Pyruvate reverses fatty-acid-induced depression of ventricular function and calcium overload after hypothermia in guinea pig hearts

Physiological Impact of Hypothermia: The Good, the Bad and the Ugly

Hypothermia is defined as a core body temperature of < 35°C, and as body temperature is reduced the impact on physiological processes can be beneficial or detrimental. The beneficial effect of hypothermia enables circulation of cooled experimental animals to be interrupted for 1-2 h without creating harmful effects, while tolerance of circulation arrest in normothermia is between 4 and 5 min. This striking difference has attracted so many investigators, experimental as well as clinical, to this field, and this discovery was fundamental for introducing therapeutic hypothermia in modern clinical medicine in the 1950's. Together with the introduction of cardiopulmonary bypass, therapeutic hypothermia has been the cornerstone in the development of modern cardiac surgery. Therapeutic hypothermia also has an undisputed role as a protective agent in organ transplantation and as a therapeutic adjuvant for cerebral protection in neonatal encephalopathy. However, the introduction of therapeutic hypothermia for organ protection during neurosurgical procedures or as a scavenger after brain and spinal trauma has been less successful. In general, the best neuroprotection seems to be obtained by avoiding hyperthermia in injured patients. Accidental hypothermia occurs when endogenous temperature control mechanisms are incapable of maintaining core body temperature within physiologic limits and core temperature becomes dependent on ambient temperature. During hypothermia spontaneous circulation is considerably reduced and with deep and/or prolonged cooling, circulatory failure may occur, which may limit safe survival of the cooled patient. Challenges that limit safe rewarming of accidental hypothermia patients include cardiac arrhythmias, uncontrolled bleeding, and "rewarming shock".

Maintaining intravenous volume mitigates hypothermia-induced myocardial dysfunction and accumulation of intracellular Ca2

NEW FINDINGS

What is the central question of this study? Detailed guidelines for volume replacement to counteract hypothermia-induced intravascular fluid loss are lacking. Evidence suggests colloids might have beneficial effects compared to crystalloids. Are central haemodynamic function and level of hypothermia-induced calcium overload, as a marker of cardiac injury, restored by fluid substitution during rewarming, and are colloids favourable to crystalloids? What is the main finding and its importance? Infusion with crystalloid or dextran during rewarming abolished post-hypothermic cardiac dysfunction, and partially mitigated myocardial calcium overload. The effects of volume replacement to support haemodynamic function are comparable to those using potent cardio-active drugs. These findings underline the importance of applying intravascular volume replacement to maintain euvolaemia during rewarming.

ABSTRACT

Previous research exploring pathophysiological mechanisms underlying circulatory collapse after rewarming victims of severe accidental hypothermia has documented post-hypothermic cardiac dysfunction and hypothermia-induced elevation of intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) in myocardial cells. The aim of the present study was to examine if maintaining euvolaemia during rewarming mitigates cardiac dysfunction and/or normalizes elevated myocardial [Ca²⁺ ]_i . A total of 21 male Wistar rats (300 g) were surface cooled to 15°C, then maintained at 15°C for 4 h, and subsequently rewarmed to 37°C. The rats were randomly assigned to one of three groups: (1) non-intervention control (n = 7), (2) dextran treated (i.v. 12 ml/kg dextran 70; n = 7), or (3) crystalloid treated (24 ml/kg 0.9% i.v. saline; n = 7). Infusions occurred during the first 30 min of rewarming. Arterial blood pressure, stroke volume (SV), cardiac output (CO), contractility (dP/dt_max ) and blood gas changes were measured. Post-hypothermic changes in [Ca²⁺ ]_i were measured using the method of radiolabelled Ca²⁺ (⁴⁵ Ca²⁺ ). Untreated controls displayed post-hypothermic cardiac dysfunction with significantly reduced CO, SV and dP/dt_max . In contrast, rats receiving crystalloid or dextran treatment showed a return to pre-hypothermic control levels of CO and SV after rewarming, with the dextran group displaying significantly better amelioration of post-hypothermic cardiac dysfunction than the crystalloid group. Compared to the post-hypothermic increase in myocardial [Ca²⁺ ]_i in non-treated controls, [Ca²⁺ ]_i values with crystalloid and dextran did not increase to the same extent after rewarming. Volume replacement with crystalloid or dextran during rewarming abolishes post-hypothermic cardiac dysfunction, and partially mitigates the hypothermia-induced elevation of [Ca²⁺ ]_i .

