Biochemical properties of membranes isolated from calcium-depleted rabbit hearts

L-leucine transport in rat heart under normal conditions and effects of a simulated hypoxia

L-leucine plays a central role in the regulation of protein metabolism in heart and has been implicated in myocardial protection, but little is known about the relationship between these phenomena and leucine transport across the cardiac sarcolemma. In this study we used sarcolemmal vesicles and ventricular myocytes isolated from rat heart to characterise L-leucine transport under normal conditions and to investigate the effect of simulated hypoxia or inhibition of protein synthesis. The Km and Vmax of leucine uptake were 5.24+/-0.65 mM and 1.43+/-1.84 nmol min(-1) mg(-1) protein in vesicles compared to 2.17+/-0.13 mM and 1.7+/-0.76 nmol min(-1) microl(-1) intracellular space in cells. Transport was not dependent on Na+ or H+ gradients. In vesicles L-leucine uptake was increased by trans-stimulation, whilst inhibition was observed with classical system L substrates including 2-aminobicyclo[2,2,1]-heptane-2-carboxylic acid (BCH) suggesting that this system mediated L-leucine transport in heart. L-Leucine uptake into isolated cardiac myocytes was inhibited after 20, 30 and 60 min of simulated hypoxia. This was not caused by reduced cell viability, although the cells underwent a rigor contracture. Inhibition of protein synthesis did not affect L-leucine transport.

Modification of heart sarcolemmal Na+/K+-ATPase activity during development of the calcium paradox

This study examined the status of sarcolemmal Na+/K+-ATPase activity in rat heart under conditions of Ca2+-paradox to explore the existence of a relationship between changes in Na+/K+-pump function and myocardial Na+ as well as K+ content. One min of reperfusion with Ca2+ after 5 min of Ca2+-free perfusion reduced Na+/K+-ATPase activity in the isolated heart by 53% while Mg2+-ATPase, another sarcolemmal bound enzyme, retained 74% of its control activity. These changes in sarcolemmal ATPase activities were dependent on the duration and Ca2+ concentration of the initial perfusion and subsequent reperfusion periods; however, the Na+/K+-ATPase activity was consistently more depressed than Mg2+-ATPase activity under all conditions. The depression in both enzyme activities was associated with a reduction in Vmax without any changes in Km values. Low Na+ perfusion and hypothermia, which protect the isolated heart from the Ca2+-paradox, also prevented reperfusion-induced enzyme alterations. A significant relationship emerged upon comparison of the changes in myocardial Na+ and K+ content to Na+/K+-ATPase activity under identical conditions. At least 60% of the control enzyme activity was necessary to maintain normal cation gradients. Depression of the Na+/K+-ATPase activity by 60-65% resulted in a marked increase and decrease in intracellular Na+ and K+ content, respectively. These results suggest that changes in myocardial Na+ and K+ content during Ca2+-paradox are related to activity of the Na+/K+-pump; the impaired Na+/K+-ATPase activity may lead to augmentation of Ca2+-overload via an enhancement of the Na+/Ca2+-exchange system.

Prevention by 7-oxo-prostacyclin of the calcium paradox in rat heart: role of the sarcolemmal (Na,K)-ATPase

It is demonstrated a fast and significant depression in the sarcolemmal (Na,K)-ATPase activity that occurs as early as 25 sec after the onset of Ca2+ depletion, and participates in the development of Ca(2+)-paradox in the rat heart. Pretreatment of the animals with 7-oxo-prostacyclin (PGI2) 24-48 h prior to the experiment prevented fairly the Ca(2+)-depletion-induced depression in (Na,K)ATPase activity and the accompanying structural and functional damage to the heart and sarcolemma during Ca(2+)-depletion as well as the development of Ca(2+)-paradox during the subsequent Ca(2+)-repletion. Pretreatment with PGI2 was chosen intentionally because previous experiments revealed, that in its late effect the drug is acting via stabilizing the membranes due induction of high activity of (Na,K)-ATPase that has increased affinity to ATP. From results obtained the following may be concluded: If during the phase of Ca(2+)-deprivation, the capability of heart sarcolemma to maintain sodium extrusion remains preserved, the expected aggravation of Ca(2+)-overload injury to Ca(2+)-paradox that would develop during Ca(2+)-repletion, may be definitely prevented. Sufficiently preserved (Na,K)-ATPase activity, hand in hand with stabilized sarcolemmal structure, may prevent an accumulation of sodium beneath the sarcolemma and consequently also an overexcessive entry of Ca2+ into the myocytes.

