The role of mitochondria and sarcoplasmic reticulum calcium handling upon reoxygenation of hypoxic myocardium

Calcium and Heart Failure: How Did We Get Here and Where Are We Going?

The occurrence and prevalence of heart failure remain high in the United States as well as globally. One person dies every 30 s from heart disease. Recognizing the importance of heart failure, clinicians and scientists have sought better therapeutic strategies and even cures for end-stage heart failure. This exploration has resulted in many failed clinical trials testing novel classes of pharmaceutical drugs and even gene therapy. As a result, along the way, there have been paradigm shifts toward and away from differing therapeutic approaches. The continued prevalence of death from heart failure, however, clearly demonstrates that the heart is not simply a pump and instead forces us to consider the complexity of simplicity in the pathophysiology of heart failure and reinforces the need to discover new therapeutic approaches.

Participation of caspase-3-like protease in necrotic cell death of myocardium during ischemia-reperfusion injury in rat hearts

This experimental study was designed to determine if caspase-3-like protease is activated during a short period of ischemia - reperfusion (I-R) that did not induce apoptosis, and whether protease-3-protease inhibitor could prevent myocardial I-R injury, especially necrotic cell death. The subjects were 20 isolated rat hearts; 10 were pretreated for 20 min with 100 micromol/L of the protease-3-protease inhibitor, peptide antagonist Asp-Glu-Val-Asp-CHO (DEVD) (Group D), and compared with the 10 no-pretreated hearts (Group C). The hearts were then subjected to 20, 30, 45, and 60 min of normothermic global ischemia followed by 30 min of reperfusion. Caspase-3-like protease was significantly elevated after 45 min and 60 min in ischemic hearts. Group D had reduced levels of caspase-3-like protease activity after 45 min and 60 min (302+/-58%, 378+/-69% of pre-ischemic control, respectively), as compared with Group C (542+/-74%, 689+/-85%, respectively) (p<0.05, p<0.05, respectively). Histological analysis also demonstrated a decrease in cellular damage in Group D, as the count ratio of necrotic cells with total cardiomyocytes was 38%, as compared with 78% in the control group (p<0.05). Caspase-3-like protease participated in I-R injury in rat hearts and inhibition of this protease resulted in a reduction of necrotic cell death.

Intracellular calcium and the relationship to contractility in an avian model of heart failure

Global contractile heart failure was induced in turkey poults by furazolidone feeding (700 ppm). Abnormal calcium regulation appears to be a key factor in the pathophysiology of heart failure, but the cellular mechanisms contributing to changes in calcium fluxes have not been clearly defined. Isolated ventricular myocytes from non-failing and failing hearts were therefore used to determine whether the whole heart and ventricular muscle contractile dysfunctions were realized at the single cell level. Whole cell current- and voltage-clamp techniques were used to evaluate action potential configurations and L-type calcium currents, respectively. Intracellular calcium transients were evaluated in isolated myocytes with fura-2 and in isolated left ventricular muscles using aequorin. Action potential durations were prolonged in failing myocytes, which correspond to slowed cytosolic calcium clearing. Calcium current-voltage relationships were normal in failing myocytes; preliminary evidence suggests that depressed transient outward potassium currents contribute to prolonged action potential durations. The number of calcium channels (as measured by radioligand binding) were also similar in non-failing and failing hearts. Isolated ventricular muscles from failing hearts had enhanced inotropic responses, in a dose-dependent fashion, to a calcium channel agonist (Bay K 8644). These data suggest that changes in intracellular calcium mobilization kinetics and longer calcium-myofilament interaction may be able to compensate for contractile failure. We conclude that the relationship between calcium current density and sarcoplasmic reticulum calcium release is a dynamic process that may be altered in the setting of heart failure at higher contraction rates.