Lethal hypothermia due to impalement

Impalement injuries in the region of large blood vessels can lead to extensive and even lethal blood loss. However, they can also lead to forced positions from which the affected persons cannot free themselves. This 85-year-old woman was found dead in a prone position in her garden. A metal bar had penetrated deeply into the front of her right thigh, while the other end of the bar was stuck firmly in the soil. The metal bar had merely displaced the woman's muscles and the larger blood vessels without causing major blood loss. There were typical findings of lethal hypothermia, including Wischnewski spots of the gastric mucosa and frost erythema on both knees and the left lower leg. The fall onto the metal bar caused an impalement injury leading to a forced position from which the woman could not free herself where she finally succumbed to lethal hypothermia.

Trimetazidine therapy for diabetic mouse hearts subjected to ex vivo acute heart failure

Acute heart failure (AHF) is the most common primary diagnosis for hospitalized heart diseases in Africa. As increased fatty acid β-oxidation (FAO) during heart failure triggers detrimental effects on the myocardium, we hypothesized that trimetazidine (TMZ) (partial FAO inhibitor) offers cardioprotection under normal and obese-related diabetic conditions. Hearts were isolated from 12-14-week-old obese male and female diabetic (db/db) mice versus lean non-diabetic littermates (db/+) controls. The Langendorff retrograde isolated heart perfusion system was employed to establish an ex vivo AHF model: a) Stabilization phase-Krebs Henseleit buffer (10 mM glucose) at 100 mmHg (25 min); b) Critical Acute Heart Failure (CAHF) phase-(1.2 mM palmitic acid, 2.5 mM glucose) at 20 mmHg (25 min); and c) Recovery Acute Heart Failure phase (RAHF)-(1.2 mM palmitic acid, 10 mM glucose) at 100 mmHg (25 min). Treated groups received 5 μM TMZ in the perfusate during either the CAHF or RAHF stage for the full duration of each respective phase. Both lean and obese males benefited from TMZ treatment administered during the RAHF phase. Sex differences were observed only in lean groups where the phases of the estrous cycle influenced therapy; only the lean follicular female group responded to TMZ treatment during the CAHF phase. Lean luteal females rather displayed an inherent cardioprotection (without treatments) that was lost with obesity. However, TMZ treatment initiated during RAHF was beneficial for obese luteal females. TMZ treatment triggered significant recovery for male and obese female hearts when administered during RAHF. There were no differences between lean and obese male hearts, while lean females displayed a functional recovery advantage over lean males. Thus TMZ emerges as a worthy therapeutic target to consider for AHF treatment in normal and obese-diabetic individuals (for both sexes), but only when administered during the recovery phase and not during the very acute stages.

Myocardial calcium overload during graded hypothermia and after rewarming in an in vivo rat model

AIM

Mechanisms underlying cardiac contractile dysfunction during and after rewarming from hypothermia remain largely unknown. We have previously reported myocardial post-hypothermic calcium overload to be the culprit. The aim of the present study was to measure changes in myocardial [Ca(2+) ](i) during graded hypothermia and after rewarming in an anesthetized, intact rat model, using the (45) Ca(2+) technique.

METHODS

Rats were randomized and cooled to 15 °C. Hearts were excised and perfusion-washed to remove extracellular calcium after 0.5 h of hypothermia (n = 9), 4 h of hypothermia (n = 8), and after 4 h of hypothermia and 2 h rewarming (n = 9). A normothermic group, kept at 37 °C for 5 h, served as control (n = 6). [Ca(2+) ](i) was determined in perchloric acid extracts of heart tissue. Spontaneous cardiac electromechanic work was maintained during hypothermia without cardiac arrest or ischaemia.

RESULTS

Between 0.5 and 4 h at 15 °C, a six-fold increase in cardiac [Ca(2+) ](i) was observed (0.55 ± 0.10 vs. 2.93 ± 0.76 μmol (g dry wt)(-1) ). Rewarming resulted in a 33% decline in [Ca(2+) ](i) , but the actual value was significantly above the value measured in control hearts.

CONCLUSION

We show that calcium overload is a characteristic feature of the beating heart during deep hypothermia, which aggravates by increasing duration of exposure. The relatively low decline in [Ca(2+) ](i) during the rewarming period indicates difficulties in recovering calcium homoeostasis, which in turn may explain cardiac contractile dysfunction observed after rewarming.

