Age-related differences in Na+-dependent Ca2+ accumulation in rabbit hearts exposed to hypoxia and acidification

Quantitative model of NMR chemical shifts of 23Na+ induced by TmDOTP: applications in studies of Na+ transport in human erythrocytes

The change in the NMR chemical shift of (23)Na(+) induced by the shift reagent TmDOTP was examined under various experimental conditions typical of cells, including changed Na(+), K(+), PO(4)(3-), and Ca(2+) concentrations, pH and temperature. A mathematical model was developed relating these factors to the observed chemical shift change relative to a capillary-sphere reference. This enabled cation concentrations to be deduced quantitatively from experimental chemical shifts, including those observed during biological time courses with cell suspensions containing TmDOTP DOTP, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis (methylenephosphonate) [corrected]. The model was applied to a (23)Na NMR time course in which monensin, a sodium ionophore, was introduced to human erythrocytes, changing the concentration of cations which may bind TmDOTP, and also resulting in cell volume changes. Using the model with experimentally determined conditions, the chemical shift was predicted and closely followed the experimental values over time. In addition to the model, parameter fitting was achieved by calculating the likelihood distribution of parameters, and seeking the maximum likelihood with a Bayesian type of analysis.

KATP channel blocker does not abolish the protective effect of Na+/H+ exchange 1 inhibition against ischaemia/reperfusion in aged myocardium

BACKGROUND AND OBJECTIVE

Ageing is associated with an increase in myocardial susceptibility to ischaemia/reperfusion (I/R) injury. Na+/H+ exchange (NHE) inhibition and anaesthetic preconditioning (APC) are shown to protect myocardium from I/R injury. We set out to investigate whether NHE inhibition can induce protection against I/R injury and whether KATP channel inhibition can enhance this effect in aged rat myocardium.

METHODS

Hearts from 24-month-old rats were assigned to four groups: control group; APC group perfused with 2.5% sevoflurane before ischaemia; HOE group perfused with (3-methylsulfonyl-4-piperidinobenzoyl) guanidine methanesulfonate (HOE-694) prior to ischaemia; and HOE+5HD group perfused with both HOE and 5-hydroxydecanoic acid before ischaemia. We measured intracellular Na+ and Ca++ to quantitate the severity of myocardial injury.

RESULTS

Both intracellular Na+ and Ca++ were significantly increased at the end of ischaemia and both were attenuated by NHE inhibition. Intracellular Na+ was 134 +/- 12 mEq kg(-1) dry weight in control group and 55 +/- 7 in HOE group (P < 0.05). Intracellular Ca++ was 1764 +/- 142 nmol l(-1) in control group and 694 +/- 213 in HOE group (P < 0.05). Infarct size was measured at 28 +/- 4% in control group vs. 17 +/- 2% in HOE group (P < 0.05). High-energy phosphates and myocardial function were better preserved in HOE group compared with control (P < 0.05). The beneficial effect of HOE on myocardial preservation was not blocked by 5HD nor were there any differences between APC and control groups.

CONCLUSION

NHE inhibition was effective in protecting myocardium from I/R injury in aged rats, whereas APC was not. 5HD failed to block the protective effect of NHE inhibition.

Na/H exchange inhibition protects newborn heart from ischemia/reperfusion injury by limiting Na+-dependent Ca2+ overload

The results of the Guardian/Expedition trials demonstrate the need for more precisely controlled studies to inhibit Na/H exchange (NHE1) during ischemia/reperfusion. This is because overwhelming evidence is consistent with the hypothesis that myocardial ischemic injury results in part from increases in intracellular Na (Nai) mediated by NHE1 that in turn promote Na/Ca exchanger-mediated increases in intracellular Ca ([Ca]i) and Ca-dependent cell damage. We used a more potent and specific NHE1 inhibitor HOE 694 (HOE) to test whether inhibition of NHE1 during ischemia limits increases in Nai and [Ca]i in newborns. NMR was used to measure pHi, Nai, [Ca]i, and ATP in isolated newborn rabbit hearts. Perfusion pressure, left ventricular developed pressure, and creatine kinase were measured. HOE was added before global ischemia. Results are reported as mean +/- SE. Nai (mEq/kg dry weight) rose from 11.6 +/- 0.9 before ischemia to 114.0 +/- 16.1 at the end of ischemia and recovered to 55.2 +/- 11.8 in the control group. During ischemia and reperfusion, the corresponding values for Nai in the HOE group (63.1 +/- 8.4 and 15.9 +/- 2.5, respectively, P < 0.05) were lower than control. In the control group [Ca]i (nM/L) rose from 331 +/- 41 to 1069 +/- 71 and recovered to 814 +/- 51, whereas in the HOE group [Ca]i rose less (P < 0.05): 359 +/- 50, 607 +/- 85, and 413 +/- 40, respectively. Total creatine kinase release was significantly reduced in the HOE group. Perfusion pressure and left ventricular developed pressure also recovered significantly better in the HOE group than in the control. In conclusion, NHE1 inhibition diminishes ischemia-induced increases in Nai and therefore [Ca], and thus diminishes myocardial injury in neonatal hearts.

