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Query: UMLS:C0022116 (ischemia)
 91,303 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

This article examines the pathophysiology of lesions caused by focal cerebral ischemia. Ischemia due to middle cerebral artery occlusion encompasses a densely ischemic focus and a less densely ischemic penumbral zone. Cells in the focus are usually doomed unless reperfusion is quickly instituted. In contrast, although the penumbra contains cells "at risk," these may remain viable for at least 4 to 8 hours. Cells in the penumbra may be salvaged by reperfusion or by drugs that prevent an extension of the infarction into the penumbral zone. Factors responsible for such an extension probably include acidosis, edema, K+/Ca++ transients, and inhibition of protein synthesis. Central to any discussion of the pathophysiology of ischemic lesions is energy depletion. This is because failure to maintain cellular adenosine triphosphate (ATP) levels leads to degradation of macromolecules of key importance to membrane and cytoskeletal integrity, to loss of ion homeostasis, involving cellular accumulation of Ca++, Na+, and Cl-, with osmotically obligated water, and to production of metabolic acids with a resulting decrease in intra- and extracellular pH. In all probability, loss of cellular calcium homeostasis plays an important role in the pathogenesis of ischemic cell damage. The resulting rise in the free cytosolic intracellular calcium concentration (Ca++) depends on both the loss of calcium pump function (due to ATP depletion), and the rise in membrane permeability to calcium. In ischemia, calcium influx occurs via multiple pathways. Some of the most important routes depend on activation of receptors by glutamate and associated excitatory amino acids released from depolarized presynaptic endings. However, ischemia also interfers with the intracellular sequestration and binding of calcium, thereby contributing to the rise in intracellular Ca++. A second key event in the ischemic tissue is activation of anaerobic glucolysis. The main reason for this activation is inhibition of mitochondrial metabolism by lack of oxygen; however, other factors probably contribute. For example, there is a complex interplay between loss of cellular calcium homeostasis and acidosis. On the one hand, a rise in intracellular Ca++ is apt to cause mitochondrial accumulation of calcium. This must interfere with ATP production and enhance anaerobic glucolysis. On the other hand, acidosis must interfere with calcium binding, thereby contributing to the rise in intracellular Ca++.
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PMID:Pathophysiology and treatment of focal cerebral ischemia. Part I: Pathophysiology. 1831 15

Although vasodilation, direct cardiac actions, or both represent the main properties of calcium channel blockers, there are further pharmacologic effects that may be therapeutically relevant. For example, hemorrheological effects, which have been demonstrated for a variety of calcium antagonists, have received relatively little attention to date. Hemorrheology describes the mechanics of blood and its components. It is of particular interest in the context of cardiovascular disease, as it has been shown that under certain conditions (reduced pump function, impaired vasomotor reserve), parameters of blood fluidity may be crucial for tissue perfusion. Whole-blood viscosity is the dominating factor in large arteries. For geometrical reasons, plasma viscosity and the rheological properties of blood cells may become of paramount importance at the microcirculatory level. In ischemic states, erythrocytes may be depleted of ATP, which they need for maintenance of normal shape and for transformation. This results in rigidification of the red blood cell and hindrance of its passage in the microcirculatory bed. Hence, blood flow deteriorates with the consequence of further unfavorable changes of the "milieu interieur," leading to the induction of a vicious cycle. Although effects on several hemorrheological parameters, for example, whole-blood viscosity, plasma viscosity, and red cell aggregation, can be demonstrated for various calcium channel blockers, the main rheological effects of these compounds are believed to consist in the improvement of erythrocyte deformability. When the ATP-dependent calcium pump is impaired in ischemia, calcium channel blockers may inhibit the slow inward transmembrane calcium flux and prevent the accumulation of intracellular calcium.
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PMID:The effects of calcium channel blockers on blood fluidity. 170 14

