ber den Stoffwechsel des menschlichen Herzens: III. Der oxydative Stoffwechsel des menschlichen Herzens unter verschiedenen Arbeitsbedingungen

Lactate and Myocadiac Energy Metabolism

The myocardium is capable of utilizing different energy substrates, which is referred to as “metabolic flexibility.” This process assures ATP production from fatty acids, glucose, lactate, amino acids, and ketones, in the face of varying metabolic contexts. In the normal physiological state, the oxidation of fatty acids contributes to approximately 60% of energy required, and the oxidation of other substrates provides the rest. The accumulation of lactate in ischemic and hypoxic tissues has traditionally be considered as a by-product, and of little utility. However, recent evidence suggests that lactate may represent an important fuel for the myocardium during exercise or myocadiac stress. This new paradigm drives increasing interest in understanding its role in cardiac metabolism under both physiological and pathological conditions. In recent years, blood lactate has been regarded as a signal of stress in cardiac disease, linking to prognosis in patients with myocardial ischemia or heart failure. In this review, we discuss the importance of lactate as an energy source and its relevance to the progression and management of heart diseases.

Increased myocardial lactate oxidation in lambs with aortopulmonary shunts at rest and during exercise

Free fatty acids are the major fuels for the myocardium, but during a higher load carbohydrates are preferred. Previously, we demonstrated that myocardial net lactate uptake was higher in lambs with aortopulmonary shunts than in control lambs. To determine whether this was caused by an increased lactate uptake and oxidation or by a decreased lactate release, we studied myocardial lactate and glucose metabolism with 13C-labeled substrates in 36 lambs in a fasting, conscious state. The lambs were assigned to two groups: a resting group consisting of 8 shunt and 9 control lambs, and an exercise group (50% of peak O2 consumption) consisting of 9 shunt and 10 control lambs. Myocardial lactate oxidation was higher in shunt than in control lambs (mean +/- SE, rest: 10.33 +/- 2.61 vs. 0. 17 +/- 0.82, exercise: 38.05 +/- 8.87 vs. 16.89 +/- 4.78 micromol. min-1. 100 g-1; P < 0.05). There was no difference in myocardial lactate release between shunt and control lambs. Oxidation of exogenous glucose, which was approximately zero at rest, increased during exercise in shunt and control lambs. The contribution of glucose and lactate to myocardial oxidative metabolism increased during exercise compared with at rest in both shunt and control lambs. We conclude that myocardial lactate oxidation is higher in shunt than in control lambs, both at rest and during exercise, and that the contribution of carbohydrates in myocardial oxidative metabolism in shunt lambs is higher than in control lambs. Thus it appears that this higher contribution of carbohydrates occurs not only in the case of pressure-overloaded hearts but also in myocardial hypertrophy due to volume overloading.

Spatial heterogeneity of blood flow in the dog heart. I. Glucose uptake, free adenosine and oxidative/glycolytic enzyme activity

The spatial heterogeneity of myocardial perfusion and metabolism was studied in 11 anaesthetized dogs under resting conditions. In each heart local myocardial blood flow was assessed using the tracer microsphere technique in 256 samples (mean mass: 83.1 mg) taken from the left anterior ventricular wall. In the same samples, the following biochemical parameters were determined: accumulation of [3H]-deoxyglucose (a measure of glucose uptake), free cytosolic adenosine (S-adenosylhomocysteine accumulation technique, a measure of tissue oxygenation and a possible mediator of blood flow regulation), and the specific activities of oxidative (citrate synthase, cytochrome-c-oxidase) and glycolytic (hexokinase, phosphoglycerate kinase) enzymes. Capillary density and mitochondrial and myofibril volume densities were determined by morphometry. Myocardial perfusion in each sample (average 0.77 ml min-1 g-1) varied between 0.1 and 2.5 times the mean (coefficient of variation 0.30+/-0.02). [3H]-deoxyglucose was deposited locally in proportion to perfusion. Samples showing low flow (<0.2 ml min-1 g-1) did not exhibit increased levels of cytosolic adenosine. The specific activities of the oxidative and glycolytic enzymes, however, were uniformly distributed between low and high flow areas. Furthermore, capillary density and mitochondrial and myofibril densities were similar in high and low flow regions. The results show firstly that local glucose metabolism in the heart occurs in proportion to local blood flow, suggesting that high flow regions have a higher than average metabolic rate. Secondly, regions of low flow are not compromized by critical oxygenation and most likely have a lower than average oxygen demand and finally, the homogeneous distribution of oxidative and glycolytic enzymes, as well as the homogeneous myocardial ultrastructure, suggest that areas with high and low blood flow under resting conditions may increase their metabolic rate to similar levels when required.