Alterations in cardiac function and subcellular membrane activities after hypervitaminosis D3

The present study was designed to induce massive accumulation of calcium in the myocardium and to evaluate the effect of calcium overload on myocardial contractile function and biochemical activity of cardiac subcellular membranes. Rats were treated with an oral administration of 500,000 units/kg of vitamin D3 for 3 consecutive days, and their hearts were sampled on the 5th day for biochemical analysis. On the 4th and 5th days, heart rate, mean aortic pressure, left ventricular systolic pressure and left ventricular dP/dt were significantly lowered in vitamin D3-treated rats, demonstrating the existence of appreciable myocardial contractile dysfunction. Marked increases in the myocardial calcium (67-fold increase) and mitochondrial calcium contents (24-fold increase) were observed by hypervitaminosis D3. Mitochondrial oxidative phosphorylation and ATPase activity were significantly reduced by this treatment. A decline in sarcolemmal Na+, K(+)-ATPase activity was also observed, while relatively minor or insignificant changes in calcium uptake and ATPase activities of sarcoplasmic reticulum were detectable. Electron microscopic examination revealed calcium deposits in the mitochondria after vitamin D3 treatment. The results suggest that hypervitaminosis D3 produces massive accumulation of calcium in the myocardium, particularly in the cardiac mitochondrial membrane, which may induce an impairment in the mitochondrial function and eventually may lead to a failure in the cardiac contractile function.

Beneficial effect of tan-shen, an extract from the root of Salvia, on post-hypoxic recovery of cardiac contractile force

The present study was undertaken to elucidate the possible effects of tanshinone VI, one of the extracts from the root of Salvia, on post-hypoxic recovery of cardiac contractile force. For this purpose, rat hearts were perfused for 45 min under reoxygenated conditions following 20-min hypoxic perfusion, and changes in tissue high-energy phosphates and calcium contents, and release of ATP metabolites and creatine kinase were examined. Post-hypoxic recovery of cardiac contractile force was augmented when hearts were treated with 42 nM tanshinone VI during hypoxia. This beneficial recovery was accompanied by enhanced restoration of myocardial high-energy phosphates, depression of hypoxia- and reoxygenation-induced increase in tissue calcium content, and suppression of release of ATP metabolites such as adenosine, inosine and hypoxanthine from the perfused heart. The results suggest that tanshinone VI is beneficial for the recovery of cardiac contractility after a certain period of oxygen-deficiency, possibly through mechanisms involving improvement of myocardial energy production upon oxygen-replenishment and/or inhibition of calcium accumulation in the cardiac cell.

No evidence of oxygen free radicals-mediated damage during the calcium paradox

Reperfusion of an isolated mammalian heart with a calcium-containing solution after a brief calcium-free perfusion results in irreversible cell damage: the calcium paradox. We investigated whether the calcium paradox is associated with oxidative damage. We measured the tissue changes of glutathione status and the release of oxidized glutathione from isolated perfused rabbit hearts as indicators of cellular oxidative events. After 10 min of calcium-free perfusion, tissue content of reduced (GSH) and oxidized (GSSG) glutathione, and of protein and non-protein sulfhydryl groups were not significantly different from control values. Restoration of the calcium concentration resulted in an immediate and massive release of GSH and a depletion of tissue content of GSH, GSSG, and non-protein sulfhydryl groups. However, only a minimal release of GSSG into the coronary effluent was observed. In addition, the characteristic features of the calcium paradox were present: development of an irreversible contracture and massive release of creatine kinase. The calcium paradox did not lead to a decrease of the tissue content of protein sulfhydryl groups. These observations indicate that the calcium paradox is not associated with oxidative damage.

