Magnesium cardioplegia reduces cytosolic and nuclear calcium and DNA fragmentation in the senescent myocardium

New Findings: Hindlimb Unloading Causes Nucleocytoplasmic Ca2+ Overload and DNA Damage in Skeletal Muscle

Disuse atrophy of skeletal muscle is associated with a severe imbalance in cellular Ca²⁺ homeostasis and marked increase in nuclear apoptosis. Nuclear Ca²⁺ is involved in the regulation of cellular Ca²⁺ homeostasis. However, it remains unclear whether nuclear Ca²⁺ levels change under skeletal muscle disuse conditions, and whether changes in nuclear Ca²⁺ levels are associated with nuclear apoptosis. In this study, changes in Ca²⁺ levels, Ca²⁺ transporters, and regulatory factors in the nucleus of hindlimb unloaded rat soleus muscle were examined to investigate the effects of disuse on nuclear Ca²⁺ homeostasis and apoptosis. Results showed that, after hindlimb unloading, the nuclear envelope Ca²⁺ levels ([Ca²⁺]_NE) and nucleocytoplasmic Ca²⁺ levels ([Ca²⁺]_NC) increased by 78% (p < 0.01) and 106% (p < 0.01), respectively. The levels of Ca²⁺-ATPase type 2 (Ca²⁺-ATPase2), Ryanodine receptor 1 (RyR1), Inositol 1,4,5-tetrakisphosphate receptor 1 (IP₃R1), Cyclic ADP ribose hydrolase (CD38) and Inositol 1,4,5-tetrakisphosphate (IP₃) increased by 470% (p < 0.001), 94% (p < 0.05), 170% (p < 0.001), 640% (p < 0.001) and 12% (p < 0.05), respectively, and the levels of Na⁺/Ca²⁺ exchanger 3 (NCX3), Ca²⁺/calmodulin dependent protein kinase II (CaMK II) and Protein kinase A (PKA) decreased by 54% (p < 0.001), 33% (p < 0.05) and 5% (p > 0.05), respectively. In addition, DNase X is mainly localized in the myonucleus and its activity is elevated after hindlimb unloading. Overall, our results suggest that enhanced Ca²⁺ uptake from cytoplasm is involved in the increase in [Ca²⁺]_NE after hindlimb unloading. Moreover, the increase in [Ca²⁺]_NC is attributed to increased Ca²⁺ release into nucleocytoplasm and weakened Ca²⁺ uptake from nucleocytoplasm. DNase X is activated due to elevated [Ca²⁺]_NC, leading to DNA fragmentation in myonucleus, ultimately initiating myonuclear apoptosis. Nucleocytoplasmic Ca²⁺ overload may contribute to the increased incidence of myonuclear apoptosis in disused skeletal muscle.

Mitochondrial Transplantation in Myocardial Ischemia and Reperfusion Injury

Ischemic heart disease remains the leading cause of death worldwide. Mitochondria are the power plant of the cardiomyocyte, generating more than 95% of the cardiac ATP. Complex cellular responses to myocardial ischemia converge on mitochondrial malfunction which persists and increases after reperfusion, determining the extent of cellular viability and post-ischemic functional recovery. In a quest to ameliorate various points in pathways from mitochondrial damage to myocardial necrosis, exhaustive pharmacologic and genetic tools have targeted various mediators of ischemia and reperfusion injury and procedural techniques without applicable success. The new concept of replacing damaged mitochondria with healthy mitochondria at the onset of reperfusion by auto-transplantation is emerging not only as potential therapy of myocardial rescue, but as gateway to a deeper understanding of mitochondrial metabolism and function. In this chapter, we explore the mechanisms of mitochondrial dysfunction during ischemia and reperfusion, current developments in the methodology of mitochondrial transplantation, mechanisms of cardioprotection and their clinical implications.

Controlled hyperkalemic reperfusion with magnesium rescues ischemic juvenile hearts by reducing calcium loading

OBJECTIVES

Our objectives were (1) to determine whether elevated Mg(2+) in controlled hyperkalemic reperfusate without intervention during ischemia protects the juvenile heart against reperfusion injury; and (2) to identify the mechanism(s) underlying any protective effect of Mg(2+).

METHODS

Langendorff-perfused hearts from juvenile (11- to 14-day-old) guinea pigs were subjected to mild (30-minute) or severe (45-minute) normothermic global ischemia and 35-minute reperfusion. Hearts were subjected to controlled hyperkalemic reperfusion without or with various concentrations of Mg(2+) (5, 10, 16, 23 mM). The mechanisms underlying the effect of Mg(2+) on intracellular Ca(2+) ([Ca(2+)]i) were also studied in isolated cardiomyocytes exposed to metabolic inhibition followed by washout using hyperkalemic solutions (reperfusion).

RESULTS

Sixteen mM Mg(2+) conferred maximal cardioprotection as assessed by improved functional recovery and reduced cardiac injury; this was associated with a significant recovery of cardiac energetics and metabolism following both mild and severe ischemia. The Mg(2+)-induced protection was additive to that of hyperkalemia following mild ischemia and conferred protection following severe ischemia when hyperkalemia alone had no significant effect. Elevated Mg(2+) in the hyperkalemic reperfusate of cardiomyocytes acutely prevented [Ca(2+)]i loading following mild metabolic inhibition and augmented the fall in [Ca(2+)]i following severe metabolic inhibition.

CONCLUSIONS

This work demonstrates for the first time in juvenile hearts that elevated Mg(2+) during controlled hyperkalemic reperfusion rescues the heart following ischemia, and that this is likely to be facilitated by reducing [Ca(2+)]i which, in turn, would aid metabolic recovery.

