Rat cardiac contractile dysfunction induced by Ca2+ overload: possible link to the proteolysis of alpha-fodrin

Resolving an inconsistency in the estimation of the energy for excitation of cardiac muscle contraction

In the excitation of muscle contraction, calcium ions interact with transmembrane transporters. This process is accompanied by energy consumption and heat liberation. To quantify this activation energy or heat in the heart or cardiac muscle, two non-pharmacological approaches can be used. In one approach using the "pressure-volume area" concept, the same estimate of activation energy is obtained regardless of the mode of contraction (either isovolumic/isometric or ejecting/shortening). In the other approach, an accurate estimate of activation energy is obtained only when the muscle contracts isometrically. If the contraction involves muscle shortening, then an additional component of heat associated with shortening is liberated, over and above that of activation. The present study thus examines the reconcilability of the two approaches by performing experiments on isolated muscles measuring contractile force and heat output. A framework was devised from the experimental data to allow us to replicate several mechanoenergetics results gleaned from the literature. From these replications, we conclude that the choice of initial muscle length (or ventricular volume) underlies the divergence of the two approaches in the estimation of activation energy when the mode of contraction involves shortening (ejection). At low initial muscle lengths, the heat of shortening is relatively small, which can lead to the misconception that activation energy is contraction mode independent. In fact, because cardiac muscle liberates heat of shortening when allowed to shorten, estimation of activation heat must be performed only under isometric (isovolumic) contractions. We thus recommend caution when estimating activation energy using the "pressure-volume area" concept.

Neutrophil-Mediated Cardiac Damage After Acute Myocardial Infarction: Significance of Defining a New Target Cell Type for Developing Cardioprotective Drugs

Significance: Acute myocardial infarction (AMI) is a leading cause of death worldwide. Post-AMI survival rates have increased with the introduction of angioplasty as a primary coronary intervention. However, reperfusion after angioplasty represents a clinical paradox, restoring blood flow to the ischemic myocardium while simultaneously inducing ion and metabolic imbalances that stimulate immune cell recruitment and activation, mitochondrial dysfunction and damaging oxidant production. Recent Advances: Preclinical data indicate that these metabolic imbalances contribute to subsequent heart failure through sustaining local recruitment of inflammatory leukocytes and oxidative stress, cardiomyocyte death, and coronary microvascular disturbances, which enhance adverse cardiac remodeling. Both left ventricular dysfunction and heart failure are strongly linked to inflammation and immune cell recruitment to the damaged myocardium. Critical Issues: Overall, therapeutic anti-inflammatory and antioxidant agents identified in preclinical trials have failed in clinical trials. Future Directions: The versatile neutrophil-derived heme enzyme, myeloperoxidase (MPO), is gaining attention as an important oxidative mediator of reperfusion injury, vascular dysfunction, adverse ventricular remodeling, and atrial fibrillation. Accordingly, there is interest in therapeutically targeting neutrophils and MPO activity in the setting of heart failure. Herein, we discuss the role of post-AMI inflammation linked to myocardial damage and heart failure, describe previous trials targeting inflammation and oxidative stress post-AMI, highlight the potential adverse impact of neutrophil and MPO, and detail therapeutic options available to target MPO clinically in AMI patients.

Mechanism underlying the negative inotropic effect in rat left ventricle in hyperthermia: the role of TRPV1

We have previously reported that the negative inotropic effects of hyperthermia (42 °C) on left ventricular (LV) mechanoenergetics using the excised, cross-circulated rat heart model. Here, we investigated the role of TRPV1 on LV mechanoenergetics in hyperthermia. We analyzed the LV end-systolic pressure–volume relation (ESPVR) and the linear relation between the myocardial oxygen consumption per beat (VO₂) and the systolic pressure–volume area (PVA; a total mechanical energy per beat) during infusion of capsazepine (CPZ) in hyperthermia, or capsaicin (Cap) under 300 bpm pacing. LV ESP decreased in each LV volume and the resultant downward-shift of LV ESPVR was suppressed by CPZ infusion in hyperthermia-hearts. In Cap-treated hearts, LV ESPVR shifted downward from the control ESPVR, similar to hyperthermia-hearts. The slopes of VO₂–PVA relationship were unchanged. The VO₂ intercepts in hyperthermia-hearts did not decrease because of decreased E–C coupling VO₂, and inversely increased basal metabolic VO₂, which was suppressed by CPZ, though the VO₂ intercepts in Cap-treated hearts significantly decreased. The levels of phosphorylated phospholamban at serine 16 decreased significantly in hyperthermia-hearts, as well as Cap-treated hearts. These results indicate that a Cap-induced decrease in the LV contractility, like in cases of hyperthermia, are due to the down-regulation of the total calcium handling in E–C coupling, suggesting that negative inotropic effect in hyperthermia-heart is, at least in part, mediated through TRPV1 signaling pathway.

Heavy Water (D2O) Containing Preservation Solution Reduces Hepatic Cold Preservation and Reperfusion Injury in an Isolated Perfused Rat Liver (IPRL) Model

Background: Heavy water (D₂O) has many biological effects due to the isotope effect of deuterium. We previously reported the efficacy of D₂O containing solution (Dsol) in the cold preservation of rat hearts. Here, we evaluated whether Dsol reduced hepatic cold preservation and reperfusion injury. Methods: Rat livers were subjected to 48-hour cold storage in University of Wisconsin (UW) solution or Dsol, and subsequently reperfused on an isolated perfused rat liver. Graft function, injury, perfusion kinetics, oxidative stress, and cytoskeletal integrity were assessed. Results: In the UW group, severe ischemia and reperfusion injury (IRI) was shown by histopathology, higher liver enzymes leakage, portal resistance, and apoptotic index, oxygen consumption, less bile production, energy charge, and reduced glutathione (GSH)/oxidized glutathione (GSSG) ratio (versus control). The Dsol group showed that these injuries were significantly ameliorated (versus the UW group). Furthermore, cytoskeletal derangement was progressed in the UW group, as shown by less degradation of α-Fodrin and by the inactivation of the actin depolymerization pathway, whereas these changes were significantly suppressed in the Dsol group. Conclusion: Dsol reduced hepatic IRI after extended cold preservation and subsequent reperfusion. The protection was primarily due to the maintenance of mitochondrial function, cytoskeletal integrity, leading to limiting oxidative stress, apoptosis, and necrosis pathways.

Left ventricular mechanoenergetics in excised, cross-circulated rat hearts under hypo-, normo-, and hyperthermic conditions

We investigated the effects of altering cardiac temperature on left ventricular (LV) myocardial mechanical work and energetics using the excised, cross-circulated rat heart model. We analyzed the LV end-systolic pressure-volume relationship (ESPVR) and linear relationship between myocardial oxygen consumption per beat (VO2) and systolic pressure-volume area (PVA; total mechanical energy per beat) in isovolumically contracting rat hearts during hypo- (32 °C), normo- (37 °C), and hyperthermia (42 °C) under a 300-beats per minute pacing. LV ESPVR shifted downward with increasing cardiac temperature. The VO2-PVA relationship was superimposable in these different thermal conditions; however, each data point of VO2-PVA shifted left-downward during increasing cardiac temperature on the superimposable VO2-PVA relationship line. VO2 for Ca2+ handling in excitation-contraction coupling decreased, which was associated with increasing cardiac temperature, during which sarcoplasmic reticulum Ca2+-ATPase (SERCA) activity was suppressed, due to phospholamban phosphorylation inhibition, and instead, O2 consumption for basal metabolism was increased. The O2 cost of LV contractility for Ca2+ also increased with increasing cardiac temperature. Logistic time constants evaluating LV relaxation time were significantly shortened with increasing cardiac temperature related to the acceleration of the detachment in cross-bridge (CB) cycling, indicating increased myosin ATPase activity. The results suggested that increasing cardiac temperature induced a negative inotropic action related to SERCA activity suppression in Ca2+ handling and increased myosin ATPase activity in CB cycling. We concluded that thermal intervention could modulate cardiac inotropism by changing CB cycling, Ca2+ handling, and basal metabolism in rat hearts.

Early dystrophin loss is coincident with the transition of compensated cardiac hypertrophy to heart failure

Hypertension causes cardiac hypertrophy, one of the most important risk factors for heart failure (HF). Despite the importance of cardiac hypertrophy as a risk factor for the development of HF, not all hypertrophied hearts will ultimately fail. Alterations of cytoskeletal and sarcolemma-associated proteins are considered markers cardiac remodeling during HF. Dystrophin provides mechanical stability to the plasma membrane through its interactions with the actin cytoskeleton and, indirectly, to extracellular matrix proteins. This study was undertaken to evaluate dystrophin and calpain-1 in the transition from compensated cardiac hypertrophy to HF. Wistar rats were subjected to abdominal aorta constriction and killed at 30, 60 and 90 days post surgery (dps). Cardiac function and blood pressure were evaluated. The hearts were collected and Western blotting and immunofluorescence performed for dystrophin, calpain-1, alpha-fodrin and calpastatin. Statistical analyses were performed and considered significant when p<0.05. After 90 dps, 70% of the animals showed hypertrophic hearts (HH) and 30% hypertrophic+dilated hearts (HD). Systolic and diastolic functions were preserved at 30 and 60 dps, however, decreased in the HD group. Blood pressure, cardiomyocyte diameter and collagen content were increased at all time points. Dystrophin expression was lightly increased at 30 and 60 dps and HH group. HD group showed decreased expression of dystrophin and calpastatin and increased expression of calpain-1 and alpha-fodrin fragments. The first signals of dystrophin reduction were observed as early as 60 dps. In conclusion, some hearts present a distinct molecular pattern at an early stage of the disease; this pattern could provide an opportunity to identify these failure-prone hearts during the development of the cardiac disease. We showed that decreased expression of dystrophin and increased expression of calpains are coincident and could work as possible therapeutic targets to prevent heart failure as a consequence of cardiac hypertrophy.