The influence of mono- and divalent cations on the cardiac metabolism of arachidonic acid

Our previous study indicated that, in the isolated rabbit heart, perfusion with Ca2+ free Krebs Henseleit buffer (KHB) results in increased conversion of exogenous arachidonic acid to PGE2 and 6-keto-PGF1 alpha, probably as the result of increased availability of substrate to cyclooxygenase. Since perfusion with Ca2+ free buffer is known to cause alterations in the cardiac content of various mono- and divalent cations, the present study was performed to determine: a) The relationship between the conversion of exogenous arachidonic acid to prostaglandins and cardiac content of Na+, K+, Ca2+ and Mg2+; and b) Whether enhanced arachidonic acid conversion to prostaglandins during Ca2+ free perfusion is due to reduced incorporation of this fatty acid into tissue lipids. Perfusion of the rabbit heart with Ca2+ free buffer produced a significant reduction in the tissue content of Na+, K+, Ca2+ and Mg2+. However, the production of 6-keto-PGF1 alpha from exogenous arachidonic acid was linearly correlated with tissue Mg2+. These observations, together with our finding that perfusion with Ca2+ free KHB reduced the incorporation of [3H] arachidonic acid into tissue lipids, suggests that Ca2+ free perfusion may, by reducing the activity of arachidonyl CoA synthetase (a Mg2+ dependent enzyme), decrease the acylation of arachidonic acid into lipids, thus increasing the availability of arachidonic acid to cyclooxygenase.

Inhibition of cyclic nucleotide phosphodiesterase and calcineurin by spermine, a calcium-independent calmodulin antagonist

Spermine binding to calmodulin and its effects on two calmodulin-dependent enzymes were studied. Spermine bound to dansylated calmodulin with an apparent Ki of 0.7 mM, and to native calmodulin with a Kd of 1.1 mM in equilibrium dialysis experiments. Its binding was found to be independent of calcium. Spermine inhibited calmodulin-activated cyclic nucleotide phosphodiesterase noncompetitively with respect to calcium (Ki = 1.1 mM). Calmodulin activation of calcineurin was inhibited at similar concentrations (Ki = 1.2 mM). Spermine had little effect on basal phosphodiesterase activity or nickel-activated calcineurin activity. Inhibition of both enzymes correlated well with spermine binding to dansylcalmodulin. These findings suggest that spermine might modulate calcium-dependent events in the cell by inactivation of calmodulin via a novel calcium-independent mechanism.

Defects in sarcolemmal Ca2+ transport in hearts due to induction of calcium paradox

Na+-Ca2+ exchange and Ca2+-pump activities were studied in sarcolemmal vesicles isolated from rat hearts subjected to "calcium paradox" on perfusion with Ca2+-free medium followed by reperfusion with medium containing 1.25 mM Ca2+. Perfusion of hearts with Ca2+-free medium for 5 minutes did not affect the Na+-dependent Ca2+ uptake, ATP-dependent Ca2+ uptake, or Ca2+-stimulated ATPase activities in sarcolemma. Reperfusion of the Ca2+-deprived hearts with medium containing Ca2+ for 1-2 minutes increased Na+-dependent Ca2+ uptake, whereas reperfusion for 5-10 minutes decreased Na+-dependent Ca2+ uptake in sarcolemmal vesicles. Both ATP-dependent Ca2+ uptake and Ca2+-stimulated ATPase activities in sarcolemma were depressed on reperfusion of Ca2+-deprived hearts for 2-10 minutes. Reperfusion of Ca2+-deprived hearts for 5 minutes, which failed to generate contractile force, resulted in contracture without any recovery of the contractile force development. These changes in sarcolemmal Ca2+ transport and contractile function were prevented when hearts were perfused with Ca2+-free medium either in the presence of low sodium (35 mM) or at a low temperature (21 degrees C) before starting the reperfusion. No alterations in the purity of the preparation or permeability of sarcolemmal vesicles with respect to Na+ or Ca2+ were detected in hearts perfused with Ca2+-free medium or on reperfusion with medium containing calcium. The results indicate abnormalities in sarcolemmal Na+-Ca2+ exchange and Ca2+-pump mechanisms on reperfusion of Ca2+-deprived hearts with medium containing Ca2+, and such changes may partly account for the occurrence of intracellular Ca2+ overload during the development of calcium paradox.

