Differential effect of global ischemia on the ryanodine-sensitive and ryanodine-insensitive calcium uptake of cardiac sarcoplasmic reticulum

Effects of zofenopril on cardiac sarcoplasmic reticulum calcium handling

Isolated rat hearts were perfused for 120 minutes in the presence or in the absence of 10 microM zofenoprilat, the active metabolite of zofenopril. At the end of perfusion, cardiac tissue was used to assay sarcoplasmic reticulum (SR) (45)Ca uptake and SR calcium release, which was determined by automatized quick filtration technique after SR vesicle loading with (45)Ca. The expression of genes involved in the control of calcium homeostasis was evaluated by polymerase chain reaction after reverse transcription. In chronic experiments, SR (45)Ca uptake and gene expression were measured in hearts derived from rats treated with 15 mg*kg(-1)*day(-1) zofenopril for 15 days. Acute or chronic zofenopril administration did not produce any change in contractile performance. In acute experiments, SR (45)Ca uptake was significantly increased after exposure to zofenoprilat. The rate constant of calcium-induced calcium release was slightly although not significantly higher, and the calcium leak measured under conditions promoting SR channel closure was significantly increased. In the chronic model, significant increase in the rate of SR (45)Ca uptake was confirmed. Gene expression was not modified, except for decreased phospholamban expression, which is observed in the acute but not in the chronic model. In conclusion, zofenopril increases SR calcium cycling and stimulates active calcium uptake into the SR.

Modulation of cardiac ionic homeostasis by 3-iodothyronamine

3-iodothyronamine (T₁AM) is a novel endogenous relative of thyroid hormone, able to interact with trace amine-associated receptors, a class of plasma membrane G protein-coupled receptors, and to produce a negative inotropic and chronotropic effect. In the isolated rat heart 20–25 μM T₁AM decreased cardiac contractility, but oxygen consumption and glucose uptake were either unchanged or disproportionately high when compared to mechanical work. In adult rat cardiomyocytes acute exposure to 20 μM T₁AM decreased the amplitude and duration of the calcium transient. In patch clamped cardiomyocytes sarcolemmal calcium current density was unchanged while current facilitation by membrane depolarization was abolished consistent with reduced sarcoplasmic reticulum (SR) calcium release. In addition, T₁AM decreased transient outward current (I_to) and I_K1 background current. SR studies involving 20 μM T₁AM revealed a significant decrease in ryanodine binding due to reduced B_max, no significant change in the rate constant of calcium-induced calcium release, a significant increase in calcium leak measured under conditions promoting channel closure, and no effect on oxalate-supported calcium uptake. Based on these observations we conclude T₁AM affects calcium and potassium homeostasis and suggest its negative inotropic action is due to a diminished pool of SR calcium as a result of increased diastolic leak through the ryanodine receptor, while increased action potential duration is accounted for by inhibition of I_to and I_K1 currents.

Time-dependent changes of the susceptibility of cardiac contractile function to hypoxia-reoxygenation after myocardial infarction in rats

In this study we analyzed the susceptibility of contractile function of the myocardium to hypoxia-reoxygenation after infarction. For this purpose, the contractility of isolated papillary muscles from rats was studied at high oxygen tension (pO2 80 kPa) and during hypoxia (pO2 3 kPa) with subsequent reoxygenation at variable intervals between 15 h and 9 weeks after permanent ligation of the left coronary artery. Hypoxic exposure reduced the contractile performance of the preparations to a similar extent in both groups. Notably, the contractility and, in particular, the relaxation rates recovered more completely from hypoxia in the hypertrophied myocardium of rats with coronary artery ligation than in sham-operated (SO) animals. The recovery of contractile function was improved maximally between 6 and 9 weeks after myocardial infarction (MI). The lower sensitivity of the (post)ischemic myocardium to hypoxia-reoxygenation correlated with enhanced left ventricular glutathione peroxidase (GSH-Px) activity (15 h to 9 weeks post-MI) and 2-3-fold increased expression levels (15 h to 6 weeks post-MI) of the 72 kDa heat shock protein (HSP72) in the papillary muscles. These findings suggest that the greater antioxidant potential and, possibly, stimulation of HSPs contribute to the sustained tolerance of the myocardium to hypoxia-reoxygenation injury after infarction.

Effects of ischemia on sarcoplasmic reticulum Ca(2+) uptake and Ca(2+) release in rat skeletal muscle

In this study, we investigated the hypothesis that prolonged ischemia would impair both sarcoplasmic reticulum (SR) Ca(2+) uptake and Ca(2+) release in skeletal muscle. To induce total ischemia (I), a tourniquet was placed around the upper hindlimb in 30 female Sprague-Dawley rats [wt = 256 +/- 6.7 (SE) g] and inflated to 350 mmHg for 4 h. The contralateral limb served as control (C). Immediately after the 4 h of ischemia, mixed gastrocnemius and tibialis anterior muscle was sampled from both limbs, and both crude muscle homogenates and SR vesicles were prepared. In another 10 control animals (CC), muscles were sampled and prepared exactly the same way, but immediately after anesthetization. Ca(2+) uptake and Ca(2+) release were measured in vitro with Indo-I on both homogenates and SR vesicles. As hypothesized, submaximal Ca(2+) uptake was lower (P < 0.05) in I compared with CC and C, by 25 and 45% in homogenates and SR vesicles, respectively. Silver nitrate (AgNO(3))-induced Ca(2+) release, which occurred in two phases (phase 1 and phase 2), was also altered in I compared with CC and C, in both muscle homogenates and SR vesicles. With ischemia, phase 1 peak Ca(2+) release was 26% lower (P < 0.05) in SR vesicles only. For phase 2, peak Ca(2+) release was 54 and 24% lower (P < 0.05) in SR vesicles and homogenates, respectively. These results demonstrate that prolonged skeletal muscle ischemia leads to a reduced SR Ca(2+) uptake in both homogenates and SR vesicles. The effects of ischemia on SR Ca(2+) release, however, depend on both the phase examined and the type of tissue preparation.