Glutamate protects against Ca(2+) paradox-induced injury and inhibits calpain activity in isolated rat hearts

This study determined the effects of glutamate on the Ca(2+) paradoxical heart, which is a model for Ca(2+) overload-induced injury during myocardial ischaemia and reperfusion, and evaluated its effect on a known mediator of injury, calpain. An isolated rat heart was retrogradely perfused in a Langendorff apparatus. Ca(2+) paradox was elicited via perfusion with a Ca(2+) -free Krebs-Henseleit (KH) solution for 3 minutes followed by Ca(2+) -containing normal KH solution for 30 minutes. The Ca(2+) paradoxical heart exhibited almost no viable tissue on triphenyltetrazolium chloride staining and markedly increased LDH release, caspase-3 activity, cytosolic cytochrome c content, and apoptotic index. These hearts also displayed significantly increased LVEDP and a disappearance of LVDP. Glutamate (5 and 20 mmol/L) significantly alleviated Ca(2+) paradox-induced injury. In contrast, 20 mmol/L mannitol had no effect on Ca(2+) paradox. Ca(2+) paradox significantly increased the extent of the translocation of μ-calpain to the sarcolemmal membrane and the proteolysis of α-fodrin, which suggests calpain activation. Glutamate also blocked these effects. A non-selective inhibitor of glutamate transporters, dl-TBOA (10 μmol/L), had no effect on control hearts, but it reversed glutamate-induced cardioprotection and reduction in calpain activity. Glutamate treatment significantly increased intracellular glutamate content in the Ca(2+) paradoxical heart, which was also blocked by dl-TBOA. We conclude that glutamate protects the heart against Ca(2+) overload-induced injury via glutamate transporters, and the inhibition of calpain activity is involved in this process.

Role of proteases in the pathophysiology of cardiac disease

Cardiovascular disease is a major cause of death and thus a great deal of effort has been made in salvaging the diseased myocardium. Although various factors have been identified as possible causes of different cardiac diseases such as heart failure and ischemic heart disease, there is a real need to elucidate their role for the better understanding of the cardiac disease pathology and formulation of strategies for developing newer therapeutic interventions. In view of the intimate involvement of different types of proteases in maintaining cellular structure, the role of proteases in various cardiac diseases has become the focus of recent research. Proteases are present in the cytosol as well as are localized in a number of subcellular organelles in the cell. These are known to use extracellular matrix, cytoskeletal, sarcolemmal, sarcoplasmic reticular, mitochondrial and myofibrillar proteins as substrates. Work from different laboratories using a wide variety of techniques has shown that the activation of proteases causes alterations of a number of specific proteins leading to subcellular remodeling and cardiac dysfunction. Inhibition of protease action by different drugs and agents, therefore, has a clinical relevance and is expected to form a part of new treatment paradigm for improving heart function. This review examines the biochemistry and localization of some of the proteases in the cardiac tissue in addition to identification of the sites of action of some protease inhibitors. (Mol Cell Biochem 263: 241-256, 2004).

Calpain-dependent cleavage of N-cadherin is involved in the progression of post-myocardial infarction remodeling

Enzymatic proteolysis by calpains, Ca(2+)-dependent intracellular cysteine proteases, has been implicated in pathological processes such as cellular degeneration or death. Here, we investigated the role of calpain activation in the hearts subjected to myocardial infarction. We produced myocardial infarction in Cast(-/-) mice deficient for calpastatin, the specific endogenous inhibitory protein for calpains, and Cast(+/+) mice. The activity of cardiac calpains in Cast(+/+) mice was not elevated within 1 day but showed a gradual elevation after 7 days following myocardial infarction, which was further pronounced in Cast(-/-) mice. Although the prevalence of cardiomyocyte death was indistinguishable between Cast(-/-) and Cast(+/+) mice, Cast(-/-) mice exhibited profound contractile dysfunction and chamber dilatation and showed a significant reduction in survival rate after myocardial infarction as compared with Cast(+/+) mice. Notably, immunofluorescence revealed that at 28 days after myocardial infarction, calpains were activated in cardiomyocytes exclusively at the border zone and that Cast(-/-) mice showed higher intensity and a broader extent of calpain activation at the border zone than Cast(+/+) mice. In the border zone of Cast(-/-) mice, pronounced activation of calpains was associated with a decrease in N-cadherin expression and up-regulation of molecular markers for cardiac hypertrophy and fibrosis. In cultured rat neonatal cardiomyocytes, calpain activation by treatment with ionomycin induced cleavage of N-cadherin and decreased expression levels of β-catenin and connexin 43, which was attenuated by calpain inhibitor. These results thus demonstrate that activation of calpains disassembles cell-cell adhesion at intercalated discs by degrading N-cadherin and thereby promotes left ventricular remodeling after myocardial infarction.

Carnitine modulates crucial myocardial adenosine triphosphatases and acetylcholinesterase enzyme activities in choline-deprived rats

Choline is an essential nutrient, and choline deficiency has been associated with cardiovascular morbidity. Choline is also the precursor of acetylcholine (cholinergic component of the heart's autonomic nervous system), whose levels are regulated by acetylcholinesterase (AChE). Cardiac contraction-relaxation cycles depend on ion gradients established by pumps like the adenosine triphosphatases (ATPases) Na(+)/K(+)-ATPase and Mg(2+)-ATPase. This study aimed to investigate the impact of dietary choline deprivation on the activity of rat myocardial AChE (cholinergic marker), Na(+)/K(+)-ATPase, and Mg(2+)-ATPase, and the possible effects of carnitine supplementation (carnitine, structurally relevant to choline, is used as an adjunct in treating cardiac diseases). Adult male albino Wistar rats were distributed among 4 groups, and were fed a standard or choline-deficient diet for one month with or without carnitine in their drinking water (0.15% w/v). The enzyme activities were determined spectrophotometrically in the myocardium homogenate. Choline deficiency seems to affect the activity of the aforementioned parameters, but only the combination of choline deprivation and carnitine supplementation increased myocardial Na(+)/K(+)-ATPase activity along with a concomitant decrease in the activities of Mg(2+)-ATPase and AChE. The results suggest that carnitine, in the setting of choline deficiency, modulates cholinergic myocardial neurotransmission and the ATPase activity in favour of cardiac work efficiency.

Chemerin suppresses ovarian follicular development and its potential involvement in follicular arrest in rats treated chronically with dihydrotestosterone

In the present study, we have investigated the cellular mechanisms of androgen-induced antral follicular growth arrest and the possible involvement of chemerin and its receptor chemokine-like receptor 1 (CMKLR1) in this process, using a chronically androgenized rat model. We hypothesize that hyperandrogenism induces antral follicle growth arrest via the action of chemerin and ovarian structural changes, resulting from granulosa cell and oocyte apoptosis and theca cell survival. Dihydrotestosterone (DHT) treatment resulted in increased expression of chemerin and CMKLR1 in antral follicles, absence of corpus luteum, and increased atypical follicles. Addition of chemerin to follicle cultures induced granulosa cell apoptosis and suppressed basal, FSH- and growth differentiation factor-9-stimulated follicular growth. DHT down-regulated aromatase expression and increased active caspase-3 content and DNA fragmentation in granulosa cells in vivo. These changes were accompanied by higher phosphatase and tensin homolog and lower phospho-Akt (Ser473) content in antral follicles and higher calpain expression and down-regulation of cytoskeletal proteins in atypical follicles, which were constituted predominantly of theca cells. DHT also activated granulosa cell caspase-3, decreased X-linked inhibitor of apoptosis protein, poly(ADP-ribose) polymerase, and phospho-Akt contents and induced apoptosis in vitro, responses readily attenuated by forced X-linked inhibitor of apoptosis protein expression. These findings are consistent with our hypothesis that antral follicular growth arrest in DHT-treated rats results from increased chemerin expression and action, as well as changes in follicular cell fate and structure, which are a consequence of dysregulated interactions of pro-survival and pro-apoptotic modulators in a cell-specific manner. Our observations suggest that this chronically androgenized rat model may be useful for studies on the long-term effects of androgens on folliculogenesis and may have implications for the female reproductive disorders associated with hyperandrogenism.

