Decreased interstitial glucose and transmural gradient in lactate during ischemia

Summary The purpose of this investigation was to assess the effects of ischemia and reperfusion on the transmural levels of glucose and lactate in the interstitium in 11 open-chest swine. Microdialysis probes were used to estimate changes in interstitial metabolities across the ventricular wall. Probes were placed in the subepicardium and the subendocardium of the left anterior descending (LAD) coronary artery perfusion bed and in the midmyocardium of the circumflex (CFX) perfusion bed. The LAD coronary artery was cannulated and perfused with blood from the femoral artery through an extracorporal perfusion circuit. Ischemia was induced in the LAD perfusion bed by reducing the flow of the LAD perfusion pump by 60% for 50 min, and was followed by 30 min of reperfusion. Regional myocardial blood flow was assessed with fluorescent microspheres. Ischemia resulted in a transmural gradient in blood flow, with the most severe reduction in flow occurring in the subendocardium (p<0.05). We found a significant reduction in interstitial glucose in both the LAD subepicardium (1.26±0.24 mM) (p=0.0009) and subendocardium (0.89±0.21 mM) (p=0.0001) during ischemia compared to the aerobic (non-ischemic) period (1.97±0.25 mM, 2.03±0.29 mM for the subepicardium and subendocardium, respectively). This coincided with a significant reduction in glucose delivery (LAD pump flow^* arterial glucose) to the LAD perfusion bed during ischemia (54.5±8.5 mol/min) compared to aerobic values (182.1±25.3 mol/min) (p<0.05). Interstitial lactate levels were significantly increased during ischemia in the LAD subendocardium (3.39±0.46 mM) compared to the aerobic values (1.73±0.46 mM) (p<0.0029). A transmural gradient in interstitial lactate levels was observed during ischemia: this gradient was not seen during the aerobic period and was negated upon reperfusion. In conclusion, ischemia resulted in a decrease in interstitial glucose in both the LAD subepicardium and subendocardium, and an increase in interstitial lactate in the LAD subendocardium. Further, a transmural gradient in interstitial lactate levels was observed during ischemia, with the highest lactate values appearing in the subendocardium.     

Recently elucidated energy catabolism pathways provide opportunities for novel treatments in hepatocellular carcinoma

It has long been known that tumors depend on energy production pathways that are different from those of normal cells. These unique pathways require the expression and function of tumor-specific enzymes. Some of these glycolytic enzymes, as well as other modulators of tumor behavior, have recently been elucidated. In theory, inhibiting such enzymes or appropriately affecting such modulators should deprive tumors of energy, while leaving nontransformed cells unaffected. These factors include certain hexokinases that catalyze glycolysis in tumors and can be inhibited by 3-bromopyruvate. 2-deoxyglucose is another modulator that depletes hexokinase stores and cannot undergo further catabolism, thus depriving tumors of their energy source. Other enzymes or modulators are under scrutiny and have shown promise. Preliminary experiments on animals with hepatocellular carcinoma have indeed shown very encouraging results. It appears that modulating the energy production pathways of tumors is poised to become a substantial research area for cancer treatment. This review will focus on the energy production pathways of transformed cells, highlight the differences between transformed and normal cells in this regard and summarize recent experiments that take advantage of these disparities in cancer treatment.     

Dynamics and regulation of glycolysis‐tricarboxylic acid metabolism in the midgut of Spodoptera litura during metamorphosis

Significant changes usually take place in the internal metabolism of insects during metamorphosis. The glycolysis‐tricarboxylic acid (glycolysis‐TCA) pathway is important for energy metabolism. To elucidate its dynamics, the mRNA levels of genes involved in this pathway were examined in the midgut of Spodoptera litura during metamorphosis, and the pyruvate content was quantified. The expression patterns of these genes in response to starvation were examined, and the interaction between protein phosphatase 1 (PP1) and phosphofructokinase (PFK) was studied. The results revealed that the expression or activities of most glycolytic enzymes was down‐regulated in prepupae and then recovered in some degree in pupae, and all TCA‐related genes were remarkably suppressed in both the prepupae and pupae. Pyruvate was enriched in the pupal midgut. Taken together, these results suggest that insects decrease both glycolysis and TCA in prepupae to save energy and then up‐regulate glycolysis but down‐regulate TCA in pupae to increase the supply of intermediates for construction of new organs. The expression of all these genes were down‐regulated by starvation, indicating that non‐feeding during metamorphosis may be a regulator of glycolysis‐TCA pathway in the midgut. Importantly, interaction between PP1 and PFK was identified and is suggested to be involved in the regulation of glycolysis.     