The recovery from trauma, whether ischemia or some other form of tissue injury, is never instantaneous; time is always required for repair and the return of normal metabolism and function. To what extent the delay in recovery of contractile activity (stunning) after a brief period of ischemia represents convalescence from ischemia-induced injury, as opposed to the expression of reperfusion-induced injury, is perhaps not as clear as the proponents of stunning would hope. Definitive evidence for a distinct reperfusion-induced pathology, which compromises the recovery of contractile function from the depressed state induced by ischemia, is elusive. If reperfusion-induced injury accounts for a significant proportion of stunning, then the molecular mechanisms responsible for initiating the event and those responsible for orchestrating the event at the level of the contractile protein are far from clear. Perturbations of calcium homeostasis are frequently cited as responsible for the depressed contractile state, however, some metabolic derangement must precede any pathologically induced ionic disturbance. In this connection, evidence indicates that free-radical-induced oxidant stress, during the early moments of reperfusion, may modify the activity of a number of thiol-regulated proteins that are directly, or indirectly, responsible for controlling the movement of calcium. Sarcolemmal sodium-calcium exchange and the calcium release channel of the sarcoplasmic reticulum may be activated, whereas the sarcolemmal calcium pump and sodium-potassium ATPase, together with the calcium pump of the sarcoplasmic reticulum, may be inhibited. Under the conditions prevailing during ischemia and reperfusion, this would be expected to promote an early intracellular calcium overload. It is difficult to reconcile such a change with the decreased inotropic state that characterizes stunning; however, it seems likely that the calcium overload is transient and that the stunned myocardium rapidly reestablishes normal levels of intracellular calcium. It is still difficult to explain adequately the reduced inotropic state; clearly, the mechanism of stunning is not quite as simple as its definition.
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PMID:Stunning: a radical re-view. 175 32

It has been proposed that oxygen free radical production is an important mediator of the myocardial dysfunction during the course of acute ischemia. We tested this hypothesis by characterizing the pathway of calcium efflux across sarcoplasmic reticulum (SR) membranes affected by oxygen free radicals. The effect of oxygen free radicals on the steady state calcium load, calcium permeability, and Ca,Mg-ATPase activity of isolated canine cardiac SR vesicles was investigated at pH 7.0. In vitro generation of oxygen free radicals by xanthine oxidase (0.09 units/ml), acting on xanthine in doses up to 50 microM as a substrate, increased the permeability of the SR vesicles to calcium, determined by measuring net efflux of calcium after stopping pump-mediated fluxes, and decreased total intravesicular calcium and free intravesicular calcium with no effect on Ca,Mg-ATPase activity. The effect of oxygen free radicals on calcium permeability was calcium gradient-dependent. Xanthine alone or xanthine plus denatured xanthine oxidase had no effect on this system. Superoxide dismutase (SOD, 56 units/ml), but not denatured SOD, significantly inhibited the effect of xanthine-xanthine oxidase reaction. The calcium permeability of the SR membrane decreased with decreasing calcium load. In addition, inasmuch as extravesicular calcium exerts only a slight effect on calcium permeability, the decrease in the permeability with calcium load is specifically related to the calcium load. Oxygen free radical-induced increase in calcium permeability was unaffected by Mg concentration between 2.1 and 21 mM. In summary, our data reveal that .O2- can produce a diminished level of accumulated calcium, which is reflected by the decreased calcium load and an increase in passive calcium permeability, and that the decreased calcium accumulation in the presence of the xanthine-xanthine oxidase system may not be mainly due to an inhibited calcium pump but due to an increased calcium permeability. Our results also suggest that increased SR membrane passive calcium permeability induced by oxygen free radicals is not carrier mediated. It is postulated that, with the oxygen free radical-mediated progressive increase in calcium permeability, free cytosolic calcium concentrations would increase in ischemic myocardium.
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PMID:The effect of oxygen free radicals on calcium permeability and calcium loading at steady state in cardiac sarcoplasmic reticulum. 284 52