Calpain inhibition reduces amplitude and accelerates decay of the late sodium current in ventricular myocytes from dogs with chronic heart failure

Calpain is an intracellular Ca²⁺ -activated protease that is involved in numerous Ca²⁺ dependent regulation of protein function in many cell types. This paper tests a hypothesis that calpains are involved in Ca²⁺ -dependent increase of the late sodium current (I_NaL) in failing heart. Chronic heart failure (HF) was induced in 2 dogs by multiple coronary artery embolization. Using a conventional patch-clamp technique, the whole-cell I_NaL was recorded in enzymatically isolated ventricular cardiomyocytes (VCMs) in which I_NaL was activated by the presence of a higher (1μM) intracellular [Ca²⁺] in the patch pipette. Cell suspensions were exposed to a cell- permeant calpain inhibitor MDL-28170 for 1–2 h before I_NaL recordings. The numerical excitation-contraction coupling (ECC) model was used to evaluate electrophysiological effects of calpain inhibition in silico. MDL caused acceleration of I_NaL decay evaluated by the two-exponential fit (τ₁ = 42±3.0 ms τ₂ = 435±27 ms, n = 6, in MDL vs. τ₁ = 52±2.1 ms τ₂ = 605±26 control no vehicle, n = 11, and vs. τ₁ = 52±2.8 ms τ₂ = 583±37 ms n = 7, control with vehicle, P<0.05 ANOVA). MDL significantly reduced I_NaL density recorded at –30 mV (0.488±0.03, n = 12, in control no vehicle, 0.4502±0.0210, n = 9 in vehicle vs. 0.166±0.05pA/pF, n = 5, in MDL). Our measurements of current-voltage relationships demonstrated that the I_NaL density was decreased by MDL in a wide range of potentials, including that for the action potential plateau. At the same time the membrane potential dependency of the steady-state activation and inactivation remained unchanged in the MDL-treated VCMs. Our ECC model predicted that calpain inhibition greatly improves myocyte function by reducing the action potential duration and intracellular diastolic Ca²⁺ accumulation in the pulse train.

A new calpain inhibitor protects left ventricular dysfunction induced by mild ischemia-reperfusion in in situ rat hearts

We have previously indicated that a new soluble calpain inhibitor, SNJ-1945 (SNJ), attenuates cardiac dysfunction after cardioplegia arrest-reperfusion by inhibiting the proteolysis of α-fodrin in in vitro study. Nevertheless, the in vivo study design is indispensable to explore realistic therapeutic approaches for clinical use. The aim of the present in situ study was to investigate whether SNJ attenuated left ventricular (LV) dysfunction (stunning) after mild ischemic-reperfusion (mI-R) in rat hearts. SNJ (60 μmol/l, 5 ml i.p.) was injected 30 min before gradual and partial coronary occlusion at proximal left anterior descending artery. To investigate LV function, we obtained curvilinear end-systolic pressure-volume relationship by increasing afterload 60 min after reperfusion. In the mI-R group, specific LV functional indices at midrange LV volume (mLVV), end-systolic pressure (ESP(mLVV)), and pressure-volume area (PVA(mLVV): a total mechanical energy per beat, linearly related to oxygen consumption) significantly decreased, but SNJ reversed these decreases to time control level. Furthermore, SNJ prevented the α-fodrin degradation and attenuated degradation of Ca(2+) handling proteins after mI-R. Our results indicate that improvements in LV function following mI-R injury are associated with inhibition of the proteolysis of α-fodrin in in situ rat hearts. In conclusion, SNJ should be a promising tool to protect the heart from the stunning.

Activation of proteases and changes in Na+-K+-ATPase subunits in hearts subjected to ischemia-reperfusion

Previous studies have shown that ischemia-reperfusion (I/R) injury is associated with cardiac dysfunction and changes in sarcolemmal Na(+)-K(+)-ATPase subunits and activity. This study was undertaken to evaluate the role of proteases in these alterations by subjecting rat hearts to different times of global ischemia, as well as reperfusion after 45 min of ischemia. Decreases in Na(+)-K(+)-ATPase activity at 30-60 min of global ischemia were accompanied by augmented activities of both calpain and matrix metalloproteinases (MMPs) and depressed protein content of β(1)- and β(2)-subunits, without changes in α(1)- and α(2)-subunits of the enzyme. Compared with control values, the activities of both calpain and MMP-2 were increased, whereas the activity and protein content for all subunits of Na(+)-K(+)-ATPase were decreased upon reperfusion for 5-40 min, except that α(1)- and α(2)-subunit content was not depressed in 5 min I/R hearts. MDL28170, a calpain inhibitor, was more effective in attenuating the I/R-induced alterations in cardiac contracture, Na(+)-K(+)-ATPase activity, and α(2)-subunit than doxycycline, an MMP inhibitor. Incubation of control sarcolemma preparation with calpain, unlike MMP-2, depressed Na(+)-K(+)-ATPase activity and decreased α(1)-, α(2)-, and β(2)-subunits, without changes in the β(1)-subunit. These results support the view that activation of both calpain and MMP-2 are involved in depressing Na(+)-K(+)-ATPase activity and degradation of its subunits directly or indirectly in hearts subjected to I/R injury.

Myocardial twitch duration and the dependence of oxygen consumption on pressure-volume area: experiments and modelling

We tested the proposition that linear length dependence of twitch duration underlies the well-characterised linear dependence of oxygen consumption (V(O(2)) ) on pressure–volume area (PVA) in the heart. By way of experimental simplification, we reduced the problem from three dimensions to one by substituting cardiac trabeculae for the classically investigated whole-heart. This allowed adoption of stress–length area (SLA) as a surrogate for PVA, and heat as a proxy for V(O(2)) . Heat and stress (force per cross-sectional area), at a range of muscle lengths and at both 1 mM and 2 mM [Ca(2+)](o), were recorded from continuously superfused rat right-ventricular trabeculae undergoing fixed-end contractions. The heat–SLA relations of trabeculae (reported here, for the first time) are linear. Twitch duration increases monotonically (but not strictly linearly) with muscle length. We probed the cellular mechanisms of this phenomenon by determining: (i) the length dependence of the duration of the Ca(2+) transient, (ii) the length dependence of the rate of force redevelopment following a length impulse (an index of Ca(2+) binding to troponin-C), (iii) the effect on the simulated time course of the twitch of progressive deletion of length and Ca(2+)-dependent mechanisms of crossbridge cooperativity, using a detailed mathematical model of the crossbridge cycle, and (iv) the conditions required to achieve these multiple length dependencies, using a greatly simplified model of twitch mechano-energetics. From the results of these four independent investigations, we infer that the linearity of the heat–SLA relation (and, by analogy, the V(O(2))–PVA relation) is remarkably robust in the face of departures from linearity of length-dependent twitch duration.

Cardiac mechanoenergetics for understanding isoproterenol-induced rat heart failure

Considering from clinical implication, it is often complained that short-term experimental diseased heart models do not mimic long-term diseased hearts that one often clinically encountered. The left ventricle (LV) function in rat cardiac hypertrophy models treated with isoproterenol (ISO) up to 16 weeks was followed up with a non-invasive echocardiography. Infusion of either ISO (1.2mgkg(-1)day(-1) for 3 days-16 weeks) or vehicle (saline 24μlday(-1) for 3 days-16 weeks; SA group) was performed by subcutaneously implanting osmotic minipump. LV and right ventricle (RV) weight ratios to body weight (mgg(-1)) in SA, ISO3d, ISO7d and ISO4w were: 1.94±0.10 and 0.54±0.04 (n=7), 2.56±0.10 and 0.66±0.05 (n=7), 2.50±0.25 and 0.64±0.07 (n=10) and 2.40±0.08 and 0.59±0.08 (n=9), respectively. From echocardiography, the LV function of the hypertrophy models at 3 days, 1 and 2 weeks was unchanged but the model at the longer-term than 4 weeks resulted in prolonged systolic failure. These results indicated that only 3-day ISO infusion induced the hypertrophy model similar in shape and function to that induced by 2-week ISO infusion; the 3-day model sufficiently represents the effects of 2-week ISO infusion. In this review, left ventricular (LV) function was compared between rat cardiac hypertrophy models treated with ISO for 3 days (ISO3d) and 7 days (ISO7d) by analyzing LV mechanical work and energetics. The LV mechanical work and energetics was unchanged in SA, ISO3d and ISO7d groups. The LV relaxation rate at 240bpm in ISO3d and ISO7d groups was significantly slower than that in SA group with unchanged contraction rate. The amounts of expression of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a), phospholamban (PLB), phosphorylated-Ser(16) PLB (p-PLB), phospholemman (PLM) and Na(+)-K(+)-ATPase (NKA) are significantly decreased in ISO3d and ISO7d groups. Furthermore, the marked collagen production (types I and III) was observed in ISO3d and ISO7d groups. These results suggested the possibility that physiological LV function is compensated, although molecular changes have been generated even in the short-term hypertrophy model. Although a novel histone deacetylase (HDAC) inhibitor, has some beneficial effects on hemodynamics, it has no effects of anti-hypertrophic modalities in ISO3d model. However, a selective sodium proton exchanger-1 (NHE-1) inhibitor normalized ISO-induced down-regulation of SERCA2a without changes in pPLB/PLB expression in the ISO7d model and ameliorates cardiac Ca(2+) handling impairment and prevents the development of cardiac dysfunction. This result indicated that SERCA2a is a key molecule in the ISO7d model. Slow LV relaxation rate in ISO7d model may be due to down-regulation of SERCA2a. In conclusion, lowering the heart rate make it possible to rescue the impairment of LV mechanical work and energetics in the ISO-induced compensatory hypertrophied rat hearts, providing basic evidence for clinical therapy for patients with some types of cardiac failure.

