Tomie Ohno

Role of glucagon in metabolic acclimation to cold and heat

Abstract Plasma glucagon concentration increased in 2 weeks cold-acclimated rats, but it returned to normal value in 4 weeks cold-acclimated ones. Plasma free fatty acid (FFA) and β-hydroxybutyrate concentrations also showed the similar pattern of changes. Plasma glucagon and FFA concentrations decreased in both 2 weeks and 4 weeks heat-acclimated animals. Plasma β-hydroxybutyrate concentration was not changed by heat acclimation. Plasma glucose concentration decreased in heat-acclimated animals, while it was not affected by cold acclimation. There was a significant positive correlation between plasma glucagon and FFA levels as a whole in 2 weeks warm-, cold- and heat-acclimated rats, and in 4 weeks warm- and heat-acclimated ones. These findings would appear to indicate that in both cold and heat acclimation glucagon is closely involved as one member of a hormonal team through regulating lipid and carbohydrate metabolism.

Cross adaption between stress and cold in rats

Three-hour immobilization stress was imposed on male adult rats of Wistar strain by restraining them on a board 6 days a week for 1–8 weeks. The stressed rats showed less body weight gain during the experiment compared to the controls. These stressed animals manifested an improved cold tolerance as shown by no significant fall in colonic temperature in the cold at −5° C for 300 min during the experimental period, while the colonic temperature of the controls fell progressively. Nonshivering thermogenesis as assessed by noradrenaline-induced increase in oxygen consumption was significantly potentiated in the stressed rats. The weight and protein content of the intercapsular brown adipose tissue (BAT) increased and BAT mitochondria were more packed in the stressed rats. Plasma insulin, insulin/glucagon molar ratio and thyroxine levels were lowered in the stressed rats, while the plasma triiodothyronine level remained unchanged. Removal of interscapular BAT led to a loss of improved cold tolerance and a significant reduction of nonshivering thermogenesis in the stressed rats.These results indicate that repetitive stress may induce cross adaptation between stress and cold through an enhanced capacity of nonshivering thermogenesis mediated, at least in part, via stimulation of BAT function.

Hormonal regulation of brown adipose tissue—with special reference to the participation of endocrine pancreas

Abstract 1. 1. The role of pancreatic hormones, especially glucagon in the thermogenic function of brown adipose tissue (BAT) was investigated in rats. 2. 2. Plasma glucagon level increased in the cold-acclimated rats, while it decreased in the heat-acclimated ones. A significant positive relationship was observed between plasma glucagon and blood free fatty acid (FFA) levels as a whole in the warm controls, cold-acclimated and heat-acclimated rats. 3. 3. Glucagon infusion provoked an immediate rise of blood FFA and glucose levels on the venous drainage from the interscapular BAT. The β-adrenergic blocker, propranolol, did not modify these lipolytic and glycogenolytic actions of glucagon. When glucagon infusion was prolonged, the elevation of blood FFA and blood glucose levels was markedly suppressed in the cold-acclimated rats. 4. 4. Glucagon exhibited a marked heat production in the isolated brown adipocytes. 5. 5. Chronic administration of glucagon caused an improved cold tolerance accompanied by enhanced activity of BAT and increased non-shivering thermogenesis. 6. 6.Cold acclimation markedly elevated the content of glucagon as well as insulin in the interscapular BAT, while heat acclimation decreased it. 7. 7.These results would appear to indicate that glucagon as well as insulin is closely involved in the metabolic temperature acclimation, possibly through activation or inhibition of thermogenic mechanism(s) residing in the BAT.