Cardiac ischemia oxidizes regulatory thiols on ryanodine receptors: captopril acts as a reducing agent to improve Ca2+ uptake by ischemic sarcoplasmic reticulum

We tested the hypothesis that ischemia alters sarcoplasmic reticulum (SR) Ca2+ transport by oxidizing regulatory thiols on ryanodine receptors (RyRs), and that membrane-permeable sulfhydryl-containing angiotensin-converting enzyme (ACE) inhibitors protect against ischemia-induced oxidation and explain in part, the therapeutic actions of captopril. Ca2+ uptake and adenosine triphosphatase (ATPase) activity was measured from SR vesicles isolated from control or ischemic dog and human ventricles and compared with or without sulfhydryl reductants. The rate and amount of Ca2+ uptake was lower for canine ischemic SR compared with control (6.5 +/- 0.2 --> 18.5 +/- 1.1 nmol Ca2+/mg/min and 123.1 +/- 4.7 --> 235.0 +/- 17.3 nmol Ca2+/mg; n = 8 each). Captopril, dithiothreitol (DTT), glutathione (GSH), and L-cysteine increased the rate and amount of Ca2+ uptake by canine and human ischemic SR vesicles by approximately 50%. Reducing agents had no effect on Ca2+- ATPase activity in either canine control or ischemic (approximately 40% less than control) SR. Captopril was as potent as DTT at reversing the oxidation of skeletal and cardiac RyRs induced by reactive disulfides (RDSs) or nitric oxide (NO). In neonatal rat myocytes, RDSs or NO triggered SR Ca2+ release and increased cytosolic Ca2+, an effect reversed by captopril and DTT but not GSH or cysteine. Pretreatment of myocytes with captopril (exposure and then wash) inhibited Ca2+ elevation elicited by RDSs or NO, indicating that captopril is an effective, membrane-permeable intracellular reducing agent. Thus, net SR Ca2+ accumulation is reduced by ischemia in part due to the oxidation of thiols that gate RyRs, an effect reversed by captopril.

Pilocarpine-induced status epilepticus causes N-methyl-D-aspartate receptor-dependent inhibition of microsomal Mg(2+)/Ca(2+) ATPase-mediated Ca(2+) uptake

Status epilepticus is associated with sustained and elevated levels of cytosolic Ca(2+). To elucidate the mechanisms associated with changes of cytosolic Ca(2+) after status epilepticus, this study was initiated to evaluate the effect of pilocarpine-induced status epilepticus on Mg(2+)/Ca(2+) ATPase-mediated Ca(2+) uptake in microsomes isolated from rat cortex, because the Ca(2+) uptake mechanism plays a major role in regulating intracellular Ca(2+) levels. The data demonstrated that the initial rate and overall Ca(2+) uptake in microsomes from pilocarpine treated animals were significantly inhibited compared with those in microsomes from saline-treated control animals. It was also shown that the inhibition of Ca(2+) uptake caused by status epilepticus was not an artifact of increased Ca(2+) release from microsomes, selective isolation of damaged microsomes from the homogenate, or decreased Mg(2+)/Ca(2+) ATPase protein in the microsomes. Pretreatment with the NMDA antagonist dizocilpine maleate blocked status epilepticus-induced inhibition of the initial rate and overall Ca(2+) uptake. The data suggest that inhibition of microsomal Mg(2+)/Ca(2+) ATPase Ca(2+) uptake is involved in NMDA-dependent deregulation of cytosolic Ca(2+) homeostasis associated with status epilepticus.

Alterations in sarcoplasmic reticulum function in female vastus lateralis with eccentric exercise

This study examined the alterations in sarcoplasmic reticulum (SR) Ca2+ sequestration function in homogenates during eccentric exercise and recovery and following additional eccentric exercise, and correlated these alterations with changes in force output. Eight healthy, untrained females, aged 20-25 years, cycled for a total of 60 min on an eccentric cycle ergometer (30 min at 66+/-3% VO2 peak and 30 min at 76+/-3% VO2 peak, determined during concentric exercise). Biopsies (extracted from the vastus lateralis) were taken before and after the exercise as well as on days 2, 6 and prior to and following identical exercise on day 14. Ca2+-uptake (nmol/min/mg protein) was unaffected (p > 0.05) following the first session of eccentric exercise; however, by day 2 a depression in uptake (p < 0.05) was observed which persisted throughout the remainder of the experiment. Maximal Ca2+-ATPase activity (nmol/min/mg protein) was elevated (p < 0.05) immediately following the first exercise session, remained elevated through day 2 and returned to pre-exercise levels by day 6 of recovery and increased again by day 14. No changes in either Ca2+-ATPase activity or Ca2+-uptake were observed with exercise on day 14. Both eccentric sessions, performed on days 0 and 14, resulted in similar depressions in force (p < 0.05) immediately following exercise. By day 2 force had recovered to pre-exercise levels. The results demonstrate that a prolonged alteration in SR Ca2+-uptake occurs following eccentric work that is unaccompanied by parallel changes in either SR Ca2+-ATPase activity or mechanical performance.