Protecting the Myocardial Cell During Coronary Revascularization

Background— Using the ischemic myocardial cell as a paradigm, competitive coronary revascularization technologies will be analyzed for their potential in causing additional myocardial cell damage during the course of therapeutic procedures.

Methods and Results— Percutaneous coronary intervention (PCI) using balloon and/or stent (bare metal or coated) approaches may be associated with myonecrosis related to atherosclerotic debris plugging the downstream coronary microcirculation as well as ischemia/reperfusion injury associated with revascularization of occluded coronary vessels. The placement of distal mechanical devices and filters during the course of PCI has not been successful in ameliorating this problem. Coronary revascularization using coronary artery bypass grafting (CABG) similarly may be associated with myocardial stunning and cell necrosis associated with ischemia/reperfusion injury. Surgically induced myocardial ischemia secondary to aortic cross clamping, results from the attenuation or cessation of coronary blood flow such that oxygen delivery to the myocardium is insufficient to meet basal myocardial requirements to preserve cellular membrane stability and viability. Recovery involves: (1) resumption of normal oxidative metabolism and the restoration of myocardial energy reserves; (2) reversal of ischemia induced cell swelling and loss of membrane ion gradients and the adenine nucleotide pool; (3) repair of damaged cell organelles such as the mitochondria and the sarcoplasmic reticulum. Despite meticulous adherence to presently known principles of surgical myocardial protection using advanced cardioplegic technologies, some patients require inotropic support and/or mechanical assist devices postoperatively, when none was required preoperatively.

Conclusions— Which method of coronary revascularization causes the least amount of myocardial cell injury and is associated with superior long-term outcomes remains an area of increasing controversy.

Opening of mitochondrial KATP channels enhances cardioprotection through the modulation of mitochondrial matrix volume, calcium accumulation, and respiration

Previously, we have shown that the pharmacological opening of the mitochondrial ATP-sensitive K channels with diazoxide (DZX) enhances the cardioprotection afforded by magnesium-supplemented potassium (K/Mg) cardioplegia. To determine the mechanisms involved in the cardioprotection afforded by K/Mg + DZX cardioplegia, rabbit hearts (n=24) were subjected to isolated Langendorff perfusion. Control hearts were perfused for 75 min. Global ischemia (GI) hearts were subjected to 30 min of equilibrium, 30 min of GI, and 15 min of reperfusion. K/Mg and K/Mg + DZX cardioplegia hearts received either K/Mg or K/Mg + DZX for 5 min before GI and reperfusion. Tissue was harvested for mitochondrial isolation and transmission electron microscopy (TEM). Mitochondrial structure, area, matrix volume, free calcium, and oxygen consumption were determined. TEM demonstrated that GI mitochondria were damaged and that K/Mg and K/Mg + DZX preserved mitochondrial structure. TEM and light scattering demonstrated separately that mitochondrial matrix and cristae area and matrix volume were significantly increased after GI and reperfusion with GI > K/Mg + DZX > K/Mg hearts (P <0.05 vs. control). Mitochondrial free calcium was significantly increased in GI and K/Mg hearts. K/Mg + DZX significantly decreased mitochondrial free calcium accumulation (P <0.05 vs. GI and K/Mg). State 3 oxygen consumption and respiratory control index in malate (complex I substrate)- and succinate (complex II substrate)-energized mitochondria were significantly decreased (P <0.05 vs. control) in the GI and K/Mg + DZX groups. These data indicate that the enhanced cardioprotection afforded by K/Mg + DZX cardioplegia occurs through the preservation of mitochondrial structure and the significant decrease in mitochondrial free calcium accumulation and mitochondrial state 3 oxygen consumption.

Mitochondrial ATP-sensitive potassium channels in surgical cardioprotection

ATP-sensitive potassium channels allow for the coupling of membrane potential to cellular metabolic status. Two K(ATP) channel subtypes coexist in the myocardium with one subtype located in the sarcolemma membrane and the other in the inner membrane of the mitochondria. The ATP-sensitive potassium channels can be pharmacologically modulated by a family of structurally diverse agents of varied potency and selectivity, collectively known as potassium channel openers and blockers. Sufficient evidence exists to indicate that the ATP-sensitive potassium channels and in particular the mitochondrial ATP-sensitive potassium channels play an important role both as a trigger and an effector in surgical cardioprotection. In this review, the biochemistry and specificity of the ATP-sensitive potassium channels is examined in relation to surgical cardioprotection.

Cardioprotection by St Thomas' solution is mediated by protein kinase C and tyrosine kinase

BACKGROUND

Intracellular signaling pathways, specifically the activation of protein kinase C and tyrosine kinase, are essential to the cardioprotection of ischemic preconditioning. We proposed that activation of PKC and TK contribute to the myocardial protection of St. Thomas' No. 2 cardioplegia solution (STC).

MATERIALS AND METHODS

Isolated rat hearts were exposed to 40 min of global ischemia followed by 120 min of reperfusion. Before ischemia, hearts received no treatment (control; n = 7), STC (n = 7), phorbol 12-myristate 13-acetate (PMA; n = 6), PMA + chelerythrine (n = 6), anisomycin (n = 6), anisomycin + genistein (n = 7), STC + chelerythrine (n = 7), STC + genistein (n = 7), PMA + genistein (n = 7) or anisomycin + chelerythrine (n = 7). Left ventricular developed pressure (LVDP) recovery, myocardial infarct size, and lactate dehydrogenase release were measured.

