CELLULAR TRANSPORT OF LONG CHAIN FATTY ACIDS

The fate of arachidonic acid and linoleic acid in isolated working rat hearts containing normal or elevated levels of coenzyme A

If myocardial levels of coenzyme A (CoA) are elevated, an increase in the rate of esterification of palmitate into myocardial triacylglycerols will occur. In this study, we determined the fate of linoleic acid and arachidonic acid in isolated working rat hearts containing normal or elevated levels of CoA. In hearts containing normal levels of CoA, oxidative rates (measured as 14CO2 production) of [14C]arachidonic acid were significantly lower than those of [14C]palmitic acid, whereas a significantly greater incorporation of [14C]arachidonic acid into myocardial neutral lipids (comprised predominantly of triacylglycerols) was seen when compared to hearts perfused with [14C]palmitic acid. In a second series of hearts, myocardial CoA levels were elevated by perfusing hearts with no carbon substrate, 15 microM pantothenate, 0.5 mM cysteine and 1 mM dithiothreitol, resulting in an increase in myocardial CoA levels from 553 +/- 2 to 918 +/- 63 nmol/g dry wt. Subsequent perfusion of hearts containing elevated CoA levels with 1.2 mM [3H]arachidonic acid or [14C]linoleic acid resulted in a significant increase in incorporation of both these fatty acids into myocardial neutral lipids compared to control hearts. Incorporation of these fatty acids into phospholipids was significantly lower than their incorporation into neutral lipids and was not affected by myocardial CoA levels. Linoleic acid oxidation was unaffected by increases in myocardial levels of CoA. If linoleic acid oxidation was inhibited by adding 5 mM pyruvate to the perfusate, no effect on the incorporation of [14C]linoleic acid into neutral lipids was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonesterified fatty acid accumulation and release during heart muscle-cell (myocyte) injury: modulation by extracellular "acceptor"

Long-chain nonesterified fatty acid (NEFA) accumulation in the heart muscle cell (myocyte) and NEFA release to the extracellular milieu are considered contributors to the pathogenesis of myocardial injury in a number of cardiovascular disease states. Reported here is a study of the factors which influence and control the interactions among NEFA formation, intracellular NEFA accumulation, and NEFA release to the extracellular compartment by the irreversibly injured myocyte. Under conditions of metabolic inhibition, neonatal rat myocytes in primary monolayer culture became virtually depleted of ATP within 8 h. The metabolically inhibited myocytes evidenced membrane phospholipid degradation and a resultant net accumulation of NEFA produced thereby in the extracellular medium. However, under conditions of nutrient deprivation, the injured myocytes retained the NEFA produced from phospholipid catabolism intracellularly and did not release it to the culture medium, although the extent of myocyte ATP depletion was the same as it had been from metabolic inhibition. Serum could elicit, in a concentration-dependent fashion, the quantitative release of NEFA from metabolically inhibited myocytes to the culture medium but did not influence the net production of NEFA by the injured cells. Similarly, NEFA release from nutrient-deprived myocytes incubated in serum-free, substrate-free medium or in physiological buffer could be induced by supplementing the medium or buffer with bovine serum albumin (BSA), and the extent of NEFA release, but not NEFA formation, was dependent upon the extracellular BSA concentration. No manipulations to media other than changing their serum content or supplementing them with BSA were found to influence the disposition of NEFA produced during phospholipid catabolism in the irreversibly injured, ATP-depleted myocyte. Therefore, although progressive metabolic compromise in the myocyte was correlated with increasing, net NEFA formation, the distribution of the NEFA between the intracellular and the extracellular compartments was not determined by the magnitude of ATP loss or by the nature or duration of at least two injury stimuli, metabolic inhibition and nutrient deprivation. Rather, the net release of NEFA from the ATP-depleted myocyte to the culture medium and the consequent reduction of intracellular myocyte NEFA overload were critically and causally dependent upon the presence and concentration of extracellular NEFA "acceptor". The influence of acceptor on the mobilization of NEFA from the injured myocyte has implications regarding the use of NEFA release as an index of myocyte pathology and could serve to modify the progression and extent of myocardial injury in vivo.