Metabolic consequences of sepsis. Correlation with altered intracellular calcium homeostasis

The availability of adequate substrate for energy homeostasis is a minimal requirement for vital organ function that normally is provided through dietary intake. When dietary sources of nutrients are inadequate, the body relies on alternate sources of energy provided by gluconeogenesis, lipolysis, and ketogenesis. Sepsis is associated with disruption of virtually all these provisional sources of energy substrate (see Fig. 4). In addition, sepsis impairs the function of the glycolytic pathway, the integrity of which is necessary for glucose to be used effectively for energy production. These abnormalities, coupled with disruption of the intracellular energy-producing machinery (e.g., glycolytic and gluconeogenic enzymes, mitochondria) eventually lead to a reduction in intracellular ATP. Furthermore, a reduction in intracellular ATP can undermine virtually all the energy-consuming functions of the cell, including energy substrate formation (e.g., failed gluconeogenesis), antioxidant production, and calcium homeostasis. High levels of intracellular calcium, in turn, are known to activate many potentially destructive enzymatic pathways (e.g., proteases, phospholipases, endonucleases) that further diminish cell function and may result in cell death. In this context, iCa2+ accumulation may play an important role in the progression from early sepsis to MODS, the most common cause of mortality in the ICU.

Ischemia-dependent efficacy of phosphodiesterase inhibition

To evaluate the inotropic efficacy of phosphodiesterase inhibition in hearts with and without ischemic injury, 27 sheep were evaluated sonomicrometrically during incremental volume loading on right heart bypass. Contractility was assessed with the preload recruitable stroke work relationship. Active relaxation rate was estimated using the time constant of isovolumic pressure decay (tau). For nonischemic assessment, groups 1 and 2 (n = 6 each) underwent 45 minutes of vented perfusion after which milrinone was administered to group 1; group 2 served as nonischemic controls. There was no detectable increase in preload recruitable stroke work or decrement in tau after milrinone administration. Groups 3 and 4 underwent 15 minutes of 37 degrees C ischemia (aortic cross-clamping) followed by 30 minutes of vented reperfusion. Milrinone was then administered to group 3 (n = 7); group 4 (n = 8) served as ischemically injured controls. Inotropic and lusitropic effects were present (group 3 preload recruitable stroke work: 35.4 +/- 5.8 mJ.beat-1.100 g-1.mL-1 before milrinone to 49.5 +/- 4.4 mJ.beat-1.100 g-1.mL-1 after milrinone [p < 0.05]; group 3 tau: 51.8 +/- 5.5 ms before milrinone to 32.2 +/- 2.5 ms after milrinone [p < 0.02]). Although milrinone restored contractility and increased the rate of active relaxation in the postischemic hearts, there was no detectable inotropic effect in nonischemic hearts. In this model, milrinone augments contractility and relaxation in postischemic myocardium but offers little inotropic benefit in non-ischemically injured hearts.

Cellular mechanisms of paired electrical stimulation in ferret ventricular myocardium: relationship between myocardial force and stimulus interval change

The subcellular mechanisms of twitch-force potentiation with paired electrical stimulation was studied in ferret ventricular myocardium using the bioluminescent calcium indicator aequorin. It is demonstrated for the first time that interpolation of an extrasystole in a train of conditioned twitches results in a beat-to-beat change in [Ca2+]i and force. Steady-state twitch force and Ca2+i were increased with paired stimulation. Increased [Ca2+]o in the setting of paired stimulation resulted in an increase in the amplitude of the postextrasystole and associated Ca2+ transient. Verapamil, a Ca2+ channel antagonist, had the opposite effect of increased [Ca2+]o. Postextrasystole potentiation was still present, but diminished in amplitude. These results indicate that postextrasystole potentiation is in part due to a verapamil-depletable store (Ca2+). Postextrasystole potentiation is therefore predominantly dependent on sarcoplasmic reticulum (SR) Ca2+ loading. Ryanodine, an alkaloid which induces Ca2+ leakage from the SR, abolished postextrasystole potentiation; however, in the presence of ryanodine the extrasystole was potentiated. Caffeine, a phosphodiesterase inhibitor which induces SR Ca2+ release and impairs uptake, also abolished postextrasystole potentiation. As with ryanodine there was resultant potentiation of the extrasystole. In the case of caffeine the calcium transient consisted of a second slow component associated with extrasystole twitch potentiation. The results are consistent with sarcolemmal Ca2+ influx playing a role in potentiation of the extrasystole in the presence of an impaired SR. These data indicate that transsarcolemmal Ca2+ influx in the presence of impaired intracellular Ca2+ buffering can directly activate the myofilaments in agreement with reports on human myocardium.