Effect of BM 17.0744, a PPARα ligand, on the metabolism of perfused hearts from control and diabetic mice

Peroxisome proliferator-activated receptor-α (PPARα) regulates the expression of fatty acid (FA) oxidation genes in liver and heart. Although PPARα ligands increased FA oxidation in cultured cardiomyocytes, the cardiac effects of chronic PPARα ligand administration in vivo have not been studied. Diabetic db/db mouse hearts exhibit characteristics of a diabetic cardiomyopathy, with altered metabolism and reduced contractile function. A testable hypothesis is that chronic administration of a PPARα agonist to db/db mice will normalize cardiac metabolism and improve contractile function. Therefore, a PPARα ligand (BM 17.0744) was administered orally to control and type 2 diabetic (db/db) mice (37.9 ± 2.5 mg/(kg·d) for 8 weeks), and effects on cardiac metabolism and contractile function were assessed. BM 17.0744 reduced plasma glucose in db/db mice, but no change was observed in control mice. FA oxidation was significantly reduced in BM 17.0744 treated db/db hearts with a corresponding increase in glycolysis and glucose oxidation; glucose and FA oxidation in control hearts was unchanged by BM 17.0744. PPARα treatment did not alter expression of PPARα target genes in either control or diabetic hearts. Therefore, metabolic alterations in hearts from PPARα-treated diabetic mice most likely reflect indirect mechanisms related to improvement in diabetic status in vivo. Despite normalization of cardiac metabolism, PPARα treatment did not improve cardiac function in diabetic hearts.Key words: PPAR, cardiac metabolism and function, diabetes.

The intermediary metabolite pyruvate attenuates stunning and reduces infarct size in in vivo porcine myocardium

The intermediary metabolite pyruvate has been shown to exert significant beneficial effects in in vitro models of myocardial oxidative stress and ischemia-reperfusion injury. However, there have been few reports of the ability of pyruvate to attenuate myocardial stunning or reduce infarct size in vivo. This study tested whether supraphysiological levels of pyruvate protect against reversible and irreversible in vivo myocardial ischemia-reperfusion injury. Anesthetized, open-chest pigs ( n = 7/group) underwent 15 min of left anterior descending coronary artery (LAD) occlusion and 3 h of reperfusion to induce stunning. Load-insensitive contractility measurements of regional preload recruitable stroke work (PRSW) and PRSW area (PRSWA) were generated. Vehicle or pyruvate (100 mg/kg iv bolus + 10 mg·kg–1·min–1 intra-atrial infusion) was administered during ischemia and for the first hour of reperfusion. In infarct studies, pigs ( n = 6/group) underwent 1 h of LAD ischemia and 3 h of reperfusion. Group I pigs received vehicle or pyruvate for 30 min before and throughout ischemia. In group II, the infusion was extended through 1 h of reperfusion. In the stunning protocol, pyruvate significantly improved the recovery of PRSWA at 1 h (50 ± 4% vs. 23 ± 3% in controls) and 3 h (69 ± 5% vs. 39 ± 3% in controls) reperfusion. Control pigs exhibited infarct sizes of 66 ± 1% of the area at risk. The pyruvate I protocol was associated with an infarct size of 49 ± 3% ( P < 0.05), whereas the pyruvate II protocol was associated with an infarct size of 30 ± 2% ( P < 0.05 vs. control and pyruvate I). These findings suggest that pyruvate attenuates stunning and decreases myocardial infarction in vivo in part by reduction of reperfusion injury. Metabolic interventions such as pyruvate should be considered when designing the optimal therapeutic strategies for limiting myocardial ischemia-reperfusion injury.

Cardiac function and metabolism in Type 2 diabetic mice after treatment with BM 17.0744, a novel PPAR-alpha activator

