Myocardial metabolism in cardiac hypoxia

CD36 as a gatekeeper of myocardial lipid metabolism and therapeutic target for metabolic disease

The multifunctional membrane glycoprotein CD36 is expressed in different types of cells and plays a key regulatory role in cellular lipid metabolism, especially in cardiac muscle. CD36 facilitates the cellular uptake of long-chain fatty acids, mediates lipid signaling, and regulates storage and oxidation of lipids in various tissues with active lipid metabolism. CD36 deficiency leads to marked impairments in peripheral lipid metabolism, which consequently impact on the cellular utilization of multiple different fuels because of the integrated nature of metabolism. The functional presence of CD36 at the plasma membrane is regulated by its reversible subcellular recycling from and to endosomes and is under the control of mechanical, hormonal, and nutritional factors. Aberrations in this dynamic role of CD36 are causally associated with various metabolic diseases, in particular insulin resistance, diabetic cardiomyopathy, and cardiac hypertrophy. Recent research in cardiac muscle has disclosed the endosomal proton pump vacuolar-type H+-ATPase (v-ATPase) as a key enzyme regulating subcellular CD36 recycling and being the site of interaction between various substrates to determine cellular substrate preference. In addition, evidence is accumulating that interventions targeting CD36 directly or modulating its subcellular recycling are effective for the treatment of metabolic diseases. In conclusion, subcellular CD36 localization is the major adaptive regulator of cellular uptake and metabolism of long-chain fatty acids and appears a suitable target for metabolic modulation therapy to mend failing hearts.

The proarrhythmic effect of hypoglycemia: evidence for increased risk from ischemia and bradycardia

Hypoglycemia increases the risk for both overall and sudden death. At a cellular level, hypoglycemia causes alterations in the physiology of myocardial tissue that are identical to proarrhythmic medications. Reduced serum glucose blocks the repolarizing K(+) channel HERG, which leads to action potential and QT prolongation and is uniformly associated with risk for torsades de pointes ventricular tachycardia. The sympathetic response induced by hypoglycemia also increases the risk of arrhythmias from Ca(2+) overload, which occur with sympathomimetic medications and excessive beta adrenergic stimulation. Thus, hypoglycemia can be considered a proarrhythmic event. This review focuses on emerging evidence for two other important changes induced by hypoglycemia that promote arrhythmias: ischemia and bradycardia. Studies of patients with "insulin shock" therapy from the early twentieth century and other more recent data strongly suggest that hypoglycemia can cause ischemia of myocardial tissue, both in association with coronary artery obstructions and by cellular mechanisms. Ischemia induces multiple proarrhythmic responses. Since ischemia itself reduces the possibility of using energy substrates other than glucose, hypoglycemia may generate positive feedback for electrophyisologic destabilization. Recent studies also show that hypoglycemia can cause bradycardia and heart block. Bradycardia is known to cause action potential prolongation and potentiate the development of torsades de pointes, particularly with low-serum K(+) which can be induced by hypoglycemic episodes. Thus, hypoglycemia-induced bradycardia may also create a dynamic, positive feedback for the development of arrhythmias and sudden death. These studies further support the hypothesis that hypoglycemia is a proarrhythmic event.

Changes in myocardial lactate metabolism during ramp exercise in patients with effort angina and microvascular angina

Changes in myocardial lactate metabolism during ramp exercise were investigated through great cardiac vein catheterization in 15 patients with effort angina (EA) and 7 patients with microvascular angina (MVA). The exercise test was performed using a supine bicycle ergometer. Blood samples were obtained from the great cardiac vein (GCV) and the radial artery each minute during exercise. Patients in the EA group showed a point at which the lactate extraction ratio (LER) and the ST level decreased rapidly during exercise. This point was clearly recognized in 12 of 13 patients, and may represent the ischemic threshold. Both the LER and ST level showed similar changes during exercise, and these values were significantly different between the ischemic threshold and peak exercise (p < 0.01). Both the LER and ST level were strongly correlated with the duration of exercise until the threshold (r = 0.703). In the MVA group, both the LER and ST level during exercise continuously decreased throughout exercise without an ischemic threshold in all of the subjects. Endomyocardial biopsy revealed sclerosis of small arteries in the myocardium in all of the patients. In both effort angina and microvascular angina, a close correlation was noted between the change in ST and that in the myocardial lactate extraction ratio.