Therapeutic potential of vitamin E against myocardial ischemic-reperfusion injury

Myocardial ischemia is a disease process characterized by reduced coronary flow such that the supply of nutritive blood to heart muscle (myocardium) is insufficient for normal myocardial aerobic metabolism. Prompt reestablishment of coronary flow by invasive and noninvasive clinical procedures is the most direct and effective means of limiting myocardial damage in ischemic heart disease patients, although reperfusion carries with it an injury component which may reflect, at least to some degree, the toxic effects of partially reduced oxygen species and their participation in degenerative cellular processes such as membrane lipid peroxidation. Vitamin E, a lipophilic, chain-breaking antioxidant, is a prominent membrane constituent in heart muscle, where it modulates/regulates various aspects of heart muscle-cell metabolism and function. Vitamin E's beneficial effects against experimentally induced oxidative damage to the heart, along with inverse epidemiological correlations between plasma vitamin E level and either anginal pain or mortality due to ischemic heart disease, suggest that vitamin E might have protective and therapeutic roles against myocardial ischemic-reperfusion injury. Laboratory investigations aimed at addressing this possibility have demonstrated that vitamin E supplementation protects isolated hearts against ischemic-reperfusion injury, and relatively more inconsistent and limited data document cardioprotective effects of vitamin E in some animal models of myocardial ischemia-reperfusion, especially when administered prior to the ischemic period. Clinical attempts to establish whether vitamin E has therapeutic benefit in ischemic heart disease patients remain inconclusive, having relied upon a variety of nonuniformly controlled protocols and a single, rather subjective endpoint (anginal pain). Consequently, although laboratory data constitute a conceptual context for and indirect support of the idea that vitamin E could be a cardioprotectant against ischemic-reperfusion injury, compelling clinical evidence regarding vitamin E's therapeutic potential in the ischemic heart-disease patient is lacking. Elective coronary revascularization would appear to provide an attractive clinical setting for evaluating the therapeutic efficacy of vitamin E in the context of cardiac ischemia-reperfusion. Further biochemical work would still be required to define how vitamin E exerts any cardioprotective effect observed in these patients.

Mitochondria, oxygen and reperfusion damage

Reperfusion of the ischaemic or hypoxic heart elicits a number of oxygen dependent processes such as cell lysis and Ca2+ uptake. It is known that the energisation of mitochondria, which requires oxygen, plays a key role in these processes and that the organelle actively sequesters Ca2+ under these circumstances. In this brief review we discuss how oxidants derived from mitochondrial electron transport may perturb mitochondrial calcium handling on reoxygenation of the hypoxic myocardium. In addition we show that the immunosuppressive agent cyclosporin has little or no effect on the oxygen dependent increase in total cell Ca2+ which occurs when hypoxic myocytes are reoxygenated. This result suggests that the Ca2+ dependent mitochondrial pore, which is known to function under conditions of oxidative stress, does not play a major role in the perturbation of Ca2+ homeostasis which occurs on reoxygenation of hypoxic hearts.

Intracellular calcium related to force development in twitch contraction of mammalian myocardium

To characterize the relationship between force production and Ca2+ occupancy of troponin C, investigators have related peak intracellular Ca2+, measured with a variety of Ca2(+)-indicators, and peak force during twitches. Inherent in the force-[Ca2+] relationship is the responsiveness of the myofilaments to Ca2+ which can be altered by different pharmacological manipulations. In this study we compared the force-[Ca2+] relationship obtained in aequorin-injected papillary muscles and saponin skinned trabeculae from control, right ventricular pressure-overload hypertrophy (POH), and hyperthyroid ferret hearts. In POH, the twitch and [Ca2+]i transient were prolonged as compared to control. Force-[Ca2+] relationships from skinned fiber preparations were superimposable between control and POH. The peak force-peak [Ca2+]i relationship in intact muscles from POH was shifted to the left as compared to control. In hyperthyroid hearts, the twitch and [Ca2+]i were abbreviated. Force-[Ca2+]i relationships from skinned fiber preparations were superimposable between control and thyrotoxic hearts. The peak force-peak [Ca2+]i relationship in intact muscles from hyperthyroid hearts was shifted to the right as compared to control. Our findings indicate that time course changes in the calcium transient artifacturally shift the peak force-peak calcium relationship in a predictable manner. Therefore, this relationship can not be used to address changes at the level of the myofilaments as previously suggested.