RESULTS

STC as well as PMA (protein kinase C activator) and anisomycin (tyrosine kinase activator) significantly reduced infarct size (6.9 +/- 2.9%, 9.6 +/- 2.1%, 14.0 +/- 4.4%) compared with controls (42.4 +/- 2.9%, P < 0.05). The infarct reduction of PMA and anisomycin were blocked by their inhibitors chelerythrine and genistein, respectively. Both chelerythrine (29.2 +/- 4.1%, P < 0.05) and genistein (40.4 +/- 4.3%, P < 0.05) attenuated the reduction of infarct size provided by STC. The recovery of LVDP improved with STC, PMA and anisomycin (72.6 +/- 1.4%, 60.4 +/- 4.7%, 57.2 +/- 4.6%) compared with control (33.8 +/- 3.6%, P < 0.05). Addition of chelerythrine or genistein to STC impaired recovery of LVDP (52.3 +/- 4.4%, 35.1 +/- 2.5%, P < 0.05) compared with STC treatment.

CONCLUSION

Administration of the pharmacologic inhibitors chelerythrine and genistein blunts the cardioprotection caused by STC treatment.

The mitochondrial K(ATP) channel and cardioprotection

Adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels allow coupling of membrane potential to cellular metabolic status. Two K(ATP) channel subtypes coexist in the myocardium, with one subtype located in the sarcolemma (sarcK(ATP)) membrane and the other in the inner membrane of the mitochondria (mitoK(ATP)). The K(ATP) channels can be pharmacologically modulated by a family of structurally diverse agents of varied potency and selectivity, collectively known as potassium channel openers and blockers. Sufficient evidence exists to indicate that the K(ATP) channels and, in particular, the mitoK(ATP) channels play an important role both as a trigger and an effector in surgical cardioprotection. In this review, the biochemistry and surgical specificity of the K(ATP) channels are examined.

Protective role of magnesium in cardiovascular diseases: a review

A considerable number of experimental, epidemiological and clinical studies are now available which point to an important role of Mg2+ in the etiology of cardiovascular pathology. In human subjects, hypomagnesemia is often associated with an imbalance of electrolytes such as Na+, K+ and Ca2+. Abnormal dietary deficiency of Mg2+ as well as abnormalities in Mg2+ metabolism play important roles in different types of heart diseases such as ischemic heart disease, congestive heart failure, sudden cardiac death, atheroscelerosis, a number of cardiac arrhythmias and ventricular complications in diabetes mellitus. Mg2+ deficiency results in progressive vasoconstriction of the coronary vessels leading to a marked reduction in oxygen and nutrient delivery to the cardiac myocytes. Numerous experimental and clinical data have suggested that Mg2+ deficiency can induce elevation of intracellular Ca2+ concentrations, formation of oxygen radicals, proinflammatory agents and growth factors and changes in membrane perrmeability and transport processes in cardiac cells. The opposing effects of Mg2+ and Ca2+ on myocardial contractility may be due to the competition between Mg2+ and Ca2+ for the same binding sites on key myocardial contractile proteins such as troponin C, myosin and actin. Stimulants, for example, catecholamines can evoke marked Mg2+ efflux which appears to be associated with a concomitant increase in the force of contraction of the heart. It has been suggested that Mg2+ efflux may be linked to the Ca2+ signalling pathway. Depletion of Mg2+ by alcohol in cardiac cells causes an increase in intracellular Ca2+, leading to coronary artery vasospasm, arrhythmias, ischemic damage and cardiac failure. Hypomagnesemia is commonly associated with hypokalemia and occurs in patients with hypertension or myocardial infarction as well as in chronic alcoholism. The inability of the senescent myocardium to respond to ischemic stress could be due to several reasons. Mg2+ supplemented K+ cardioplegia modulates Ca2+ accumulation and is directly involved in the mechanisms leading to enhanced post ischemic functional recovery in the aged myocardium following ischemia. While many of these mechanisms remain controversial and in some cases speculative, the beneficial effects related to consequences of Mg2+ supplementation are apparent. Further research are needed for the incorporation of these findings toward the development of novel myocardial protective role of Mg2+ to reduce morbidity and mortality of patients suffering from a variety of cardiac diseases.

Effects of NHE-1 inhibition on cardioprotection and impact on protection by K/Mg cardioplegia

BACKGROUND

Cardiac sodium hydrogen exchanger isoform-1 (NHE-1) activity during ischemia/reperfusion contributes to myocardial injury. The effects of NHE-1 inhibition during ischemia or reperfusion and on the protection afforded by K/Mg cardioplegia was unknown.

METHODS

Rabbit hearts were used for Langendorff perfusion. Control hearts were perfused for 180 minutes. Global ischemia (GI) hearts received 30 minutes normothermic global ischemia and 120 minutes reperfusion. K/Mg hearts received cardioplegia 5 minutes before ischemia. Separate groups of GI and K/Mg hearts received the NHE-1 inhibitor, HOE-642, before ischemia (HOE-642-I), at the immediate start of reperfusion (HOE-642-R), or both before ischemia and at the immediate start of reperfusion (HOE-642-IR).

RESULTS

Left ventricular peak developed pressure was significantly increased in HOE-I, HOE-R, and HOE-IR throughout reperfusion (p < 0.05 versus GI). Infarct size was significantly decreased (p < 0.05 versus GI) in all groups, but was significantly increased in HOE-R as compared with HOE-IR (p < 0.05). NHE-1 inhibition with K/Mg cardioplegia significantly decreased left ventricular peak developed pressure after 90 minutes of reperfusion (p < 0.05 versus K/Mg), with no significant effect on infarct size.

CONCLUSIONS

NHE-1 inhibition used alone provides cardioprotection with optimal effects being observed with HOE-IR. NHE-1 inhibition with K/Mg cardioplegia decreases postischemic functional recovery during late reperfusion.