Methods for the metabolic quantification of regional myocardial ischemia

An adequate balance between oxygen supply and demand is a basic requirement for normal cardiac function. When oxygen supply does not meet the demand, progressive cellular damage occurs leading to cardiac dysfunction and, ultimately, tissue death. While traditionally "ischemia" has been defined as decreased oxygen supply secondary to a decrease in blood flow, and "hypoxia" as decreased oxygen supply secondary to a decrease in oxygen tension, this review defines ischemia in its broader sense, namely as a pathophysiologic state in which there is a lack of oxygen relative to the demand for it. In a large number of experimental studies involving the heart, there is need to promptly recognize the ischemic state, to monitor its course in vivo, and to quantify it. Because of cardiac autoregulatory mechanisms, research methods which attempt to quantify supply (e.g., measurement of myocardial blood flow) and/or demand (e.g., measurement of myocardial oxygen consumption) do not necessarily reflect the status of the balance between supply and demand. An imbalance between myocardial supply and demand is more likely to be reflected by metabolic fluxes and by the accumulation of products specific to the ischemic state. Thus, the purpose of this review is to summarize the various methods available to the cardiac surgical investigator today for the metabolic quantification of myocardial ischemia. Due to the complexity of the heart and its inherent regional differences, myocardial ischemic changes are frequently regional in nature. Thus, this review will address metabolic methods for the regional quantification of myocardial ischemia.

Uptake and transport of lipid substrates in the heart

Between 1955-1960 it was realized that the fatty acids circulating in the blood, after transport into the cardiac cells, then beta-oxidation in the myocyte mitochondria, were the major source of energy behind the impressive hydraulic performance of the heart. Only albumin-bound fatty acids and, to a lesser extent, cholesterol ester and triglyceride fatty acids have access to the cardiac cells. Circulating phospholipid fatty acids are excluded. From experiments with isolated perfused hearts it was concluded that fatty acid uptake by the myocardium was essentially an energy independent process. An important question still pending in the literature concerns the mechanisms of fatty acid transport through the capillary endothelium, through the cardiac cell plasma membrane and then through the intracellular compartments. The most plausible model now emerging considers that specific fatty acid-binding proteins, sequentially disposed along this cascade of barriers, might facilitate and drive the flux of fatty acids entering the cardiac cells.

Metabolism of lipids in experimental hypertrophic hearts of rabbits

Cardiac hypertrophy was induced in rabbits by subcutaneous injection of thyroxine or isoprenaline or by surgically constricting the abdominal aorta. Alterations in lipid metabolism were observed in these hypertrophic hearts. Thyroxine or isoprenaline treatment increased the fatty acids in the serum and stimulated a marked increase in total lipids, triglycerides, and fatty acids in the hypertrophied myocardium. Coarctation of the aorta, in contrast, induced a significant increase in these lipids without significantly affecting serum free fatty acids. Histochemical and morphological studies confirmed an increase in neutral lipids. It is suggested that the observed increase in fatty acids in the heart following thyroxine or isoprenaline treatment is related to the increase in serum free fatty acids, which is followed by an increase in the removal of serum fatty acids by the heart. However, the amount of serum fatty acids that is removed exceeds the amount that is oxidized, which leads to an increase in lipid stores. The increase in lipid stores in the heart following coarctation of the aorta probably corresponds to the decrease in myocardial concentrations of carnitine. Serum lipid levels following coarctation were not significantly different from those of controls.