Effects of isoproterenol infusion on the myocardial uptake of fatty acids and other substrates in lambs with an aortopulmonary shunt

Isoproterenol, used in the management of infants with left-to-right shunts and circulatory congestion, increases myocardial work load and oxygen consumption. In addition, it may selectively enhance myocardial fatty acid utilization. The less efficient oxidation of FFA could induce an oxygen wasting effect and thus further increase myocardial oxygen consumption. The combination of such an oxygen wasting effect and the chrono- and inotropic effects of isoproterenol could induce an imbalance between myocardial oxygen supply and demand in hearts of which resting oxygen consumption is already elevated. We studied myocardial substrate uptake (FFA, triglycerides, glucose, lactate, pyruvate, beta-OH-butyrate, and acetoacetate) in 10 7-wk-old lambs with an aortopulmonary left-to-right shunt (57 +/- 4% of left ventricular output, mean +/- SEM) and 9 control lambs during isoproterenol infusion (0.1 mumol.min-1.kg-1). Myocardial blood flow and oxygen consumption increased in both groups but less in shunt than in control lambs because of the smaller rise in heart rate in the shunt lambs. The arterial FFA concentration increased 3-fold in both groups and was not different between the two groups. The FFA arteriocoronary sinus difference, however, was not affected by the isoproterenol infusion. The myocardial FFA uptake thus followed the changes in myocardial blood flow and did not increase more in shunt than in control lambs. Isoproterenol infusion does, in spite of a 3-fold increase in arterial FFA concentration, not induce a shift toward a greater percentage uptake of fatty acids compared with other substrates in lambs with aortopulmonary left-to-right shunt, so that the possibility of an oxygen wasting effect can be ruled out as an unwanted side effect.

Iodine-123-labelled fatty acids for myocardial single-photon emission tomography: current status and future perspectives

Renewed interest in the clinical use of iodine-123-labelled fatty acids is currently primarily focused on the use of iodine-123-labelled 15-(p-iodophenyl)pentadecanoic acid (IPPA) and "modified" fatty acid analogues such as 15-(p-iodophenyl)-3-R,S-methylpentadecanoic acid (BMIPP) which show delayed myocardial clearance, thus permitting single-photon emission tomographic imaging. Interest in the use of BMIPP and similar agents results from the differences which have often been observed in various types of heart disease between regional myocardial uptake patterns of [123I]BMIPP and flow tracer distribution. Although the physiological basis is not completely understood, differences between regional fatty acid and flow tracer distribution may reflect alterations in important parameters of metabolism which can be useful for patient management or therapy planning. These tracers may also represent unique metabolic probes for correlation of energy substrate metabolism with regional myocardial viability. The two agents currently most widely used clinically are 123I-labelled IPPA and BMIPP. While [123I]IPPA is commercially available as a radiopharmaceutical in Europe (Cygne) and Canada (Nordion), multicenter trials are in progress in the United States as a prelude to approval for broad use. [123I]BMIPP was recently introduced as Cardiodine for commercial distribution in Japan (Nihon Medi-Physics, Inc.). [123I]BMIPP is also being used in clinical studies on an institutional approval basis at several institutions in Europe and the United States. In this review, the development of a variety of radioiodinated fatty acids is discussed. The results of clinical trials with [123I]IPPA and [123I]BMIPP are discussed in detail, as are the future prospects for fatty acid imaging.

Interrelationship between lactate and cardiac fatty acid metabolism

This overview is presented, in the main, to summarize the following aspects of lactate and cardiac fatty acid metabolism: 1. The utilization of exogenous carbohydrates and fatty acids by the heart. 2. The competition between lactate and fatty acids in cardiac energy metabolism. 3. The effect of lactate on endogenous triacylglycerol homeostasis. 4. Lactate-induced impairment of functional recovery of the post-ischemic heart. 5. The effect of lactate on lipid metabolism in the ischemic and post-ischemic heart. 6. The consequences of hyperlactaemia for cardiac imaging.

Myocardial turnover of plasma free fatty acids during angina pectoris induced by atrial pacing

Myocardial extraction of free fatty acids (FFA), together with glucose, lactate, pyruvate, glycerol and oxygen was determined by simultaneous sampling of blood from an artery (a) and the coronary sinus (cs) at rest and during chest pains induced by atrial pacing in seven fasting male patients with ischaemic heart disease. Results were compared to those, at rest and during pacing at heart rate 140 beats min-1, in ten healthy men of similar age. A continuous i.v. infusion of 14C oleate and 3H palmitate enabled the calculation of simultaneous myocardial uptake and release of FFA as well as of the fraction of extracted FFA which underwent direct oxidation. During chest pain lactate net extraction decreased to become, in some patients, negative. FFA extraction, as estimated from the fractional extraction of labelled fatty acid was likewise decreased, while the a-cs O2 difference was not significantly altered. The fractional oxidation of extracted FFA was increased, whereas the calculated fatty acid release from the heart was unaltered. The increase in fractional oxidation was quantitatively correlated with the decrease in lactate extraction suggesting that it was related to the degree or extent of ischaemia. It was also proportional to the decrease in FFA extraction. Thus, in patients with angina pectoris the ischaemic myocardium may be subjected to a limitation not only with regard to oxygen but also substrate flux into the myocardial cells.