Improved cold tolerance in glucagon-treated rats

Abstract In glucagon-treated rats (50 μg/100g, twice a day, 4 wks, sc) (GTR), the weights of liver and interscapular brown adipose tissue (BAT), and the level of plasma glucagon increased as compared with those in the vehicle-treated controls (VC). Mitochondria of BAT were markedly developed in size and cristae. Cold tolerance as assessed by the rate of fall in colonic temperature at −5 °C was improved. Elevations of colonic temperatures by noradrenaline (40 μg/100g, im) were significantly enhanced in GTR. After cold exposure, blood free fatty acids (FFA) and plasma glucagon levels increased, but blood glucose and β-hydroxybutyrate levels were not changed in VC. Both blood FFA and β-hydroxybutyrate levels increased and blood glucose level decreased, but plasma glucagon levels was not affected by cold exposure in GTR. These results suggest that glucagon is involved in cold acclimation by means of enhanced nonshivering thermogenesis, possibly due to an activation of BAT as well as increased production and utilization of ketone bodies in the liver.

Fasting-induced modifications of fatty acids composition in brown adipose tissue

Abstract 1. 1.|Effect of fasting for 72 h on fatty acid (FA) composition was studied in triglyceride and phospholipid fractions of rat interscapular brown adipose tissue (BAT) 2. 2.|Fasting decreased the body and BAT weights, and tissue DNA was significantly decreased by fasting. The tissue triglyceride and phospholipid levels were not affected by fasting 3. 3.|In triglyceride FA or BAT the extent of unsaturation was increased as assessed by mol% of polyunsaturated FA, the ratio of polyunsaturated FA to saturated FA and the unsaturation index in the fasted state 4. 4.|In phospholipid FA or BAT the extent of unsaturation also increased as assessed by the same indices for TG. 5. 5.|These results, especially in phospholipid FA of BAT, were in good agreement with the previous report in the cold-acclimated rats, suggesting that BAT function is, at least in part, stimulated during fasting.

Postnatal changes in fatty acids composition of brown adipose tissue

It has been demonstrated that thermogenic activity of brown adipose tissue (BAT) is higher during the early postnatal period, decreasing towards a low adult level. The present study examined postnatal changes in the lipid composition of BAT. BAT from pre-weaning rats at 4 and 14 days old showed the following differences in lipid composition compared to that from adults of 12 weeks old. (i) Relative weight of interscapular BAT to body weight was markedly greater. (ii) BAT-triglyceride (TG) level was lower, while BAT-phospholipid (PL)level was higher. (iii) In TG fatty acids (FA) polyunsaturated fatty acids (PU; mol %), arachidonate index (AI), unsaturation index (UI) and PU/saturated FA (SA) were higher; rare FA such as eicosadienoate, bishomo-γ-linolenic acid and lignoceric acid in mol % were also higher. (iv) In PL-FA monounsaturated FA (MU) in mol % was lower; PU mol %, AI and UI were higher. These features in BAT of pre-weaning rats resembled those in the cold-acclimated adults, suggesting a close relationship of the PL-FA profile to high activity of BAT.

Comparison of in vitro thermogenesis of brown adipose tissue in cold-acclimated rats and guinea pigs

Abstract 1. 1. Effects of noradrenaline (NA) and glucagon (G) were investigated on in vitro brown adipose tissue (BAT) thermogenesis by measuring oxygen consumption with an oxygen electrode on finely minced tissue blocks incubated in Krebs-Ringer phosphate buffer at 37°C. 2. 2. The interscapular BAT weight increased in rats, while it rather decreased in guinea pigs by cold acclimation. Fat percentage in BAT decreased in the cold-acclimated rats and guinea pigs. 3. 3. Na- and G-stimulated oxygen consumptions of BAT expressed per mg tissue weight were suppressed in the cold-acclimated rats, but were enhanced in the cold-acclimated guinea pigs as compared with those in the warm control animals. Adenosine deaminase did not affect the result in the cold-acclimated rats However, the oxygen consumption per BAT pad did not differ between the cold-acclimated and warm control animals. 4. 4. These findings reveal species differences in the in vitro responsiveness of BAT of cold-acclimated animals to the thermogenic factors. It is further suggested that some compensatory mechanisms such as hyperplasia and/or increased blood flow induce an enhanced thermogenesis in BAT during cold acclimation.