Relation of myocardial oxygen consumption and function to high energy phosphate utilization during graded hypoxia and reoxygenation in sheep in vivo

This study investigates the relation between myocardial oxygen consumption (MVO2), function, and high energy phosphates during severe hypoxia and reoxygenation in sheep in vivo. Graded hypoxia was performed in open-chested sheep to adjust PO2 to values where rapid depletion of energy stores occurred. Highly time-resolved 31P nuclear magnetic resonance spectroscopy enabled monitoring of myocardial phosphates throughout hypoxia and recovery with simultaneous MVO2 measurement. Sheep undergoing graded hypoxia (n = 5) with an arterial PO2 nadir of 13.4 +/- 0.5 mmHg, demonstrated maintained rates of oxygen consumption with large changes in coronary flow as phosphocreatine (PCr) decreased within 4 min to 40 +/- 7% of baseline. ATP utilization rate increased simultaneously 59 +/- 20%. Recovery was accompanied by marked increases in MVO2 from 2.0 +/- 0.5 to 7.2 +/- 1.9 mumol/g per min, while PCr recovery rate was 4.3 +/- 0.6 mumol/g per min. ATP decreased to 75 +/- 6% of baseline during severe hypoxia and did not recover. Sheep (n = 5) which underwent moderate hypoxia (PO2 maintained 25-35 mmHg for 10 min) did not demonstrate change in PCr or ATP. Functional and work assessment (n = 4) revealed that cardiac power increased during the graded hypoxia and was maintained through early reoxygenation. These studies show that (a) MVO2 does not decrease during oxygen deprivation in vivo despite marked and rapid decreases in high energy phosphates; (b) contractile function during hypoxia in vivo does not decrease during periods of PCr depletion and intracellular phosphate accumulation, and this may be related to marked increases in circulating catecholamines during global hypoxia. The measured creatine rephosphorylation rate is 34 +/- 11% of predicted (P < 0.01) calculated from reoxygenation parameters, which indicates that some mitochondrial respiratory uncoupling also occurs during the rephosphorylation period.

Effect of hypoxia on phosphatidylcholine biosynthesis in the isolated hamster heart

In hamster heart, the majority of the phosphatidylcholine is synthesized via the CDP-choline pathway, and the rate-limiting step of this pathway is catalysed by CTP:phosphocholine cytidylyltransferase (EC 2.7.7.15). We have shown previously [Choy (1982) J. Biol. Chem. 257, 10928-10933] that, in the myopathic heart, the level of cardiac CTP was diminished during the development of the disease. In order to maintain the level of CDP-choline, and consequently the rate of phosphatidylcholine biosynthesis, cardiac cytidylyltransferase activity was increased. However, it was not clear if the same compensatory mechanism would occur when the cardiac CTP level was decreased rapidly. In this study, hypoxia of the hamster heart was produced by perfusion with buffer saturated with 95% N2. The heart was pulse-labelled with radioactive choline and then chased with non-radioactive choline for various periods under hypoxic conditions. There was a severe decrease in ATP and CTP levels within 60 min of hypoxic perfusion, with a corresponding fall in the rate of phosphatidylcholine biosynthesis. Analysis of the choline-containing metabolites revealed that the lowered ATP level did not affect the phosphorylation of choline to phosphocholine, but the lower CTP level resulted in the decreased conversion of phosphocholine to CDP-choline. Determination of enzyme activities revealed that hypoxic treatment resulted in the enhanced translocation of cytidylyltransferase from the cytosolic to the microsomal form. This enhanced translocation was probably caused by the accumulation of fatty acids in the heart during hypoxia. We postulate that the enhancement of translocation of the cytidylyltransferase to the microsomal form (a more active form) is a mechanism by which the heart can compensate for the decrease in CTP level during hypoxia in order to maintain phosphatidylcholine biosynthesis.