Ruthenium red modifies the cardiac and skeletal muscle Ca(2+) release channels (ryanodine receptors) by multiple mechanisms

The effects of ruthenium red (RR) on the skeletal and cardiac muscle ryanodine receptors (RyRs) were studied in vesicle-Ca(2+) flux, [(3)H]ryanodine binding, and single channel measurements. In vesicle-Ca(2+) flux measurements, RR was more effective in inhibiting RyRs at 0.2 microM than 20 microM free Ca(2+). [(3)H]Ryanodine binding measurements suggested noncompetitive interactions between RR inhibition and Ca(2+) regulatory sites of RyRs. In symmetric 0.25 M KCl with 10-20 microM cytosolic Ca(2+), cytosolic RR decreased single channel activities at positive and negative holding potentials. In close to fully activated skeletal (20 microM Ca(2+) + 2 mM ATP) and cardiac (200 microM Ca(2+)) RyRs, cytosolic RR induced a predominant subconductance at a positive but not negative holding potential. Lumenal RR induced a major subconductance in cardiac RyR at negative but not positive holding potentials and several subconductances in skeletal RyR. The RR-related subconductances of cardiac RyR showed a nonlinear voltage dependence, and more than one RR molecule appeared to be involved in their formation. Cytosolic and lumenal RR also induced subconductances in Ca(2+)-conducting skeletal and cardiac RyRs recorded at 0 mV holding potential. These results suggest that RR inhibits RyRs and induces subconductances by binding to cytosolic and lumenal sites of skeletal and cardiac RyRs.

Alterations in sarcoplasmic reticulum function and gene expression in ischemic-reperfused rat heart

In view of the critical role of sarcoplasmic reticular (SR) Ca(2+) release and the Ca(2+) pump in cardiac contraction-relaxation, this study was undertaken to assess the status of SR function, protein content, and gene expression in isolated rat hearts subjected to global ischemia for 30 min followed by 60 min of reperfusion (I/R). Attenuated recovery of contractile function in the I/R hearts was associated with reduced SR Ca(2+) uptake, Ca(2+) release, and ryanodine-binding activities. mRNA levels and protein contents for SR Ca(2+) pump ATPase and Ca(2+) release channels were markedly depressed in the I/R hearts. Perfusion of hearts with superoxide dismutase plus catalase, well-known scavengers of oxyradicals, prevented the I/R-induced alterations in cardiac function and partially prevented SR Ca(2+) transport activities and mRNA abundance. In hearts perfused with xanthine plus xanthine oxidase or H(2)O(2), changes similar to those in the I/R hearts were observed. These results indicate that oxyradicals may participate in depressing the SR Ca(2+) handling and gene expression in the I/R heart. It is suggested that treatment of hearts with antioxidants may improve the recovery of cardiac function by preserving the SR function and partially protecting the SR gene expression.

Global ischemia-induced inhibition of the coupling ratio of calcium uptake and ATP hydrolysis by rat whole brain microsomal Mg(2+)/Ca(2+) ATPase

Ischemia is associated with a loss of cytosolic calcium homeostasis. Intracellular stores, particularly in endoplasmic reticulum, are critical for the maintenance of calcium homeostasis. Recent studies have shown that ischemia significantly inhibited microsomal calcium uptake mediated by Mg(2+)/Ca(2+) ATPase, the major mechanism of endoplasmic reticulum calcium sequestration. This study was initiated to determine whether the decreased calcium uptake caused by ischemia was the result of inhibition of Mg(2+)/Ca(2+) ATPase activity or an uncoupling of calcium uptake from ATP hydrolysis. The microsomal Mg(2+)/Ca(2+) ATPase specific inhibitor thapsigargin partially inhibited ATPase activity and completely inhibited calcium uptake. ATPase inhibited by thapsigargin was considered microsomal Mg(2+)/Ca(2+) ATPase. Ischemia from 5 to 60 min had no significant effect on thapsigargin sensitive ATPase activity. However, under identical conditions, increasing ischemia from 5 to 60 min significantly inhibited microsomal calcium uptake. Comparing calcium uptake to ATP hydrolysis as ischemia increased from 5 to 60 min revealed that the coupling ratio of calcium molecules sequestered to ATP molecules hydrolyzed became significantly decreased. The results demonstrated that the effect of ischemia on microsomal calcium uptake was mediated by an uncoupling of calcium transport from Mg(2+)/Ca(2+) ATPase activity.

Protection of cardiomyocytes by pinacidil during metabolic inhibition and hyperkalemia

The objective of this study is to understand the mechanism underlying the cardioprotective effects of pinacidil, an ATP-sensitive K+ channel (K(ATP)) opener. We examined the effects of 10 microM pinacidil in cultured chicken cardiomyocytes. Pinacidil caused a concentration-dependent delay in metabolic inhibition-induced increase in intracellular calcium concentration ([Ca2+]i) and creatine phosphokinase release, and this action was antagonized by glyburide, a K(ATP) blocker. Neither verapamil, an L-type Ca2+ channel blocker, nor bepridil, a Na+-Ca2+ exchange inhibitor, affected the time course of increase in [Ca2+]i induced by metabolic inhibition. Pinacidil did not have an effect on the amplitude of K+-induced increase in [Ca2+]i, but accelerated the rate of decline following peak stimulation. In contrast, glyburide reduced the amplitude of K+-induced increase in [Ca2+]i and prolonged the rate of decline. These results provide direct evidence that pinacidil protects cardiomyocytes from metabolic inhibition-induced injury by cyanide (CN) through a delay in the onset of increase in [Ca2+]i, rather than by inhibition of the L-type Ca2+-channels or by alteration of Na+-Ca2+ exchange.

