Fatty acids suppress recovery of heart function after hypothermic perfusion

Myocardial fatty acid metabolism in health and disease

There is a constant high demand for energy to sustain the continuous contractile activity of the heart, which is met primarily by the beta-oxidation of long-chain fatty acids. The control of fatty acid beta-oxidation is complex and is aimed at ensuring that the supply and oxidation of the fatty acids is sufficient to meet the energy demands of the heart. The metabolism of fatty acids via beta-oxidation is not regulated in isolation; rather, it occurs in response to alterations in contractile work, the presence of competing substrates (i.e., glucose, lactate, ketones, amino acids), changes in hormonal milieu, and limitations in oxygen supply. Alterations in fatty acid metabolism can contribute to cardiac pathology. For instance, the excessive uptake and beta-oxidation of fatty acids in obesity and diabetes can compromise cardiac function. Furthermore, alterations in fatty acid beta-oxidation both during and after ischemia and in the failing heart can also contribute to cardiac pathology. This paper reviews the regulation of myocardial fatty acid beta-oxidation and how alterations in fatty acid beta-oxidation can contribute to heart disease. The implications of inhibiting fatty acid beta-oxidation as a potential novel therapeutic approach for the treatment of various forms of heart disease are also discussed.

Perioperative blood glucose control during adult coronary artery bypass surgery

Coronary artery bypass graft (CABG) procedures are among the most frequently performed surgical procedures in the United States. People with cardiovascular disease who also have diabetes have a greater risk of poor outcomes after CABG procedures than patients who do not have diabetes. This literature review examines current information regarding perioperative blood glucose (BG) control. It emphasizes BG control in adults during the hypothermic period of cardiopulmonary bypass. Hyperglycemia, not the diagnosis of diabetes, significantly increases the risk of adverse clinical outcomes, longer hospitalizations, and increased health care costs.

Effects of hypothermia on myocardial substrate selection

BACKGROUND

Hypothermia lowers the metabolic rate and increases ischemic tolerance but the effects of temperature on myocardial substrate selection are not well defined.

METHODS

Isolated rat hearts were perfused with physiologic concentrations of 13C labeled lactate, pyruvate, acetoacetate, mixed long-chain fatty acids, and glucose. Hearts were cooled over 5 to 10 minutes to one of four target temperatures (37 degrees, 32 degrees, 27 degrees, or 17 degrees C), then perfused for an additional 30 minutes, freeze-clamped, and extracted. 13C NMR spectra were obtained and substrate oxidation patterns were determined by isotopomer analysis.

RESULTS

Although hearts in all groups were supplied with identical substrates, the percentage of acetyl-CoA oxidized within the citric acid cycle that arose from fatty acids decreased significantly from 53.8% +/- 0.8% in the 37 degrees C group to 33.1% +/- 3.3% in the 17 degrees C group. Lactate or pyruvate utilization increased from 3.3% +/- 0.5% to 25.7% +/- 3.6%, respectively (p < 0.05 by one-way ANOVA).

CONCLUSIONS

These data suggest that moderate hypothermia suppresses fatty acid oxidation and deep hypothermia significantly increases utilization of lactate and pyruvate. These effects may result from relative inhibition of catabolism of complex molecules such as fatty acids, or stimulation of pyruvate dehydrogenase. These effects on substrate metabolism may play a role in myocardial protection afforded by hypothermia.

Malonyl CoA control of fatty acid oxidation in the ischemic heart

Abnormally high rates of fatty acid metabolism is an important contributor to the severity of ischemic heart disease. During and following myocardial ischemia a number of alterations in fatty acid oxidation occur that result in an excessive amount of fatty acids being used as a fuel source by the heart. This contributes to a decrease in cardiac efficiency both during and following the ischemic episode. Central to the regulation of fatty acid oxidation in the heart is malonyl CoA, which is a potent endogenous inhibitor of mitochondrial fatty acid uptake. The levels of malonyl CoA are regulated both by its synthesis by acetyl CoA carboxylase (ACC) and its degradation by malonyl CoA decarboxylase (MCD). ACC is in turn controlled by AMP-activated protein kinase (AMPK), which acts as a fuel gauge in the heart. The control of these enzymes are altered during ischemia, such that malonyl CoA levels in the heart decrease, resulting in an increased relative contribution of fatty acids to oxidative metabolism. Activation of AMPK during and following ischemia appears to be centrally involved in this decrease in malonyl CoA. Clinical evidence is now accumulating that show that inhibition of fatty acid oxidation is an effective approach to treating ischemic heart disease. As a result, modulation of fatty acid oxidation by targeting the enzymes controlling malonyl CoA may be a novel approach to treating angina pectoris and acute myocardial infarction. This paper will discuss some of the molecular changes that occur in fatty acid oxidation in the ischemic heart and will include a discussion of the important role of malonyl CoA in this process.