Hemodynamics and metabolism during surface-induced hypothermia in the dog: a comparison of pH management strategies

The management of blood pH during hypothermia remains controversial. The present study was designed to determine whether hemodynamics and oxygen consumption during hypothermia are different between the alpha-stat and pH-stat strategies. Theoretical considerations of enzyme kinetics suggest that the alpha-stat strategy would result in a higher oxygen consumption during hypothermia. Because hypothermia is used to decrease oxygen consumption for protection during ischemia, a pH scheme that results in a greater oxygen demand for any level of ischemia would be detrimental. The core temperature of 22 dogs was lowered to 26 degrees C by combined surface cooling and gastric irrigation. Either the alpha-stat (N = 9) or the pH-stat (N = 13) pH strategy was used. The arterial pressure was different between the two groups at 26 degrees C (65 +/- 6 vs 85 +/- 6 mm Hg, alpha-stat vs pH-stat, respectively, P less than 0.05). Neither systemic oxygen consumption nor the Q10 was different between groups. There were no differences in any other hemodynamic parameters. In summary, during moderate hypothermia alpha-stat pH management results in an arterial pressure lower than that of pH-stat management, possibly resulting in improved peripheral perfusion. Despite theoretical predictions, the alpha-stat pH scheme does not result in an oxygen consumption higher than that of the pH-stat scheme.

Metabolic and ultrastructural aspects of experimental diabetic glomerulopathy

The effects of alloxan induced diabetes and insulin treatment on rat kidney glomerular ion transport and the oxidative phosphorylation were investigated in order to correlate metabolic and ultrastructural alterations. In alloxan diabetic rats an early decrease of Na+K+ ATPase activity in isolated glomeruli preparations and the subsequent uncoupling of oxidative phosphorylation were observed. These metabolic alterations are associated with structural changes such as the thickening of the basement membrane and the disorganization of both endothelial and mesangial cells. Insulin treatment induces a slow and only partial recovery of metabolic changes. Ultrastructural features of the kidney cortex were almost completely normalized after only two months of insulin treatment.

Response of isolated guinea pig myocardium to insulin therapy during normothermia and graded hypothermia

The discovery of insulin in 1922 aroused immediate clinical interest in its use in heart disease. In severe heart failure, insulin release is suppressed by the combined effect of poor pancreatic perfusion and by increased sympathetic activity. In these circumstances, myocardial metabolism of glucose may break down through the deficiency of insulin. Because of this, glucose, insulin and potassium solution (GIK solution) has been used in cardiopulmonary resuscitation. However, its mechanism is not yet fully known. This study was designed to determine the effect of insulin on cardiac muscle at various temperatures. The mechanical response of papillary muscle isolated from guinea pig ventricle was observed under various thermal conditions (23-37 degrees C). Twitch tension was increased by the administration of 0.2 I.U./ml insulin under each thermal condition. In all circumstances, the increase in contractile force was noted about 2 min after the administration of insulin. The effect of insulin on 20 preparations demonstrated the mean maximum contractile force was 226% ( +/- 34 S.D., n = 5) in 37 degrees C, 194% ( +/- 36 S.D., n = 5) in 30 degrees C, 190% ( +/- 30 S.D., n = 5) in 27 degrees C and 200% ( +/- 36 S.D., n = 5) in in 23 degrees C. The differences between different temperatures was not significant. The effect of insulin during depression Na-K pump by high concentration of ouabain (g-strophanthin, 10(-5) M) was also observed. Insulin (0.2 I.U./ml) was administered when the papillary muscle showed no response to electrical stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Decrease by naloxone of some electrocardiographic and biochemical changes following endotoxin induced shock in rats

Administration of endotoxin, a lipopolysaccharide extracted from cell walls of gram negative bacteria, elicited alterations in various metabolic parameters and in the electrocardiogram of rats. Cardiac glycogen and serum glucose were decreased, while serum pyruvate and acid phosphatase levels were increased. There was initial tachycardia followed by significant bradycardia and elevation of the ST segment in the animals with shock. Erythrocyte count, haemoglobin and haematocrit were not changed after shock. Treatment with naloxone caused significant decreases in the metabolic and electrocardiographic changes induced by endotoxin.