Calpain inhibition preserves talin and attenuates right heart failure in acute pulmonary hypertension

Right heart failure from right ventricular (RV) pressure overload is a major cause of morbidity and mortality, but its mechanism is incompletely understood. We tested the hypothesis that right heart failure during 4 hours of RV pressure overload is associated with alterations of the focal adhesion protein talin, and that the inhibition of calpain attenuates RV dysfunction and preserves RV talin. Anesthetized open-chest pigs treated with the calpain inhibitor MDL-28170 (n = 20) or inactive vehicle (n = 23) underwent 4 hours of RV pressure overload by pulmonary artery constriction (initial RV systolic pressure, 64 ± 1 and 66 ± 1 mm Hg in MDL-28170 and vehicle-treated pigs, respectively). Progressive RV contractile dysfunction was attenuated by MDL-28170: after 4 hours of RV pressure overload, RV systolic pressure was 44 ± 4 mm Hg versus 49 ± 6 mm Hg (P = 0.011), and RV stroke work was 72 ± 5% of baseline versus 90 ± 5% of baseline, (P = 0.027), in vehicle-treated versus MDL-28170-treated pigs, respectively. MDL-28170 reduced the incidence of hemodynamic instability (death or systolic blood pressure of < 85 mm Hg) by 46% (P = 0.013). RV pressure overload disrupted talin organization. MDL-28170 preserved talin abundance in the RV free wall (P = 0.039), and talin abundance correlated with the maintenance of RV free wall stroke work (r = 0.58, P = 0.0039). α-actinin and vinculin showed similar changes according to immunohistology. Right heart failure from acute RV pressure overload is associated with reduced talin abundance and disrupted talin organization. Calpain inhibition preserves the abundance and organization of talin and RV function. Calpain inhibition may offer clinical utility in treating acute cor pulmonale.

Evaluation of left ventricular mechanical work and energetics of normal hearts in SERCA2a transgenic rats

Cardiac sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA2a) is responsible for most of the Ca(2+) removal during diastole and a larger Ca(2+) handling energy consumer in excitation-contraction (E-C) coupling. To understand the cardiac performance under long-term SERCA2a overexpression conditions, we established SERCA2a transgenic (TG) Wistar rats to analyze cardiac mechanical work and energetics in normal hearts during pacing at 300 beats/min. SERCA2a protein expression was increased in TGI and TGII rats (F2 and F3 of the same father and different mothers). Mean left ventricular (LV) end-systolic pressure (ESP) and systolic pressure-volume area (PVA; a total mechanical energy per beat) at midrange LV volume (mLVV) were significantly larger in TGI rats and were unchanged in TGII rats, compared to those in non-TG [wildtype (WT)] littermates. Mean myocardial oxygen consumption per minute for E-C coupling was significantly increased, and the mean slope of myocardial oxygen consumption per beat (VO(2))-PVA (systolic PVA) linear relation was smaller, but the overall O(2) cost of LV contractility for Ca(2+) is unchanged in all TG rats. Mean Ca(2+) concentration exerting maximal ESP(mLVV) in TGII rats was significantly higher than that in WT rats. The Ca(2+) overloading protocol did not elicit mitochondrial swelling in TGII rats. Tolerance to higher Ca(2+) concentrations may support the possibility for enhanced SERCA2a activity in TGII rats. In conclusion, long-term SERCA2a overexpression enhanced or maintained LV mechanics, improved contractile efficiency under higher energy expenditure for Ca(2+) handling, and improved Ca(2+) tolerance, but it did not change the overall O(2) cost of LV contractility for Ca(2+) in normal hearts of TG rats.

Calcium-mediated cell death during myocardial reperfusion

Reperfusion may induce additional cell death in patients with acute myocardial infarction receiving primary angioplasty or thrombolysis. Altered intracellular Ca(2+) handling was initially considered an essential mechanism of reperfusion-induced cardiomyocyte death. However, more recent studies have demonstrated the importance of Ca(2+)-independent mechanisms that converge on mitochondrial permeability transition (MPT) and are shared by cardiomyocytes and other cell types. This article analyses the importance of Ca(2+)-dependent cell death in light of these new observations. Altered Ca(2+) handling includes increased cytosolic Ca(2+) levels, leading to activation of calpain-mediated proteolysis and sarcoplasmic reticulum-driven oscillations; this can induce hypercontracture, but also MPT due to the privileged Ca(2+) transfer between sarcoplasmic reticulum and mitochondria through cytosolic Ca(2+) microdomains. In the opposite direction, permeability transition can worsen altered Ca(2+) handling and favour hypercontracture. Ca(2+) appears to play an important role in cell death during the initial minutes of reperfusion, particularly after brief periods of ischaemia. Developing effective and safe treatments to prevent Ca(2+)-mediated cardiomyocyte death in patients with transient ischaemia, by targeting Ca(2+) influx, intracellular Ca(2+) handling, or Ca(2+)-induced cell death effectors, is an unmet challenge with important therapeutic implications and large potential clinical impact.

Extracellular and intracellular proteases in cardiac dysfunction due to ischemia-reperfusion injury

Various procedures such as angioplasty, thrombolytic therapy, coronary bypass surgery, and cardiac transplantation are invariably associated with ischemia-reperfusion (I/R) injury. Impaired recovery of cardiac function due to I/R injury is considered to be a consequence of the occurrence of both oxidative stress and intracellular Ca(2+)-overload in the myocardium. These changes in the ischemic myocardium appear to activate both extracellular and intracellular proteases which are responsible for the cleavage of extracellular matrix and subcellular structures involved in the maintenance of cardiac function. It is thus intended to discuss the actions of I/R injury on several proteases, with a focus on calpain, matrix metalloproteinases, and cathepsins as well as their role in inducing alterations both inside and outside the cardiomyocytes. In addition, modifications of subcellular organelles such as myofibrils, sarcoplasmic reticulum and sarcolemma as well as extracellular matrix, and the potential regulatory effects of endogenous inhibitors on protease activities are identified. Both extracellular and intracellular proteolytic activities appear to be imperative in determining the true extent of I/R injury and their inhibition seems to be of critical importance for improving the recovery of cardiac function. Thus, both extracellular and intracellular proteases may serve as potential targets for the development of cardioprotective interventions for reducing damage to the heart and retarding the development of contractile dysfunction caused by I/R injury.

The cellular and molecular origin of reactive oxygen species generation during myocardial ischemia and reperfusion

Myocardial ischemia-reperfusion injury is an important cause of impaired heart function in the early postoperative period subsequent to cardiac surgery. Reactive oxygen species (ROS) generation increases during both ischemia and reperfusion and it plays a central role in the pathophysiology of intraoperative myocardial injury. Unfortunately, the cellular source of these ROS during ischemia and reperfusion is often poorly defined. Similarly, individual ROS members tend to be grouped together as free radicals with a uniform reactivity towards biomolecules and with deleterious effects collectively ascribed under the vague umbrella of oxidative stress. This review aims to clarify the identity, origin, and progression of ROS during myocardial ischemia and reperfusion. Additionally, this review aims to describe the biochemical reactions and cellular processes that are initiated by specific ROS that work in concert to ultimately yield the clinical manifestations of myocardial ischemia-reperfusion. Lastly, this review provides an overview of several key cardioprotective strategies that target myocardial ischemia-reperfusion injury from the perspective of ROS generation. This overview is illustrated with example clinical studies that have attempted to translate these strategies to reduce the severity of ischemia-reperfusion injury during coronary artery bypass grafting surgery.

Calpain protects the heart from hemodynamic stress

Calpains make up a family of Ca(2+)-dependent intracellular cysteine proteases that include ubiquitously expressed μ- and m-calpains. Both are heterodimers consisting of a distinct large catalytic subunit (calpain 1 for μ-calpain and calpain 2 for m-calpain) and a common regulatory subunit (calpain 4). The physiological roles of calpain remain unclear in the organs, including the heart, but it has been suggested that calpain is activated by Ca(2+) overload in diseased hearts, resulting in cardiac dysfunction. In this study, cardiac-specific calpain 4-deficient mice were generated to elucidate the role of calpain in the heart in response to hemodynamic stress. Cardiac-specific deletion of calpain 4 resulted in decreased protein levels of calpains 1 and 2 and showed no cardiac phenotypes under base-line conditions but caused left ventricle dilatation, contractile dysfunction, and heart failure with interstitial fibrosis 1 week after pressure overload. Pressure-overloaded calpain 4-deficient hearts took up a membrane-impermeant dye, Evans blue, indicating plasma membrane disruption. Membrane repair assays using a two-photon laser-scanning microscope revealed that calpain 4-deficient cardiomyocytes failed to reseal a plasma membrane that had been disrupted by laser irradiation. Thus, the data indicate that calpain protects the heart from hemodynamic stresses, such as pressure overload.

Lack of beta3 integrin signaling contributes to calpain-mediated myocardial cell loss in pressure-overloaded myocardium

Although cardiac hypertrophy initially ensues as a compensatory mechanism, it often culminates in congestive heart failure. Based on our earlier studies that calpain and beta3 integrin play cell death and survival roles, respectively, during pressure-overload (PO) hypertrophy, we investigated if the loss of beta3 integrin signaling is a potential mechanism for calpain-mediated cardiomyocyte death during PO. beta3 Integrin knockout (beta3) and wild-type mice were used to induce either moderate or severe PO in vivo for short-term (72-hour) and long-term (4-week) transverse aortic constriction. Whereas wild-type mice showed no changes during moderate PO at both time points, beta3 mice exhibited both enrichment of the mu-calpain isoform and programmed cell death of cardiomyocytes after 4-week PO. However, with severe PO that caused increased mortality in both mice groups, cell death was observed in wild-type mice also. To study calpain's role, calpeptin, a specific inhibitor of calpain, was administered through an osmotic mini-pump at 2.5 mg/kg per day beginning 3 days before moderate transverse aortic constriction or sham surgery. Calpeptin administration blocked both calpain enrichment and myocardial cell death in the 4-week PO beta3 mice. Because beta3 integrin contributes to cardioprotective signaling, these studies indicate that the loss of specific integrin function could be a key mechanism for calpain-mediated programmed cell death of cardiomyocytes in PO myocardium.