The recognition of histopathologic substrates of myocardial contractile damage in human acute ischemia is still very poor, notwithstanding the impressive advances in the inherent clinical diagnostic technology and concepts. The first and foremost inotropic abnormality ensuing ischemia, easily taken for atonic in origin, actually consists of a pathologic contracture of the injured myocardium, depending upon abrupt fall of ATP, and defective extrusion calcium pump with persistence of actomyosin rigor-complexes. In sustained ischemia, further membrane damage exposes the myocell to massive calcium intrusion, with eventual precipitation of it and cell death (reperfusion stone-heart). In case of transient, "hit and run" ischemia, the "stunned" myocardium undergoes prolonged contractile abnormalities. In keeping with fundamentals in pathophysiology of contraction, ischemic myofibrils in human hyperacute infarct, showed spare I bands, accounting for contracture and followed by loss of the regular cross-striation register; then, groups of adjacent sarcomeres were seen to join into true "contraction" bands, with Z lines impinging upon A bands and obliterating the I bands. Coagulative denaturation of contractile proteins follows, presenting as irregular, amorphous degeneration stripes astride irreversibly damaged myocells. As such, these cells can be passively overstretched by the nearby functioning muscle. In turn, the fixed waviness of viable, acutely ischemic myocardium was thought to configure, histologically, the loss of ATP-dependent "plasticity" of myofilaments, in a state of contracture. The "relaxant effect" of inotropic-chronotropic-positive catecholamines, favoring diastole, has been also pointed out. The present microscopic findings are cogent to clinicopathologic problems of coronary ischemia-reperfusion, and sudden death from cardiogenic shock.
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PMID:[Myocardial mechanical injury in acute ischemia: a pathophysiologic and histopathologic review]. 287 93

Normothermic global ischemia of 7, 10, 15 and 60 min was found to depress oxalate supported calcium uptake rate measured either in unfractionated homogenates or isolated sarcoplasmic reticulum. The degree of depression increased with the duration of ischemia. Comparison of the isolated sarcoplasmic reticulum with unfractionated homogenates showed that the isolated sarcoplasmic reticulum was more damaged by ischemia than the unfractionated homogenate. The cause of this discrepancy was not due to inactivation of sarcoplasmic reticulum during isolation but was due to the discard of greater portions of undamaged sarcoplasmic reticulum as the ischemic period increased. Ischemia preferentially affected that sarcoplasmic reticulum most easily fragmented by homogenization. To determine if the depression of sarcoplasmic reticulum function is uniform throughout the isolated fraction, we compared several properties of the isolated fractions. After 10 min of ischemia, extensive properties such as calcium oxalate uptake rate, calcium ATPase rate, calcium oxalate capacity and steady-state calcium loading were depressed 50, 41, 48 and 24% respectively. In contrast, intensive properties such as permeability, calcium-ATPase turnover rate, and ratio of forward nucleotide flux to reverse nucleotide flux were unaffected by ischemia. However, one intensive property, the coupling ratio, was depressed 20%. We conclude from this difference in the effects of ischemia on extensive and intensive properties that the major effect of ischemia is to inactivate the Ca-ATPase.
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PMID:Effects of ischemia on the isolation and function of canine cardiac sarcoplasmic reticulum. 294 2

Of all tissues of the extremities, muscle is the least tolerant of ischemia. Hypothermia of tissue is considered beneficial for the maintenance of viability of muscle in amputated limbs before surgical replantation, but it has never been established that conventional cooling in an ice bath or its equivalent (temperature of tissue, approximately 1 degree Celsius) is the optimum level of hypothermia for minimizing metabolic derangement in ischemic muscle. In this study, we first defined the time course and level of metabolic derangement of muscle in twenty-eight ischemic hind limbs in cats at 22, 15, 10, 5, and 1 degree Celsius. The levels of adenosine triphosphate and phosphocreatine and the mean intracellular pH of the muscles in the lateral aspect of the thigh in each limb were monitored with phosphorus nuclear magnetic-resonance spectroscopy over time. The excised muscles from six freshly amputated legs of live humans were then similarly studied to determine whether muscles from cats and from humans exhibit comparable bioenergetic responses to hypothermic ischemia. A final series of ten ischemic hind limbs from cats was studied by nuclear magnetic resonance and muscle biopsy for direct biochemical assay of tissue energy metabolites to compare the metabolic benefits of two different methods of preserving limbs: continuous cooling in an ice bath, and a newly devised protocol for the rapid induction and maintenance of so-called intermediate (10 +/- 5 degrees Celsius) hypothermia of tissue. Ischemic skeletal muscle in cats exhibited a paradoxical metabolic response to extreme cold (1 degree Celsius). The rate of metabolic deterioration progressively declined with decreasing temperature of tissue to 10 degrees Celsius. However, at 5 degrees Celsius, no additional benefit was detected, and at 1 degree Celsius, there was a significant acceleration in the rates of degradation of adenosine triphosphate and phosphocreatine and in the production of lactate. The rate of degradation of adenosine triphosphate in human ischemic muscle was also faster at 1 degree Celsius than at 10 degrees Celsius. This paradoxical response is apparently due to a severe inhibition of the calcium pump of the sarcoplasmic reticulum of the muscle cell at temperatures of less than 5 degrees Celsius. The inhibition permits an efflux of calcium to the myofibrils, which stimulates both glycolysis and the degradation of adenosine triphosphate by myofibrillar adenosine triphosphatase.
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PMID:The bioenergetics of preservation of limbs before replantation. The rationale for intermediate hypothermia. 319 76