Intraluminal pCO2: a reliable indicator of intestinal ischemia

A reliable, objective method to determine small bowel ischemia intraoperatively has not been developed. These experiments examined the relationship between intraluminal pCO2 (IL pCO2), intestinal blood flow, and degree of ischemic mucosal injury. IL pCO2 was measured with a clinical mass spectrometer using Teflon catheters calibrated in tissue mode; transmural intestinal blood flow was measured with radioactive microspheres. Anesthetized rabbits (N = 24) were cannulated for microsphere injections and mass spectrometer catheters were placed in the lumen of the small bowel. Blood flow was determined prior to superior mesenteric artery occlusion and then at 30, 60, or 180 min after occlusion. In control animals the superior mesenteric artery was not clamped. Intestinal biopsies were taken at the time of each blood flow determination and microscopic injury was graded from 1 (normal) to 4 (complete epithelial slough). There was a strong linear correlation between the IL pCO2 and the histologic grade of injury (r = 0.778, P less than 0.001). These results show that intestinal ischemia due to superior mesenteric artery occlusion causes a rapid, sustained rise in small bowel IL pCO2 that correlates with the degree of mucosal injury. These experiments suggest that this technology may provide a superior method to assess intestinal perfusion.

The border zone of the early myocardial infarction in dogs; its characteristics and viability

In open-chest dogs, the left anterior descending coronary artery was ligated for 150 min. The heart was semiserially cut on a cryomicrotome and areas of ischemic damage were visualized by means of glycogen depletion (PAS reaction) and tissue acidosis (a "sandwich" technique with pH indicator dispersed in a layer of gel). The extent of myocardial damage was determined morphometrically. The mass of the glycogen-depleted heart muscle was greater than the mass of the ischemic tissue detected by means of decreased pH (p less than 0.01). The border zone was characterized by glycogen depletion without acidosis. Circulation studies using intravital fluorescein staining have shown that perfusion is partially retained in the border zone; it is assumed that the hypoperfusion triggers glycogenolysis. Nevertheless, the level of perfusion suffices to wash out the acidic end products. Comparison of contrapulsation-treated dogs and untreated dogs shows that the amount of damaged tissue comprising the border zone can be reduced by this therapeutic intervention (p less than 0.02) - in contrast to the acidotic tissue, the amount of which is not significantly influenced. Therefore the border zone contains damaged but still viable muscle cells.

Effects of propranolol and hydrocortisone pretreatment on radiation-induced myocardial injury in rats

Earlier studies in our laboratory (23) showed evidence of dose-related acute injury to the myocardium after exposure of rats to ionizing radiation. Biochemical, histological, and electrocardiographic parameters were studied. In further continuation of this study, the effects of intervention by pretreatment with propranolol (10 mg/kg body weight) and hydrocortisone (10 mg/kg body weight) have been studied. The above drugs were administered to male albino rats weighing 150 to 200 g 30 min before exposure to 6000-rad single-dose gamma radiation over the precordial area. The parameters observed were cardiac glycogen, serum enzymes, lactate, pyruvate, blood sugar, adrenal ascorbic acid, and histology of the myocardium. The beneficial effects of this procedure are discussed.

Structure of the freeze-fractured sarcolemma in the normal and anoxic rabbit myocardium

The purpose of this study was to examine the ultrastructure of the sarcolemma in the normal and severely anoxic rabbit heart with the technique of freeze-fracture. Severe anoxia and subsequent reoxygenation cause a significant decrease (31%) in intramembranous particles (IMP) in the P face of the membrane and a 25% decrease in the E face. P face IMP's are severely aggregated. The decrease in density and the redistribution of IMP's indicate a severely altered lipoprotein structure of the sarcolemma. In addition, the necks of caveolae open and the caveolae become flattened in the plane of the membrane. With reoxygenation, many rupture. Spherical projections of cytoplasmic vesicles appear in the membrane (possibly of sarcoplasmic reticulum or lysosomal origin) and also can be seen to rupture after reoxygenation. When glucose is present in the perfusate, it affords some protection against these structural defects. We propose that the fragmentation or holes in the sarcolemma reported in severe anoxia are directly related to the structural changes reported in this study.

The evaluation of ischemic heart disease thallium-210 with comments on radionuclide angiography

Coronary artery disease causing myocardial ischemia and infarction is the leading cause of death in America. Methods that can be used to diagnose and follow the response to therapy of coronary artery disease or its effect on myocardial ischemia should help control the morbidity and mortality of ischemic heart disease. The use of ECG monitoring is less sensitive and specific for ischemia than thallium (TI) imaging or the use of radionuclide angiography (RNA). In large patient populations, the findings of a positive ECG and TI or RNA study will be highly predictive for the presence of coronary artery disease, while negative test results make the disease unlikely. A combined approach to the patient with possible ischemic heart disease is presented.