Analysis of sarcoplasmic reticulum Ca2+ transport and Ca2+ ATPase enzymatic properties using mouse cardiac tissue homogenates

Recent studies have focused on developing transgenic mouse models to explore the physiological roles of sarcoplasmic reticulum (SR) calcium handling proteins. The goal of this study was to develop methodology to measure SR Ca2+ transport function and enzymatic properties of SR Ca2+ ATPase (SERCA) in individual mouse hearts. We describe here the procedures to specifically measure SR Ca2+ uptake, the formation and decomposition of SERCA phosphoenzyme intermediate (E-P) in mouse cardiac homogenates. The specificity of SERCA enzymatic activity in cardiac homogenates was established by (a) the selective inhibition of SERCA enzyme by inhibitor-thapsigargin, and (b) comparison of the kinetic parameters of SERCA activity between homogenates and isolated microsomes. Here we show that the apparent affinity of SERCA for Ca2+ and ATP, the time to reach steady-state levels of E-P, and the rate of E-P decomposition (turnover rate of SERCA enzyme) are similar in homogenates and microsomes. These studies demonstrate that SERCA Ca2+ transport and enzymatic properties can be accurately measured in mouse cardiac tissue homogenates. Additionally, we show that frozen cardiac homogenates can be used without significant loss of enzymatic activity. In conclusion, we have developed and established the methods to employ tissue homogenates to study SR Ca2+ transport function in individual mouse hearts.

Cyclosporin A treatment alters characteristics of Ca2+-release channel in cardiac sarcoplasmic reticulum

Chronic treatment with cyclosporin A (CsA) has been reported (H. S. Banijamali, M. H. ter Keurs, L. C. Paul, and H. E. ter Keurs. Cardiovasc. Res. 27: 1845-1854, 1993; I. Kingma, E. Harmsen, H. E. ter Keurs, H. Benediktsson, and L. C. Paul. Int. J. Cardiol. 31: 15-22, 1991) to induce reversible alterations of contractile properties in rat hearts. To define the molecular mechanisms underlying the physiological alterations, the Ca2+-release channel (CRC) and Ca2+-ATPase from sarcoplasmic reticulum in rats were examined. Ryanodine binding to whole homogenates of rat hearts shows time- and dose-dependent alterations in CRC properties by CsA. On 3 wk of treatment with 15 mg CsA. kg body wt-1. day-1, 1) maximal ryanodine binding (Bmax) decreased, 2) the dissociation constant of ryanodine (Kd) increased, 3) caffeine sensitivity of CRC increased, and 4) ruthenium red sensitivity of CRC decreased. On the other hand, Bmax and Kd of ryanodine binding in rat skeletal muscles were not changed. Ryanodine-sensitive oxalate-supported Ca2+ uptake in whole homogenates was lower in CsA-treated rat hearts than in control hearts, whereas total Ca2+ uptake in the presence of 500 M ryanodine was not changed. Functional experiments with rapamycin and Western blot analysis suggest that the CsA-induced alteration of ryanodine binding is due at least in part to an upregulation of calcineurin. The heart muscle-specific alterations of CRC could be responsible for the previously reported contractile changes of CsA-treated rat hearts.

Ryanodine-sensitive, thapsigargin-insensitive calcium uptake in rat ventricle homogenates

Thapsigargin is a natural product that specifically inhibits all known SERCA calcium pumps with high affinity. We investigated the effects of thapsigargin on cardiac sarcoplasmic reticulum (SR) by measuring the oxalate-supported calcium uptake rate in the unfractionated homogenate and in the isolated SR fraction. The uptake rate in both the isolated SR and unfractionated homogenate are stimulated about two-fold by preincubation with high concentrations of ryanodine, which closes the SR efflux channel. Thapsigargin stoichiometrically and completely inhibited the calcium uptake rate in the isolated SR, both in the presence and absence of SR channel blockade. In contrast, thapsigargin nearly completely inhibited the homogenate calcium uptake only in the absence of SR channel blockade; in the presence of blockade, about 20% of the uptake activity was insensitive to thapsigargin. This result unmasks a thapsigargin-insensitive, ryanodine-sensitive component of calcium uptake in the heart. This activity is in an oxalate-permeable pool and is inhibited by cyclopiazonic acid, another inhibitor of the SERCA calcium pumps. There was no TG-insensitive activity in the rat EDL muscle homogenate. The absence of thapsigargin-insensitive uptake activity in the isolated SR can be attributed to its inactivation during the isolation of the SR. The oxalate permeability and ryanodine sensitivity suggest that the TG-insensitive calcium uptake activity is closely related to the classical SR. The different thapsigargin sensitivities suggests the existence of two kinds of intracellular calcium pumps in the heart.

Effects of Ca2+ channel blockers on Ca2+ loading induced by metabolic inhibition and hyperkalemia in cardiomyocytes

The effects of the L-type (nifedipine and verapamil) and the T-type (mibefradil) Ca2+ channel blockers on the increase in intracellular Ca2+ concentration ([Ca2+]i) induced by NaCN metabolic inhibition and hyperkalemia were examined in chicken cardiomyocytes using fluorescence imaging with Fura-2. NaCN induced a slow and sustained rise in [Ca2+]i, which was not affected by pretreating the cells for 5 min with nifedipine, verapamil, or mibefradil at 100 nM or 10 microM. Pretreatment of the cells with 10 microM nifedipine, verapamil, or mibefradil for 5 min remarkably inhibited the K+-induced increase in [Ca2+]i. These inhibitory effects diminished after 48-h pretreatment with nifedipine or verapamil but not with mibefradil. Ryanodine also induces an increase in [Ca2+]i, and this effect was enhanced by 48-h pretreatment of the cells with 10 microM verapamil but not with 10 microM mibefradil. We conclude that the NaCN-induced increase in [Ca2+]i is independent of the Ca2+ influx though the L-type or T-type Ca2+ channels. Chronic inhibition of the L-type Ca2+ channels but not T-type channels may enhance the ryanodine receptor-mediated Ca2+ release, which may be responsible for the development of tolerance to their inhibitory effects on K+-induced increase in [Ca2+]i.