Efficient limitation of intracellular edema and sodium accumulation by cardioplegia is dissociated from recovery of rat hearts from cold ischemic storage

Energy deficiency and disturbances of sodium and water homeostasis are considered as mechanisms of injury during hypothermic preservation of cardiac muscle. The present study attempts to characterize the effect of potassium (K+) and magnesium (Mg2+) cardioplegia on these mechanisms. Cellular parameters were measured by multinuclear NMR spectroscopy in isolated rat hearts during 12 h of ischemia at 4 degrees C and 2 h of normothermic reperfusion with an isoosmotic Krebs-Henseleit (KH) solution. Potassium and magnesium cardioplegia (a) reduced the rate of ATP hydrolysis and cellular acidification during early stages of ischemia; (b) caused an early cessation of the phase of fast sodium influx after 40 min (P<0.001 vs 120 min with KH); (c) reduced intracellular sodium accumulation to 148-165 micromol/gdw after 12 h (P<0.01 vs 268+/-15 micromol/gdw with KH); (d) decreased ischemic volumes to 2.7+/-0.1 and 2.8+/-0.1 ml/gdw after 8 and 12 h of storage, respectively (P<0.005 v 3.0 and 3.3 ml/gdw with KH). Quantitative analysis of these parameters showed that both hypothermia and cardioplegia increased the relative contribution of sodium to intracellular water accumulation by a factor of 2-2.5. In view of the marked reduction in absolute sodium and water contents, the data indicate that cold cardioplegia limits the increase in intracellular osmolarity. Myocardial mechanical and metabolic recoveries, and cellular viability deteriorated during prolongation of the ischemic period from 8 to 12 h in all experimental groups (P<0.005). Reperfusion was efficient in reversing intracellular sodium and water accumulation in hearts stored with cardioplegia, in contrast to hearts stored in KH. Magnesium, but not potassium cardioplegia, lowered interstitial water contents (P<0.01 v KH), increased intracellular magnesium concentrations (P<0.001), improved mechanical and metabolic recoveries (P<0.01) and cellular viability (P<0.001). These results indicate (a) cardioplegia reduces intracellular sodium (by approximately 46%) and water accumulation (by 66%) during cold ischemia; (b) both hypothermia and cardioplegia limit the rise in intracellular osmolarity and increase the contribution of sodium to cellular swelling; (c) intracellular sodium and water contents were dissociated from myocardial viability and recovery from cold ischemia in potassium and magnesium cardioplegic solutions. It is concluded that intracellular sodium and water accumulation are not dominant factors in determination of cardiac outcome from ischemia.

Glucose and fatty acid oxidation by the in situ dog heart during experimental cooling and rewarming

BACKGROUND

Reduced myocardial function after hypothermia may be metabolic in origin, but the relationship between myocardial metabolism and the various components of hypothermia-mediated dysfunction has not been thoroughly investigated.

METHODS

In the present study we measured myocardial uptake and oxidation of glucose and oleate in mongrel dogs undergoing cooling to 25 degrees C followed by rewarming to 37 degrees C, using radiolabeled substrates.

RESULTS

Segment work index declined from 39.3 +/- 5.1 to 15.1 +/- 2.4 mm Hg in response to cooling from 37 degrees to 25 degrees C and did not recover completely on rewarming (27.2 +/- 4.2 mm Hg, p < 0.05). Oleate uptake declined from 3,251 +/- 619 to 1,043 +/- 356 nmol.min-1.100 g-1 (p < 0.05) when the dogs were cooled from 37 degrees to 25 degrees C. Simultaneously, oxidation rate fell from 1,089 +/- 158 to 354 +/- 83 nmol.min-1.100 g-1 (p < 0.05). On rewarming, oleate uptake was restored to prehypothermic values, whereas its rate of oxidation remained depressed (480 +/- 129 nmol.min-1.100 g-1; p < 0.05). Uptake and oxidation of glucose also declined significantly during cooling. However, both uptake and oxidation of glucose recovered fully on rewarming.