Hearts from diabetic db/db mice, a model of Type 2 diabetes, exhibit left ventricular failure and altered metabolism of exogenous substrates. Peroxisome proliferator-activated receptor-alpha (PPAR-alpha) ligands reduce plasma lipid and glucose concentrations and improve insulin sensitivity in db/db mice. Consequently, the effect of 4- to 5-wk treatment of db/db mice with a novel PPAR-alpha ligand (BM 17.0744; 25-38 mg x kg(-1) x day(-1)), commencing at 8 wk of age, on ex vivo cardiac function and metabolism was determined. Elevated plasma concentrations of glucose, fatty acids, and triacylglycerol (34.0 +/- 3.6, 2.0 +/- 0.4, and 0.9 +/- 0.1 mM, respectively) were reduced to normal after treatment with BM 17.0744 (10.8 +/- 0.6, 1.1 +/- 0.1, and 0.6 +/- 0.1 mM). Plasma insulin was also reduced significantly in treated compared with untreated db/db mice. Chronic treatment of db/db mice with the PPAR-alpha agonist resulted in a 50% reduction in rates of fatty acid oxidation, with a concomitant increase in glycolysis (1.7-fold) and glucose oxidation (2.3- fold). Correction of the diabetes-induced abnormalities in systemic and cardiac metabolism after BM 17.0744 treatment did not, however, improve left ventricular contractile function.

Myocardial protection by insulin at reperfusion requires early administration and is mediated via Akt and p70s6 kinase cell-survival signaling

The "metabolic cocktail" comprising glucose-insulin-potassium administrated at reperfusion reduces infarct size in the in vivo rat heart. We propose that insulin is the major component mediating this protection and acts via Akt prosurvival signaling. This hypothesis was studied in isolated perfused rat hearts (measuring infarct size to area of risk [%]) subjected to 35 minutes regional myocardial ischemia and 2 hours reperfusion. Insulin administered at the onset of reperfusion attenuated infarct size by >/=45% versus control hearts (P<0.001). Insulin-mediated cardioprotection was found to be independent of the presence of glucose at reperfusion. Moreover, the cell survival benefit of insulin is temporally dependent, in that insulin administration from the onset of reperfusion and maintained for either 15 minutes or for the duration of reperfusion reduced infarct size. In contrast, protection was abrogated if insulin administration was delayed until 15 minutes into reperfusion. Pharmacological inhibition of both upstream and downstream signals in the Akt prosurvival pathway abolished the cardioprotective effects of insulin. Here coadministration of insulin with the tyrosine kinase inhibitor lavendustin A, the phosphatidylinositol3-kinase (PI3-kinase) inhibitor wortmannin, and mTOR/p70s6 kinase inhibitor rapamycin abolished cardioprotection. Steady-state levels of activated/phosphorylated Akt correlated with insulin administration. Finally, downstream prosurvival targets of Akt including p70s6 kinase and BAD were modulated by insulin. In conclusion, insulin administration at reperfusion reduces myocardial infarction, is dependent on early administration during reperfusion, and is mediated via Akt and p70s6 kinase dependent signaling pathway. Moreover, BAD is maintained in its inert phosphorylated state in response to insulin therapy.

Microperfusion techniques for long-term hypothermic preservation

BACKGROUND

The aim of this study was to compare several methods of hypothermic heart preservation.

METHODS

We preserved isolated pig hearts for 24 hours in cold cardioplegia (4 degrees C), using either continuous microperfusion (Group I) or simple storage (Group II), and with a new preservative solution (NPS, groups IA and IIA) vs St. Thomas' solution (groups IB and IIB). The main characteristics of the NPS include (1) prevention of cell swelling with polyethelene glycol (PEG), (2) low calcium and magnesium, and (3) presence of metabolic substrates, such as glucose, insulin, pyruvate, aspartate, alanyl-glutamine, and membrane stabilization compounds such as ethanol and chlorpromazine.

RESULTS

The 4 above groups were compared with hearts harvested and immediately reperfused (control group). During preservation, only Group IB showed significant edema (40% +/- 8.4% water gain). Adenylate charge was 25% to 50% higher in microperfused Groups IA and IB (0.678 +/- 0.049 and 0.795 +/- 0.071, respectively) as compared with simple-storage groups IIA and IIB (0.605 +/- 0.048 and 0.524 +/- 0.160, respectively). Ultrastructural analysis showed that tissue injury occurred mainly in Group IIB (altered mitochondria, chromatin clumping). Functional data showed better recovery of NPS groups as compared with St. Thomas groups: coronary flow was identical in Group IB and control (57.8 +/- 22 and 56.6 +/- 14 ml/min/100 g, respectively), and in IA > IB (p < 0.001) and IIA > IIB (p < 0.01); the rate pressure products were higher in NPS groups compared with St. Thomas groups (IA > IB, p < 0.01); IIA > IIB, p < 0.05).

CONCLUSIONS

The microperfusion method associated with the NPS provides excellent protection in long-term hypothermic heart preservation.