Control of metabolism in brown adipose tissue

Neonatal capsaicin treatment in rats induces chronic hyperthermia resulting in infectious disease

Treatment of neonatal animals with capsaicin has previously been associated with long-lasting hyperthermia and severe cutaneous lesions. The present study analyzed the effects of capsaicin-induced hyperthermia on the occurrence of infectious disease and pruritic dermatitis in a rat model. Pregnant Sprague-Dawley (SD) rats were obtained 1 week prior to parturition. Pups from each litter were randomly assigned to the following experimental groups: Capsaicin-treated (cap-treated; n=10) or vehicle-treated (n=5). Capsaicin (50 mg/kg) or vehicle were systemically administered to the SD rat pups (age, 48 h), after which body temperature was measured using a biotelemetry system, and the effects of hyperthermia on the ability of the rat pups to resist bacterial infection were analyzed. Furthermore, pruritus-induced scratching behavior and dermatitis were assessed, and changes in interleukin (IL)-4- and IL-13-induced immunoglobulin E expression were measured. Treatment of neonatal rats with capsaicin resulted in chronic hyperthermia, which had negative effects on the host immune defense response. The expression levels of T-helper type 2 cell-associated cytokines were significantly increased (P<0.01) in the cap-treated rats following bacterial infection with Staphylococcus aureus or Streptococcus agalactiae. Furthermore, cap-treated rats exhibited pruritus-induced scratching behavior and dermatitis. The results of the present study suggested that treatment of neonatal rats with capsaicin induces chronic hyperthermia and decreases the effectiveness of the host defense system. Therefore, a cap-treated neonatal rat model may be considered useful when investigating the association between hyperthermia and infectious disease.

Neonatal capsaicin treatment in rats induces chronic hyperthermia resulting in infectious disease

Treatment of neonatal animals with capsaicin has previously been associated with long-lasting hyperthermia and severe cutaneous lesions. The present study analyzed the effects of capsaicin-induced hyperthermia on the occurrence of infectious disease and pruritic dermatitis in a rat model. Pregnant Sprague-Dawley (SD) rats were obtained 1 week prior to parturition. Pups from each litter were randomly assigned to the following experimental groups: Capsaicin-treated (cap-treated; n=10) or vehicle-treated (n=5). Capsaicin (50 mg/kg) or vehicle were systemically administered to the SD rat pups (age, 48 h), after which body temperature was measured using a biotelemetry system, and the effects of hyperthermia on the ability of the rat pups to resist bacterial infection were analyzed. Furthermore, pruritus-induced scratching behavior and dermatitis were assessed, and changes in interleukin (IL)-4- and IL-13-induced immunoglobulin E expression were measured. Treatment of neonatal rats with capsaicin resulted in chronic hyperthermia, which had negative effects on the host immune defense response. The expression levels of T-helper type 2 cell-associated cytokines were significantly increased (P<0.01) in the cap-treated rats following bacterial infection with Staphylococcus aureus or Streptococcus agalactiae. Furthermore, cap-treated rats exhibited pruritus-induced scratching behavior and dermatitis. The results of the present study suggested that treatment of neonatal rats with capsaicin induces chronic hyperthermia and decreases the effectiveness of the host defense system. Therefore, a cap-treated neonatal rat model may be considered useful when investigating the association between hyperthermia and infectious disease.

The role of Coenzyme Q in mitochondrial electron transport

In mitochondria, most Coenzyme Q is free in the lipid bilayer; the question as to whether tightly bound, non-exchangeable Coenzyme Q molecules exist in mitochondrial complexes is still an open question. We review the mechanism of inter-complex electron transfer mediated by ubiquinone and discuss the kinetic consequences of the supramolecular organization of the respiratory complexes (randomly dispersed vs. super-complexes) in terms of Coenzyme Q pool behavior vs. metabolic channeling, respectively, both in physiological and in some pathological conditions. As an example of intra-complex electron transfer, we discuss in particular Complex I, a topic that is still under active investigation.

Membrane lateral pressure as a modulator of glycerol-3-phosphate dehydrogenase activity

Michaelis-Menten kinetics of glycerol-3-phosphate dehydrogenase activity in proteoliposomes from brown adipose tissue mitochondria with exogenously added phospholipids or cholesterol was measured. It was shown that changes in membrane lipid composition affected the membrane lateral pressure and therefore modulated the enzyme activity, namely Vmax value. Contrarily, changes in surface charge caused by minute amounts of phosphatidylserine or charged organic substances influenced only the apparent Km value. The role of bulk phospholipids in regulation of glycerol-3-phosphate dehydrogenase is discussed.

Metabolic consequences of limited substrate anion permeability in brown fat mitochondria from a hibernator, the golden hamster

Brown fat mitochondria obtained from a hibernator, the golden hamster, were investigated in order to elucidate the significance of membrane permeability for metabolic functioning at different temperatures. The mitochondria were shown to have active permeases for phosphate and pyruvate, but very poorly developed permeases for di- and tricarboxylate substrate anions. This was shown with both osmotic swelling techniques and respiration-driven uptake studies. It was shown that the very limited malate permeation observed was compatible with it being a non-carrier-mediated diffusion process. The role of malate transport in supporting fatty-acid oxidation in vitro as a function of temperature was studied in detail. The results support our earlier suggestion that physiologically pyruvate carboxylase probably functions to generate oxaloacetate when high concentrations of condensing partner are needed during thermogenesis. They may also explain earlier observations that acetate was produced from palmitoyl-carnitine at low temperatures even when malate was present; this is here shown to be due to the limited malate permeability at these low temperatures. Thus, even at the body temperature of the hibernating hamster (4-5 degrees C), brown fat is probably able to combust fatty acids totally.