A cardioprotective agent of a novel calpain inhibitor, SNJ-1945, exerts beta1 actions on left ventricular mechanical work and energetics

We have previously shown that a newly developed calpain inhibitor, SNJ-1945 (SNJ), with good aqueous solubility prevents the heart from KCl arrest-reperfusion injury associated with the impairment of total Ca(2+) handling by inhibiting the proteolysis of alpha-fodrin as a cardioplegia. The aim of the present study was to investigate certain actions of this calpain inhibitor, SNJ, on left ventricular (LV) mechanical work and energetics in cross-circulated excised rat hearts undergoing blood perfusion with 40 microM SNJ. Mean end-systolic pressure at midrange LV volume and systolic pressure-volume area (PVA) at mLVV (a total mechanical energy/beat) were significantly increased by SNJ perfusion (P < 0.01). Mean myocardial oxygen consumption per beat (Vo(2)) intercepts (Vo(2) for the total Ca(2+) handling in excitation-contraction coupling and basal metabolism) of Vo(2)-PVA linear relations were significantly increased (P < 0.01) with unchanged mean slopes of Vo(2)-PVA linear relations. Pretreatment with the selective beta(1)-blocker landiolol (10 microM) blocked these effects of SNJ perfusion. There were no significant differences in mean basal metabolic oxygen consumption among normal, 40 microM SNJ, and 10 microM landiolol + 40 microM SNJ groups. Our results indicate that water-soluble SNJ exerted positive actions on mechanical work and energetics mediated via beta(1)-adrenergic receptors associated with the enhancement of total Ca(2+) handling in excitation-contraction coupling and with unchanged contractile efficiency. In clinical settings, this pharmacological action of SNJ is beneficial as an additive agent for cardioplegia.

Cardioprotective effects of a novel calpain inhibitor SNJ-1945 for reperfusion injury after cardioplegic cardiac arrest

We have previously indicated that calpain inhibitor-1 prevents the heart from ischemia- reperfusion injury associated with the impairment of total Ca(2+) handling by inhibiting the proteolysis of alpha-fodrin. However, this inhibitor is insoluble with water and inappropriate for clinical application. The aim of the present study was to investigate the protective effect of a newly developed calpain inhibitor, SNJ-1945 (SNJ), with good aqueous solubility on left ventricular (LV) mechanical work and energetics in the cross-circulated rat hearts. SNJ (150 microM) was added to KCl (30 meq) cardioplegia (CP). Mean end-systolic pressure at midrange LV volume (ESP(mLVV)) and systolic pressure-volume area (PVA) at mLVV (PVA(mLVV); a total mechanical energy per beat) were hardly changed after CP plus SNJ arrest-reperfusion (post-CP + SNJ), whereas ESP(mLVV) and PVA(mLVV) in post-CP group were significantly (P < 0.01) decreased. Mean myocardial oxygen consumption for the total Ca(2+) handling in excitation-contraction coupling did not significantly decrease in post-CP + SNJ group, whereas it was significantly (P < 0.01) decreased in post-CP group. The mean amounts of 145- and 150-kDa fragments of alpha-fodrin in the post-CP group were significantly larger than those in normal and post-CP + SNJ groups. In contrast, the mean amounts of L-type Ca(2+) channel and sarcoplasmic reticulum Ca(2+)-ATPase were not significantly different among normal, post-CP, and post-CP + SNJ groups. Our results indicate that soluble SNJ attenuates cardiac dysfunction due to CP arrest-reperfusion injury associated with the impairment of the total Ca(2+) handling in excitation-contraction coupling by inhibiting the proteolysis of alpha-fodrin.

Left ventricular function of isoproterenol-induced hypertrophied rat hearts perfused with blood: mechanical work and energetics

We investigated left ventricular (LV) mechanical work and energetics in the cross-circulated (blood-perfused) isoproterenol [Iso 1.2 mg x kg(-1).day(-1) for 3 days (Iso3) or 7 days (Iso7)]-induced hypertrophied rat heart preparation under isovolumic contraction-relaxation. We evaluated pressure-time curves per beat, end-systolic pressure-volume and end-diastolic pressure-volume relations, and myocardial O(2) consumption per beat (Vo(2))-systolic pressure-volume area (PVA; a total mechanical energy per beat) linear relations at 240 beats/min, because Iso-induced hypertrophied hearts failed to completely relax at 300 beats/min. The LV relaxation rate at 240 beats/min in Iso-induced hypertrophied hearts was significantly slower than that in control hearts [saline 24 microl/day for 3 and 7 days (Sa)] with unchanged contraction rate. The Vo(2)-intercepts (composed of basal metabolism and Ca(2+) cycling energy consumption in excitation-contraction coupling) of Vo(2)-PVA linear relations were unchanged associated with their unchanged slopes in Sa, Iso3, and Iso7 groups. The oxygen costs of LV contractility were also unchanged in all three groups. The amounts of expression of sarcoplasmic reticulum Ca(2+)-ATPase, phospholamban (PLB), phosphorylated-Ser(16) PLB, phospholemman, and Na(+)-K(+)-ATPase are significantly decreased in Iso3 and Iso7 groups, although the amount of expression of NCX1 is unchanged in all three groups. Furthermore, the marked collagen production (types I and III) was observed in Iso3 and Iso7 groups. These results suggested the possibility that lowering the heart rate was beneficial to improve mechanical work and energetics in isoproterenol-induced hypertrophied rat hearts, although LV relaxation rate was slower than in normal hearts.

Ventricular performance and Na+-K+ ATPase activity are reduced early and late after myocardial infarction in rats

Myocardial infarction leads to compensatory ventricular remodeling. Disturbances in myocardial contractility depend on the active transport of Ca2+ and Na+, which are regulated by Na+-K+ ATPase. Inappropriate regulation of Na+-K+ ATPase activity leads to excessive loss of K+ and gain of Na+ by the cell. We determined the participation of Na+-K+ ATPase in ventricular performance early and late after myocardial infarction. Wistar rats (8-10 per group) underwent left coronary artery ligation (infarcted, Inf) or sham-operation (Sham). Ventricular performance was measured at 3 and 30 days after surgery using the Langendorff technique. Left ventricular systolic pressure was obtained under different ventricular diastolic pressures and increased extracellular Ca2+ concentrations (Ca2+e) and after low and high ouabain concentrations. The baseline coronary perfusion pressure increased 3 days after myocardial infarction and normalized by 30 days (Sham 3 = 88 +/- 6; Inf 3 = 130 +/- 9; Inf 30 = 92 +/- 7 mmHg; P < 0.05). The inotropic response to Ca2+e and ouabain was reduced at 3 and 30 days after myocardial infarction (Ca2+ = 1.25 mM; Sham 3 = 70 +/- 3; Inf 3 = 45 +/- 2; Inf 30 = 29 +/- 3 mmHg; P < 0.05), while the Frank-Starling mechanism was preserved. At 3 and 30 days after myocardial infarction, ventricular Na+-K+ ATPase activity and contractility were reduced. This Na+-K+ ATPase hypoactivity may modify the Na+, K+ and Ca2+ transport across the sarcolemma resulting in ventricular dysfunction.

Effects of formaldehyde on cardiovascular system in in situ rat hearts

The aim of the present study was to examine the effects of formaldehyde solution on rat left ventricular function and compare it with those in hypertrophic hearts treated with isoproterenol by pressure-volume measurements with the catheter method. After 20-30 min. of intravenous infusion of 3.7% formaldehyde solution (FA) at 10 μl (3.7 mg)/kg/min, normal and hypertrophic hearts showed significant decreases in left ventricle end-systolic pressure (ESP), heart rate and cardiac output per minute, indicating an acute pumping failure. Hypertrophic hearts showed significantly smaller ESP, stroke volumes and cardiac output than those in normal hearts. Systolic pressure-volume area at midrange left ventricular volume (PVA(mLVV) : a mechanical work capability index) was significantly smaller than that in normal hearts and per cent of mean PVA(mLVV) versus pre-infusion mean value in hypertrophic hearts was significantly decreased compared to normal hearts 30 min. after FA infusion. The marked decrease in pH, base excess and no changes in PaO₂ and PaCO₂ suggest metabolic acidosis. The correction of metabolic acidosis with 9% NaHCO₃ did not influence on the acute pumping failure, indicating that metabolic acidosis did not cause it. Ultrastructural observations revealed marked dilation of the sarcoplasmic reticulum with intact sarcolemmal membranes and no disintegration of muscle myofibrils. Ryanodine receptors and calcium (Ca²⁺) pumps (SERCA2A) located in the sarcoplasmic reticulum have major roles in the cytosolic Ca²⁺ handling. Taken together, acute pumping failure by FA may derive from the impairment of Ca²⁺ handling in the cardiac excitation-contraction coupling.