Beta-adrenergic receptors and enzymes in rat myocardial membranes: implications of fractionation procedures and beta-adrenoceptor antagonists

1. We performed an enzymatic characterization of two different fractionation procedures of ventricles from rat hearts. The enzymatic assays covered succinic dehydrogenase as a marker for inner mitochondrial membranes, monoamine oxidase as a marker for outer mitochondrial membranes, NADPH-cytochrome c reductase and RNA as endoplasmatic reticular markers, acid phosphatase as a lysosomal marker, and lactic dehydrogenase as a marker for the "soluble" compartment; DNA was estimated for nuclear contamination. 2. The plasma membrane markers 5'-nucleotidase, Ca2+-ATPase, Mg2+-ATPase, Na+-K+-ATPase, and adenylate cyclase were determined. 3. The roughly prepared membrane fractions showed increased yields of the membrane markers; the number of beta receptors, determined with (-)-[3H] dihydroalprenolol and DL-propranolol, amounted to 68 +/- 6 fmol/mg protein (KD = 3390 +/- 450 pmol, Hill coefficient = 1.5). 4. The membrane fraction prepared with a linear sucrose gradient showed an increased inner mitochondrial membrane marker; presumably the outer mitochondrial membrane was stripped off. The beta-receptor number was 39 +/- 3 fmol/mg protein (KD = 6250 +/- 300 pmol; Hill coefficient = 1.2).

Phospholipid composition and amphiphile content of isolated sarcolemma from normal and autolytic rat myocardium

Sarcolemmal vesicles were purified to a similar extent, 50- to 60-fold on a protein basis, from normal rat hearts and hearts subjected to 30 or 60 min of autolysis at 37 degrees C (total ischemia in vitro). Electron microscopic examination of the autolytic hearts revealed sarcolemmal discontinuities and other morphological characteristics typical of irreversible cell injury. Total contents and percentage composition of phospholipid classes did not differ between normal and autolytic hearts or between sarcolemmal preparations from these hearts. There was no increase in lysophospholipid contents of whole hearts or of purified sarcolemma after autolysis. Long chain acyl-CoAs or acylcarnitines did not accumulate in autolytic hearts under our experimental conditions. The molar long chain acyl-CoA: phospholipid ratio in isolated sarcolemma was extremely low (1:100,000). It increased 3-fold after autolysis but the increase was most probably the result of an increase in mitochondrial contamination of the sarcolemmal preparations from autolytic hearts. The molar long chain acylcarnitine: phospholipid ratio of isolated sarcolemma was much larger (1:100), but it did not change after autolysis. Experiments, in which radioactive amphiphiles were incorporated in isolated sarcolemma that was subsequently repeatedly washed, indicated that the lysophospholipid and acylcarnitine contents of isolated sarcolemma reflect the contents of sarcolemma in situ, but that sarcolemmal acyl-CoA is used for re-acylation reactions during purification, explaining the low acyl-CoA content of isolated sarcolemma. Na/K-ATPase and Na/Ca-exchange activities were markedly depressed in isolated sarcolemma from autolytic hearts. Our results suggest that sarcolemmal phospholipid breakdown and sarcolemmal amphiphile accumulation are not responsible for the structural and functional defects of the sarcolemma after autolysis.

