Alteration of intracellular Na+ during ischemia in diabetic rat hearts: the role of reduced activity in Na+/H+ exchange against stunning

Functional and structural impact of pirfenidone on the alterations of cardiac disease and diabetes mellitus

A synthetic compound, termed pirfenidone (PFD), is considered promising for the treatment of cardiac disease. It leads to beneficial effects in animal models of diabetes mellitus (DM); as well as in heart attack, atrial fibrillation, muscular dystrophy, and diabetic cardiomyopathy (DC). The latter is a result of alterations linked to metabolic syndrome as they promote cardiac hypertrophy, fibrosis and contractile dysfunction. Although reduced level of fibrosis and stiffness represent an essential step in the mechanism of PFD action, a wide range of functional effects might also contribute to the therapeutic benefits. For example, PFD stimulates L-type voltage-gated Ca(2+) channels (LTCCs), which are pivotal for a process known as excitation-contraction coupling (ECC). Recent evidence suggests that these two types of actions - namely structural and functional - aid in treating both cardiac disease and DM. This view is supported by the fact that in DC, for example, systolic dysfunction arises from both cardiac stiffness linked to fibrosis and down-regulation of ECC. Thus, not surprisingly, clinical trials have been conducted with PFD in the settings of DM, for treating not only cardiac but also renal disease. This review presents all these concepts, along with the possible mechanisms and pathophysiological consequences.

Intracellular levels of Na(+) and TTX-sensitive Na(+) channel current in diabetic rat ventricular cardiomyocytes

Intracellular Na(+) ([Na(+)](i)) is an important modulator of excitation-contraction coupling via regulating Ca(2+) efflux/influx, and no investigation has been so far performed in diabetic rat heart. Here, we examined whether any change of [Na(+)](i) in paced cardiomyocytes could contribute to functional alterations during diabetes. Slowing down in depolarization phase of the action potential, small but significant decrease in its amplitude with a slight depolarized resting membrane potential was traced in live cardiomyocytes from diabetic rat, being parallel with a decreased TTX-sensitive Na(+) channel current (I(Na)) density. We recorded either [Na(+)](i) or [Ca(2+)](i) by using a fluorescent Na(+) indicator (SBFI-AM or Na-Green) or a Ca(2+) indicator (Fura 2-AM) in freshly isolated cardiomyocytes. We examined both [Na(+)](i) and [Ca(2+)](i) at rest, and also [Na(+)](i) during pacing with electrical field stimulation in a range of 0.2-2.0 Hz stimulation frequency. In order to test the possible contribution of Na(+)/H(+) exchanger (NHE) to [Na(+)](i), we examined the free cytoplasmic [H(+)](i) changes from time course of [H(+)](i) recovery in cardiomyocytes loaded with SNARF1-AM by using ammonium prepulse method. Our data showed that [Na(+)](i) in resting cells from either diabetic or control group was not significantly different, whereas the increase in [Na(+)](i) was significantly smaller in paced diabetic cardiomyocytes compared to that of the controls. However, resting [Ca(2+)](i) in diabetic cardiomyocytes was significantly higher than that of the controls. Here, a lower basal pH(i) in diabetics compared with the controls correlates also with a slightly higher but not significantly different NHE activity and consequently a similar Na(+) loading rate at resting state with a leftward shift in pH sensitivity of NHE-dependent H(+)-flux. NHE protein level remained unchanged, while protein levels of Na(+)/K(+) ATPase and Na(+)/Ca(2+) exchanger were decreased in the diabetic cardiomyocytes. Taken together, the present data indicate that depressed I(Na) plays an important role in altered electrical activity with less Na(+) influx during contraction, and an increased [Ca(2+)](i) load in these cells seems to be independent of [Na(+)](i). The data with insulin treatment suggest further a recent balance between Na(+) influx and efflux proteins associated with the [Na(+)](i), particularly during diabetes.