Mechanical work and energetic analysis of eccentric cardiac remodeling in a volume overload heart failure in rats

Eccentric cardiac remodeling seen in dilated cardiomyopathy or regurgitant valvular disease is a well-known process of heart failure progression, but its mechanoenergetic profile has not been yet established. We made a volume overload (VO) heart failure model in rats and for the first time investigated left ventricular (LV) mechanical work and energetics in cross-circulated whole heart preparations. Laparotomy was performed in 14 Wistar male rats, and abdominal aortic-inferior vena caval shunt was created in seven rats (VO group). Another seven rats underwent a sham operation without functional shunt (Sham group). LV dimensions changes were followed with weekly transthoracic echocardiography. Three months after surgery, we measured LV pressure and volume and myocardial O(2) consumption in isolated heart cross circulation. LV internal dimensions in both systolic and diastolic phases were significantly increased in the VO group versus the Sham group (P < 0.05). LV pressure was markedly decreased in the VO group versus in the Sham group (P < 0.05). LV end-systolic pressure-volume relation shifted downward, and myocardial O(2) consumption related to Ca(2+) handling significantly decreased. The contractile response to Ca(2+) infusion was attenuated. Nevertheless, the increase in Ca(2+) handling-related O(2) consumption per unit change in LV contractility in the VO group was significantly higher than that in the Sham group (P < 0.05). The levels of sarco(endo)plasmic reticulum Ca(2+)-ATPase 2a protein were reduced in the VO group (P < 0.01). In conclusion, VO failing rat hearts had a character of marked contractile dysfunction accompanied with less efficient energy utilization in the Ca(2+) handling processes. These results suggest that restoring Ca(2+) handling in excitation-contraction coupling would improve the contractility of the myocardium after eccentric cardiac remodeling.

Rescue of Ca2+ overload-induced left ventriclur dysfunction by targeted ablation of phospholamban

In failing hearts, a deficiency in sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)2a results in abnormal Ca2+ handling and diminished contraction. In addition, a decrease in the phosphorylation of phospholamban (PLB) has been reported. Gene transfer of antisense PLB (asPLB) can improve contractile function in the failing human myocardium. Gene transfer of SERCA2a improves survival and the energy potential in failing hearts. The aim of present study was to evaluate whether enhancement of SERCA2a function prevents acute Ca2+ overload-induced left ventricular (LV) dysfunction in rat hearts. We ablated PLB using adenoviral gene transfer of asPLB by a new and less invasive gene delivery method, which involved a percutaneous technique. Experiments were performed on 13 excised cross-circulated rat hearts: 5 rats underwent sham operations, 4 rats underwent gene transfer of the reporter gene β-galactosidase (Ad.β-gal), and 4 rats underwent gene transfer of asPLB (Ad.asPLB). After clearance of high Ca2+ infused into the coronary, there was LV contractile dysfunction associated with the decreased myocardial O2 consumption per beat (Vo2) intercept (equal to decreased Vo2 for Ca2+ handling in excitation-contraction coupling) of the Vo2-systolic pressure-volume area (PVA; total mechanical energy per beat) linear relation in the hearts that underwent sham operation and had been infected with Ad.β-gal. Hearts that had been infected with Ad.asPLB were rescued from LV contractile dysfunction associated with an unchanged Vo2 intercept of the Vo2-PVA linear relation. We conclude that SERCA2a function enhanced by adenoviral gene transfer of asPLB prevents Ca2+ overload-induced LV contractile dysfunction in terms of mechanical work and especially energetics.

Pursuing enigmas on ischemic heart disease and sudden cardiac death

This article reviews what our colleagues have found as to how ischemic injury or cell death develop in myocardium through Ca(2+)-dependent protease calpain and how compensatory responses evolve through activation of intracellular signaling molecules including PKC isoforms, MAP kinase family enzymes and PI3 kinase. We also addressed how restraint or other psychological stress evokes hypertension and cardiovascular responses in signaling molecules or genes. Unexpectedly, carbon monoxide protects heart and cardiogenic cells against ischemia-resperfusion injury. When I think back, the unresolved cases of autopsies provided ideas for experimental study, which then taught us how the other cases died.

Overexpression of TRPC3 increases apoptosis but not necrosis in response to ischemia-reperfusion in adult mouse cardiomyocytes

An increase in cytosolic Ca2+ via a capacitative calcium entry (CCE)-mediated pathway, attributed to members of the transient receptor potential (TRP) superfamily, TRPC1 and TRPC3, has been reported to play an important role in regulating cardiomyocyte hypertrophy. Increased cytosolic Ca2+ also plays a critical role in mediating cell death in response to ischemia-reperfusion (I/R). Therefore, we tested the hypothesis that overexpression of TRPC3 in cardiomyocytes will increase sensitivity to I/R injury. Adult cardiomyocytes isolated from wild-type (WT) mice and from mice overexpressing TRPC3 in the heart were subjected to 90 min of ischemia and 3 h of reperfusion. After I/R, viability was 51 +/- 1% in WT mice and 42 +/- 5% in transgenic mice (P < 0.05). Apoptosis assessed by annexin V was significantly increased in the TRPC3 group compared with WT (32 +/- 1% vs. 21 +/- 3%; P < 0.05); however, there was no significant difference in necrosis between groups. Treatment of TRPC3 cells with the CCE inhibitor SKF-96365 (0.5 microM) significantly improved cellular viability (54 +/- 4%) and decreased apoptosis (15 +/- 4%); in contrast, the L-type Ca2+ channel inhibitor verapamil (10 microM) had no effect. Calpain-mediated cleavage of alpha-fodrin was increased approximately threefold in the transgenic group following I/R compared with WT (P < 0.05); this was significantly attenuated by SKF-96365. The calpain inhibitor PD-150606 (25 microM) attenuated the increase in both alpha-fodrin cleavage and apoptosis in the TPRC3 group. Increased TRPC3 expression also increased sensitivity to Ca2+ overload stress, but it did not affect the response to TNF-alpha-induced apoptosis. These results suggest that CCE mediated via TRPC may play a role in cardiomyocyte apoptosis following I/R due, at least in part, to increased calpain activation.

Increased O-GlcNAc levels during reperfusion lead to improved functional recovery and reduced calpain proteolysis

We have previously shown that preischemic treatment with glucosamine improved cardiac functional recovery following ischemia-reperfusion, and this was mediated, at least in part, via enhanced flux through the hexosamine biosynthesis pathway and subsequently elevated O-linked N-acetylglucosamine (O-GlcNAc) protein levels. However, preischemic treatment is typically impractical in a clinical setting; therefore, the goal of this study was to investigate whether increasing protein O-GlcNAc levels only during reperfusion also improved recovery. Isolated perfused rat hearts were subjected to 20 min of global, no-flow ischemia followed by 60 min of reperfusion. Administration of glucosamine (10 mM) or an inhibitor of O-GlcNAcase, O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc; 200 microM), during the first 20 min of reperfusion significantly improved cardiac functional recovery and reduced troponin release during reperfusion compared with untreated control. Both interventions also significantly increased the levels of protein O-GlcNAc and ATP levels. We also found that both glucosamine and PUGNAc attenuated calpain-mediated proteolysis of alpha-fodrin as well as Ca(2+)/calmodulin-dependent protein kinase II during reperfusion. Thus two independent strategies for increasing protein O-GlcNAc levels in the heart during reperfusion significantly improved recovery, and this was correlated with attenuation of calcium-mediated proteolysis. These data provide further support for the concept that increasing cardiac O-GlcNAc levels may be a clinically relevant cardioprotective strategy and suggest that this protection could be due, at least in part, to inhibition of calcium-mediated stress responses.

The extrinsic caspase pathway modulates endotoxin-induced diaphragm contractile dysfunction

The mechanisms by which infections induce diaphragm dysfunction remain poorly understood. The purpose of this study was to determine which caspase pathways (i.e., the extrinsic, death receptor-linked caspase-8 pathway, and/or the intrinsic, mitochondrial-related caspase-9 pathway) are responsible for endotoxin-induced diaphragm contractile dysfunction. We determined 1) whether endotoxin administration (12 mg/kg IP) to mice induces caspase-8 or -9 activation in the diaphragm; 2) whether administration of a caspase-8 inhibitor (N-acetyl-Ile-Glu-Thr-Asp-CHO, 3 mg/kg iv) or a caspase-9 inhibitor (N-acetyl-Leu-Glu-His-Asp-CHO, 3 mg/kg iv) blocks endotoxin-induced diaphragmatic weakness and caspase-3 activation; 3) whether TNF receptor 1-deficient mice have reduced caspase activation and diaphragm dysfunction following endotoxin; and 4) whether cytokines (TNF-alpha or cytomix, a mixture of TNF-alpha, interleukin-1beta, interferon-gamma, and endotoxin) evoke caspase activation in C(2)C(12) myotubes. Endotoxin markedly reduced diaphragm force generation (P < 0.001) and induced increases in caspase-3 and caspase-8 activity (P < 0.03), but failed to increase caspase-9. Inhibitors of caspase-8, but not of caspase-9, prevented endotoxin-induced reductions in diaphragm force and caspase-3 activation (P < 0.01). Mice deficient in TNF receptor 1 also had reduced caspase-8 activation (P < 0.001) and less contractile dysfunction (P < 0.01) after endotoxin. Furthermore, incubation of C(2)C(12) cells with either TNF-alpha or cytomix elicited significant caspase-8 activation. The caspase-8 pathway is strongly activated in the diaphragm following endotoxin and is responsible for caspase-3 activation and diaphragm weakness.

Calpains and human disease

Calpains, particularly conventional dimeric calpains, have claimed to be involved in the cell degeneration processes that characterize numerous disease conditions linked to dysfunctions of cellular Ca2+ homeostasis. The evidence supporting their involvement has traditionally been indirect and circumstantial, but recent work has added more solid evidence supporting the role of ubiquitous dimeric calpains in the process of neurodegeneration. The only disease condition in which a calpain defect has been conclusively involved concerns an atypical monomeric calpain: the muscle specific calpain-3, also known as p94. Inactivating defects in its gene cause a muscular dystrophy termed LGMD-2A. The molecular mechanism by which the absence of the proteolytic activity of calpain-3 causes the dystrophic process is unknown. Another atypical calpain, which has been characterized recently as a Ca2(+)-dependent protease, calpain 10, appears To be involved in the etiology of type 2 diabetes. The involvement has been inferred essentially from genetic evidence. Also in the case of type 2 diabetes the molecular mechanisms that could link the disease to calpain 10 are unknown.