After local restriction by 70% and 90% of coronary circulation the properties of calcium pump were altered in myocardial sarcoplasmic reticulum (SR), since the activity of Ca2+-dependent ATPase and binding of 45Ca2+ by the SR membranes in the ischemic zone and out of it were decreased. Increase in the ischemic state made these alterations more distinct. Under conditions of the experiment content of ATP decreased and the metabolism of electrolytes was altered. A decrease in potassium and an increase in intracellular sodium content as well as a marked decrease in the ratio Ki/Ke were noted. Content of Ca2+ was simultaneously increased 2-2.5-fold in myocardium. These alterations might be responsible for impairment of normal ratios of systole/diastole cycles as well as for a decrease in the contractile functions of myocardium in ischemia.
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PMID:[ATPase activity and binding of calcium by sarcoplasmic reticulum of the myocardium in coronary insufficiency]. 622 47

It has been clearly established that ischemic heart disease, hypertension and ageing affect diastolic function before any change is observed in contractile function. Though an increasingly recognised clinical entity, cardiac failure with normal systolic function still does not have any specific treatment. Phosphodiesterase inhibitors which increase AMPc, in addition to their inotropic and vasodilator effects, accelerate relaxation. Major and isolated abnormalities of relaxation have been demonstrated in vitro in non necrosed tissues of both the dilated and hypertrophic forms of advanced cardiomyopathy. The myocardium seems unable to restore rapidly the low cytosolic calcium concentrations required for the deactivation of the contractile proteins. The underlying mechanisms are probably very complex but a deficit in AMPc production has been demonstrated in very advanced stages of cardiomyopathy. In ischemia, however, the abnormalities of relaxation seem to be directly related to a defect in free energy production inhibiting the sarcoplasmic reticulum calcium pump. If abnormalities of relaxation due to ischemia and those due essentially to a passive mechanism are excluded, phosphodiesterase inhibitors would seem to have pharmacological effects likely to improve diastolic function. Clinical studies confirm the beneficial effects of Milrinone and Enoximone on relaxation and the rapid phase of diastolic filling, both in acute and chronic studies. However, it has not yet been clearly established whether improved diastolic function is due to a direct action on the myocardium or an indirect action due to improved conditions of load. In order to determine the specific effects of phosphodiesterase inhibitors on diastolic function, further research is required.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:[Effects of phosphodiesterase inhibitors on diastolic function]. 821 93

Possible mechanisms of myocardial stunning at the cellular level include oxygen free radical production, neutrophil-induced damage, accelerated degradation of adenosine triphosphate (ATP), ischemia-induced myofibrillar damage, and calcium overload due to abnormal sodium/calcium exchange. Although the role of free radicals is generally accepted, their source(s) and clinical importance remain controversial. Similarly, an association of a declining ATP pool with myocardial stunning is well established but may not have clinical relevance as treatments that result in functional recovery do not always increase ATP levels. Little evidence exists for a strong link between myofibril damage and myocardial stunning. In contrast to these mechanisms, altered calcium homeostasis is an attractive hypothesis because: normal hearts perfused with high levels of calcium develop contractile dysfunctions similar to stunning; stunned hearts perfused with low calcium have increased function; calcium overload inhibition attenuates stunning; and reduced calcium ATPase activities have been found in sarcoplasmic reticulum from stunned hearts. Future research efforts should strive to simultaneously evaluate flow, function, and metabolic changes that occur in the stunned heart. Changes in the latter may eventually be studied using the techniques of molecular biology, integrating knowledge at the molecular level with clinical needs.
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PMID:Cellular and subcellular aspects of myocardial stunning. 846 18


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