Adenosine-enhanced ischemic preconditioning modulates necrosis and apoptosis: effects of stunning and ischemia-reperfusion

BACKGROUND

Adenosine-enhanced ischemic preconditioning extends the protection of ischemic preconditioning by both significantly decreasing infarct size and significantly enhancing postischemic functional recovery.

METHODS

The effects of adenosine-enhanced ischemic preconditioning on necrosis and apoptosis were investigated in the sheep heart using models of stunning (15 minutes regional ischemia, 120 minutes reperfusion) and ischemia-reperfusion (30 and 60 minutes regional ischemia, 120 minutes reperfusion). Ischemic preconditioned hearts received 5 minutes regional ischemia, 5 minutes reperfusion before ischemia. Adenosine-enhanced ischemic preconditioned hearts received a 10 mmol/L adenosine bolus (10 mL) through the left atrium coincident with ischemic preconditioning. Adenosine hearts received a 10 mmol/L bolus (10 mL) of adenosine. Regional ischemic hearts received no pretreatment.

RESULTS

Minimal apoptosis (< 45 per 3,000 myocytes) was observed in the stunning models but was significantly increased with ischemia-reperfusion in regional ischemic hearts after 30 minutes (p < 0.05 versus ischemic preconditioning, adenosine, or adenosine-enhanced ischemic preconditioning) and in adenosine and ischemic preconditioned hearts after 60 minutes ischemia (p < 0.05 versus adenosine-enhanced ischemic preconditioning). DNA laddering was apparent after 60 minutes ischemia in regional ischemia, adenosine, and ischemic preconditioning but not in adenosine-enhanced ischemic preconditioned hearts.

CONCLUSIONS

Adenosine-enhanced ischemic preconditioning significantly ameliorates necrosis and apoptosis in the regional ischemic blood-perfused heart.

Inhibition of RNA transcription modulates magnesium-supplemented potassium cardioplegia protection

BACKGROUND

Previously we reported that decreased postischemic functional recovery was associated with increased DNA fragmentation in the aged myocardium. Magnesium-supplemented potassium (K/Mg) cardioplegia ameliorated DNA fragmentation and enhanced post-ischemic functional recovery. We hypothesized that K/Mg cardioprotection might involve either an RNA- or a protein-dependent mechanism.

METHODS

Aged rabbit hearts underwent Langendorff perfusion. Global ischemia hearts (GI) received 30 minutes of global ischemia and 60 minutes of reperfusion; K/Mg hearts received cardioplegia before global ischemia. To investigate the role of RNA and protein synthesis, K/Mg hearts were treated with alpha-amanitin or cycloheximide to inhibit RNA or protein synthesis. We also determined the quantity of DNA fragmentation and RNA/DNA ratio.

RESULTS

Inhibition of RNA but not protein synthesis significantly decreased K/Mg cardioprotection and was associated with significantly decreased postischemic functional recovery (p < 0.05 versus K/Mg), increased DNA fragmentation, and decreased RNA/DNA ratio (p < 0.05 versus K/Mg).

CONCLUSIONS

These results indicate that K/Mg cardioprotection in the aged myocardium was modulated by an RNA-dependent mechanism.

Superiority of magnesium cardioplegia in neonatal myocardial protection

BACKGROUND

We have shown that magnesium can offset the detrimental effects of normocalcemic cardioplegia in hypoxic neonatal hearts. It is not known, however, whether magnesium offers any additional benefit when used in conjunction with hypocalcemic cardioplegia.

METHODS

Twenty neonatal piglets underwent 60 minutes of ventilator hypoxia (FiO2 8% to 10%) followed by 20 minutes of normothermic ischemia on cardiopulmonary bypass (hypoxic-ischemic stress). They then underwent 70 minutes of multidose blood cardioplegic arrest. Five (Group 1), received a hypocalcemic (Ca+2 0.2 to 0.4 mM/L) cardiologic solution without magnesium, whereas in 10, magnesium was added at either a low dose (5 to 6 mEq/L, Group 2) or high dose (10 to 12 mEq/L, Group 3). In the last 5 (Group 4), magnesium (10 to 12 mEq/L) was added to a normocalcemic cardioplegic solution. Function was assessed using pressure volume loops and expressed as percentage of control.

RESULTS

Compared to hypocalcemia cardioplegic solution without magnesium (Group 1), both high- and low-dose magnesium enrichment (Groups 2 and 3) improved myocardial protection resulting in complete return of systolic (40% vs 101% vs 102%) (p < 0.001 vs Groups 2 and 3) and global myocardial function (39% vs 102% vs 101%) (p < 0.001 vs Groups 2 and 3), and reduced diastolic stiffness (267% vs 158% vs 154%) (p < 0.001 vs Groups 2 and 3). Conversely, even high-dose magnesium supplementation could not offset the detrimental effects of normocalcemic cardioplegia resulting in depressed systolic (End Systolic Elastance [EES] 41%+/-1%) (p < 0.001 vs Groups 2 and 3) and global myocardial function (40%+/-1%) (p < 0.001 vs Groups 2 and 3), and a marked rise in diastolic stiffness (258%+/-5%) (p < 0.001 vs Groups 2 and 3). Hypocalcemic magnesium cardioplegia has now been used successfully in 247 adult and pediatric patients.

CONCLUSIONS

Magnesium enrichment of hypocalcemic cardioplegic solutions improves myocardial protection resulting in complete functional preservation. However, magnesium cannot prevent the detrimental effects of normocalcemic cardioplegia when the heart is severely stressed. This study, therefore, strongly supports using both a hypocalcemic cardioplegic solution and magnesium supplementation as their benefits are additive.