A mouse model for monitoring calpain activity under physiological and pathological conditions

Calpains are Ca(2+)-dependent cysteine proteases known to be important for the regulation of cell functions and which aberrant activation causes cell death in a number of degenerative disorders. To provide a tool for monitoring the status of calpain activity in vivo under physiological and pathological conditions, we created a mouse model that expresses ubiquitously a fluorescent reporter consisting of eCFP and eYFP separated by a linker cleavable by the ubiquitous calpains. We named this mouse CAFI for calpain activity monitored by FRET imaging. Our validation studies demonstrated that the level of calpain activity correlates with a decrease in FRET (fluorescence resonance energy transfer) between the two fluorescent proteins. Using this model, we observed a small level of activity after denervation and fasting, a high level of activity during muscle regeneration and ischemia, and local activity in damaged myofibers after exercise. Finally, we crossed the CAFI mouse with the alpha-sarcoglycan-deficient model, demonstrating an increase of calpain activity at the steady state. Altogether, our results present evidence that CAFI mice could be a valuable tool in which to follow calpain activity at physiological levels and in disease states.

Caspase activation contributes to endotoxin-induced diaphragm weakness

Infections produce significant respiratory muscle weakness, but the mechanisms by which inflammation reduces muscle force remain incompletely understood. Recent work suggests that caspase 3 releases actin and myosin from the contractile protein lattice, so we postulated that infections may reduce skeletal muscle force by activating caspase 3. The present experiments were designed to test this hypothesis by determining 1) diaphragm caspase 3 activation in the diaphragm after endotoxin and 2) the effect of a broad-spectrum caspase inhibitor, Z-Val-Ala-Asp(OCH3)-fluoromethylketone (zVAD-fmk), and a selective caspase 3 inhibitor, N-acetyl-Asp-Glu-Val-Asp-al (DEVD-CHO), on endotoxin-induced diaphragm weakness. Caspase 3 activation was assessed by measuring caspase protein levels and by measuring cleavage of a fluorogenic substrate. Diaphragm force was measured in response to electrical stimulation (1-150 Hz). Caspase-mediated spectrin degradation was assessed by Western blotting. Parameters were compared in mice given saline, endotoxin (12 mg/kg ip), endotoxin plus zVAD-fmk (3 mg/kg iv), zVAD-fmk alone, or endotoxin plus DEVD-CHO (3 mg/kg iv). Endotoxin increased diaphragm active caspase 3 protein (P<0.003), increased caspase 3 activity (P<0.002), increased diaphragm spectrin degradation (P<0.001), and reduced diaphragm force (P<0.001). Administration of zVAD-fmk or DEVD-CHO prevented endotoxin-induced weakness (e.g., force in response to 150-Hz stimulation was 23.8+/-1.4, 12.1+/-1.3, 23.5+/-0.8, 22.7+/-1.3, and 24.4+/-0.8 N/cm2, respectively, for control, endotoxin, endotoxin plus zVAD-fmk, endotoxin plus DEVD-CHO, and zVAD-fmk alone treated groups, P<0.001). Caspase inhibitors also prevented spectrin degradation. In conclusion, endotoxin administration elicits significant diaphragm caspase 3 activation and caspase-mediated diaphragmatic weakness.

Detrimental effects after dobutamine infusion on rat left ventricular function: mechanical work and energetics

We have previously reported that continuous infusion of dobutamine into the coronary artery induces positive inotropic effects but induces no detrimental effects in cross-circulated, excised normal rat hearts and even in Ca2+ overload-induced contractile failing rat hearts. However, we hypothesized that some detrimental effects on left ventricular (LV) function are induced after continuous dobutamine infusion and the following clearance of blood dobutamine, as is the case after beta-adrenergic receptor stimulation. To test this hypothesis, we investigated LV mechanical work and energetics in the same type of preparations that underwent continuous dobutamine infusion and clearance of blood dobutamine. We found that both mean end-systolic pressure and systolic pressure-volume area (PVA; a measure of total mechanical energy per beat) at midrange LV volume were significantly (P < 0.01) decreased. The mean myocardial oxygen consumption per beat intercept, which is composed of for the total Ca2+ handling in excitation-contraction coupling and basal metabolism, of the and PVA linear relation was also significantly (P < 0.05) decreased (n=8). The mean slope of the linear relation was unchanged in such hearts. Post-dobutamine basal metabolism was unchanged (n = 5 of the 8 hearts). The moderate proteolysis of a cytoskeleton protein, alpha-fodrin was identified (n = 7 of the 8 hearts with the decreased intercept), after clearance of blood dobutamine. In agreement with our hypothesis, the detrimental effect of the post-beta-adrenergic receptor stimulation was induced by a moderate concentration of dobutamine; we found systolic dysfunction due to the impairment of Ca2+ handling in excitation-contraction coupling in the rat LV and proteolysis of a cytoskeleton protein, alpha-fodrin.

Na+/Ca2+ exchange inhibition protects the rat heart from ischemia-reperfusion injury by blocking energy-wasting processes

We have recently reported that exposure of rat hearts to high Ca2+ produces a Ca2+ overload-induced contractile failure in rat hearts, which was associated with proteolysis of α-fodrin. We hypothesized that contractile failure after ischemia-reperfusion (I/R) is similar to that after high Ca2+ infusion. To test this hypothesis, we investigated left ventricular (LV) mechanical work and energetics in the cross-circulated rat hearts, which were subjected to 15 min global ischemia and 60 min reperfusion. Sixty minutes after I/R, mean systolic pressure-volume area (PVA; a total mechanical energy per beat) at midrange LV volume (mLVV) (PVAmLVV) was significantly decreased from 5.89 ± 1.55 to 3.83 ± 1.16 mmHg·ml·beat−1·g−1 ( n = 6). Mean myocardial oxygen consumption per beat (Vo2) intercept of (Vo2-PVA linear relation was significantly decreased from 0.21 ± 0.05 to 0.15 ± 0.03 μl O2·beat−1·g−1 without change in its slope. Initial 30-min reperfusion with a Na+/Ca2+ exchanger (NCX) inhibitor KB-R7943 (KBR; 10 μmol/l) significantly reduced the decrease in mean PVAmLVV and Vo2 intercept ( n = 6). Although Vo2 for the Ca2+ handling was finally decreased, it transiently but significantly increased from the control for 10–15 min after I/R. This increase in Vo2 for the Ca2+ handling was completely blocked by KBR, suggesting an inhibition of reverse-mode NCX by KBR. α-Fodrin proteolysis, which was significantly increased after I/R, was also significantly reduced by KBR. Our study shows that the contractile failure after I/R is similar to that after high Ca2+ infusion, although the contribution of reverse-mode NCX to the contractile failure is different. An inhibition of reverse-mode NCX during initial reperfusion protects the heart against reperfusion injury.

Calpain inhibitor-1 protects the rat heart from ischemia-reperfusion injury: analysis by mechanical work and energetics

We hypothesized that calpain inhibitor-1 protected left ventricular (LV) function from ischemia-reperfusion injury by inhibiting the proteolysis of α-fodrin. To test this hypothesis, we investigated the effect of calpain inhibitor-1 on LV mechanical work and energetics in the cross-circulated rat hearts that underwent 15-min global ischemia and 60-min reperfusion ( n = 9). After ischemia-reperfusion with calpain inhibitor-1, mean end-systolic pressure at midrange LV volume and systolic pressure-volume area (PVA) at midrange LV volume (total mechanical energy per beat) were hardly changed, although they were significantly ( P < 0.01) decreased after ischemia-reperfusion without calpain inhibitor-1. Mean myocardial oxygen consumption per beat (Vo2) intercepts (PVA-independent Vo2; Vo2 for the total Ca2+ handling in excitation-contraction coupling and basal metabolism) of Vo2-PVA linear relations were also unchanged after ischemia-reperfusion with calpain inhibitor-1, although they were significantly ( P < 0.01) decreased after ischemia-reperfusion without calpain inhibitor-1. There were no significant differences in O2 costs of LV PVA and contractility among the hearts in control (or normal) postischemia-reperfusion and postischemia-reperfusion with calpain inhibitor-1. Western blot analysis of α-fodrin and the immunostaining of 150-kDa products of α-fodrin confirmed that calpain inhibitor-1 almost completely protected the proteolysis of α-fodrin. Our results indicate that calpain inhibitor-1 prevents the heart from ischemia-reperfusion injury associated with the impairment of total Ca2+ handling by directly inhibiting the proteolysis of α-fodrin.

