With the ever-increasing burden of obesity and Type 2 diabetes, it is generally acknowledged that there remains a need for developing new therapeutics. One potential mechanism to combat obesity is to raise energy expenditure via increasing the amount of uncoupled respiration from the mitochondria-rich brown and beige adipocytes. With the recent appreciation of thermogenic adipocytes in humans, much effort is being made to elucidate the signaling pathways that regulate the browning of adipose tissue. In this review, we focus on the ligand–receptor signaling pathways that influence the cyclic nucleotides, cAMP and cGMP, in adipocytes. We chose to focus on G-protein–coupled receptor (GPCR), guanylyl cyclase and phosphodiesterase regulation of adipocytes because they are the targets of a large proportion of all currently available therapeutics. Furthermore, there is a large overlap in their signaling pathways, as signaling events that raise cAMP or cGMP generally increase adipocyte lipolysis and cause changes that are commonly referred to as browning: increasing mitochondrial biogenesis, uncoupling protein 1 (UCP1) expression and respiration.

Fatty liver diseases are complications of the metabolic syndrome associated with obesity, insulin resistance and low grade inflammation. Our aim was to uncover mechanisms contributing to hepatic complications in this setting. We used foz/foz mice prone to obesity, insulin resistance and progressive fibrosing non-alcoholic steatohepatitis (NASH). Foz/foz mice are hyperphagic but wild-type (WT)-matched calorie intake failed to protect against obesity, adipose inflammation and glucose intolerance. Obese foz/foz mice had similar physical activity level but reduced energy expenditure. Thermogenic adaptation to high-fat diet (HFD) or to cold exposure was severely impaired in foz/foz mice compared with HFD-fed WT littermates due to lower sympathetic tone in their brown adipose tissue (BAT). Intermittent cold exposure (ICE) restored BAT function and thereby improved glucose tolerance, decreased fat mass and liver steatosis. We conclude that failure of BAT adaptation drives the metabolic complications of obesity in foz/foz mice, including development of liver steatosis. Induction of endogenous BAT function had a significant therapeutic impact on obesity, glucose tolerance and liver complications and is a potential new avenue for therapy of non-alcoholic fatty liver disease (NAFLD).

1. Intravenous infusion of amino acid mixtures stimulates human oxidative heat production more effectively under general anaesthesia than in the unanaesthetized state. To analyse the splanchnic and extra-splanchnic regional distribution of this stimulation, whole body and splanchnic oxygen uptake, blood flow and blood temperatures were measured by a catheterization technique in 14 patients undergoing isoflurane anaesthesia for abdominal surgery. During the anaesthesia period, a mixture of 19 amino acids (240 kJ/h) was infused intravenously into seven of the patients while the others served as controls, receiving isovolumic infusions of a nutrient-free saline solution. 2. Whole body oxygen consumption fell by ≈ 40% during anaesthesia and surgery in the controls. Approximately 80–95% of the reduction occurred in the extra-splanchnic tissues. The splanchnic oxygen uptake fell by 31 ± 6% in the controls during the initial phase of anaesthesia, after which it returned to the pre-anaesthesia level. The initial reduction of the controls' splanchnic oxygen consumption accounted for only ≈23% of the simultaneous anaesthesia-induced reduction in pulmonary oxygen uptake. No initial reduction of the splanchnic oxygen uptake was observed in the patients treated with amino acid. 3. The amino acid infusion stimulated the whole body oxidative heat production by ≈18 W during anaesthesia and surgery and by ≈70 W at the emergence from anaesthesia. Approximately 74% of the stimulation occurred in the extra-splanchnic tissues. At awakening, the splanchnic oxygen uptake rose to ≈64% above the pre-anaesthesia level in the amino acid group. 4. During the entire period of anaesthesia, the whole body heat content fell by 282 ± 68 kJ in the controls and by 57 ± 25 kJ in the amino acid group. Amino acid treatment thus prevented ≈80% of the anaesthesia-induced reduction in whole body heat content. 5. During anaesthesia and surgery, cardiac output was ≈25% and ≈6% below the baseline, pre-anaesthesia levels in the control and amino acid groups, respectively. At awakening, it rose to ≈44% above baseline in the amino acid group while in the controls it remained unchanged. In both groups the splanchnic blood flow was unaffected by anaesthesia or by amino acid infusions.