Intracellular free calcium accumulation in ferret vascular smooth muscle during crystalloid and blood cardioplegic infusions

OBJECTIVE

The effects of magnesium- and potassium-based crystalloid and blood-containing cardioplegic solutions on coronary smooth muscle intracellular free calcium ([Ca2+]i) accumulation and microvascular contractile function were examined.

METHODS

Isolated ferret hearts were subjected to hyperkalemic (25 mmol/L K+) blood cardioplegic infusion, hypermagnesemic (25 mmol/L Mg2+, K+-free) crystalloid cardioplegic infusion, or hyperkalemic crystalloid cardioplegic infusion for 1 hour. Coronary arterioles were isolated, cannulated, and loaded with fura 2. Reactivity and [Ca2+]i were assessed with videomicroscopy. [Ca2+]i was measured at baseline and after application of 50 mmol/L KCl. In addition, [Ca2+]i and vascular contraction were measured during exposure to Mg2+ and K+ cardioplegic solution at both 4 degrees C and 37 degrees C.

RESULTS

From a baseline [Ca2+]i of 177 +/- 52 nmol/L, K+ cardioplegic infusion (302 +/- 80 nmol/L potassium) markedly increased [Ca2+]i, whereas blood cardioplegic infusion (214 +/- 53 nmol/L) and Mg2+ cardioplegic infusion (180 +/- 42 nmol/L) did not alter [Ca2+]i. Although a difference between groups in percentage contraction after application of 50 mmol/L KCl was not observed, [Ca2+]i increased significantly more in vessels in the control group (764 +/- 327 nmol/L) and the K+ crystalloid cardioplegic infusion group (698 +/- 215 nmol/L) than in vessels in the blood cardioplegic infusion group (402 +/- 45 nmol/L) and the Mg2+ cardioplegic infusion group (389 +/- 80 nmol/L). Mg2+ cardioplegic solution induced no microvascular contraction at either 4 degrees C or 37 degrees C, nor was an increase in [Ca2+]i observed. K+ cardioplegic solution induced microvascular contraction at 37 degrees C but not at 4 degrees C; it increased [Ca2+]i at both 4 degrees C and 37 degrees C.

CONCLUSION

An Mg2+-based cardioplegic solution, or appropriate Mg2+ or blood supplementation of a K+ crystalloid cardioplegic solution, may decrease the accumulation of [Ca2+]i in the vascular smooth muscle during ischemic arrest.

[Ischemic preconditioning in the aged heart--myocardial protective effect as compared with the mature heart]

It is now well established that pre-treatment with sublethal ischemia, followed by reperfusion, will delay myocardial necrosis during a later sustained ischemic episode, termed ischemic preconditioning (IPC); this has been confirmed experimentally and clinically. However, the effects for the senescent heart differ from those of the mature heart at both functional and cellular levels which have not yet been determined. Comparisons were made between aged (> 135 weeks, n = 18) and mature (15 approximately 20 weeks, n = 8) rabbit hearts which underwent 30 min. normothermic global ischemia with 120 min reperfusion in a buffer-perfused isolated, paced heart model, and the effects of IPC on post-ischemic functional recovery and infarct size were investigated. Ischemic preconditioned hearts (n = 6) were subjected to one cycle of 5 min. global ischemia and 5 min. reperfusion prior to global ischemia. Global ischemic hearts (n = 6) were subjected to 30 min. global ischemia without intervention. Control hearts (n = 6) were subjected to perfusion without ischemia. Post-ischemic functional recovery was better in the ischemic preconditioned hearts than in the global ischemic hearts in both aged and mature hearts. However, in the aged hearts, post-ischemic functional recovery was slightly reduced compared to that of the mature hearts, and only the coronary flow was well-preserved. In the mature hearts, myocardial infarction in the ischemic preconditioned hearts (14.9 +/- 1.3%) and in the control hearts (1.0 +/- 0.3%) was significantly decreased (p < 0.01) compared to that of the global ischemic hearts (32.9 +/- 5.1%). In the aged hearts, myocardial infarction in the ischemic preconditioned hearts (18.9 +/- 2.7%) and in the control hearts (1.1 +/- 0.6%) was significantly decreased (p < 0.001) compared to that of the global ischemic hearts (37.6 +/- 3.7%). The relationship between infarct size and post-ischemic functional recovery of left ventricularpeak developed pressure (LVDP) was linear and the correlation negative, with r = -0.934 (p < 0.001) and -0.875 (p < 0.001) for mature and aged hearts respectively. The data suggest that, in the senescent myocardium, the cellular pathways involved ischemic preconditioning responses that were post-ischemic, and that functional recovery was worse as compared to that of the mature myocardium. Furthermore, the effects of post-ischemic functional recovery became consistently weaker during the control period of 120 min. reperfusion after a prolonged ischemic insult in a buffer perfused isolated rabbit model. However, the effects of infarct size limitation were well-preserved in both senescent and mature myocardia.