Left ventricular mechanoenergetics after hyperpolarized cardioplegic arrest by nicorandil and after depolarized cardioplegic arrest by KCl

We hypothesized that there are no differences in left ventricular (LV) mechanoenergetics between after hyperpolarized cardioplegic arrest by nicorandil (nicorandil arrest) and after depolarized one by high potassium chloride (KCl arrest). The aim of the present study was to test this hypothesis using LV curved end-systolic pressure-volume relation (ESPVR) and linear pressure-volume area (PVA)-myocardial oxygen consumption per beat (Vo2) relation. All hearts underwent 30 min global ischemia (30°C) after infusion of 5 ml of cardioplegia. Cardioplegia consisted of either 30 mmol/l KCl (7 hearts) or nicorandil (100 μmol/l) in Tyrode solution (6 hearts). After a 30-min blood reperfusion, ESPVR and Vo2-PVA relation were assessed again. Mean end-systolic pressure (ESPmLVV) and mean PVA at midrange LV volume (PVAmLVV) significantly ( P < 0.05) decreased to 79.1 ± 13.4% and 85.4 ± 17.1% of control after KCl arrest and to 85.3 ± 14.8% and 86.4 ± 16.9% of control after nicorandil arrest. There were no significant differences in both decreases of mean ESPmLVV and PVAmLVV between each arrest. The slopes of Vo2-PVA relations were also unchanged after each arrest. There was a significant ( P < 0.005) difference in the decreases of mean Vo2 intercepts of Vo2-PVA relations between post-KCl arrest (73.9 ± 8.2% of control) and post-nicorandil arrest (99.2 ± 10.1% of control), however. Proteolysis of α-fodrin due to Ca2+ overload was significantly marked after KCl arrest. The present results indicate that the total calcium handling in excitation-contraction coupling is transiently impaired after KCl arrest, whereas it is unchanged after nicorandil arrest. This suggests the possibility that nicorandil is a better cardioplegia than KCl.

[Energy utility of failing heart]

We investigated left ventricular (LV) mechanoenergetics in acute and chronic failing hearts, induced by high Ca(2+), ischemic-reperfusion injury, diabetes mellitus (DM), and hypothyroidism, using cross-circulated excised rat heart preparations. After high Ca(2+) or ischemic-reperfusion, there was a contractile failure associated with a parallel downward shift of the linear relation between myocardial O(2) consumption per beat (VO(2)) and systolic pressure-volume area (PVA). This result indicated a decrease in VO(2) for total Ca(2+) handling in E-C coupling. We found proteolysis of a cytoskeletal protein, alpha-fodrin. A calpain inhibitor significantly suppressed contractile failure, decreased VO(2) for total Ca(2+) handling, and membrane alpha-fodrin degradation. In DM, the LV relaxation rate was significantly slower, resulting in the decreased O(2) consumption per min for total Ca(2+) handling in E-C coupling. In hypothyroidism, there were systolic and diastolic failures associated with the decreased O(2) consumption per beat for total Ca(2+) handling in E-C coupling. The protein level of sarcoplasmic reticulum Ca(2+) ATPase (SERCA2) was significantly lower in DM and hypothyroidism. We conclude that suppression of O(2) consumption for total Ca(2+) handling, mainly utilized by SERCA2, is a major cause of failing hearts, mediated through degradation of membrane alpha-fodrin via activation of calpain or suppressed expression of SERCA2.

Left Ventricular Mechanoenergetics in Small Animals

Studies on left ventricular mechanical work and energetics in rat and mouse hearts are reviewed. First, left ventricular linear end-systolic pressure-volume relation (ESPVR) and curved end-diastolic pressure-volume relation (EDPVR) in canine hearts and left ventricular curved ESPVR and curved EDPVR in rat hearts are reviewed. Second, as an index for total mechanical energy per beat in rat hearts as in canine hearts, a systolic pressure-volume area (PVA) is proposed. By the use of our original system for measuring continuous oxygen consumption for rat left ventricular mechanical work, the linear left ventricular myocardial oxygen consumption per beat (VO2)-PVA relation is obtained as in canine hearts. The slope of VO2-PVA relation (oxygen cost of PVA) indicates a ratio of chemomechanical energy transduction. VO2 intercept (PVA-independent VO2) indicates the summation of oxygen consumption for Ca2+ handling in excitation-contraction coupling and for basal metabolism. An equivalent maximal elastance (eEmax) is proposed as a new left ventricular contractility index based on PVA at the midrange left ventricular volume. The slope of the linear relation between PVA-independent VO2 and eEmax (oxygen cost of eEmax) indicates changes in oxygen consumption for Ca2+ handling in excitation-contraction coupling per unit changes in left ventricular contractility. The key framework of VO2-PVA-eEmax can give us a better understanding for the biology and mechanisms of physiological and various failing rat heart models in terms of mechanical work and energetics.

The calpain system

The calpain system originally comprised three molecules: two Ca2+-dependent proteases, mu-calpain and m-calpain, and a third polypeptide, calpastatin, whose only known function is to inhibit the two calpains. Both mu- and m-calpain are heterodimers containing an identical 28-kDa subunit and an 80-kDa subunit that shares 55-65% sequence homology between the two proteases. The crystallographic structure of m-calpain reveals six "domains" in the 80-kDa subunit: 1). a 19-amino acid NH2-terminal sequence; 2). and 3). two domains that constitute the active site, IIa and IIb; 4). domain III; 5). an 18-amino acid extended sequence linking domain III to domain IV; and 6). domain IV, which resembles the penta EF-hand family of polypeptides. The single calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to 85 kDa by use of different promoters and alternative splicing events. The physiological significance of these different calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of the sites is Ca2+ dependent. Since 1989, cDNA cloning has identified 12 additional mRNAs in mammals that encode polypeptides homologous to domains IIa and IIb of the 80-kDa subunit of mu- and m-calpain, and calpain-like mRNAs have been identified in other organisms. The molecules encoded by these mRNAs have not been isolated, so little is known about their properties. How calpain activity is regulated in cells is still unclear, but the calpains ostensibly participate in a variety of cellular processes including remodeling of cytoskeletal/membrane attachments, different signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2+ homeostasis results in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma.

Cardiac dysfunction in terms of left ventricular mechanical work and energetics in hypothyroid rats

We hypothesized that cardiac dysfunction in hypothyroidism is mainly caused by the impairment of Ca(2+) handling in excitation-contraction coupling. To prove this hypothesis, we investigated left ventricular (LV) mechanical work and energetics without interference of preload and afterload in an excised, blood-perfused whole heart preparation from hypothyroid rats. We found that LV inotropism and lusitropism were significantly depressed, and these depressions were causally related to decreased myocardial oxygen consumption for Ca(2+) handling and for basal metabolism. The oxygen costs of LV contractility for Ca(2+) and for dobutamine in the hypothyroid rats did not differ from those in age-matched normal rats. The expression of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2) significantly decreased and that of phospholamban significantly increased. The present results revealed that changes in LV energetics associated with decreased mechanical work in hypothyroid rats are mainly caused by the impairment of Ca(2+) uptake via SERCA2. We conclude that the impairment of Ca(2+) uptake plays an important role in the pathogenesis of cardiac dysfunction in hypothyroidism.

Oxygen costs of left ventricular contractility for dobutamine and Ca(2+) in normal rat hearts and the cost for dobutamine in Ca(2+) overload-induced failing hearts

The aim of the present study was to compare the oxygen (O(2)) cost of left ventricular (LV) contractility (equivalent maximal elastance; eEmax, an index for contractility) for dobutamine (a beta-receptor stimulant) with that for calcium (Ca(2+)) in normal rat hearts and to assess the O(2) cost of LV eEmax for dobutamine in Ca(2+) overload-induced failing rat hearts. The mean O(2) cost of LV eEmax (x10(-4) microl O(2) x beat(-1) x mmHg(-1) x ml x g(-2)) for Ca(2+) was 1.30+/-0.37 in 12 normal hearts, and for dobutamine it was 1.26+/-0.30 in eight different normal hearts. In the same five normal hearts, the mean O(2) cost of LV eEmax for dobutamine was 1.15+/-0.27, and for Ca(2+) it was 0.81+/-0.36. These mean values showed no significant differences between Ca(2+) and dobutamine. In five Ca(2+) overload-induced failing hearts, the mean O(2) cost of LV eEmax for Ca(2+) could not be assessed, but the mean O(2) cost of LV eEmax for dobutamine was 1.04+/-0.40. This mean value for dobutamine did not differ significantly from those (see above 1.26+/-0.30 or 1.15+/-0.27) in the normal hearts. The present results indicate, in terms of the coupling of mechanical work and energetics of the heart, that the total Ca(2+) handling VO(2) in excitation-contraction coupling against unit LV contractility change for dobutamine in the contractile failing hearts does not differ from that in the normal hearts. This suggests that in the Ca(2+) overload-induced contractile failing hearts, there were no changes in the sensitivity of the contractile machinery for Ca(2+), in the Ca(2+)/ATP in the total Ca(2+) handling, and in the ATP/VO(2) in the mitochondria.

Left ventricular diastolic dysfunction in type 2 diabetes mellitus model rats

To gain insight into the pathogenesis of diabetic cardiomyopathy, we investigated cardiac function in terms of the coupling of left ventricular mechanical work and the energetics in Otsuka Long-Evans Tokushima Fatty rats, which are well known as a model of type 2 diabetes mellitus (DM). Neither left ventricular systolic function and mean coronary flow nor coronary flow reserve differed even in late DM rats. The amount of oxygen required for mechanical work and contraction was unaltered, although myosin isozyme was finally transformed from V(1) to V(3). The maximum pacing rate was decreased from 300 to 240 beats/min, and the left ventricular relaxation rate was significantly (P < 0.05) slower only in late DM rats, resulting in decreased oxygen consumption per minute for total Ca(2+) handling in excitation-contraction coupling mainly consumed by sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2) without significant changes in basal metabolism or in mitochondrial oxidative phosphorylation. The protein level of SERCA2 in membranes was significantly (P < 0.001) lower in severe DM rats. We conclude that the only lusitropic dysfunction due to the depressed expression of SERCA2 is related to generating diabetic cardiomyopathy even in the present type 2 diabetic rats.