Overexpression of the Na+/H+exchanger and ischemia-reperfusion injury in the myocardium

In the myocardium, the Na+/H+exchanger isoform-1 (NHE1) activity is detrimental during ischemia-reperfusion (I/R) injury, causing increased intracellular Na+(Nai+) accumulation that results in subsequent Ca2+overload. We tested the hypothesis that increased expression of NHE1 would accentuate myocardial I/R injury. Transgenic mice were created that increased the Na+/H+exchanger activity specifically in the myocardium. Intact hearts from transgenic mice at 10–15 wk of age showed no change in heart performance, resting intracellular pH (pHi) or phosphocreatine/ATP levels. Transgenic and wild-type (WT) hearts were subjected to 20 min of ischemia followed by 40 min of reperfusion. Surprisingly, the percent recovery of rate-pressure product (%RPP) after I/R improved in NHE1-overexpressing hearts (64 ± 5% vs. 41 ± 5% in WT; P < 0.05). In addition, NMR spectroscopy revealed that NHE1 overexpressor hearts contained higher ATP during early reperfusion (levels P < 0.05), and there was no difference in Na+accumulation during I/R between transgenic and WT hearts. HOE642 (cariporide), an NHE1 inhibitor, equivalently protected both WT and NHE1-overexpressing hearts. When hearts were perfused with bicarbonate-free HEPES buffer to eliminate the contribution of HCO3−transporters to pHiregulation, there was no difference in contractile recovery after reperfusion between controls and transgenics, but NHE1-overexpressing hearts showed a greater decrease in ATP during ischemia. These results indicate that the basal activity of NHE1 is not rate limiting in causing damage during I/R, therefore, increasing the level of NHE1 does not enhance injury and can have some small protective effects.

Reduction of myocardial infarct size by SM-197378, a novel Na+/H+ exchange inhibitor, in rabbits

The effects of SM-197378, 2-[[[amino(imino)methyl]amino]carbonyl]-1-methyl-4-trifluoromethyl-1H-indol-7-yl=hydrogen=sulfate monohydrate, a novel potent Na+/H+exchange inhibitor, on heart injury were studied using a rabbit model involving 30 min of myocardial ischemia and 5 h of reperfusion. Intravenous administration of SM-197378 before ischemia reduced the infarct size by approximately 30-50% in a dose-dependent manner. This anti-necrotic effect was achieved without significant hemodynamic changes. Moreover, administration of SM-197378 before reperfusion also resulted in a significant, approximately 30-40%, reduction in the infarct size. The anti-necrotic effect of pre-ischemic bolus treatment with SM-197378 was compared with that of nicorandil, a K+channel opener with nitrate-like activity, and ischemic preconditioning. With 30 and 60 min of ischemia, the anti-necrotic effects of SM-197378 and ischemic preconditioning were similar and superior to that of nicorandil. With 90 min of ischemia, the anti-necrotic effect of SM-197378 was superior to that of ischemic preconditioning.

Reduction of myocardial infarct size by SM-198110, a novel Na+/H+ exchange inhibitor, in rabbits