CONCLUSIONS

The results of the present study demonstrate a reduced capacity to oxidize fatty acids by the myocardium during rewarming after hypothermia.

Effects of dichloroacetate on mechanical recovery and oxidation of physiologic substrates after ischemia and reperfusion in the isolated heart

The effects of dichloroacetate (DCA) on fatty acid oxidation and flux through pyruvate dehydrogenase (PDH) were studied in ischemic, reperfused myocardium supplied with glucose, long-chain fatty acids, lactate, pyruvate, and acetoacetate. The oxidation rates of all substrates were determined by combined 13C nuclear magnetic resonance (NMR) spectroscopy and oxygen-consumption measurements, and PDH flux was assessed by lactate plus pyruvate oxidation. In nonischemic control hearts, DCA increased PDH flux more than eightfold (from 0.68 +/- 0.28 to 5.81 +/- 1.16 micromol/min/g dry weight; n = 8 each group; p < 0.05) and significantly inhibited the oxidation of acetoacetate and fatty acids. DCA also improved mechanical recovery after 30 min of ischemia plus 30 min of reperfusion but did not significantly increase PDH flux measured at the end of the reperfusion period (1.35 +/- 0.42 micromol/min/g dry weight) compared with untreated ischemic hearts (0.87 +/- 0.28 micromol/min/g dry weight; n = 8 each group; p = NS). Although DCA had a modest effect on functional recovery in the reperfused myocardium, this beneficial effect was not associated with either marked stimulation of PDH flux or inhibition of fatty acid oxidation.

Effects of hypothermia on energy metabolism in rat and Richardson's ground squirrel hearts

Glycolysis, glucose oxidation, palmitate oxidation, and cardiac function were measured in isolated working hearts from ground squirrels and rats subjected to a hypothermia-rewarming protocol. Hearts were perfused initially for 30 min at 37 degrees C, followed by 2 h of hypothermic perfusion at 15 degrees C, after which hearts were rewarmed to 37 degrees C and further perfused for 30 min. Functional recovery in ground squirrel hearts was greater than in rat hearts after rewarming. Hypothermia-rewarming had a similar general effect on the various metabolic pathways in both species. Despite these similarities, total energy substrate metabolic rates were greater in rat than ground squirrel hearts during hypothermia despite a lower level of work being performed by the rat hearts, indicating that rat hearts are less efficient than ground squirrel hearts during hypothermia. After rewarming, energy substrate metabolism recovered completely in both species, although cardiac work remained depressed in rat hearts. The difference in functional recovery between rat and ground squirrel hearts after rewarming cannot be explained by general differences in energy substrate metabolism during hypothermia or after rewarming.

Free fatty acid metabolism during myocardial ischemia and reperfusion

Long chain free fatty acids (FFA) are the preferred metabolic substrates of myocardium under aerobic conditions. However, under ischemic conditions long chain FFA have been shown to be harmful both clinically and experimentally. Serum levels of free fatty acids frequently are elevated in patients with myocardial ischemia. The proposed mechanisms of the detrimental effects of free fatty acids include: (1) accumulation of toxic intermediates of fatty acid metabolism, such as long chain acyl-CoA thioesters and long chain acylcarnitines, (2) inhibition of glucose utilization, particularly glycolysis, during ischemia and/or reperfusion, and (3) uncoupling of oxidative metabolism from electron transfer. The relative importance of these mechanisms remains controversial. The primary site of FFA-induced injury appears to be the sarcolemmal and intracellular membranes and their associated enzymes. Inhibitors of free fatty acid metabolism have been shown experimentally to decrease the size of myocardial infarction and lessen postischemic cardiac dysfunction in animal models of regional and global ischemia. The mechanism by which FFA inhibitors improve cardiac function in the postischemic heart is controversial. Whether the effects are dependent on decreased levels of long chain intermediates and/or enhancement of glucose utilization is under investigation. Manipulation of myocardial fatty acid metabolism may prove beneficial in the treatment of myocardial ischemia, particularly during situations of controlled ischemia and reperfusion, such as percutaneous transluminal coronary angioplasty and coronary artery bypass grafting.