A subconvulsive dose of kainate selectively compromises astrocytic metabolism in the mouse brain in vivo

Despite the well-established use of kainate as a model for seizure activity and temporal lobe epilepsy, most studies have been performed at doses giving rise to general limbic seizures and have mainly focused on neuronal function. Little is known about the effect of lower doses of kainate on cerebral metabolism and particularly that associated with astrocytes. We investigated astrocytic and neuronal metabolism in the cerebral cortex of adult mice after treatment with saline (controls), a subconvulsive or a mildly convulsive dose of kainate. A combination of [1,2-¹³C]acetate and [1-¹³C]glucose was injected and subsequent nuclear magnetic resonance spectroscopy of cortical extracts was employed to distinctively map astrocytic and neuronal metabolism. The subconvulsive dose of kainate led to an instantaneous increase in the cortical lactate content, a subsequent reduction in the amount of [4,5-¹³C]glutamine and an increase in the calculated astrocytic TCA cycle activity. In contrast, the convulsive dose led to decrements in the cortical content and ¹³C labeling of glutamate, glutamine, GABA, and aspartate. Evidence is provided that astrocytic metabolism is affected by a subconvulsive dose of kainate, whereas a higher dose is required to affect neuronal metabolism. The cerebral glycogen content was dose-dependently reduced by kainate supporting a role for glycogen during seizure activity.     

Neonatal de-afferentation of capsaicin-sensitive sensory nerves increases in vivo insulin sensitivity in conscious adult rats

Summary Sensory neuropeptides, released from the peripheral nervous system, might modulate glucose homeostasis by antagonizing insulin action. The effects of de-afferentation of functional small diameter unmyelinated C-fibres (sensory nerves) on in vivo insulin-mediated intracellular glucose metabolism were investigated by using euglycaemic insulin (6 and 18 mU/kg.min) clamps with [3-³H]-glucose infusion in 24 adult rats, treated neonatally with either capsaicin (CAP) (50 mg/kg) or vehicle (CON). Following the clamp, skeletal muscle groups, liver and adipose tissue were freeze-clamped. At plasma insulin levels of approximately 90 mU/l, CAP-rats showed a 21 % increase in whole body glucose uptake compared with CON (24.4 ± 1.6 vs 20.1 ± 0.8 mg/kg · min, p < 0.02), which was paralleled by a 20 % increase in whole body glycolysis (12.6 ± 0.8 vs 10.5 ± 0.5 mg/kg.min p < 0.05) (concentration of ³H₂O in plasma). Whole body skeletal muscle glycogenesis was increased by 80 % in CAP-rats (5.7 ± 0.7 vs 3.1 ± 0.7 mg/kg · min, p < 0.05) with increased muscle glycogen synthase activity. Whole body (muscle, liver and adipose tissue combined) de novo lipogenesis also was increased in CAP-rats compared with CON (0.69 ± 0.10 vs 0.44 ± 0.06 mg/kg · min, p < 0.05) (incorporation of [3-³H]-glucose counts into glycogen or fat). Hepatic glucose production was lower in CAP-rats compared with CON (0.6 ± 0.6 vs 2.1 ± 0.7 mg/kg · min, p < 0.05). Plasma glucagon, corticosterone, epinephrine and norepinephrine levels were reduced in CAP-rats: 43 ± 2 compared with 70 ± 6 pg/ml, 855 ± 55 compared with 1131 ± 138 nmol/l, 513 ± 136 compared with 1048 ± 164 pmol/l and 928 ± 142 compared with 1472 ± 331 pmol/l, respectively, p < 0.05. At plasma insulin levels of approximately 400 mU/l, CAP-rats showed no differences in peripheral and hepatic insulin action compared with CON. We conclude that the removae of endogenous sensory neuropeptides, by de-afferentation of capsaicin-sensitive sensory nerves, increases in vivo insulin sensitivity, but not responsiveness: 1) primarily through an increased sensitivity of skeletal muscle glycogen synthesis to insulin; 2) through a reduction in the levels of counter-regulatory hormones, thereby creating a milieu which favours overall in vivo insulin sensitivity with respect to glucose uptake, glucose production, glycolysis, glycogenesis and lipogenesis. [Diabetologia (1998) 41: 813–820] Received: 10 November 1997 and in revised form 4 March 1998     