The effects of 3-[2-({[amino(imino)methyl]amino}carbonyl)-4-chloro-1H-indol-1-yl]-1-propanesulphonic acid monohydrate (SM-198110), a novel potent Na+/H+ exchange inhibitor, and cariporide (Hoe642), another Na+/H+ exchange inhibitor, were studied in a myocardial ischaemia and reperfusion injury model. Anaesthetized rabbits were subjected to occlusion of the coronary artery for 30 min followed by reperfusion for 5 h. SM-198110 or cariporide was administered before ischaemia and before reperfusion. We also assessed the anti-necrotic effect of SM-198110 when given before reperfusion, both alone and together with glibenclamide, a K(ATP) channel blocker, 5-hydroxydecanoate (5-HD), a mitochondrial K(ATP) channel-selective blocker and 8-(p-sulphophenyl)-theophylline (8-SPT), an adenosine receptor blocker. The infarct size was reduced dose-dependently by i.v. administration of SM-198110 before ischaemia, with a significant reduction in serum creatine phosphokinase activity. Infarct sizes, normalized to the size of the area-at-risk (means+/-SE) were: vehicle 56.6+/-3.7%; low-dose SM-198110 39.2+/-6.3%; mid-dose 32.8+/-7.4% (P < 0.05); high-dose 22.1+/-6.7% (P < 0.01). This anti-necrotic effect of SM-198110 was achieved without significant haemodynamic changes. Cariporide given before ischaemia also reduced infarct size significantly and dose-dependently. SM-198110 administered before reperfusion also resulted in a dose-dependent reduction in the infarct size. Infarct sizes were: vehicle 56.6+/-3.7%; low-dose SM-198110 44.5+/-5.7%; mid-dose 36.3+/-6.6% (P < 0.01); high-dose 34.7+/-3.8% (P < 0.01). In contrast, cariporide given before reperfusion did not reduce infarct sizes significantly. The anti-necrotic effect of SM-198110 was observed even when given 10 min after the beginning of reperfusion. Glibenclamide and 5-HD abolished the anti-necrotic effect of treatment before reperfusion with SM-198110. However, the co-administration of 8-SPT with SM-198110 did not affect infarct size. These results suggest that, in addition to Na+/H+ exchange inhibition, mitochondrial and/or sarcolemmal K(ATP) channels contribute to the anti-necrotic effect of SM-198110 when the latter is given before reperfusion.

Different pathways for sodium entry in cardiac cells during ischemia and early reperfusion

A number of data are consistent with the hypothesis that increases in intracellular Na+ concentration (Na+i) during ischemia and early reperfusion lead to calcium overload and exacerbation of myocardial injury. However, the mechanisms underlying the increased Na+i remain unclear. 23Na nuclear magnetic resonance spectroscopy was used to monitor Na+i in isolated rat hearts perfused with a high concentration of fatty acid as can occur under some pathological conditions. Whole-cell patch-clamp experiments were also performed on isolated cardiomyocytes in order to investigate the role of voltage-gated sodium channels. Na+i increased to substantially above control levels during no-flow ischemia. The results show that a pharmacological reduction of Na+i increase by cariporide (1 micromol/L, a Na+/H+ exchange blocker) is not the only protection against ischemia-reperfusion damage, but that such protection may also be brought about by metabolic action aimed at reducing fatty acid utilization by myocardial cells. This action was obtained in the presence of etomoxir (0.1 micromol/L), an inhibitor of carnitine palmitoyltransferase-1 (the key enzyme involved in fatty acid uptake by the mitochondria) which also decreases long-chain acyl carnitine accumulation. The possibility of Na+ channels participating in Na+i increase as a consequence of alterations in cardiac metabolism was studied in isolated cells. Sustained I(Na) was stimulated by the presence of lysophosphatidylcholine (LPC, 10 micromol/L) whose accumulation during ischemia is, at least partly, dependent on increased long-chain acyl carnitine. Current activation was particularly significant in the range of potentials between -60 and -20 mV. This may have particular relevance in ischemia. The quantity of charge carried by sustained I(Na) was reduced by 24% in the presence of 1 micromol/L cariporide. Therefore, limitation of long-chain fatty acid metabolism, and consequent limitation of ischemia-induced long-chain acyl carnitine accumulation, may contribute to reducing intracellular Na+ increase during ischemia-reperfusion.