[Effects of myocardial protection with ryanodine, measured with the intracellular calcium fluprescent indicator Fura-2]

The effects of Ryanodine, an inhibitor of salcoplasmic reticulum function, was investigated in isolated hearts of the Wistar rat strain. The cytosolic calcium was measured with the intracellular Ca2+ fluorescent indicator Fura-2. After 3 minutes perfusion of various cardioplegic solution which were added potassium (the concentration; 20 mmol/L) and four Ryanodine groups (1, 4, 10, and 20 nmol/L), these were obtained cardiac arrest. The arrested hearts were kept at 37 degrees C (normothermia) measuring hemodynamic studies. Hemodynamic parameters were heart rate, LVDP, LV dp/dt, coronary flow, and the intracellular calcium fluorescents which were calculated intracellular Ca2+. The significant difference was not noted in LVDP, but comparable improvement was achieved with Ryanodine groups (p < 0.05; 20 nM vs 0 nM). Other cardiac functions were likely same as above. The cytosolic calcium concentration of Ryanodine groups was depressed during cardiac arrest, it was 97.4 +/- 17.2% at the end of cardiac arrest, and it was slowly increased to 161.9 +/- 46.9% after 40 minutes reperfusion. On the other hand, that of the control group was higher than Ryanodine groups at every measuring points about two times. There was significant difference between both groups (p < 0.01). Concequently these phenomena caused that Ca2+ handling in the salcoplasmic reticulum were supressed by Ryanodine and the contractile function was recovered for that reason.

[Decreasing sarcoplasmic reticular calcium gives rise to myocardial protection--the effect of thapsigargin for myocardial protection under conditions of normothermia]

Deceasing sarcoplasmic reticular (SR) calcium may contribute to the myocardiac protection against ischemia and reperfusion-induced injury. Therefore, using the isolated working rat heart model, we investigated the effect of Thapsigargin (TH)-induced SR calcium diminution on the myocardial protection when added either before onset of ischemia or at time of reperfusion under conditions of normothermic ischemia. Hearts (n = 6/group) from male Wistar rats were aerobically (37 degrees C) perfused (20 min) with bicarbonate buffer. In the experimental protocol A, this was followed by a 3 min infusion of St. Thomas' Hospital cardioplegic solution No. 2 (STS) containing various concentrations of TH. Hearts were then subjected to 34 min of normothermic (37 degrees C) global ischemia and 35 min of reperfusion (15 min Langendorff, 20 min working). Reperfusion cardiac functions at 20 min of working perfusion was measured and compared with the preischemia values. STS added to 0.1 and 0.25 mumol/L TH improved recovery of aortic flow after 20 min reperfusion from 47 +/- 3% in the TH free controls to 62 +/- 3, 63 +/- 2% (n = 6) (p < 0.05). There was no difference in creatine kinase (CK) leakage during Langendorff reperfusion between the TH treated groups and the control group. In the experimental protocol B, 3 min of cardioplegia without TH and 34 min of ischemia (37 degrees C) were followed by a 10 min Langendorff reperfusion with various concentrations of TH, then 10 min Langendroff reperfusion for washing out, and 20 min working reperfusion. When TH was added to reperfusate the recovery of aortic flow did not change, 0.5 mumol/L TH group had the detelious effect. Thus, TH, when added to the cardioplegia, enhanced myocardial protection. We conclude that lessened uptake of Ca2+ into sarcoplasmic reticulum by inhibitors of the Ca(2+)-ATPase pump can decrease ischemia and reperfusion-induced injury.

Injury to the Ca2+ ATPase of the sarcoplasmic reticulum in anesthetized dogs contributes to myocardial reperfusion injury

OBJECTIVE

Sarcoplasmic reticulum dysfunction may contribute to calcium (Ca2+) overload during myocardial reperfusion. The aim of this study was to investigate its role in reperfusion injury.

METHODS

Open chest dogs undergoing 15 min of left anterior descending coronary artery occlusion and 3 h of reperfusion were randomized to intracoronary infusions of 0.9% saline, vehicle, or the Ca2+ channel antagonist, nifedipine (50 micrograms/min from 2 minutes before to 5 minutes after reperfusion). After each experiment, transmural myocardial biopsies were removed from ischemic/reperfused and nonischemic myocardium in the beating state and analyzed for (i) sarcoplasmic reticulum protein content (Ca2+ ATPase, phospholamban, and calsequestrin) by immunoblotting and (ii) Ca2+ uptake by sarcoplasmic reticulum vesicles with and without 300 micromolar ryanodine or the Ca2+ ATPase activator, antiphospholamban (2D12) antibody.

RESULTS

Contractile function did not recover in controls and vehicle-treated dogs after ischemia and reperfusion (mean systolic shortening, -2 +/- 2%), but completely recovered in nifedipine-treated dogs (17 +/- 2%, p = NS vs. baseline, p < 0.01 vs. control). Ventricular fibrillation occurred in 50% of controls and vehicle dogs and 0% of nifedipine-treated dogs (p < 0.01). Ca2+ uptake by the sarcoplasmic reticulum vesicles was severely reduced in ischemic/reperfused myocardium of controls and vehicle dogs (p < 0.01 vs. nonischemic). Ryanodine and the 2D12 antibody improved, but did not reverse the low Ca2+ uptake. Protein content was similar in ischemic/reperfused and nonischemic myocardium. In contrast, Ca2+ uptake and the responses to ryanodine and 2D12 antibody were normal in ischemic/reperfused myocardium from nifedipine-treated dogs.

CONCLUSION

Dysfunction of the sarcoplasmic reticulum Ca2+ ATPase pump correlates with reperfusion injury. Reactivation of Ca2+ channels at reperfusion contributed to Ca2+ pump dysfunction. Ca2+ pump injury may be a critical event in myocardial reperfusion injury.