Effect of thyroid hormone on mitochondrial properties and oxidative stress in cells from patients with mtDNA defects

Mitochondrial (mt)DNA mutations contribute to various disease states characterized by low ATP production. In contrast, thyroid hormone [3,3',5-triiodothyronine (T(3))] induces mitochondrial biogenesis and enhances ATP generation within cells. To evaluate the role of T(3)-mediated mitochondrial biogenesis in patients with mtDNA mutations, three fibroblast cell lines with mtDNA mutations were evaluated, including two patients with Leigh's syndrome and one with hypertrophic cardiomyopathy. Compared with control cells, patient fibroblasts displayed similar levels of mitochondrial mass, peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha), mitochondrial transcription factor A (Tfam), and uncoupling protein 2 (UCP2) protein expression. However, patient cells exhibited a 1.6-fold elevation in ROS production, a 1.7-fold elevation in cytoplasmic Ca2+ levels, a 1.2-fold elevation in mitochondrial membrane potential, and 30% less complex V activity compared with control cells. Patient cells also displayed 20-25% reductions in both cytochrome c oxidase (COX) activity and MnSOD protein levels compared with control cells. After T(3) treatment of patient cells, ROS production was decreased by 40%, cytoplasmic Ca2+ was reduced by 20%, COX activity was increased by 1.3-fold, and ATP levels were elevated by 1.6-fold, despite the absence of a change in mitochondrial mass. There were no significant alterations in the protein expression of PGC-1alpha, Tfam, or UCP2 in either T(3)-treated patient or control cells. However, T(3) restored the mitochondrial membrane potential, complex V activity, and levels of MnSOD to normal values in patient cells and elevated MnSOD levels by 21% in control cells. These results suggest that T(3) acts to reduce cellular oxidative stress, which may help attenuate ROS-mediated damage, along with improving mitochondrial function and energy status in cells with mtDNA defects.

RESUSCITATION WITH 21% OR 100% OXYGEN IS EQUALLY EFFECTIVE IN RESTORING PERFUSION AND OXYGEN METABOLISM IN THE LIVER OF HYPOXIC NEWBORN PIGLETS

The differential effects of the use of high or low oxygen levels during resuscitation on the neonatal liver are unknown. We compared the hepatic hemodynamics and oxygen metabolism in hypoxic newborn piglets resuscitated with 21% or 100% oxygen. Twenty-seven piglets (age, 1-3 days; weight, 1.5-2.0 kg) were acutely instrumented to measure cardiac output, hepatic artery, and portal venous blood flows (hepatic artery flow index [HAFI] and portal venous flow index [PVFI], respectively). The animals underwent 2 h of hypoxia (fraction of inspired oxygen, 0.10-0.15), then reoxygenation with 21% (n = 9) or 100% (n = 9) oxygen for 1 h, then 1 h with 21% oxygen. The controls (n = 9) were sham-operated without hypoxia-reoxygenation. Oxygen transport and plasma lactate concentrations were studied. Hypoxic animals had hypotension and decreased cardiac index with metabolic acidosis (mean pH, 7.00-7.02; P < 0.05 vs. controls). The PVFI and the total hepatic blood flow (THFI = PVFI + HAFI), despite the absence of significant change in HAFI, decreased to 16 +/- 2 mL/min/kg and 19 +/- 3 mL/min/kg, respectively (versus 24 +/- 2 mL/min/kg and 28 +/- 2 mL/min/kg of controls; P < 0.05). Fifteen minutes after reoxygenation, the cardiac index improved, PVFI recovered, HAFI was maintained, and THFI was not different between the groups. The hepatic oxygen consumption decreased (59%; P < 0.05) and the extraction increased (89%; P < 0.001) during hypoxia. Similarly, on reoxygenation, the hepatic oxygen consumption improved; however, extraction decreased versus controls on 100% but not on 21% oxygen (P < 0.05). The plasma lactate concentrations increased in both groups with hypoxia and were not different during reoxygenation between the group administered with 21% oxygen and the group administered with 100% oxygen. The hypoxic neonatal liver has reduced hepatic blood flow but has relatively preserved HAFI, and oxygen consumption recovered similarly on reoxygenation with 21% and 100% oxygen. The increased oxygen extraction during hypoxia normalized in 21% but reduced in 100% reoxygenation, with no differences in plasma lactate concentrations.