Is timing everything? Therapeutic potential of modulators of cardiac Na(+) transporters

Sodium ion (Na(+)) transporters have roles in the modulation of cardiomyocyte pH and Na(+) and Ca(2+) handling. Activation of the cardiac Na(+)-H(+) exchanger 1 (NHE1) during ischaemia induces arrhythmias, myocardial stunning and irreversible cell injury. As the benefits of NHE1 inhibitors (e.g., amiloride, cariporide) in models of myocardial infarction are usually much greater when used as pretreatment, rather than during or after ischaemia, it is probably not surprising that clinical trials with cariporide in ischaemia have shown little shortterm benefit. NHE1 inhibitors have been shown to be beneficial in animal models of ventricular fibrillation and resuscitation, cardioplegia, hypertrophy and heart failure, and their therapeutic potential in these conditions should be further developed. The Na(+)-HCO(3)(-) cotransporter (NBC) is also stimulated by intracellular acidification, and part of the benefit of angiotensin-converting enzyme inhibitors after myocardial infarction may be due to inhibition of the NBC. Selective inhibitors of the NBC are required to determine the therapeutic potential of this mechanism. The Na(+)-Ca(2+) exchanger (NCX) has a major role in cardiac Na(+) and Ca(2+) homeostasis and influences cardiac electrical activity. The NCX also has a role in ischaemia/infarction, arrhythmias, hypertrophy and heart failure. NCX inhibitors may have beneficial effects in animal models of ischaemia and reperfusion injury and the therapeutic benefit of these should be further studied in animal models.

Effects of epinephrine on underperfusion-reperfusion injuries in diabetic and non-diabetic rat hearts

The sympathetic nervous systems may bear relevance to the increased incidence of heart failure in diabetes (DM). In our isolated rat hearts perfused at constant low flow rate, norepinephrine dose-dependently enhanced diabetic myocardial damage, particularly during underperfusion. The purpose of this investigation is to examine the effects of epinephrine on the ischemic injury and on the reperfusion injury in DM and non-DM rat hearts, and to clarify whether the cardiac states during underperfusion at constant low pressure are similar to those at constant low flow rate. Isolated streptozotocin-induced 6-week DM and non-DM rat hearts with a balloon in the left ventricle (LV) were paced and normal perfused at 75 cm H2O with normoxic Krebs-Henseleit solution. Then the hearts were underperfused at 35 cm H2O, a constant low pressure with below one-third of the pre-ischemic coronary perfusion flow (CPF) level. Four min after the start of underperfusion, the perfusate was changed to that containing epinephrine 10(-6) M. After 45 min underperfusion with or without epinephrine, all of the hearts were reperfused without epinephrine at 75 cm H2O for 45 min. To detect changes in LV stiffness, the isometric tension along the longitudinal direction of the whole heart and the LV isovolumic pressure were monitored simultaneously. In DM hearts, the underperfusion alone caused a slight increase in LV stiffness, and all the changes recovered to the pre-ischemic levels during reperfusion. Epinephrine during underperfusion accelerated the start of increase in LV stiffness and the decrease in CPF. During reperfusion the changes recovered partly to the control levels. In non-DM hearts, epinephrine during underperfusion caused only a slight increase in LV stiffness though a similar low CPF to DM hearts. However, the reperfusion caused a marked increase in LV stiffness and a lower recovery of CPF. Epinephrine at constant low pressure, as well as norepinephrine at constant low flow rate, enhanced the ischemic injury, particularly in DM hearts, while aggravated the reperfusion injury in non-DM hearts.

Protection of ischemic myocardium in diabetics by inhibition of electroneutral Na+-K+-2Cl- cotransporter