Myocardial substrate oxidation during warm continuous blood cardioplegia

BACKGROUND

Although long-chain fatty acids are a major energy substrate utilized by the myocardium, changes in the substrate balance toward a predominating fatty acid utilization could jeopardize the myocardium during cardiac operative procedures.

METHODS

In the present study myocardial substrate utilization was examined during warm continuous blood cardioplegia (4 hours, 37 degrees C), using pigs undergoing cardiopulmonary bypass. Hearts were perfused antegradely in a closed extracorporeal circuit in which cardioplegic donor blood (hematocrit, 22%) containing 14C-glucose and 3H-oleate was delivered to the heart. Arterial and coronary sinus blood samples were taken at intervals for determination of plasma concentrations of energy substrates, as well as glucose and oleate oxidation rates (14CO2 and 3HOH production).

RESULTS

The concentration of fatty acids in the cardioplegic perfusate did not change significantly during the cardiac arrest period. The mean concentration of glucose showed a 30% decline (not significant), whereas the lactate concentration increased from a starting value of 3.12 +/- 0.27 to 6.31 +/- 0.72 mmol/L at the end (mean +/- standard error of the mean; n = 8; p < 0.05). Only fatty acid levels showed a significant (positive) arterial-coronary sinus difference. Myocardial oxidation of oleate varied between 302 +/- 71 and 650 +/- 66 nmol.min-1.heart-1, whereas the range of variation for glucose oxidation was 144 +/- 64 to 355 +/- 107 nmol.min-1.heart-1. However, the changes in fatty acid levels and glucose oxidation rates during the cardiac arrest period were not statistically significant. We calculated that overall glucose oxidation accounted for less than 5% of the total aerobic energy production.

CONCLUSIONS

The present results demonstrate overreliance on fatty acids as a source of energy during warm continuous blood cardioplegia, consistent with a condition of myocardial insulin resistance.

Developmental differences in myocyte contractile response after cardioplegic arrest

Although developmental differences in left ventricular function after cardioplegic arrest and rewarming have been postulated, whether differences exist at the level of the myocyte remains unexplored. This project tested the hypothesis that there is a differential effect of hypothermic hyperkalemic cardioplegic arrest with subsequent rewarming on contractile function of immature compared with adult ventricular myocytes. Myocytes were isolated from the left ventricular free wall of five immature and five adult rabbits and incubated for 2 hours in hyperkalemic modified Ringer's solution at 4 degrees C (cardioplegia) or for 2 hours in cell culture medium at 37 degrees C (normothermia). Myocytes were resuspended ("rewarmed") in 37 degrees C cell culture medium after the incubation protocol. Normothermic baseline contractile performance was lower in immature, compared with adult, myocytes. Specifically, myocyte shortening velocity was 62 +/- 4 microm/sec in immature and 112 +/-6 microm/sec in adult myocytes (p < 0.01). After cardioplegia and rewarming, immature myocyte contractile function was unchanged, whereas adult myocyte contractile function was significantly diminished. For example, myocyte shortening velocity was 65 +/- 4 microm/sec in immature and 58 +/- 3 microm/sec in adult myocytes (p < 0.01 versus normothermic). Myocyte surface area, which reflects myocyte volume, was increased after cardioplegia and rewarming in adults (3582 +/- 55 versus 3316 +/- 46 microm2, p < 0.01), but remained unchanged in immature myocytes (2212 +/- 27 versus 2285 +/- 28 microm2, P = not significant). These unique findings demonstrate a preservation of myocyte contractile function and volume regulation in immature myocytes after cardioplegic arrest and rewarming. Thus this study directly demonstrates that developmental differences exist in myocyte responses to hypothermic hyperkalemic cardioplegic arrest with subsequent rewarming.