Depression of intramyocardial oxyhemoglobin dissociation by angiographic contrast media

The effect of the addition of radiographic contrast material (Renografin) to blood on the oxyhemoglobin dissociation curve and P50 was measured by a metabolic deoxygenation technique in a strongly buffered red cell suspension. With incubation time constant, increasing doses produced progressive decreases in P50. With incubation time varied at a constant dose, a decrease in P50 was seen after only one minute. In addition, in vivo studies were performed on 11 patients undergoing cardiac catheterization. Simultaneous proximal coronary sinus and aortic samples were drawn as controls, and then at one minute and five minutes after injection of the left coronary artery. In eight patients studies were performed after, and in three prior to left ventriculography. At one minute after left coronary injection there was a significant decrease of coronary sinus as compared to aortic P50 (p less than .10) (only when left ventriculography was performed prior to coronary arteriography). The magnitude of these effects in vivo is unknown, but they would be expected to be more severe in areas distal to a critical coronary lesion due to stasis of blood flow and ischemic metabolic changes.

Effect of isoproterenol on regional myocardial perfusion and tissue oxygenation in acute myocardial infarction

The effect of isoproterenol infusion on regional myocardial perfusion and tissue oxygenation during acute myocardial infarction was investigated in anesthetized dogs. Measurements of regional flow with radioactive microspheres and myocardial lactate and adenosine triphosphate from analysis of myocardial biopsies were compared in normal, marginal, and infarcted tissue in dogs with a ligated coronary artery. After 10 minutes of isoproterenol 0.15 microgram/Kg./minute, flow was unchanged in the marginal and infarcted regions, and, although rises occurred in most dogs, changes were inconsistent in the normal regions. In the marginal regions, tissue lactate rose by 5.6 mumoles/g (97 per cent) and adenosine triphosphate fell by 2.4 mumoles/g (46 per cent) after isoproterenol. No consistent changes occurred in the normal or infarcted regions of the dogs given isoproternol or in any regions of control dogs given saline. It is concluded that beta-adrenergic stimulation with isoproterenol increases tissue ischemia in experimental acute myocardial infarction.

Protective effect of methylprednisolone on ischaemic myocardium assessed by ventricular function

Intracardiac surgical procedures are best carried out when the heart is still and bloodless. This condition, however, produces myocardial cellular damage with loss of contractility and compliance unless some protection can be provided. Myocardial contractility and compliance is best studied by isovolumic ventricular function tests, which were used to evaluate the protective effect of methylprednisolone on the isolated cross-perfused canine heart made ischaemic for 2 hours. Control experiments included 2 hours of ischaemia without methylprednisolone, and 2 hours of continuous normothermic cross-perfusion. The methylprednisolone-treated hearts had probably significantly better ventricular function after 2 hours of ischaemia than did hearts without the methylprednisolone, while the cross-perfused hearts were best overall. This work suggests that methylprednisolone may have a protective effect on the ischaemic myocardium of the intact canine heart.

Beneficial metabolic effects of methylprednisolone sodium succinate in acute myocardial ischemia

The metabolic and hemodynamic effects of methylprednisolone sodium succinate (40 mg/kg body weight) after acute myocardial ischemia were determined in 24 heparinized mongrel dogs. Myocardial ischemia was produced by ligation of the left anterior descending coronary artery. Catheters in the coronary sinus and the vein draining the left anterior descending coronary arterial area were used to collect blood samples from nonischemic and ischemic myocardium. Lactate, pyruvate, glucose, free fatty acids and oxygen were measured in arterial and venous blood from ischemic and nonischemic areas before and 3, 30 and 60 minutes after myocardial ischemia in animals with (Group II) and without (Group I) steroid treatment. In both Groups I and II glucose, lactate, free fatty acids, oxygen and coronary blood flow in nonischemic areas were not significantly changed, whereas glucose uptake in ischemic areas was significantly increased with myocardial ischemia and remained elevated. In Group I lactate uptake in ischemic areas became negative after coronary arterial ligation and remained so; in Group II, it increased after 30 (70%) and 60 (111%) minutes. Free fatty acid uptake in ischemic areas was reduced after myocardial ischemia in Group I, but in Group II it increased after 30 (224%) and 60 minutes (173%), and there was a concomitant increase in oxygen uptake. Pyruvate uptake in nonischemic areas decreased after 60 minutes in Group I, whereas it was reduced after 30 (68%) and 60 minutes (513%) in Group II. The changes were similar in ischemic myocardium. There were no significant changes in hemodynamic indexes. Coronary blood flow in ischemic areas decreased in Group I after myocardial ischemia and further after 30 and 60 minutes, but in Group II it increased after 30 (82%) and 60 minutes (53%). The data indicate that administration of methylprednisolone results in improved collateral blood flow into the infarcted area and a significantly improved metabolic response of ischemic myocardium. The glucocorticoid may also have a direct benefical effect on carbohydrate metabolism and cause the increased pyruvate neccesary to maintain the generation of energy-producing substrates. The results also suggest that methylprednisolone increases cell survival time and results in greater salvage of ischemic myocardium.