Functional and metabolic effects of extracellular magnesium in normoxic and ischemic myocardium

Metabolic and functional responses to extracellular Mg2+ concentration ([Mg2+]o) were studied in perfused rat heart. Elevations of [Mg2+]o from 1.2 to 2.4, 5.0, and 8.0 mM dose dependently reduced contractile function and myocardial oxygen consumption (MVO2) up to 80%. Intracellular Mg2+ concentration ([Mg2+]i) remained stable (0.45-0.50 mM) during perfusion with 1.2-5. 0 mM [Mg2+]o but increased to 0.81 +/- 0.14 mM with 8.0 mM [Mg2+]o. Myocardial ATP was unaffected by [Mg2+]o, phosphocreatine (PCr) increased up to 25%, and Pi declined by up to 50%. Free energy of ATP hydrolysis (DeltaGATP) increased from -60 to -64 kJ/mol. Adenosine efflux declined in parallel with changes in MVO2 and [AMP]. At comparable workload and MVO2, the effects of [Mg2+]o on cytosolic free energy were mimicked by reduced extracellular Ca2+ concentration ([Ca2+]o) or Ca2+ antagonism with verapamil. Moreover, functional and energetic effects of [Mg2+]o were reversed by elevated [Ca2+]o. Despite similar reductions in preischemic function and MVO2, metabolic and functional recovery from 30 min of global ischemia was enhanced in hearts treated with 8.0 mM [Mg2+]o vs. 2.0 microM verapamil. It is concluded that 1) 1.2-8.0 mM [Mg2+]o improves myocardial cytosolic free energy indirectly by reducing metabolic rate and Ca2+ entry; 2) [Mg2+]i does not respond rapidly to elevations in [Mg2+]o from 1.2 to 5.0 mM and is uninvolved in acute functional and metabolic responses to [Mg2+]o; 3) adenosine formation in rat heart is indirectly reduced during elevated [Mg2+]o; and 4) 8.0 mM [Mg2+]o provides superior protection during ischemia-reperfusion compared with functionally equipotent Ca2+ channel blockade.

Adenosine-enhanced ischemic preconditioning provides enhanced postischemic recovery and limitation of infarct size in the rabbit heart

OBJECTIVE

The purpose of this study was to determine the effect of an intracoronary bolus injection of adenosine used in concert with ischemic preconditioning on postischemic functional recovery and infarct size reduction in the rabbit heart and to compare adenosine-enhanced ischemic preconditioning with ischemic preconditioning and magnesium-supplemented potassium cardioplegia.

METHODS

New Zealand White rabbits (n = 36) were used for Langendorff perfusion. Control hearts were perfused at 37 degrees C for 180 minutes; global ischemic hearts received 30 minutes of global ischemia and 120 minutes of reperfusion; magnesium-supplemented potassium cardioplegic hearts received cardioplegia 5 minutes before global ischemia; ischemic preconditioned hearts received 5 minutes of zero-flow global ischemia and 5 minutes of reperfusion before global ischemia; adenosine-enhanced ischemic preconditioned hearts received a bolus injection of adenosine just before the preconditioning. To separate the effects of adenosine from adenosine-enhanced ischemic preconditioning, a control group received a bolus injection of adenosine 10 minutes before global ischemia.

RESULTS

Infarct volume in global ischemic hearts was 32.9% +/- 5.1% and 1.03% +/- 0.3% in control hearts. The infarct volume decreased (10.23% +/- 2.6% and 7.0% +/- 1.6%, respectively; p < 0.001 versus global ischemia) in the ischemic preconditioned group and control group, but this did not enhance postischemic functional recovery. Magnesium-supplemented potassium cardioplegia and adenosine-enhanced ischemic preconditioning significantly decreased infarct volume (2.9% +/- 0.8% and 2.8% +/- 0.55%, respectively; p < 0.001 versus global ischemia, p = 0.02 versus ischemic preconditioning and p = 0.05 versus control group) and significantly enhanced postischemic functional recovery.

CONCLUSIONS

Adenosine-enhanced ischemic preconditioning is superior to ischemic preconditioning and provides equal protection to that afforded by magnesium-supplemented potassium cardioplegia.

The relationship between calcium and magnesium in pediatric myocardial protection

OBJECTIVE

We previously demonstrated that calcium can be harmful to the hypoxic neonatal heart. Despite the fact that magnesium inhibits membrane transport of calcium, few studies have examined whether magnesium can prevent the deleterious effects of calcium in cardioplegic solutions.

METHODS

Twenty neonatal piglets (5 to 18 days old) underwent 60 minutes of ventilator hypoxia (inspired oxygen fraction 8% to 10%) followed by reoxygenation with the use of cardiopulmonary bypass before cardioplegic arrest to produce a clinically relevant hypoxic "stress" injury. The aorta was then crossclamped for 70 minutes with multidose blood cardioplegia. Ten piglets received a hypocalcemic (0.2 to 0.4 mmol/L) cardioplegic solution without (group 1, n = 5) or with magnesium (10 mEq/L) (group II, n = 5) supplementation. Ten other piglets were protected with a normocalcemic (1.0 to 1.2 mmol/L) cardioplegic solution without (group III, n = 5) or with magnesium (group IV, n = 5). Myocardial function was assessed by means of pressure volume loops and expressed as a percentage of control. Coronary vascular resistance was assessed during each cardioplegic infusion.

RESULTS

Adding magnesium to a hypocalcemic cardioplegic solution (groups I and II) had no effect: Both groups had complete preservation of postbypass systolic function (end-systolic elastance 101% vs 104%) and preload recruitable stroke work (101% vs 102%), minimal increase in diastolic stiffness (159% vs 153%), and no difference in myocardial tissue edema (78.8% vs 78.9%) or coronary vascular resistance. Conversely, when a normocalcemic cardioplegic solution was administered without magnesium supplementation (group III), the results were markedly poorer than results obtained with magnesium supplementation (group IV). Without magnesium, there was a marked reduction in postbypass systolic function (end-systolic elastance 49% vs 101%; p < 0.05), increased diastolic stiffness (276% vs 162%; p < 0.05), decreased preload recruitable stroke work (53% vs 102%; p < 0.05), increased myocardial tissue edema (80.0% vs 78.9%; p < 0.05), and a rise in coronary vascular resistance (p < 0.05). Magnesium supplementation of the normocalcemic cardioplegic solution, by contrast, resulted in complete functional recovery.