Blood flow and transport of free fatty acids in striated muscle under chronic ischemia

Three weeks of ischemia of the right hind limb in rat produced by ligature and severing of the right common iliac artery, resulted in changes in FFA transport and blood flow in the regenerating tibialis muscle. FFA transport was measured using i.v. administered palmitate-1-14C bound to albumin. Uptake of palmitate-1-14C or 86Rb 90 s after i.v. administration was decreased in ischemic tibialis muscle, whereas no apparent effect of ischemia on the contralateral muscle was seen. Plasma FFA outflow rate measured according Beker and Rostami was not affected by the presence of an ischemic extremity in comparison with control animals. The plasma FFA inflow rate into the ischemic muscle was markedly decreased to values of 6.5 mmumoles FFA/min/g whereas in control animals values were 18.8 mmumoles FFA/min/g wet tissue. Decreased FFA inflow rate to the ischemic tibialis muscle was due to the decreased radioactivity of palmitate-1-14C in the muscle under the conditions of a constant plasma FFA pool. Differences in the uptake of palmitate-1-14C in tibialis muscle were dependent upon the muscle blood flow, which was markedly decreased in the ischemic muscle on indirect measurement with 86Rb or with the 133Xe clearance.

External detection and visualization of myocardial ischemia with 11C-substrates in vitro and in vivo

To characterize externally detectable changes in myocardial metabolism of free fatty acids (FFA) and glucose associated with ischemia, isovolumically beating rabbit hearts were perfused under conditions of selected flows with cyclotron-produced, short-lived (t1/2 - 20.4 minutes), 11C-labeled isotopes of glucose and FFA. Tension-time index decreased 83% and lactate production increased from 0.5 +/- 1.9 (SE) to 5.3 +/- 2.1 mumol/min per g of dry weight reflecting myocardial ischemia after flow was reduced from 20 to 5 ml/min. After 30 minutes of low flow the myocardial accumulation of 11C-octanoate, expressed as the extraction fraction, declined from 56 +/- 15% to 30 +/- 3%, reflecting metabolic suppression of FFA extraction during low flow. Effects attributable exclusively to prolonged residence time were excluded. Similar results were obtained with 11C-palmitate. The myocardial avidity for 11C-palmitate was demonstrable by rectilinear whole body scanning in dogs given 5 mCi of the agent intravenously. Diminished 11C-palmitate uptake in zones of myocardium rendered ischemic for 20 minutes prior to reflow in intact dogs was delineated by electrocardiographically gated positron-emission transaxial computer reconstruction tomography. Thus, diminished 11C-FFA extraction, externally detectable, accompanies decreased perfusion in isolated perfused hearts, and decreased 11C-FFA uptake reflecting myocardial ischemia in vivo can be evaluated noninvasively by positron-emission transaxial tomography.

Effects of glucose-insulin-potassium on myocardial substrate availability and utilization in stable coronary artery disease. Studies on myocardial carbohydrate, lipid and oxygen arterial-coronary sinus differences in patients with coronary artery disease

To assess the metabolic effects of myocardial substrate alteration in patients with coronary artery disease, glucose-insulin-potassium solution was administered intravenously for 30 minutes to 14 men with stable angiographically documented coronary artery disease. The glucose-insulin-potassium solution (300 g of glucose, 50 units of regular insulin and 80 mEq of potassium chloride per liter of water) was infused at a constant rate in each patient, but individual infusion rates ranged from 0.013 to 0.032 ml/kg per min (4 to 10 mg glucose/kg per min) in the 14 patients. Simultaneous arterial and coronary sinus samples were obtained at 15 minute intervals during a stable 30 minute control period and again at 15 minute intervals during the infusion; samples were assayed for glucose, lactate, free fatty acid and oxygen content. In all 14 patients, during the glucose-insulin-potassium infusion, arterial glucose and lactate increased and arterial free fatty acid levels fell; the magnitude of the changes in arterial lactate and free fatty acids was related to the rate of infusion. Arterial-coronary sinus differences (A-Cs) for glucose, lactate and free fatty acid levels correlated with the arterial concentrations of these substrates (r = 0.66, 0.87 and 0.79, respectively). Regression analyses demonstrated myocardial thresholds for the uptake of these substrates as follows: glucose 79 mg/100 ml; lactate 300 mu mole/liter; and free fatty acids 100 to 200 mu Eq/liter. Finally and most importantly, the reduction in A-Cs oxygen values after glucose-insulin-potassium infusion correlated with the reduction in A-Cs free fatty acid levels (r = 0.64, P less than 0.0001). This observation suggests that, in patients with coronary artery disease, glucose-insulin-potassium infusion may significantly diminish myocardial oxygen requirements by reduction of myocardial free fatty acid utilization and simultaneous enhancement of myocardial carbohydrate utilization. Myocardial substrate availability may be an important determinant of myocardial oxygen demand in patients with coronary artery disease. Infusion of glucose-insulin-potassium solution has the potential to alter myocardial substrate availability, thus improving the balance between myocardial oxygen demand and supply.