Compensatory up-regulation of cardiac SR Ca2+-pump by heat-shock counteracts SR Ca2+-channel activation by ischemia/reperfusion

We tested the hypothesis that heat-shock protected myocardial Ca2+-cycling by sarcoplasmic reticulum from ischemia and reperfusion (I/R) injury. Twenty-four hours after increasing body temperature to 42 degrees C for 15 min, rat hearts were isolated, Langendorff-perfused, and subjected to 30 min ischemia then 30 min reperfusion. Left ventricles were homogenized and their ionized Ca2+ concentration monitored with indo- during Ca2+-uptake in the presence and absence of the Ca2+-release channel (CRC) modulator ryanodine. Tissue content of heat-shock protein 72 (HSP 72) was analyzed. Exposure to I/R resulted in a 37% enhancement of CRC activity but no effect on Ca2+-pumping activity, resulting in 25% decreased net Ca2+-uptake activity. Pre-exposure to heat-shock resulted in a 10-fold increase in HSP 72, and a 25% enhancement of maximal Ca2+-pumping activity which counteracted the effect of I/R on CRC and net Ca2+-uptake activities. This protection of SR Ca2+-cycling was associated with partial protection of myocardial physiological performance. Net Ca2+-uptake activity was correlated with the left ventricular developed pressure and its rate of change. We conclude that one of the mechanisms by which heat-shock protects myocardium from I/R injury is to upregulate SR Ca2+-pumping activity to counteract the enhanced SR Ca2+-release produced by I/R.

The effects of ryanodine on calcium uptake by the sarcoplasmic reticulum of ischemic and reperfused rat myocardium

The effects of ischemia and reperfusion on sarcoplasmic reticulum (SR) calcium uptake were measured in crude heart homogenates of rats and were compared to published results for rabbit hearts. Isolated rat hearts (n = 5 in each group) were Langendorff-perfused at 37 degrees C and were either kept normally perfused (control group), or submitted to 15 min normothermic ischemia (ischemic group), or reperfused for 10 min after 15 min ischemia (reperfused group). Mechanical function recovered to 50-60% of control after 10 min reperfusion following ischemia. Ca uptake (control Vmax: 23.0 +/- 2.20 nmol.min.1.mg of protein-1) decreased during ischemia (Vmax: 15.7 +/- 1.60 nmol.min-1.mg-1) but recovered to control level on reperfusion (Vmax: 20.8 +/- 2.02 nmol.min-1.mg-1). An increased Ca uptake was obtained when the measurements were carried out in the presence of ryanodine (430 microM) to block Ca leakage through SR Ca-release channels. The relative magnitude of ryanodine effect in the ischemic myocardium (increase: 77.2 +/- 18.20%) was more marked than in control (32.0 +/- 8.22%) or reperfused myocardium (39.0 +/- 10.66%). This result is different from that of rabbit myocardium where similar ryanodine effect is present in all groups (56.7 +/- 13.76%, 50.0 +/- 13.56% and 54.2 +/- 6.88% in control, ischemic and reperfused hearts, respectively) and suggests that a component of cytosolic Ca overload via SR Ca-release channels is present during ischemia in rat, but not in rabbit myocardium.

Ischemia-induced alterations in sarcoplasmic reticulum Ca(2+)-ATPase activity in rat soleus and EDL muscles

To investigate the time-dependent effects of ischemia, as modified by muscle fiber type composition, on sarcoplasmic reticulum (SR) function, Ca(2+)-ATPase activity (total minus basal) was measured in homogenates prepared from samples obtained from rat soleus and extensor digitorum longus (EDL) muscle of ischemic and contralateral controls. Ischemia was induced by occlusion of blood flow to one hindlimb for periods of 1, 2, and 3 h (n = 10 per group). In EDL, maximal Ca(2+)-ATPase activity (expressed in mumol.g wet wt-1.min-1) was higher (P < 0.05) in ischemic than in control at 1 h (80 +/- 10 vs. 56.5 +/- 5.3) and increased progressively with ischemia at both 2 h (88 +/- 4.6 vs. 53.1 +/- 2.8) and 3 h (116 +/- 3.8 vs. 67.8 +/- 3.2). In contrast, in soleus, increases (P < 0.05) in Ca(2+)-ATPase activity with ischemia were observed at 2 h (19.2 +/- 0.86 vs. 14.0 +/- 0.56) and 3 h (19.9 +/- 1.4 vs. 12.4 +/- 0.62) but not at 1 h (10.7 +/- 1.5 vs. 10.0 +/- 0.83). In both EDL and soleus, basal Mg(2+)-ATPase was unchanged with ischemia. On the basis of these findings, it can be concluded that ischemia results in an increase in the maximal SR Ca(2+)-ATPase activity but that the time course of the change is dependent on the fiber type composition of the muscle.

Mechanisms of subcellular remodelling in post-infarct heart failure

Occlusion of a coronary artery results in myocardial ischemia and subsequent myocardial infarction. Whenever the infarct size is more than 30% of the ventricular wall, the remaining myocardium attempts to compensate for the loss of muscle mass by changing the size and shape of cardiocytes in addition to developing cardiac hypertrophy, cardiac dilatation and congestive heart failure. This remodeling of the heart is associated with changes in the extracellular matrix including collagen proteins and is most probably due to the activation of both sympathetic nervous system and renin-angiotensin system as well as increased formation of various growth factors. Alterations in contractile function of the infarcted heart are associated with remodelling of the sarcoplasmic reticulum with respect to Ca(2+)-pump and Ca(2+)-release channels as well as contractile and regulatory proteins of the myofibrils. Myocardial infarction has also been shown to result in remodelling of the sarcolemmal membrane with respect to Ca(2+)-channels, Ca(2+)-transport systems, cardiac receptors and signal transduction mechanisms. Although information regarding remodelling of mitochondria in the infarcted heart is limited, alterations in energy yielding and Ca(2+)-accumulating systems are suspected. Accordingly, it is suggested that changes in cardiac contractile dysfunction due to myocardial infarction are associated with remodeling of both extracellular matrix and subcellular organelles in the heart.