Myocardial metabolism during graded intraportal verapamil infusion in awake dogs

Verapamil produces comparatively greater in vivo left ventricular (LV) depression than other calcium channel antagonists produce, possibly because of myocardial metabolic derangements in addition to L-channel antagonism. Therefore, we studied myocardial lipid and carbohydrate usage and the effect of insulin treatment during progressive verapamil toxicity. Verapamil was infused through the portal vein to simulate oral overdose. Eighteen mongrel dogs were instrumented to measure multiple hemodynamic and metabolic parameters. After 1-week recovery, dogs underwent control euglycemic insulin dose-response studies (n = 6) in the conscious state: at 1,000 mU/mm insulin infusion rate, myocardial glucose and lactate extraction increased seven- and threefold, respectively with no change in coronary artery blood flow or ventricular elasticity and end-systole (Ees). In 12 separate dogs, intraportal graded verapamil toxicity was induced in 3 h by increasing the infusion rate hourly: 0.04 -- 0.08 -- 0.1 mg/kg/mm. At the end of hour 3, myocardial extraction of free fatty acids decreased from 33 +/- 4 to 9 +/- 3% (mean +/- SEM, p < 0.05), without significant change in myocardial blood flow or arterial free fatty acid concentration. Verapamil toxicity increased arterial glucose from 3.5 +/- 0.16 to 6.1 +/- 1.1 mM; simultaneously, myocardial glucose extraction doubled, although endogenous insulin concentrations did not increase. Arterial lactate concentrations and net myocardial lactate uptake both increased (p < 0.05 vs basal blue). Ees decreased from 28 +/- 1 mm Hg/mm (basal) to 20 +/- 2 mm Hg/mm (end of hour 3, p <0.05). Animals were randomized into two treatment groups; either (a) insulin-glucose (1,000 mU/mm, n 6; arterial glucose was clamped +/- 10% with 50% dextrose), or (b) saline controls (n = 6) that received equivalent volume of saline. After 1-h insulin treatment, Ees increased to 34 + 3 mm Hg; in controls, Ees was 15 +/- 3 mm Hg/mm (p < 0.05). With insulin-glucose treatment, neither myocardial glucose nor lactate extraction increased significantly (p = 0.06 for lactate). Verapamil therefore inhibits myocardial fatty acid uptake and impedes insulin-stimulated myocardial glucose uptake; under these conditions, insulin-glucose treatment increases myocardial contractile function independent of increased sugar transport. These findings indicate that verapamil toxicity produces myocardial insulin resistance and, potentially, nutrient deprivation that may contribute to clinically relevant negative inotropy.

Substrate preference of isolated perfused rat hearts during hypothermia and rewarming

Fatty acid and glucose oxidation rates were measured in isolated rat hearts undergoing hypothermia and rewarming. The hearts were perfused in the Langendorff mode with Krebs-Henseleit bicarbonate buffer containing 11.1 mM glucose plus 0.6 mM albumin-bound oleic acid as energy substrates. The hearts were stabilized at 37 degrees C and thereafter cooled progressively to 15 degrees C over a period of 60 min. The hearts were kept at this temperature for 10 min and then rewarmed to 37 degrees C during the next 30 min. Control hearts were perfused at 37 degrees C throughout the whole perfusion period. Trace amounts of [14C]glucose or [14C]oleic acid were included in the perfusate, and the rate of substrate oxidation was determined on the basis of the radioactive CO2 production. In normothermic hearts steady state oxidation rates of glucose and oleate were found to be 0.17 +/- 0.01 and 0.51 +/- 0.07 mumol min-1 g-1 dry wt, respectively (mean +/- SEM). In response to hypothermia (15 degrees C) glucose oxidation was reduced by 76% (from 0.17 +/- 0.01 to 0.04 +/- 0.01 mumol min-1 g-1 dry wt) and oleate oxidation by 47% (from 0.51 +/- 0.07 to 0.27 +/- 0.02 mumol min-1 g-1 dry wt). Upon rewarming glucose and fatty acid oxidation rates returned to essentially the same values (0.12 +/- 0.02 and 0.45 +/- 0.04 mumol min-1 g-1 dry wt) as those observed under steady state normothermic conditions. The molar ratio between glucose and fatty acid oxidation was, however, significantly (P < 0.05) lower in hypothermic than in normothermic hearts.(ABSTRACT TRUNCATED AT 250 WORDS)