Metabolism of free fatty acids, glucose and catecholamines in acute myocardial infarction. Relation to myocardial ischemia and infarct size

The myocardial metabolism of free fatty acids, glucose and catecholamines is reviewed in relation to current trends in the therapy of experimental myocardial infarction. Major modifications in the metabolism of free fatty acids, glucose and catecholamines have already been found after acute myocardial infarction in man, and animal experimental data suggest that such metabolic changes might play a role in the modification of infarct size and sometimes in the development of arrhythmias. However, animal experiments often represent extreme situations and the therapeutic use in man of agents to modify the metabolism of free fatty acids, glucose or catecholamines after myocardial infarction needs intensive investigation before general application. The sum total of the evidence from animal experiments suggests that increased circulating concentrations of free fatty acids and catecholamines, if sufficiently high, may be harmful rather than helpful to the outcome of acute myocardial infarction, and that increased provision of glucose (as glucose, insulin and potassium) may be beneficial. Reservations to these conclusions are that the concentrations used appear to be important factors in catecholamine and free fatty acid effects, and that the mechanism of action of glucose-insulin-potassium is more complex than originally thought.

Changes in enzyme activities in tissues of rats exposed to hypoxia

Rats were exposed to various degrees of hypoxia and enzyme activities in their tissues were determined. In general, oxidative metabolism was not increased in response to hypoxia, nor was anaerobic metabolism. Physiological and anatomical changes were concluded to be more important than changes in cellular enzyme activities in the overall adaptation to acute hypoxia.

Comparison of the effects of anoxia and whole heart ischemia on carbohydrate utilization in isolated working rat hearts

The rates of utilization of glycogen and exogenous glucose by hearts perfused at low coronary flows and high perfusate oxygen tension (ischemia) and by hearts perfused at high coronary flows and low perfusate oxygen tension (anoxia) were studied in the isolated, working rat heart. Ischemic tissue had a glycolytic rate that was 25% of the anoxic rate and 50% of the control rate. Inhibition of carbohydrate utilization during ischemia was due to a lower flux through the glycolytic pathway and not to a lower rate of glucose transport or substrate availability. Insulin and elevated perfusate glucose increased glucose transport and caused accumulation of free intracellular glucose, but ischemia still inhibited glucose utilization. Insulin failed to maintain tissue levels of high-energy phosphates or to prevent mechanical failure in ischemic hearts. In the absence of insulin, tissue lactate increased tenfold during 20 minutes of ischemia, although it increased only threefold in anoxic tissue. The increased tissue lactate in ischemic hearts corresponded to the inhibition of glycolysis and to the failure of mechanical performance. Insulin caused a further increase in tissue lactate during ischemia. These findings illustrate several important differences between anoxia and ischemia and suggest that insulin may be more harmful than it is beneficial in severely ischemic tissue.