Involvement of the sarcoplasmic reticulum calcium pump in myocardial contractile dysfunction: comparison between chronic pressure-overload and stunning

Acute as well as chronic forms of heart failure involve mechanical dysfunction during systole and/or diastole. The rapid Ca2+ release from and Ca2+ reuptake into the tubuli of the sarcoplasmic reticulum are processes that critically determine normal systolic and diastolic myocardial function, which explains why in the last fifteen years so much attention has been paid to understand the performance of the sarcoplasmic reticulum Ca2+ pump during myocardial contractile dysfunction. In this communication we have reviewed the literature data on sarcoplasmic reticulum Ca2+ pump function in the chronically pressure-overloaded hypertrophied and stunned (post-ischemic reversibly injured) myocardium in the light of some new data from our laboratory. Results on the pressure-overloaded hypertrophied myocardium provide evidence that impaired relaxation is most likely due to a low capacity of the sarcoplasmic reticulum to pump Ca2+, a consequence of a lower density of Ca(2+)-pumping sites within the sarcotubular membranes. Contractile dysfunction in stunned myocardium is accompanied by an upregulation of the sarcoplasmic reticulum Ca2+ ATPase gene resulting in a slight increase of the Ca2+ pumping activity. The latter increase is likely an adaptive response of the reversibly injured myocardium which may contribute to the slow recovery of contractile function.

Left ventricular diastolic function of the reperfused postischemic donor heart

This study examined the pathophysiological relationship between left ventricular diastolic function and myocardial biochemical changes during reperfusion following hypothermic cardioplegic preservation of the donor heart. Isolated canine hearts (n = 47) were preserved for 6 h at 5 degrees C, followed by normothermic reperfusion for 2 h. Regression analysis demonstrated a highly significant correlation between: Left ventricular maximum -dp/dt and the left ventricular end-diastolic pressure (r = -0.56, P = 0.001); myocardial concentrations of adenosine triphosphate (ATP) and Ca2+ (r = -0.59, P = 0.0001); maximum -dp/dt and myocardial concentrations of: (1) ATP, (2) Ca2+, and (3) total adenine nucleotide with left ventricular volume loading (r = -0.53, P = 0.003; r = 0.51, P = 0.002; and r = 0.52, P = 0.002, respectively); and left ventricular end-diastolic pressure and myocardial Ca2+ (r = 0.66, P = 0.0001). These results suggest that left ventricular relaxation, as assessed by maximum -dp/dt, has a negative correlation with left ventricular stiffness, as determined by the end-diastolic pressure in preserved donor hearts. Furthermore, increased myocardial Ca2+ concentrations reflect exhaustion of myocardial ATP. Thus, the myocardial Ca2+ concentration correlates directly with wall stiffness and inversely with ventricular relaxation, while ATP concentration correlates directly with ventricular relaxation.

Ryanodine wastes oxygen consumption for Ca2+ handling in the dog heart. A new pathological heart model

Ryanodine (RYA) at a low concentration (several tens of nM) is known to selectively bind to Ca2+ release channels in sarcoplasmic reticulum (SR) and to fix them open. The present study was designed to investigate the effects of the selective change in Ca2+ release channel activity on cardiac mechanoenergetics as a model of Ca(2+)-leaky SR observed in pathological hearts. We analyzed the negative inotropic effect of RYA at a low concentration (up to 30 +/- 13 nM) on left ventricular (LV) mechanoenergetics using frameworks of LV Emax (a contractility index) and the myocardial oxygen consumption (LV VO2)-systolic pressure-volume area (PVA) (a measure of total mechanical energy) relation in 11 isolated, blood-perfused dog hearts. RYA significantly decreased Emax by 42%, whereas PVA-independent VO2 remained disproportionately high (93% of control). This oxygen-wasting effect of RYA was quite different from ordinary inotropic drugs, which alter Emax and PVA-independent VO2 proportionally. The present result suggests that RYA suppresses force generation of cardiac muscle for a given amount of total sequestered Ca2+ by SR in a similar way to myocardial ischemia and stunning. We speculate about the underlying mechanism that RYA makes SR leaky for Ca2+ and thereby wastes energy for Ca2+ handling by SR.

Comparison of sarcoplasmic reticulum characteristics in aortic smooth and ventricular muscles using chemically skinned preparations

The active Ca2+ transport and the kinetics of Ca2+ efflux in sarcoplasmic reticulum (SR) of skinned aortic smooth muscle cells are compared with those in SR of skinned ventricular muscle cells. 45Ca uptake was measured at pCa 6.6 in the presence of 10 mM of K oxalate from 2 to 30 min. Unstimulated and Ca2+ stimulated Ca2+ efflux were assessed on Ca2+ desaturation curves performed in skinned cells preloaded with 45Ca in the absence of K oxalate. Results demonstrate that SR in aortic smooth muscle cells compared with SR in ventricular cells has: a) 2 times lower initial phase of Ca2+ uptake, b) 10 times lower enclosed volume, c) higher rate of unstimulated Ca2+ efflux and d) higher plus more sustained release of Ca2+ induced by Ca2+. It is also proved that the stimulated release of Ca2+ can be suppressed by 0.6 mM of ryanodine in aortic smooth muscle and by 50 microM of ruthenium red in ventricular muscle. This work provides direct measurements of Ca2+ transport capacity and of Ca2+ release of in situ sarcoplasmic reticulum. It quantifies the functional differences between vascular smooth and ventricular muscles.