Inhaled Sevoflurane Produces Better Delayed Myocardial Protection at 48 Versus 24 Hours After Exposure

Ischemia preconditioning produces a delayed window of cardioprotection against subsequent ischemia and reperfusion injury. Contradictory results have been reported regarding the ability of inhaled anesthetics to produce similar effects. Our investigation was designed to test whether inhaled sevoflurane is capable of producing a delayed window of anesthetic preconditioning and to compare the differences at 24 and 48 h after exposure. Male Fischer-344 rats, 2-4 mo old, were exposed to sevoflurane (2.5% for 60 min). Twenty-four or 48 h after exposure, the hearts were isolated and perfused for 30 min (equilibration) followed by 25 min of ischemia and then 60 min of reperfusion. Control hearts received no treatment before ischemia. Left ventricular (LV) function, creatine kinase (CK), and infarct size (IS) were measured. Nuclear magnetic resonance was used to measure Na+(i), [Ca2+]i, and pH(i). There was improved LV function and significant reduction in IS and CK and in both the 24- and 48-h delayed groups compared with the controls. There was also a significant recovery of LV function and reduction in IS and CK in the 48-h group when compared with the 24-h group. There was significant adenosine triphosphate preservation in both the 24- and 48-h groups, as well as a significant reduction in acidosis, [Ca2+]I, and Na+(i) in response to ischemia in both the groups versus the control. Sevoflurane is capable of producing a delayed window of preconditioning, and it takes more than 24 h to produce maximal protective effects.

Effects of cold cardioplegia on pH, Na, and Ca in newborn rabbit hearts

Many studies suggest myocardial ischemia-reperfusion (I/R) injury results largely from cytosolic proton (Hi)-stimulated increases in cytosolic Na (Nai), which cause Na/Ca exchange-mediated increases in cytosolic Ca concentration ([Ca]i). Because cold, crystalloid cardioplegia (CCC) limits [H]i, we tested the hypothesis that in newborn hearts, CCC diminishes Hi, Nai, and Cai accumulation during I/R to limit injury. NMR measured intracellular pH (pHi), Nai, [Ca]i, and ATP in isolated Langendorff-perfused newborn rabbit hearts. The control ischemia protocol was 30 min for baseline perfusion, 40 min for global ischemia, and 40 min for reperfusion, all at 37°C. CCC protocols were the same, except that ice-cold CCC was infused for 5 min before ischemia and heart temperature was lowered to 12°C during ischemia. Normal potassium CCC solution (NKCCC) was identical to the control perfusate, except for temperature; the high potassium (HKCCC) was identical to NKCCC, except that an additional 11 mmol/l KCl was substituted isosmotically for NaCl. NKCCC and HKCCC were not significantly different for any measurement. The following were different ( P < 0.05). End-ischemia pHi was higher in the CCC than in the control group. Similarly, CCC limited increases in Nai during I/R. End-ischemia Nai values (in meq/kg dry wt) were 115 ± 16 in the control group, 49 ± 13 in the NKCCC group, and 37 ± 12 in the HKCCC group. CCC also improved [Ca]i recovery during reperfusion. After 40 min of reperfusion, [Ca]i values (in nmol/l) were 302 ± 50 in the control group, 145 ± 13 in the NKCCC group, and 182 ± 19 in the HKCCC group. CCC limited ATP depletion during ischemia and improved recovery of ATP and left ventricular developed pressure and decreased creatine kinase release during reperfusion. Surprisingly, CCC did not significantly limit [Ca]i during ischemia. The latter is explained as the result of Ca release from intracellular buffers on cooling.

Cardiac function, myocardial glutathione, and matrix metalloproteinase-2 levels in hypoxic newborn pigs reoxygenated by 21%, 50%, or 100% oxygen

After asphyxia, it is standard to resuscitate the newborn with 100% oxygen, which may create a hypoxia-reoxygenation process that may contribute to subsequent myocardial dysfunction. We examined the effects of graded reoxygenation on cardiac function, myocardial glutathione levels, and matrix metalloproteinase (MMP)-2 activity during recovery. Thirty-two piglets (1-3 days old, weighing 1.5-2.1 kg) were anesthetized and instrumented for continuous monitoring of cardiac index, and systemic and pulmonary arterial pressures. After 2 h of hypoxia, piglets were randomized to receive reoxygenation for 1 h with 21%, 50%, or 100% oxygen (n = 8 each), followed by 3 h at 21% oxygen. At 2 h of hypoxemia (PaO2 32-34 mmHg), the animals had hypotension, decreased cardiac index, and elevated pulmonary arterial pressure (P < 0.001 vs. controls). Upon reoxygenation, cardiac function recovered in all groups with higher cardiac index and lower systemic vascular resistance in the 21% group at 30 min of reoxygenation (P < 0.05 vs. controls). Pulmonary artery pressure normalized in an oxygen-dependent fashion (100% = 50% > 21%), despite an immediate recovery of pulmonary vascular resistance in all groups. The hypoxia-reoxygenated (21%-100%) hearts had similarly increased MMP-2 activity and decreased glutathione levels (P < 0.05, 100% vs. controls), which correlated significantly with cardiac index and stroke volume during reoxygenation, and similar features of early myocardial necrosis. In neonatal resuscitation, if used with caution because of a slower resolution of pulmonary hypertension, 21% reoxygenation results in similar cardiac function and early myocardial injury as 50% or 100%. The significance of higher oxidative stress with high oxygen concentration is unknown, at least in the acute recovery period.