Non-thermal stress-induced modifications of fatty acids profiles in rat brown adipose tissue

Abstract 1. 1.|The changes in fatty acids (FA) composition were studied in triglyceride and phospholipid fractions of interscapular brown adipose tissue (BAT) in cold-acelimated (5°C, 4 wk) (CA), repetitively immobilized (3 hr/day, 4 wk) (IM) and treadmill-running trained (T) (30 m/min, 30 min/day under 8° inclination, 4 wk) rats. 2. 2.|Fatty acids composition in triglyceride: in CA rats the saturated FA (SA) and polyunsaturated FA (PU) were higher, while monounsaturated FA (MU) was lower. In IM rats SA was lower, while MU and PU were higher. Unsaturation index (UI) and PU/SA were higher in IM rats. In T rats SA and MU were lower, while PU was higher, UI and PU/SA were higher in T rats. 3. 3.|Fatty acids composition in phospholipid: in CA rats SA and PU were higher, while MU was lower. UI and PU/SA were higher. In IM rats SA and MU were lower, while PU was higher. UI and PU/SA were higher. In T rats MU was lower, but no changes were observed in UI and PU/SA. 4. 4.|These findings have indicated that non-thermal stress such as immobilization caused similar changes in FA composition of rat BAT, especially an increase in the extent of unsaturation in FA of phospholipid, to those previously reported and presently confirmed in CA animals. On the other hand, exercise training did not affect the extent of unsaturation in FA of phospholipid as assessed by UI and PU/SA.

Effect of sucrose-induced overfeeding on brown adipose tissue—With special reference to in vitro thermogenesis and fatty acids compositions

Effect of overfeeding induced by sucrose solution (32%) was investigated on in vitro oxygen consumption and fatty acids (FA) compositions of interscapular brown adipose tissue (BAT) in rat. Sucrose group (S) was fed standard diet with sucrose solution as drinking water. Food intake was greater by 10–24% in S as compared with that in standard diet control group (C). Colonic and skin temperatures were higher in S. The weight and DNA content of BAT were greater in S. In vitro basal and noradrenaline- or glucagon-stimulated oxygen consumption did not differ between the groups when expressed per DNA, while they were significantly greater in S in terms of whole tissue pad. In S BAT triglyceride (TG) and phospholipid (PL) levels as well as tissue contents were higher. In TG-FA of S saturated FA and monounsaturated FA were higher, while polyunsaturated FA were lower. In PL-FA of S monounsaturated FA were higher, while saturated FA and polyunsaturated FA were lower. These findings imply that sucrose-induced overfeeding increases BAT thermogenesis through tissue hyperplasia and higher PL-FA unsaturation as indicated by higher proportions of monounsaturates.

Brown Adipose Tissue and Nonshivering Thermogenesis in Stressful States

Adaptive transition of thermoregulatory thermogenesis from shivering to more efficient non-shivering thermogenesis in cold has been well documented. Nonshivering thermogenic mechanisms have been explored in the major nonshivering thermogenic organ, brown adipose tissue (BAT). Here we review our recent work on some regulatory elements (phospholipid fatty acids, nitric oxide, uncoupling protein, inhibitory central structure) of this tissue under various physiological conditions. The following points are suggested. (1) The docosahexaenoic acid (DHA) level in BAT phospholipids is closely associated with BAT in vitro thermogenesis; increase in DHA enhances, while decrease in DHA suppresses, thermogenesis of BAT. DHA is also involved in BAT hyperplasia leading to increased BAT thermogenesis. (2) The tonic inhibitory mechanism for BAT thermogenesis resides in the midbrain site in a hibernator, the hamster, as well as in the rat, a nonhibernator. (3) Nitric oxide (NO) produced by endothelial NO synthase in BAT is involved in an increase in BAT blood flow and possibly in BAT thermogenesis induced by the sympathetic outflow stimulated by the ventromedial hypothalamus (VMH). (4) Nonthermal stress of repetitive immobilization enhances BAT thermogenesis by increasing both capacity and activity of uncoupling protein 1 (UCP-1), leading to an acquisition of cross-adaptability to cold.