Diabetes increases both the incidence of cardiovascular disease and complications of myocardial infarction and heart failure. Studies using diabetic animals have shown that changes in myocardial sodium transporters result in alterations in intracellular sodium (Na(i)) homeostasis. Because the changes in sodium homeostasis can be due to increased entry of Na+ via the electroneutral Na+-K+-2Cl- cotransporter (NKCC), we conducted experiments in acute diabetic hearts to determine if 1) net inward cation flux via NKCC is increased, 2) this cotransporter contributes to a greater increase in Na(i) during ischemia, and 3) inhibition of NKCC limits injury and improves function after ischemia-reperfusion. These issues were investigated in perfused type I diabetic and nondiabetic rat hearts subjected to ischemia and 60 min of reperfusion. A group of diabetic and nondiabetic hearts was perfused with 5 microM of bumetanide, an inhibitor of NKCC. Flux via NKCC, Na(i), and ATP was measured in each group with the use of radiotracer 86Rb, 23Na, and 31P nuclear magnetic resonance spectroscopy, respectively, whereas ischemic injury was assessed by measuring creatine kinase release on reperfusion. Cation flux via NKCC, as measured by 86Rb uptake, was significantly increased in diabetic hearts. Inhibition of NKCC significantly reduced ischemic injury in diabetic hearts, improved functional recovery on reperfusion, attenuated the ischemic rise in Na(i), and conserved ATP during ischemia-reperfusion. Parallel studies in nondiabetic hearts showed that NKCC inhibition was not cardioprotective. These findings demonstrate that flux via NKCC is increased in type I diabetic hearts and that inhibition with bumetanide attenuates changes in Na(i) and ATP during ischemia and protects against ischemic injury. The data suggest a therapeutic role for pharmacological agents that inhibit flux via NKCC in diabetic patients with myocardial ischemia.

Role of intracellular Na(+) kinetics in preconditioned rat heart

To elucidate the role of intracellular Na(+) kinetics in the mechanism for ischemic preconditioning (IPC), we measured intracellular Na(+) concentration ([Na(+)](i)) using (23)Na-magnetic resonance spectroscopy in isolated rat hearts. IPC significantly delayed the initial [Na(+)](i) increase (d[Na(+)](i)/dt) compared with non-IPC control, resulting in attenuation of Na(+) accumulation (Delta[Na(+)](i)) during 27 minutes of ischemia with better functional recovery. [Na(+)](i) in IPC, but not in control, recovered to preischemic level during a 6-minute reperfusion. The Na(+)-H(+) exchange inhibitor further suppressed d[Na(+)](i)/dt in both control and IPC hearts with concomitant improvement of functional recovery, suggesting little contribution to the mechanism of IPC. The mitochondrial ATP-sensitive K(+) (mito K(ATP)) channel activator diazoxide (30 micromol/L) completely mimicked both [Na(+)](i) kinetics and functional recovery in IPC without any additive effects to IPC. The mito K(ATP) channel blocker 5-hydroxydecanoic acid (100 micromol/L) lost protective effect as well as the attenuation of d[Na(+)](i)/dt and [Na(+)](i) recovery induced by diazoxide. However, 5-hydroxydecanoic acid also lost IPC-induced protection, but incompletely abolished the alteration of d[Na(+)](i)/dt and the [Na(+)](i) recovery. The Na(+)/K(+)-ATPase inhibitor ouabain (200 micromol/L) did not change d[Na(+)](i)/dt in non-IPC hearts, but it abolished the IPC- or diazoxide-induced reduction of d[Na(+)](i)/dt and the [Na(+)](i) recovery, whereas IPC followed by ouabain treatment showed partial functional recovery with smaller Delta[Na(+)](i) than other ouabain groups. In conclusion, alteration of Na(+) kinetics by preserving Na(+) efflux via Na(+)/K(+)-ATPase mediated by mito K(ATP) channel activation mainly contributes to functional protection in IPC hearts. The contribution of mito K(ATP) channel-independent pathway relating to Na(+) kinetics including reduced Na(+) influx is limited in functional protection of IPC.