Sevoflurane preconditioning limits intracellular/mitochondrial Ca2+ in ischemic newborn myocardium

Sevoflurane preconditioning (SPC) in adult hearts reduces myocardial ischemia/reperfusion (I/R) injury, an effect that may be mediated by reductions in intracellular Ca(2+) ([Ca(2+)](i)) and/or mitochondrial Ca(2+) ([Ca(2+)](m)) accumulation during ischemia and reperfusion. Because the physiology, pharmacology, and metabolic responses of the newborn differ from adults, we tested the hypothesis that SPC protects newborn myocardium by limiting [Ca(2+)](i) and [Ca(2+)](m) by a K(ATP) channel-dependent mechanism. Fluorescence spectrofluorometry and nuclear magnetic resonance spectroscopy were used to measure [Ca(2+)](i), [Ca(2+)](m), and adenosine triphosphate (ATP) in 4- to 7-day-old Langendorff-perfused rabbit hearts. Three experimental groups were used to study the effect of SPC on [Ca(2+)](m)/[Ca(2+)](i), ATP, as well as hemodynamics and ischemic injury. The role of mitochondrial K(ATP) channels was assessed by exposing the SPC hearts to the mitochondrial K(ATP) channel blocker 5-hydroxydecanoic acid. Our results show that SPC significantly decreased [Ca(2+)](i) and [Ca(2+)](m) during I/R, as well as decreased creatine kinase release during reperfusion and resulted in higher ATP. 5-Hydroxydecanoic acid abolished the effect of SPC on [Ca(2+)], hemodynamics, ATP, and creatine kinase release. In conclusion, decreased [Ca(2+)](i) and [Ca(2+)](m) observed with SPC is associated with greater ATP recovery as well as diminished cell injury. Mitochondrial K(ATP) channel blockade attenuates the SPC effect during I/R, suggesting that these channels are involved in the protective effects of SPC in the newborn.

IMPLICATIONS

The results of this study support the hypothesis that sevoflurane preconditioning protects newborn hearts from calcium overload and ischemic injury via a mechanism dependent on mitochondrial KATP channels.

Acute effects of 17β-estradiol on myocardial pH, Na+, and Ca2+ and ischemia-reperfusion injury

Evidence suggests that 1) ischemia-reperfusion injury is due largely to cytosolic Ca2+ accumulation resulting from functional coupling of Na+/Ca2+ exchange (NCE) with stimulated Na+/H+ exchange (NHE1) and 2) 17β-estradiol (E2) stimulates release of NO, which inhibits NHE1. Thus we tested the hypothesis that acute E2 limits myocardial Na+ and therefore Ca2+ accumulation, thereby limiting ischemia-reperfusion injury. NMR was used to measure cytosolic pH (pHi), Na+ (Na[Formula: see text]), and calcium concentration ([Ca2+]i) in Krebs-Henseleit (KH)-perfused hearts from ovariectomized rats (OVX). Left ventricular developed pressure (LVDP) and lactate dehydrogenase (LDH) release were also measured. Control ischemia-reperfusion was 20 min of baseline perfusion, 40 min of global ischemia, and 40 min of reperfusion. The E2 protocol was identical, except that 1 nM E2 was included in the perfusate before ischemia and during reperfusion. E2 significantly limited the changes in pHi, Na[Formula: see text] and [Ca2+]i during ischemia ( P < 0.05). In control OVX vs. OVX+E2, pHi fell from 6.93 ± 0.03 to 5.98 ± 0.04 vs. 6.96 ± 0.04 to 6.68 ± 0.07; Na[Formula: see text] rose from 25 ± 6 to 109 ± 14 meq/kg dry wt vs. 25 ± 1 to 76 ± 3; [Ca2+]i changed from 365 ± 69 to 1,248 ± 180 nM vs. 293 ± 66 to 202 ± 64 nM. E2 also improved recovery of LVDP and diminished release of LDH during reperfusion. Effects of E2 were diminished by 1 μM Nω-nitro-l-arginine methyl ester. Thus the data are consistent with the hypothesis. However, E2 limitation of increases in [Ca2+]i is greater than can be accounted for by the thermodynamic effect of reduced Na[Formula: see text] accumulation on NCE.