Effect of brief regional ischemia followed by reperfusion with or without superoxide dismutase and catalase administration on myocardial sarcoplasmic reticulum and contractile function

The effect of reperfusion with and without free radical scavengers on sarcoplasmic reticulum and contractile function was examined in a canine model of 15-minute coronary artery occlusion followed by reperfusion. Dogs were reperfused with (n = 13) or without (n = 16) superoxide dismutase and catalase or were killed at 15 minutes of ischemia (n = 17). Superoxide dismutase and catalase were administered as a bolus (20,000 and 12,500 U/kg, respectively) beginning 1.25 minutes before reperfusion followed by infusion of 16,000 and 12,500 U/kg/hr, respectively. Sarcoplasmic reticulum function was evaluated from the rate of calcium uptake of unfractionated subepicardial, subendocardial, and transmural homogenates determined with and without ruthenium red to close the calcium release channel. Mechanical function was evaluated by means of sonomicrometry. Fifteen minutes of ischemia significantly (p less than 0.05) depressed the sarcoplasmic reticulum calcium uptake rate only in the subendocardium (from 25 +/- 2 to 14 +/- 1 nmol/min/mg without ruthenium red and from 60 +/- 3 to 49 +/- 3 nmol/min/mg with ruthenium red). Reperfusion with or without superoxide dismutase and catalase restored homogenate calcium uptake rates to normal, although severe contractile dysfunction persisted. This indicates that damage to the sarcoplasmic reticulum may not be the major cause of postreperfusion contractile dysfunction. Ischemia-reperfusion caused a decrease in systolic shortening from 19 +/- 2% to 1 +/- 2% with and from 18 +/- 1% to 4 +/- 1% without free radical scavengers (p = NS between groups). Thus administration of superoxide dismutase and catalase beginning shortly before reperfusion had no effect on postreperfusion contractile dysfunction or sarcoplasmic reticulum function.

Seasonal variations in the rate and capacity of cardiac SR calcium accumulation in a hibernating species

The rate of calcium uptake and the level of calcium accumulation was measured in cardiac muscle SR from hibernating and nonhibernating Richardson's ground squirrels. In whole heart homogenates, the rate of calcium uptake was higher (P less than 0.05) in hibernating animals than it was in active animals. Further purification of homogenates into sacroplasmic reticulum (SR) preparations showed that the hibernating animals had the highest rate of calcium uptake and the greatest level of calcium accumulation. These results could not be explained by variations in non-SR membrane contaminants nor by changes in the maximal activity or total amount of a SR marker enzyme, the Ca(2+)-ATPase. The addition of ryanodine to the calcium uptake medium increased the level of calcium accumulation in all groups by a similar amount. It is concluded that the high rate of calcium uptake by isolated cardiac SR vesicles from hibernating ground squirrels reflects the activity of the organelle in vivo, and that the ability of the ryanodine-insensitive population of SR vesicles to accumulate calcium is affected by hibernation.

Role of calcium and other ions in reperfusion injury

Calcium-related mechanisms may play a role in several aspects of reperfusion injury. One proposal is that internal cytosolic calcium concentration is elevated early in the reperfusion period and that excess oscillations of calcium can very significantly contribute to reperfusion ventricular arrhythmias. Alternate hypotheses, such as those involving free radicals and the local tissue renin-angiotensin system, can be married to the existing hypothesis. Furthermore, the hypothesis allows for a role of the sodium-calcium exchange system and the proton-sodium exchanger. The hypothesis also provides an explanation for "stunning," as it is proposed that early excessive cytosolic calcium damages the organelles regulating the contractile cycle, which subsequently develops into imperfect functioning of the contractile apparatus. Calcium antagonist drugs given during the ischemic period may lessen reperfusion injury by decreasing the severity of ischemic damage. When given at the time of reperfusion, results are complex and to some extent conflicting. The biggest challenge is to understand how relatively low doses of calcium antagonists given after the onset of reperfusion help to decrease delayed reperfusion "stunning." A logical but untested proposal is that calcium antagonists help to prevent delayed contraction-band necrosis, one of the causes of delayed no-reflow.

Stabilization of rat cardiac sacroplasmic reticulum Ca2+ uptake activity and isolation of vesicles with improved calcium uptake activity

The Ca2+ uptake activity of rat cardiac sacroplasmic reticulum (CSR) in ventricular homogenates is highly unstable, and this instability probably accounts for the low specific activity of Ca2+ uptake in previously reported fractions of isolated rat CSR. The instability was observed at either 0 degrees or 37 degrees, but the Ca2+ uptake activity was relatively stable at 25 degrees. The decay of Ca2+ uptake activity at 0 degrees could not be prevented by either PMSF or leupeptin, but dithiothreitol exerted some protective effects. Sodium metabisulfite prevented decay of the Ca2+ uptake activity of homogenates kept on ice but not of homogenates kept at 37 degrees. We also found that release of the CSR from the cellular debris required homogenization in high KCl. This distinguishes rat CSR from canine CSR. Isolated CSR was produced by a combination of differential centrifugation and discontinuous sucrous gradient centrifugation. The average rate of the sustained oxalate-supported calcium uptake in the resulting CSR fraction was 0.36 mumol/min-mg in the absence of CSR calcium channel blockers and 0.67 mumol/min/mg in the presence of 10 microM ruthenium red. Thus, this preparation has the advantage of containing both the releasing and non-releasing fractions of the CSR. The Ca2(+)-ATPase rates averaged 1.07 mumol/min/mg and 0.88 mumol/min-mg in the absence and presence of ruthenium red, respectively. Although these rates are higher than previously reported rates, this CSR preparation should still be considered a 'crude' preparation. A major distinction between the rat CSR and dog CSR was the lower content of Ca2(+)-ATPase in rat CSR, as judged by SDS-PAGE. Preparations of CSR isolated by this method may be useful in evaluating alterations in CSR function.