Cancer‐like metabolism of the mammalian retina

The retina, like many cancers, produces energy from glycolysis even in the presence of oxygen. This phenomenon is known as aerobic glycolysis and eponymously as the Warburg effect. In recent years, the Warburg effect has become an explosive area of study within the cancer research community. The expanding knowledge about the molecular mechanisms underpinning the Warburg effect in cancer promises to provide a greater understanding of mammalian retinal metabolism and has motivated cancer researchers to target the Warburg effect as a novel treatment strategy for cancer. However, if the molecular mechanisms underlying the Warburg effect are shared by the retina and cancer, treatments targeting the Warburg effect may have serious adverse effects on retinal metabolism. Herein, we provide an updated understanding of the Warburg effect in mammalian retina.     

Contrasting action of short- and long-term adrenaline infusion on dog skeletal muscle glucose metabolism

Summary There are important differences between the short- and long-term effects of adrenaline on determinants of glucose tolerance. To assess this metabolic adaptation at tissue level, the present study examined the effect of acute and prolonged in vivo elevation of adrenaline on glycogen metabolism and glycolysis in skeletal muscle. Adrenaline (50 ng · kg⁻¹ · min⁻¹) was infused for 2 h or 74 h and the results compared with 1 h 0.9% NaCl infusion in six trained dogs. Muscle glycogen content was reduced by long-term adrenaline (161 ± 17 vs NaCl 250 ± 24 μmol/g dry weight;p < 0.05) but not short-term adrenaline (233 ± 21) indicating a sustained effect of adrenaline on glycogen metabolism. Acutely, glycogen synthase I was reduced (short-term adrenaline 12 ± 6 vs NaC122 ± 7μmol glycosyl units · g⁻¹ · min⁻¹;p < 0.05) but returned to normal with prolonged adrenaline infusion (20 ± 5). In contrast, K_m for glycogen phosphorylasea was not changed acutely (short-term adrenaline 31 ± 6 vs NaCl 27 ± 7 mmol/1 inorganic phosphate) but was reduced during long-term infusion (19 ± 4;p < 0.05 vs short-term adrenaline). Thus, with short- and long-term adrenaline infusion, there were different enzyme changes, although likely to promote glycogenolysis in both cases. In the glycolytic pathway the substrates glucose 6-phosphate and fructose 6-phosphate did not change significantly and hexokinase was not inhibited. Acutely, phosphofructokinase had reduced V_max (short-term adrenaline 34 ± 6 vs NaCl 44 ± 5 U/g; p < 0.05) but was still above the maximal operating rate in vivo. With prolonged adrenaline infusion, the K_m for phosphofructokinase was reduced (long-term adrenaline 0.32 ± 0.03 vs NaCl 0.44 ± 0.07 mmol/l fructose 6-phosphate;p < 0.05). In this situation of relatively low glycolytic flux, the sustained glycogenolytic effect of prolonged adrenaline infusion mediated by increased glycogen phosphorylase a ctivity occurs without a significant accumulation of hexose monophosphates or impairment of glycolysis.     

Metabolic Abnormalities in the Diabetic Heart

Congestive heart failure is a major health problem in the diabetic. Diabetics have a high incidence of heart disease, including an increased incidence and severity of congestive heart failure than the non-diabetic. Progression to heart failure after an acute myocardial infarction is also more frequent in diabetics then non-diabetics. While atherosclerosis and ischemic injury are important contributing factors to this high in incidence of heart failure, another important factor is diabetes-induced changes within the heart itself. A prominent change that occurs in the diabetic is a switch in cardiac energy metabolism. Increases in fatty acid oxidation accompanied by decreases in glucose metabolism can result in the myocardium becoming almost entirely reliant on fatty acid oxidation as a source of energy. This switch in energy metabolism contributes to congestive heart failure by increasing the severity of injury following an acute myocardial infarction, and by having direct negative effects on contractile function. This paper will review the evidence linking alterations in energy metabolism to alterations in contractile function in the diabetic.