Changes in cyclic nucleotides during the calcium paradox in the isolated rat heart

Reperfusion of hearts with a Ca2+-containing medium after a perfusion period in Ca2+-free medium results in irreversible cell damage (calcium paradox). In this investigation we have studied coronary flow and cyclic AMP and cyclic GMP levels after several periods of Ca2+-free perfusion in isolated rat hearts. We also investigated the effects of papaverine (Pap), noradrenaline (NA), acetylcholine (ACh) and absence of inorganic phosphate during Ca2+-free perfusion on coronary flow (CF) and cyclic nucleotide levels. Inability of the heart to recover contractile activity with development of contracture during the reperfusion period was accepted as indicative of the calcium paradox. Ca2+-free perfusion alone and NA and absence of inorganic phosphate during the Ca2+-free perfusion period increased CF, whereas Pap and ACh decreased it. However, only Ca2+-free perfusion and NA elevated cyclic AMP. On the other hand, Pap and ACh increased cyclic GMP (with a transient rise of cyclic AMP in Pap infusion), and absence of inorganic phosphate decreased both cyclic AMP and cyclic GMP. Pap, ACh and absence of phosphate prevented the calcium paradox. Our study suggests that increased cyclic AMP during the Ca2+-free perfusion may contribute, with the other factors, to the occurrence of the calcium paradox.

Polyamines mediate uncontrolled calcium entry and cell damage in rat heart in the calcium paradox

Brief perfusion of heart with calcium-free medium renders myocardial cells calcium-sensitive so that readmission of calcium results in uncontrolled Ca2+ entry and acute massive cell injury (calcium paradox). We investigated the hypothesis that polyamines may be involved in the mediation of abnormal Ca2+ influx and cell damage in the calcium paradox. The isolated perfused rat heart was used for these studies. Calcium-free perfusion promptly (less than 5 min) decreased the levels of polyamines and the activity of their rate-regulating synthetic enzyme, ornithine decarboxylase (ODC), and calcium reperfusion abruptly (less than 15-180 s) increased these components. alpha-Difluoromethylornithine (DFMO), a specific suicide inhibitor of ODC, suppressed the calcium reperfusion-induced increase in polyamines and the concomitant increase in myocardial cellular 45Ca influx, loss of contractility, release of cytosolic enzymes, myoglobin, and protein, and structural lesions. Putrescine, the product of ODC activity, nullified DFMO inhibition and restored the calcium reperfusion-induced increment in polyamines and the full expression of the calcium paradox. Putrescine itself enhanced the reperfusion-evoked release of myoglobin and protein in the absence of DFMO. Hypothermia blocked the changes in heart ODC and polyamines induced by calcium-free perfusion and calcium reperfusion and prevented the calcium paradox. These results indicate that rapid Ca2+-directed changes in ODC activity and polyamine levels are essential for triggering excessive transsarcolemmal transport of Ca2+ and explosive myocardial cell injury in the calcium paradox.

Effects of Ca2+/Mg2+ removal on aiNa, aiK, and tension in cardiac Purkinje fibers

Sheep cardiac Purkinje fibers were exposed to solutions free of divalent cations for hour-long periods, while intracellular Na+ and K+ activities were measured using ion-sensitive microelectrodes. Intracellular Na+ activity (aiNa) increased to 50.1 +/- 8.1 mM, and intracellular K+ activity (aiK) decreased to 76.7 +/- 3.5 mM. These ionic changes could be blocked by the presence of Mg2+ or the Ca2+ channel blocking agents D 600 and nifedipine. The rise in aiNa and the fall in aiK was accentuated by the inhibition of the Na+-K+ pump with acetylstrophanthidin or by removal of extracellular K+. These results demonstrate that in cardiac Purkinje fibers removal of divalent cations produces intracellular loading of Na+ by Na+ entry through the Ca2+ channel. On reexposure to Ca2+-containing solutions, the cells become loaded with Ca2+, and the fibers exhibit large contractures. These observations implicate Na+-Ca2+ exchange in the entry of Ca2+ into these cells during Ca2+ repletion and in the etiology of the calcium paradox.