CONCLUSIONS

This study demonstrates that (1) magnesium does not alter the cardioprotective effects of a hypocalcemic cardioplegic solution, (2) a normocalcemic cardioplegic solution is detrimental to neonatal myocardium subjected to a previous hypoxic stress, and (3) magnesium supplementation of normocalcemic cardioplegic solutions prevents the deleterious effects of calcium.

Potassium channel openers prevent potassium-induced calcium loading of cardiac cells: possible implications in cardioplegia

Hyperkalemic solutions that are used as cardioplegic agents, while effective in inducing electromechanical arrest, are only partially cardioprotective, and ventricular dysfunction has been observed. The underlying pathophysiology of cardioplegia-associated ventricular dysfunction is complex and not fully understood, but it could be related, in part, to intracellular Ca2+ loading induced by high K+ concentrations present in cardioplegic solutions. Yet no effective cytoprotective means against possible intracellular Ca2+ loading, under these conditions, has been described. Recently, potassium channel openers, which open adenosine triphosphate-sensitive K+ channels, have been reported to possess cardioprotective properties under global ischemic conditions. However, it is not known whether these novel agents could prevent intracellular Ca2+ loading that could occur during cardioplegia. Intracellular Ca2+ was monitored in ventricular myocytes, loaded with the Ca(2+)-sensitive fluorescent probe Fluo-3AM, using epifluorescent digital imaging and laser confocal microscopy. Exposure of a myocyte to a 16 mmol/L concentration of K+, a concentration of K+ commonly used in cardioplegic solutions, induced a nonhomogeneous increase in intracellular Ca2+. Potassium channel opening drugs, such as aprikalim or nicorandil, effectively prevented these solutions from increasing intracellular Ca2+. The preventive effect of potassium channel opening drugs was antagonized by glyburide, a selective blocker of adenosine triphosphate-sensitive K+ channels. This study demonstrates, at the single cardiac cell level, that solutions containing a 16 mmol/L concentration of K+ promote intracellular Ca2+ loading, which can be prevented by potassium channel opening drugs. Therefore, potassium channel opening drugs should be considered to prevent intracellular Ca2+ loading associated with the use of cardioplegic solutions.

Developmental differences in cytosolic calcium accumulation associated with surgically induced global ischemia: optimization of cardioplegic protection and mechanism of action

OBJECTIVE

The effect of cardioplegic solutions with high concentrations of potassium or magnesium (or both) on cytosolic calcium accumulation was investigated with fura-2 in isolated perfused mature (n = 24) and aged (n = 24) rabbit hearts.

METHODS

We compared cytosolic calcium accumulation before ischemia (control), during 30 minutes of ischemia and 30 minutes of reperfusion under global ischemia, or after treatment with potassium (20 mmol/L), magnesium (20 mmol/L), or both.

RESULTS

Cytosolic calcium accumulation was increased during global ischemia in the mature heart (from 178.7 +/- 24.2 in the control group to 393.6 +/- 25.5 nmol/L; p < 0.005) and in the aged heart (from 187.4 +/- 18.7 in the control group to 501.0 +/- 46.1 nmol/L; p < 0.005). Potassium reduced cytosolic calcium accumulation during ischemia in both the mature and aged hearts (300.9 +/- 23.2 and 365.2 +/- 27.7 nmol/L, respectively; p < 0.05 vs global ischemia). Magnesium and potassium/magnesium completely controlled cytosolic calcium accumulation in the mature heart (198.7 +/- 27.5 nmol/L; p < 0.01 vs global ischemia and p < 0.05 vs potassium: 182.3 +/- 22.7 nmol/L; p < 0.05 vs global ischemia and potassium, respectively). Magnesium and potassium/magnesium attenuated cytosolic calcium accumulation in the aged heart (261.3 +/- 26.7, 262.3 +/- 25.2 nmol/L, respectively; p < 0.01 vs global ischemia). These changes in cytosolic calcium accumulation correlated with improved post-ischemic ventricular function. To investigate the mechanism(s) of magnesium-supplemented cardioplegic inhibition of cytosolic calcium accumulation, we performed parallel studies (n = 43) using nifedipine, ryanodine, and dimethylthiourea. Nifedipine with or without ryanodine reduced cytosolic calcium accumulation. Dimethylthiourea did not alter cytosolic calcium accumulation during global ischemia. Our results suggest that cytosolic calcium accumulation during global ischemia was mainly increased via the sarcolemmal 1-type calcium channel and the sarcoplasmic reticulum calcium-release channel. The modulating action of potassium/magnesium cardioplegia on cytosolic calcium accumulation during ischemia would appear to act through the inhibition of the myocardial 1-type calcium channel and the sarcoplasmic reticulum calcium-release channel.

CONCLUSION

Senescent cardiac dysfunction correlates with increased ischemia-induced cytosolic calcium accumulation. Magnesium-supplemented potassium cardioplegia ameliorates this age-related phenomenon at normothermia and may have important implications in myocardial protection in the elderly population.

Is the preconditioning response conserved in senescent myocardium?

BACKGROUND

Senescent myocardium differs from adult myocardium at both functional and cellular levels. To adjudicate the efficacy of ischemic preconditioning as an alternative or adjuvant myoprotective strategy a reproducible, age-independent, intact laboratory model is necessary.

METHODS

Adult (0.5 to 1.0 years) and senescent (5.7 to 8.0 years) sheep underwent 60 minutes of normothermic regional ischemia with 150 minutes of reperfusion. Group II (adult-ischemic preconditioning) and group IV (aged-ischemic preconditioning) underwent preconditioning with three 5-minute episodes of normothermic regional ischemia. Group I (adult-control) and group III (aged-control) were not preconditioned.