The effect of Oxfenicine on cardiac carbohydrate metabolism in intact dogs

Measurement of cardiac glucose oxidation (as a percentage of CO2 production) was made using the technique of infusion of 14C-D-glucose, together with measurement of 14CO2 and total CO2 produced by the myocardium. The measurements were made in 16 dogs under chloralose anaesthesia, before and after an i.v. injection of S-4-hydroxyphenylglycine (16.7 mg X kg-1, Oxfenicine: Pfizer). In one group of dogs circulating free-fatty-acid (FFA) levels were raised by infusion of intralipid heparin; in the other, circulating lactate was increased by infusion of 5 MNa-lactate (pH 7.0). In the last group of dogs the action of the drug was studied in cardiac denervated dogs. In the dogs with normal circulating substrate levels, Oxfenicine increased the glucose oxidation from 17.3 to 39.9% of total substrate oxidized. This was also the case in those dogs with high circulating FFA (9.0 to 32.3%). However, in dogs with high circulating lactate (over 5.0 mmol X l-1) the oxidation of glucose was relatively unaffected (2.0 to 7.1%). In cardiac denervated dogs, with a known inhibition of glycolysis, Oxfenicine increased glucose oxidation from 4.8 to 23.5%. These results show that Oxfenicine is able to switch the heart from the oxidation of fat to glucose or lactate as fuel.

Substrate utilization in the myocardium

Substrate extraction is the disappearance of a substance from arterial blood into the myocardium, substrate utilization being the combustion of that substrate. The terms used for extraction and utilization are defined. There are many problems of accurately measuring glucose and free fatty acid oxidation. Lactate is the preferred substrate of the normal, blood-perfused heart. If the substrate oxidized affects the myocardial oxygen consumption, this could be of considerable importance under conditions of limited oxygen supply. At present there is conflicting evidence. It may be beneficial to switch the heart from one substrate to another, e.g., from fat to carbohydrate fuels. The conditions of ischaemia and hypoxaemia are not the same, and future investigations should be made under carefully controlled conditions.

Oxygen consumption and substrate uptake of the hypertrophied rat heart in situ

A new heart preparation was developed which permits in situ measurements of myocardial oxygen consumption and substrate uptake in small animals. Using the new method the mechanical activity, as well as oxygen consumption and substrate uptake of the heart, was measured in Goldblatt rats with left ventricular hypertrophy of about 40%. 1. In agreement with former investigations on the hypertrophied rat heart, this model also shows that both the performance of the whole ventricle, as well as the contractile force per unit of cross-sectional area, is increased in the state of stable hypertrophy. 2. The absolute values of oxygen consumption and substrate uptake are increased in the hypertrophied hearts. However, oxygen consumption and substrate uptake as related to muscle mass and to wall stress were largely identical in hypertrophied and control hearts. 3. Hypertrophied hearts and controls utilize substrates according to their respective arterial blood concentration. Under our experimental conditions approximately 50% of the total energy in both groups is obtained from glucose, 30% from lactate, and 20% from fat. The relatively high consumption of lactate could be explained by the glucose uptake and lactate release of the erythrocytes.

Studies on the regulation of myocardial blood flow in man. I.: Training effects on blood flow and metabolism of the healthy heart at rest and during standardized heavy exercise

In a comparative study 11 athletes and 11 untrained students were investigated at rest, of these 6 trained and 5 untrained individuals during exercise as well. Myocardial blood flow was measured by the argon method. Myocardial oxygen consumption, myocardial substrate uptake of glucose, lactate, and free fatty acids and cardiac output were determined by the direct Fick principle. Exercise was standardized according to 65% of an individual's maximal oxygen uptake (delta VO2 max). Coronary flow reserve was determined by dipyridamole injections. All measurements were made during hemodynamic and respiratory steady-state conditions with the subject in a supine position. At rest, myocardial blood flow and myocardial oxygen consumption were significantly lower in trained subjects compared to the untrained ones. These differences were more pronounced during heavy exercise. They cannot be explained completely by hemodynamic parameters. - During exercise, myocardial substrate uptake shifted to a predominant lactate uptake of almost 90% of total substrate uptake. Total substrate uptake as well as lactate uptake correlated significantly with myocardial oxygen. - Coronary flow reserve was lower in the trained group. It is concluded that the heart muscle of a trained individual requires less energy at a given work load than in the untrained state.