Effects of catecholamines on rat myocardial metabolism. I. Influence of catecholamines on energy-rich nucleotides and phosphorylated fraction contents

1. The influence of catecholamines (adrenaline and noradrenaline) on energy metabolism of the rat myocardium has been studied by incubating slices of this tissue with these hormones and by following the levels of the different phosphorylated fractions and adenylic nucleotides. 2. Similar effects are obtained with both hormones, adrenaline being more effective. 3. Catecholamines decrease significantly the total amount of phosphate while Pi content increases during the first 10 minutes of incubation; labile and residual phosphate contents increase at the beginning of incubation and decrease to the initial values afterwards. 4. ATP and ADP levels decrease significantly with both hormones; however, the effect of noradrenalin on the ATP level needs a longer time of incubation. The ATP/ADP ratios decrease after 5 minutes incubation and the total adenylic nucleotide content is severely decreased (35 per cent with adrenalin, after 20 minutes incubation). 5. Similar results have been obtained with other tissues; these results can explain the decrease of aerobic metabolism we observed under the same conditions.

Myocardial ultrastructural changes during extracorporeal circulation wtih anoxic cardiac arrest and its prevention by coronary perfusion. Experimental study

This experimental work has been carried out with the aim of studying the ultrastructural myocardial changes caused by prolonged anoxic cardiac arrest during cardiopulmonary bypass, and their prevention by means of two different techniques of coronary perfusion--systemic-pressure continuous and low-pressure intermittent perfusion. After 30 minutes of cardiac anoxia, the ultrastructural changes of the myocardial cell were reverted to normal by coronary perfusion; when anoxic cardiac arrest was prolonged up to 60 minutes there was severe myocardial damage, with marked mitochondrial changes and dehiscence of intercalated discs, which persisted in spite of restoring coronary flow. These morphological data were in accordance with the fact that no dog which underwent anoxic cardiac arrest for 60 minutes recovered. Both intermittent and continuous coronary perfusion were effective in preventing anoxic damage; cardiac muscle cells were better preserved by low-pressure intermittent perfusion than by systemic-pressure continuous perfusion, which caused intracellular and intramitochondrial oedema.

Contractions induced by a calcium-triggered release of calcium from the sarcoplasmic reticulum of single skinned cardiac cells