Reduction of myocardial infarct size by SM-20550, a novel Na(+)/H(+) exchange inhibitor, in rabbits

The effects of N-(aminoiminomethyl)-1, 4-dimethyl-1H-indole-2-carboxamide methanesulfonic acid (SM-20550), a novel potent Na(+)/H(+) exchanger, and nicorandil, a K(+) channel opener with nitrate-like activity, were studied in a myocardial ischemia and reperfusion injury model. Anesthetized rabbits underwent occlusion of the coronary artery (30 min) followed by reperfusion (5 h). Intravenous administration of SM-20550 before ischemia reduced the infarct size by approximately 30-70% in a dose-dependent manner, with a significant reduction in serum creatine phosphokinase activity. Similarly, intravenous administration of nicorandil before ischemia reduced the infarct size by 33% with a significant reduction in serum creatine phosphokinase activity. Moreover, intravenous administration of SM-20550 after ischemia resulted in a significant, approximately 20-40% reduction in the infarct size, but the administration of nicorandil after ischemia did not reduce the infarct size. These results indicate that SM-20550 reduced myocardial necrosis when administered either before or after ischemia.

Slowly inactivating component of sodium current in ventricular myocytes is decreased by diabetes and partially inhibited by known Na(+)-H(+)Exchange blockers

Recent evidence has suggested a major role for a slowly inactivating component of Na(+)current (I(NaL)) as a contributor to ischemic Na(+)loading. The purposes of this study were to investigate veratrine and lysophosphatidylcholine (LPC)-induced I(NaL)in single ventricular myocytes of normal and diabetic rats and to analyse the effects on this current of three pharmacological agents, known as Na(+)/H(+)exchange inhibitors, whose selectivity has been questioned in several studies. A decrease in Na(+)/H(+)exchange activity has been previously shown to be associated with diabetes, and this has been found to confer some protection to the diabetic heart after an episode of ischemia/reperfusion. Recordings were made using the whole-cell patch-clamp technique. I(NaL)was stimulated either by veratrine (100 mg/ml) or by LPC (10 micromol/l) applied extracellularly. Veratrine as well as LPC-induced I(NaL)was found to be significantly decreased in ventricular myocytes isolated from diabetic rat hearts. Veratrine- and LPC-induced I(NaL)in ventricular myocytes of normal rats was significantly (in the range 10(-7)to 10(-4)mol/l) inhibited by the Na(+)/H(+)exchange blockers HOE 694, EIPA and HOE 642. HOE 694 was the most potent inhibitor, followed by the amiloride derivative EIPA and HOE 642. The sensitivity of veratrine-induced I(NaL)to inhibition by HOE 694 and EIPA was markedly reduced in diabetic ventricular myocytes, with no observed inhibition by HOE 642. These data may have important implications as to the protection that may be afforded against ischemic and reperfusion injury, especially during ischemia and when ischemia occurs in a diabetic situation.

Contribution of the Na(+) channel and Na(+)/H(+) exchanger to the anoxic rise of [Na(+)] in ventricular myocytes

The aim of this study was to quantify the contribution of the Na(+)/H(+) exchanger (NHE) and the Na(+) channel to the rise in cytosolic Na(+) concentration ([Na(+)]) that is seen in anoxic guinea pig ventricular myocytes. [Na(+)] was measured with the use of microfluorometry and was found to rise to 44 mM after prolonged anoxia. This rise was partially sensitive to either TTX or HOE-642, selective inhibitors of the Na(+) channel and NHE1, respectively. [Na(+)] did not significantly rise when both drugs were present, suggesting that other routes of Na(+) entry were insignificant. However, the relative contributions of the NHE and the Na(+) channel were found to be remarkably sensitive to ionic conditions expected to occur during ischemia. The Na(+) channel was the dominant Na(+) source during acidic anoxia. However, the NHE was the dominant Na(+) source during both hyperkalemic anoxia and simulated ischemia (hyperkalemia, low pH, and anoxia). The data suggest that the NHE may prove to be the best pharmacological target to reduce Na(+) entry during true ischemia and that inhibition of Na(+) influx could contribute strongly to the cardioprotective effects of NHE inhibitors.