Preparation and metabolic characterization of isolated rat lung cells

Collagenase digestion of minced lung tissue yielded isolated cells, functionally viable as judged by several metabolic and morphological criteria, representative of all the cell species normally present in the tissue. The efficiency of the isolation procedure was about 25 per cent. Aerobic metabolism was not affected by most of the substrates tested except by succinate which increased oxygen utilization, and glucose, fructose and octanoate which significantly decreased oxygen uptake. Since no significant changes have been observed in the cellular adenine nucleotide content during glucose depression of aerobic metabolism it is concluded that the glycolytic flux had to be sufficient as to account for the decrease in the mitochondrial energy production. The mechanism responsible for these effects as well as their physiological significance are discussed herewith.

CO2-mediated control of fatty acid metabolism in isolated hamster brown-fat cells during norepinephrine stimulation

1. Addition of norepinephrine to isolated hamster brown-fat cells suspended in Krebs-Ringer phosphate buffer induces a pronounced, but temporary increase in respiratory rate. 2. If Krebs-Ringer phosphate buffer is bubbled with CO2 prior to the addition of cells and norepinephrine, the respiratory capacity of the cells is further potentiated and most important, the respiration is maintained at a high rate until the medium becomes depleted of oxygen. 3. This respiratory pattern cannot be obtained in CO2-bubbled Krebs-Ringer bicarbonate buffer. 4. The results indicate that CO2 has a regulatory effect on fatty acid metabolism in isolated hamster brown-fat cells.

Gylcerol-3-phosphate shuttle and its function in intermediary metabolism of hamster brown-adipose tissue

1. Brown adipose tissue of the hamster possesses high specific activities of soluble, cytoplasmic NAD-linked, as well as mitochondrial flavin-coupled, glycerol-3-phosphate dehydrogenases. The ratio of the two enzyme activities is high (close to 1), when compared with other tissues of the hamster. 2. In the presence of rotenone, NADH is oxidised very poorly by homogenates of brown adipose tissue. A high rate of oxidation is obtained upon further addition of dihydroxyacetone phosphate, which itself is negligible oxidised. When followed fluorimetrically glycerol 3-phosphate can also be observed to induce NADH oxidation, but only after a significant lag time. Similar results are obtained with isolated mitochondria plus high-speed supernatant. With high-speed supernatant alone, only dihydroxyacetone phosphate has any effect, whereas with isolated mitochondria neither dihydroxyacetone phosphate nor glycerol 3-phosphate induce any NADH disappearance. 3. Respiration induced by NADH plus dihydroxyacetone phosphate in homogenates equals 56% of the respiration induced by glycerol 3-phosphate alone. 4. Respiration induced by NADH plus dihydroxyacetone phosphate, as well as that induced by glycerol 3-phosphate, is inhibited by the same concentrations of inhibitors as are required for inhibition of the mitochondrial dehydrogenase i.e. EDTA, long-chain unsaturated fatty acids, long-chain fatty acyl CoA esters. 5. In isolated brown adipocytes in the presence of rotenone, norepinephrine significantly inhibits respiration induced by glycerol 3-phosphate. 6. The results obtained are discussed with respect to the role of glycerol 3-phosphate as an electron sink for cytosolic reducing equivalents to maintain a low level of extramitochondrial NADH. A means of maintaining a level of glycerol 3-phosphate adequate for triglyceride synthesis is also considered.

The regulation of glycerol 3-phosphate oxidase of rate brownadipose tissue mitochondria by long-chain free fatty acids

Added free fatty acids inhibit oxidation of glycerol 3-phosphate, succinate and NADH in brown-adipose tissue mitochondria from 10-day-old rats. The most pronounced is the inhibitory effect of glycerol 3-phosphate cytochrome c reductase (GP-cyto. c reductase). Contrary to other reductases, GP-cyto. c reductase activity of freshly isolated mitochondria is already inhibited by the fraction of endogenous free fatty acids. Both added and endogenous free fatty acids inhibition of GP-cyto. c reductase is fully reversible by the removal of free fatty acids by bovine serum albumine treatment. The inhibition of GP-cyto. c reductase is of strictly non-competitive type. The most inhibitory are unsaturated long-chain free fatty acids-oleic and linoleic acid. Results are discussed with regards to the regulatory importance of free fatty acids in brown-adiposetissue during intensive non-shivering thermogenesis.

The effect of noradrenaline on glyceride synthesis and oxidative metabolism in vitro in the brown fat of newborn rabbits

1. The effect of noradrenaline on the synthesis of glyceride from [U-(14)C]glucose and on gas exchange in the brown fat of newborn rabbits in vitro was investigated. 2. The specific radioactivity of l-glycerol 3-phosphate was lower than that of lactate, presumably because glycerol derived from glyceride was rephosphorylated by glycerokinase. 3. In the basal state more than 25% of the total respiration was due to pyruvate oxidation. Noradrenaline stimulated glyceride synthesis and total respiration without changing the proportion of the total respiration due to pyruvate oxidation. 4. The extra ADP released by noradrenaline stimulation of glyceride synthesis could not have supported more than 2% of the observed increase in substrate oxidation if mitochondria from brown-fat-cells remain fully coupled in the stimulated state, but could have supported about one-third of the observed increase if they become uncoupled in the presence of noradrenaline.