Effect of chronic myocardial ischemia on the activity of carnitine palmitylcoenzyme A transferase of isolated canine heart mitochondria

The effect of chronic ischemia on the activity of carnitine palmitylcoenzyme A transferase (PCoAT) was examined. Control mitochondria were isolated from the left ventricle of normal hearts and from the uninvolved areas of ischemic hearts. Mitochondria from ischemic tissue were isolated from the left ventricle of chronically ischemic hearts. Formation of the enzyme product, palmityl- 14 C-carnitine, was markedly depressed in control mitochondria isolated from ischemic hearts; however, the pH optimums of the enzymes from control and ischemic mitochondria were in the same range. After 1 day of chronic ischemia, the kinetics of the enzyme from ischemic mitochondria (compared with that of the enzyme from control mitochondria) showed marked changes in Vmax. Periods of ischemia longer than 1 day significantly decreased the K m of carnitine PCoAT for carnitine from 2.59 ± 0.476 mM to 0.713 ± 0.148 mM, and Vmax was still depressed (15 to 3 nmoles/min mg -1 ). The K m for palmitylCoA was not significantly affected. Arrhenius plots for the enzyme from ischemic (32 hours) mitochondria revealed a single phasic response to temperature change. Two nonintersecting slopes were characteristic of the normal 1-day ischemic and the normal sonicated activity. As a result of ischemia, changes in the lipid components in the membrane containing carnitine PCoAT were postulated: alterations in the hydrophobic environment of the enzyme produce interference in the binding of palmitylCoA to the second substrate site and may result in a decrease in the K m of the enzyme for carnitine. The single phasic response of the enzyme to temperature change may be additional evidence for membrane lipid alterations. A defect in the transport of long-chain fatty acids may be an early change associated with myocardial ischemia.

Metabolic response during impending myocardial infarction. I. Relevance of studies of glucose and fatty acid metabolism in animals

Both glucose and free fatty acids (FFA) are major fuels for the normally oxygenated heart with the dominant contribution being derived from glucose in the resting, fed state and FFA in the resting, fasted state. In anoxia, all energy must be produced anaerobically from glycogen or glucose. Anoxia by itself accelerates glycolysis, but further increases may follow an increased circulating glucose concentration and the addition of insulin. Even maximal rates of anaerobic glycolysis, achieved at high coronary flow rates, can only sustain the energy needs of the K + -arrested heart but not of the working heart. FFA cannot be utilized for energy in anoxia and may accumulate intracellularly as triglyceride or FFA. From these and other animal data have grown the concepts that glucose is "good" for the survival of the ischemic heart, and that FFA is "bad." However, glucose-fatty acid interaction has not been well studied in the infarcting, ischemic myocardium. While a "toxic" effect of FFA has been shown in many experimental models, there are other reports of increased FFA concentrations having no harmful effect or even a beneficial effect on the infarcting myocardium. The possible benefits of administration of glucose (with insulin) to patients with acute myocardial infarction could only be assessed by a controlled therapeutic trial.

Metabolic effects of ethanol on the rabbit heart

Ethanol in saline solution (15%, v/v) was infused into anesthetized rabbits at a rate of 0.494 ml/min for the first 12 minutes and then at 0.247 ml/min for 108 minutes. Three hours after the infusion, heart triglyceride and lipoprotein lipase were assayed. Oxidation and esterification of fatty acids (palmitate -14 C as an indicator) were assessed by using either tissue homogenates or perfused hearts taken from the rabbits. Oxidation-reduction states of the perfused hearts were examined by measuring the tissue levels of dehydrogenase-linked substrates. The infusion of ethanol resulted in 180% increase in heart triglyceride content, but the infusion of norepinephrine (3 µg/kg/min) did not change the content. No change in plasma free fatty acids and triglyceride or heart lipoprotein lipase activity was detected. Addition of ethanol had little effect on the distribution of palmitate -14 C in the lipids of tissue slices and homogenates. On the other hand, prior infusion of ethanol resulted in depression of 14 CO 2 production (70 and 50%) and enhanced fatty acid esterification into triglyceride (270 and 170%) both in homogenates and perfused hearts. Mitochondrial and cytoplasmic redox states were shifted to more oxidized states by ethanol infusion. It is postulated from these results that an accumulation of triglyceride in the rabbit heart in response to ethanol administration is a result of decreased fatty acid oxidation rather than of increased triglyceride uptake or increased fatty acid synthesis.