1. Fragments of single cardiac cells were obtained by homogenization of ventricular tissue from adult rats. Remaining pieces of sacrolemma were removed by micro-dissection. Tension was recorded from the ends of the skinned (sarcolemma-free) cells with a photodiode force transducer. 2. In the presence of a strong buffering of the free [Ca2+] with 4-0 mM total EGTA, a tonic tension was obtained that increased according to t sigmoid curve when the free ([Ca2+] was increased from 10(-6-75)M to 10(-5-0)M. This curve was not modified by the destruction of the sarcoplasmic reticulum (SR) by the detergent Brij 58. Therefore, the tonic tension corresponded to the direct effect of the free [Ca2+] present in the buffer on the myofilaments. 3. In the presence of a slight buffering of the free [Ca2+] with 0-050 mM total EGTA, cyclic contractions were observed that were attributed to cyclic releases and re-sequestrations of Ca2+ by the SR. The absence of effect of azide and ruthenium red on the cyclic contractions obtained at a free [Ca2+] lower than 10(-6-50)M demonstrated that the mitochondria played no role in the triggering of these contractions. 4. Cyclic contractions were induced by a slight variation of free [Ca2+] in the buffer from 10(-7-65)M to 10(-7-40)M. Their amplitude at 10(-7-40)M free Ca2+ was equal to the tonic tension developed by a free [Ca2+] 20 times higher applied to the myofilaments when the SR was destroyed by detergent or functionally inhibited by high total [EGTA]. It was concluded that these cyclic contractions corresponded to a Ca2+-triggered release of Ca2+ from the SR. 5. The cyclic contractions were induced by the filling of the SR with Ca2+ to a critical level at which it released a fraction of the Ca2+ it contained. Each contraction was followed by a re-sequestration of Ca2+, the kinetics of which conditioned the duration of the cycles. 6. The amplitude of the cyclic contractions increased when the free [Ca2+] that triggered them was increased. This gradation was deemed incompatible with a simple regenerative process, which should produce an all-or-nothing response. Additional process, such as a modulation of the Ca2+ release by free [Mg2+] and [ADP] may help to explain the gradation of the contractions. 7. It was concluded that a Ca2+-triggered release of Ca2+ from the SR of rat ventricular cells may amplify the Ca2+ flux crossing the sarcolemma during the plateau of the action potential, thereby permitting the activation of the myofilaments.

Protection of the myocardium with high-energy solutions

An approach to intraoperative protection of the myocardium is described that attempts to increase glucose utilization by infusion of high-energy solutions during aortic cross-clamping. Infusion of hypertonic glucose or glucose plus insulin prior to aortic cross-clamping has enhanced contractility and increased high-energy phosphate moieties in animals with induced ischemia. Recent pilot experiments in our laboratory suggest that infusions of creatine may result in increased production of creatine phosphate, which in turn induces phosphorylation of adenosine diphosphate to adenosine triphosphate, possibly enhancing myocardial contractility. The intraoperative clinical benefits of these infusions remain to be proved, however.

Myocardiac changes in experimental renal failure--a light and electron microscopic study

The experiments we described in this paper demonstrated that the myocardiac lesions without hypertension could be produced by renal failure in rabbits; after this experimental renal failure, increase in blood urea nitrogen and various functional and morphological changes suggestive of heart lesions appeared. The main structural changes in the heart were cellular edema with dilatation of the sarcotubular system, destructive changes of the mitochondria and contractile elements, and coagulative degeneration. These myocardiac lesions are induced by renal failure, and are probably caused by electrolyte imbalance, metabolic disorder, and/or hemodynamic abnormality rather than by hypertensive or toxic factors.

The effect of hypoglycemia on myocardial ischemic injury during acute experimental coronary artery occlusion

To determine the effect of hypoglycemia on myocardial ischemic injury following coronary artery occlusion epicardial electrograms were recorded 15 minutes after two 20-minute coronary artery occlusions in seven anesthetized dogs. The first occlusion was a control (blood glucose 85 plus or minus 5(sd) mg per cent). Before the second occlusion hypoglycemia was induced (blood glucose 40 plus or minus 5 mg per cent) by the intravenous administration of insulin (2 units/kg). The average ST-segment elevation in leads during control was 3.5 plus or minus 1.0 mV which rose to 6.1 plus or minus 1.4 mV during the second occlusion (P smaller than 0.05). The number of sites showing ST-segment elevation exceeding 2 mV increased from 7.6 plus or minus 1.6 during control to 10.6 plus or minus 1.4 (P smaller than 0.05) during the occlusion with hypoglycemia. In other dogs, a coronary artery was occluded for 24 hours. Epicardial ST-segment elevations were compared to creatine phosphokinase (CPK) activity and histological appearance from the same sites. CPK activity in sites with normal ST segments (0-2 mV) was 33.1 plus or minus 6.0 IU/mg protein. Six additional dogs received insulin following the 15 minute epicardial map and blood sugar was maintained at a level of 46 plus or minus 6 mg per cent for the 24 hours. These dogs showed more myocardial necrosis than predicted by the ST-segment elevation prior to insulin administration. Forty-six percent of sites, which in control dogs would have been expected to have normal CPK and histological appearance, showed depressed CPK activity and histological evidence of early myocardial necrosis. Thus, hypoglycemia increases myocardial damage, as reflected by enzymatic and histological analyses.