The effect of hypothermia during the period of calcium repletion on the calcium paradox

Reperfusion of an isolated heart with a calcium-containing solution after a short calcium-free perfusion may result in irreversible cell damage: the calcium paradox. In this investigation the effect of hypothermia during reperfusion with calcium-containing solution on the calcium paradox damage in the isolated rat heart was studied. In addition, the effect of pre-cooling the heart during the calcium-free period was investigated. Creatine kinase release was used to define cell damage. Normothermic (37 degrees C) calcium-free perfusion followed by normothermic reperfusion with calcium-containing solution resulted in a massive release of CK. When the normothermic calcium-free perfusion was followed by hypothermic (10 degrees C) calcium-containing reperfusion, CK release was reduced by 20% (P less than 0.005). This CK release during reperfusion was further reduced by 55% and 80% when the normothermic calcium-free perfusion was followed by 5 or 10 min respectively of hypothermic calcium-free perfusion prior to the hypothermic calcium-containing reperfusion. The results show that hypothermia during the period of calcium repletion retards the sequence of events which ultimately results in release of large amounts of intracellular components.

Effects of calcium channel blocking agents on calcium and centrilobular necrosis in the liver of rats treated with hepatotoxic agents

Carbon tetrachloride, chloroform, dimethylnitrosamine, thioacetamide or acetaminophen was each administered to rats in a single hepatotoxic dose. Nifedipine, verapamil or chlorpromazine was administered in association with the hepatotoxic agents to determine if calcium channel blocking agents would prevent an increase in liver cell calcium associated with hepatotoxicity and to determine if these agents would protect against the development of centrilobular necrosis. Following a latent period different for each toxic agent, a 4- to 18-fold increase in liver cell calcium content had occurred by 24 hr. The calcium increase and the centrilobular necrosis (mean histologic score) were correlated. A relatively high calcium to necrosis ratio was obtained with dimethylnitrosamine, thioacetamide and acetaminophen. A lesser calcium to necrosis ratio was obtained with chloroform and carbon tetrachloride, the two toxic agents that destroyed the intracellular calcium sequestration activity of the liver endoplasmic reticulum. Nifedipine or chlorpromazine, administered prior to and 7 hr after the toxic agent, completely prevented the centrilobular necrosis caused by thioacetamide, carbon tetrachloride and acetaminophen; almost completely prevented necrosis with dimethylnitrosamine; and provided partial protection against chloroform toxicity. Two doses of verapamil provided partial protection against necrosis when carbon tetrachloride was the toxic agent and provided almost complete protection with dimethylnitrosamine. A reduction in liver cell calcium was associated with the protective action of the three calcium channel blocking agents. These findings are compared with earlier studies of the protective effects of calcium channel blocking agents in cardiac ischemia.

Suppression of cellular injury during the calcium paradox in rat heart by factors which reduce calcium uptake by mitochondria

Isolated Langendorff perfused rat hearts were used to study changes in the Ca, Na and K content, contractile force and the loss of cellular material during the Ca paradox. Five minutes perfusion with Ca-free solution containing 1 mM EGTA, followed by 10 min of reperfusion in 1.8 mM Ca causes irreversible contracture, K loss, increase in Na and Ca and a massive release of myoglobin and other cellular material into the perfusate (the calcium paradox). During the Ca-free perfusion the ventricles gain Na but the K content decreases slightly. The size of the Na gain appears to depend upon the buffer used and is larger in bicarbonate than in Tris. When HCO3- or H2PO4- ions are omitted from the bathing solution (in Tris, HEPES, or TES buffered salines) the adverse effects of Ca readmission are reduced. Tris buffer gives the best protection. Metabolic inhibition with FCCP (5 X 10(-7) M), or with CN-(2 X 10(-3) M) together with iodoacetic acid (2 X 10(-3) M), decreases Ca uptake during the Ca paradox and inhibits the release of cellular material. In both cases a contracture is observed. Ruthenium red (10(-4) M) does not inhibit the Ca readmission contracture but reduces the release of cellular material and the gain of Ca and Na. The results suggest that the loss of cellular constituents during the calcium paradox, is related to an active uptake of Ca by the mitochondria and may lead to massive changes in the cellular ion concentration, during Ca-repletion.