RESULTS

Risk size and infarct size weights were delineated by monastryl blue pigment infusion and buffered tetrazolium solution. Ischemic preconditioning was evidenced by an infarct size reduction of 54% for adult sheep and 47% for senescent sheep (p < 0.01 versus age-matched controls; p = not significant for adult versus senescent).

CONCLUSIONS

The data suggest that the cellular pathways involved with the preconditioning response are well preserved in senescent myocardium and support the utility of the ovine heart model to investigate the clinical relevance of ischemic preconditioning for the increasingly aged population presently undergoing cardiac operations.

Magnesium cardioplegia enhances mRNA levels and the maximal velocity of cytochrome oxidase I in the senescent myocardium during global ischemia

BACKGROUND

The aged myocardium accumulates significantly more cytosolic calcium [Ca2+]i during ischemia, and functional recovery is more severely compromised as compared with the mature heart. Cardioplegia ameliorates these phenomena. The mechanism by which increased calcium accumulation reduces functional recovery in the senescent myocardium is unknown, but it has been suggested that futile calcium cycling in the mitochondria leading to depletion of ATP stores during normothermic global ischemia may be involved.

METHODS AND RESULTS

To investigate the effect of cardioplegia on mitochondrial calcium ([Ca2+]mt) accumulation and the expression of cytochrome oxidase I (COX I) during global ischemia, mitochondria were isolated from mature (age, 15 to 20 weeks) and aged (age > 130 weeks) rabbit hearts after Langendorff perfusion. Five perfused heart groups were investigated: 30 minutes of global ischemia without treatment (control), with potassium (K, 20 mmol/L), magnesium (Mg, 20 mmol/L), or potassium and magnesium (K/Mg) cardioplegia. No significant difference in [Ca2+]mt was evident in mature hearts with any protocol. In aged hearts, [Ca2+]mt was increased in global ischemia but was ameliorated with Mg and K/Mg cardioplegia. COX I mRNA levels in aged hearts were lower in both control and global ischemia but were increased with cardioplegia. Maximal velocities for COX I were significantly increased with Mg cardioplegia both in the mature and the aged myocardium.

CONCLUSIONS

K and/or Mg cardioplegia ameliorates [Ca2+]mt accumulation in aged hearts during normothermic global ischemia and increases COX I mRNA levels to a level not significantly different from that found in mature hearts.

Myocardial mitochondrial calcium accumulation modulates nuclear calcium accumulation and DNA fragmentation

BACKGROUND

Previously, we have shown that normothermic global ischemia increases cytosolic calcium accumulation in both the mature and aged heart. Increased nuclear and mitochondrial calcium accumulation was shown to occur in the aged but not the mature heart, and these age-related differences were associated with increased DNA fragmentation and decreased cellular viability only in the aged heart.

METHODS

To investigate the relationship between increased mitochondrial and nuclear calcium and DNA fragmentation, mature and aged rabbit hearts were subjected to normothermic global ischemia with and without the addition of ruthenium red to block mitochondrial calcium influx. Cytosolic calcium accumulation was measured in a parallel experiment using fura-2.

RESULTS

Ruthenium red ameliorated mitochondrial calcium accumulation and was associated with both decreased DNA fragmentation and decreased nuclear calcium accumulation.

CONCLUSIONS

Nuclear calcium accumulation was correlated with increased mitochondrial calcium accumulation but not increased cytosolic calcium accumulation in the aged heart. Modulation of mitochondrion "futile calcium cycling" may be of significance in the modulation of ischemic myocardial injury.

[Indications for the use of magnesium in anesthesia and intensive care]

Magnesium (Mg), a cofactor in numerous enzymatic reactions, is often ignored by clinicians, as the symptomatology of Mg depletion is not specific and usually associated with that of the cause of the depletion. Furthermore, the plasma Mg concentration (0.8 to 1.1 mmol.L-1) is only equivalent to one percent of the total body content. A Mg deficit may exist while plasma Mg concentration is normal. Therefore other techniques for Mg assessment, such as the repletion test, as well as red blood cell and lymphocyte concentrations have been used. A renewed interest for Mg occurred as numerous studies have shown the therapeutic efficiency of Mg and as the mechanisms of its haemodynamic effects have been recognized. Mg regulates Na-K-ATPase activity, K channels activity and, most of all, it is a natural calcium channel blocking agent. These properties explain its important place in electrophysiology of myocardial cells and the effects on the tension of smooth muscles, resulting in a vasodilation and a bronchodilation respectively. The antagonistic effect of Mg on calcium decreases the presynaptic release of acetylcholine at the neuromuscular junction and the release of epinephrine at the peripheral sympathetic nerves and the adrenals. Mg potentiates the effect of non-depolarizing muscle relaxants. A Mg deficiency occurs often in ICU patients, in alcoholics and during use of diuretics. Simultaneous administration of Mg is often required for treatment of potassium deficiency. Mg has an anti-arrhythmic effect towards digoxin-mediated dysrhythmias and torsades de pointes, and can be efficient in other arrhythmias. Systematic use of Mg seems to decrease mortality of acute myocardial infarction and is justified during cardiac surgery, often associated with hypomagnesemia, because of vasodilation of coronary arteries and in order to prevent occurrence of arrhythmias. Mg, because of its calcium channel blocking properties and as it lowers the release of epinephrine, is indicated for surgery of pheochromocytoma. In eclamptic and pre-eclamptic patients, the use of Mg is valuable, but not as an anti-epileptic agent. Other clinical uses of Mg have been proposed, but they are either anecdotal or of uncertain efficiency.