Free fatty acid metabolism of the human heart at rest

Myocardial substrate metabolism was studied in 13 subjects at the time of diagnostic cardiac catheterization by means of palmitic acid-(14)C infusion with arterial and coronary sinus sampling. Two subjects were considered free of cardiac pathology and all, with one exception, demonstrated lactate extraction across the portion of heart under study. Data for this single lactate-producing subject were treated separately.The fractional extraction of (14)C-labeled free fatty acids (FFA) (44.4+/-9.5%) was nearly twice that of unlabeled FFA (23.2+/-7.8%) and raised the possibility of release of FFA into the coronary sinus. FFA uptake, based on either the arterial minus coronary sinus concentration difference or the FFA-(14)C fractional extraction, was directly proportional to the arterial FFA concentration. Gas-liquid chromatography failed to demonstrate selective handling of any individual FFA by the heart. Fractional oxidation of FFA was 53.5+/-12.7%, accounting for 53.2+/-14.4% of the heart's oxygen consumption while nonlipid substrates accounted for an additional 30.0+/-17.3%. Determinations of both labeled and unlabeled triglycerides suggested utilization of this substrate by the fasting human heart. Direct measurement of FFA fractional oxidation as well as FFA uptake, exclusive of possible simultaneous FFA release, would appear necessary in studies concerned with human myocardial FFA metabolism.

Metabolism of triglyceride fatty acid by the perfused rat heart

1. Chyle lipids, labelled with (14)C, are taken up and oxidized by the isolated perfused rat heart. 2. In recirculatory perfusions, when chyle lipids are the sole exogenous energy source, about 24% of the total oxygen uptake is accounted for by their oxidation. This proportion is not changed by starvation of the rats for 48hr. and falls when an external work load is imposed on the left ventricle. 3. With albumin in the perfusion medium, the rate of (14)CO(2) output is reduced by half and there is a rise in the proportion of (14)C-labelled free fatty acids in the medium. 4. Clearing-factor lipase appears in the perfusion medium when chyle lipids are perfused through the heart. In the absence of albumin, the activity of the medium enzyme is low and only a small proportion of the (14)CO(2) output can be accounted for by the oxidation of free fatty acids released by it. In the presence of albumin, the enzyme is more active in the medium. 5. When a substantial proportion of the total clearing-factor lipase is removed from the heart by a prior perfusion with heparin, (14)C-labelled chyle lipid perfused subsequently is oxidized at only half the normal rate.

Use of Radioiodinated Fatty Acid for Photoscans of the Heart

Radioiodinated fatty acid bound to albumin (RIFA) was given by intravenous injection to dogs and moribund human subjects in order to label heart muscle.

Photoscans of the thorax of dogs given RIFA revealed concentration of radioactivity in the ventricular region of the heart due to incorporation of the labeled fatty acid into myocardial triglyceride and phospholipid.

Photoscans of the thorax following intravenous injection of radioiodinated serum albumin or NaI 131 showed a different density pattern in the region of the heart due to radioactivity in blood within the cardiac chambers.

Areas of myocardial infarction were detected in photoscans of the thorax in 11 of 14 dogs in which one coronary artery was ligated. At necropsy the concentration of radioactivity was much higher in healthy myocardium than in infarcted muscle. The same distribution of radioactivity was observed in hearts from three human subjects with myocardial infarction given RIFA shortly before death. It is proposed that areas of myocardial infarction might be demonstrated during life by photoscans of the thorax following intravenous injection of RIFA.