumulating that cardiotonic agents with Ca2+ sensitizing action are more effective than agents that act purely via the upstream mechanism in clinical settings. Further clinical trials are required to establish the effectiveness of Ca2+ sensitizers in long-term therapy for congestive heart failure patients.

p38 Mitogen-activated protein kinase mediates a negative inotropic effect in cardiac myocytes

p38 Mitogen-activated protein kinase (MAPK) is one of the most ancient signaling molecules and is involved in multiple cellular processes, including cell proliferation, cell growth, and cell death. In the heart, enhanced activation of p38 MAPK is associated with ischemia/reperfusion injury and the onset of heart failure. In the present study, we investigated the function of p38 MAPK in regulating cardiac contractility and its underlying mechanisms. In cultured adult rat cardiomyocytes, activation of p38 MAPK by adenoviral gene transfer of an activated mutant of its upstream kinase, MKK3bE, led to a significant reduction in baseline contractility, compared with uninfected cells or those infected with a control adenoviral vector (Adv-beta-galactosidase). The inhibitory effect of MKK3bE on contractility was largely prevented by coexpressing a dominant-negative mutant of p38 MAPK or treating cells with a p38 MAPK inhibitor, SB203580. Conversely, inhibition of endogenous p38 MAPK activity by SB203580 rapidly and reversibly enhanced cell contractility in a dose-dependent manner, without altering L-type Ca(2+) currents or Ca(2+)(i) transients. MKK3bE-induced p38 activation had no significant effect on pH(i), whereas SB203580 had a minor effect to elevate pH(i). Furthermore, activation of p38 MAPK was unable to increase troponin I phosphorylation. Thus, we conclude that the negative inotropic effect of p38 MAPK is mediated by decreasing myofilament response to Ca(2+), rather than by altering Ca(2+)(i) homeostasis and that the reduced myofilament Ca(2+) sensitivity is unlikely attributable to troponin I phosphorylation or alterations in pH(i). These findings reveal a novel function of p38 MAPK and shed a new light on our understanding of the coincidence of p38 MAPK activation and the onset of heart failure.

Expression of slow skeletal troponin I in adult transgenic mouse heart muscle reduces the force decline observed during acidic conditions

1. Acidosis in cardiac muscle is associated with a decrease in developed force. We hypothesized that slow skeletal troponin I (ssTnI), which is expressed in neonatal hearts, is responsible for the observed decreased response to acidic conditions. To test this hypothesis directly, we used adult transgenic (TG) mice that express ssTnI in the heart. Cardiac TnI (cTnI) was completely replaced by ssTnI either with a FLAG epitope introduced into the N-terminus (TG-ssTnI) or without the epitope (TG-ssTnI) in these mice. TG mice that express cTnI were also generated as a control TG line (TG-cTnI). Non-transgenic (NTG) littermates were used as controls. 2. We measured the force-calcium relationship in all four groups at pH 7.0 and pH 6.5 in detergent-extracted fibre bundles prepared from left ventricular papillary muscles. The force-calcium relationship was identical in fibre bundles from NTG and TG-cTnI mouse hearts, therefore NTG mice served as controls for TG-ssTnIand TG-ssTnI mice. Compared to NTG controls, the force generated by fibre bundles from TG mice expressing ssTnI was more sensitive to Ca(2+). The shift in EC(50) (the concentration of Ca(2+) at which half-maximal force is generated) caused by acidic pH was significantly smaller in fibre bundles isolated from TG hearts compared to those from NTG hearts. However, there was no difference in the force-calcium relationship between hearts from the TG-ssTnIand TG-ssTnI groups. 3. We also isolated papillary muscles from the right ventricle of NTG and TG mouse hearts expressing ssTnI and measured isometric force at extracellular pH 7.33 and pH 6.75. At acidic pH, after an initial decline, twitch force recovered to 60 +/- 3 % (n = 7) in NTG papillary muscles, 98 +/- 2 % (n = 5) in muscles from TG-ssTnIand 96 +/- 3 % (n = 7) in muscles from TG-ssTnI hearts. Our results indicate that TnI isoform composition plays a crucial role in the determination of myocardial force sensitivity to acidosis.