The myocardial Na(+)-H(+) exchange: structure, regulation, and its role in heart disease

The Na(+)-H(+) exchange (NHE) is a major mechanism by which the heart adapts to intracellular acidosis during ischemia and recovers from the acidosis after reperfusion. There are at least 6 NHE isoforms thus far identified with the NHE1 subtype representing the major one found in the mammalian myocardium. This 110-kDa glycoprotein extrudes protons concomitantly with Na(+) influx in a 1:1 stoichiometric relationship rendering the process electroneutral, and its activity is regulated by numerous factors, including phosphorylation-dependent processes. There is convincing evidence that NHE mediates tissue injury during ischemia and reperfusion, which probably reflects the fact that under conditions of tissue stress, including ischemia, Na(+)-K(+) ATPase is inhibited, thereby limiting Na(+) extrusion, resulting in an elevation in [Na(+)](i). The latter effect, in turn, will increase [Ca(2+)](i) via Na(+)-Ca(2+) exchange. In addition, NHE1 mRNA expression is elevated in response to injury, which may further contribute to the deleterious consequence of pathological insult. Extensive studies using NHE inhibitors have consistently shown protective effects against ischemic and reperfusion injury in a large variety of experimental models and has led to clinical evaluation of NHE inhibition in patients with coronary artery disease. Emerging evidence also implicates NHE1 in other cardiac disease states, and the exchanger may be particularly critical to postinfarction remodeling responses resulting in development of hypertrophy and heart failure.

Intracellular sodium accumulation during ischemia as the substrate for reperfusion injury

To elucidate the role of intracellular Na+ kinetics during ischemia and reperfusion in postischemic contractile dysfunction, intracellular Na+ concentration ([Na+]i) was measured in isolated perfused rat hearts using 23Na nuclear magnetic resonance spectroscopy. The extension of the ischemic period from 9 minutes to 15, 21, and 27 minutes (at 37 degrees C) increased [Na+]i at the end of ischemia from 270.0+/-10.4% of preischemic level (mean+/-SE, n=5) to 348.4+/-12.0% (n=5), 491.0+/-34.0% (n=7), and 505.3+/-12.1% (n=5), respectively, whereas the recovery of developed pressure worsened with the prolongation of the ischemic period (95.1+/-4.2%, 84.3+/-1. 2%, 52.8+/-13.7%, and 16.9+/-6.4% of preischemic level). The kinetics of [Na+]i recovery during reperfusion was analyzed by the fitting of a monoexponential function. When the hearts were reperfused with low-[Ca]o (0.15 mmol/L) solution, the time constants of the recovery (tau) after 15-minute (8.07+/-0.85 minutes, n=5) and 21-minute ischemia (6.44+/-0.90, n=5) were significantly extended, with better functional recovery (98.5+/-1.4% for 15-minute [P<0.05]; 98.0+/-1.0% for 21-minute [P<0.05]) compared with standard reperfusion ([Ca]o=2.0 mmol/L, tau=3.58+/-0.28 minutes for 15-minute [P<0.0001]; tau=3.02+/-0.20 for 21-minute [P<0.0001]). A selective inhibitor of Na+/Ca2+ exchanger also decelerated the [Na+]i recovery, which suggests that the recovery reflects the Na+/Ca2+ exchange activity. In contrast, high-[Ca]o reperfusion (5 mmol/L) accelerated the [Na+]i recovery after 9-minute ischemia (tau=2.48+/-0.11 minute, n=5 [P<0.0001]) and 15-minute ischemia (tau=2.10+/-0.07, n=6 [P<0. 05]), but functional recovery deteriorated only in the hearts with 15-minute ischemia (29.8+/-9.4% [P<0.05]). [Na+]i recovery after 27-minute ischemia was incomplete and decelerated by low-[Ca]o reperfusion, with limited improvement of functional recovery (42. 5+/-7.9%, n=5 [P<0.05]). These results indicate that intracellular Na+ accumulation during ischemia is the substrate for reperfusion injury and that the [Na+]i kinetics during reperfusion, which is coupled with Ca2+ influx, also determines the degree of injury.