1. Sympatho-adrenal activity was measured after the consumption of a 3.15 MJ mixed meal. Whole-body noradrenaline spillover rates, forearm plasma noradrenaline spillover and adrenaline secretion rates were derived using isotope dilution methodology. Heart rate and blood pressure spectral analysis measurements were also made. The relation of sympatho-adrenal activity to thermogenic and cardiovascular events was studied. Sympathetic nervous and thermogenic responses were measured for 120 min after the single 3.15 MJ meal and compared with those after three 1.05 MJ meals, given 30 min apart. 2. Whole-body and forearm plasma noradrenaline spillover, and the 0.1 Hz component of systolic pressure power all increased significantly postprandially, while the 0.1 Hz component of heart rate variability, an indirect index of cardiac sympathetic nervous activity, remained unaltered. Adrenaline secretion was unaltered postprandially. Whole-body plasma noradrenaline spillover and thermogenesis during the 120 min postprandial period were 37% and 36% higher after the single meal as compared with the multiple meals, although this was not statistically significant. 3. The sympathetic neural responses were delayed in relation to peak plasma insulin levels and sustained in the face of declining insulin levels. Energy expenditure increased significantly postprandially, but there was no direct quantitative relationship to plasma noradrenaline spillover. Forearm oxygen consumption did not increase postprandially despite significant increases in regional noradrenaline spillover. Thus, no close relation was demonstrated between postprandial sympathetic nervous activation and either insulin secretion or thermogenesis.

1. Intravenous infusion of amino acids stimulates energy expenditure and heat accumulation in normal man. To find out whether such stimulation also occurs during general anaesthesia, thermogenesis was measured in 21 patients before, during and after anaesthesia and surgery. 2. Ten patients received a mixture of 19 amino acids (240 kJ/h) infused intravenously throughout the anaesthesia. The other 11 patients, serving as controls, received saline. Using catheters previously inserted into the pulmonary and a systemic artery, cardiac output, arteriovenous oxygen difference, pulmonary oxygen uptake and mixed blood temperature were measured. 3. During anaesthesia and surgery, the blood temperature fell by 0.67 ± 0.09 °C/h in the control patients and by 0.38 ± 0.06 C/h in the amino acid-treated patients. Anaesthesia during 34 ± 4 min before surgery reduced the pulmonary oxygen uptake by 145 ± 9 ml/min in the control patients and by 81 ± 10 ml/min in the amino acid-treated patients, corresponding to reductions in total energy expenditure of 47 W in the control group and 26 W in the amino acid-treated group. The difference, 21 W, illustrates the thermogenic action of the amino acids. This value may be compared with that of 4 W, observed in unanaesthetized individuals subjected to 30 min of identical amino acid infusions. 4. At awakening after the anaesthesia, the oxygen consumption rose to 71 ± 21% above the pre-anaesthesia level in the amino acid-treated patients, who, without shivering, rapidly returned to normothermia, whereas in the control patients the oxygen uptake remained slightly below the pre-anaesthesia level, despite sustained hypothermia and vigorous shivering. 5. It is concluded that general anaesthesia augments five-fold the thermic effect of amino acids and that peroperative infusion of amino acids effectively prevents anaesthesia-induced hypothermia.

1. Resting energy expenditure and the metabolic responses to adrenaline (infusion rate: 0.03 μg min −1 kg −1 fat-free mass for 1 h) were investigated in 25 patients with liver cirrhosis. The patient group was heterogeneous and varied with respect to the aetiology of cirrhosis, the clinical condition (i.e. Child A or B), the nutritional status and the degree of hyper-insulinaemia. 2. When compared with 10 healthy control subjects the basal plasma adrenaline and noradrenaline concentrations were both increased in cirrhosis and remained elevated during adrenaline infusion (+39% and +31%, respectively; P <0.05). Concomitantly, the peripheral plasma insulin concentration and the molar C-peptide/insulin ratio were increased in liver cirrhosis (+96% and + 30%, respectively; P <0.05). Hyperinsulinaemia was more pronounced in patients with ethanol-induced liver cirrhosis. 3. When expressed per kg fat-free mass, resting energy expenditure was enhanced in liver cirrhosis (+21%; P <0.05) and was more pronounced (i.e. resting energy expenditures of +35% to +49% above estimated values) in patients with ethanol-induced cirrhosis, at advanced stages of the disease and in association with decreased body cell mass. 4. Infusion of adrenaline increased heart rate, O 2 consumption and the plasma concentrations of glucose, lactate, free fatty acids, glycerol and 3-hydroxybutyrate, and similar transient increases and subsequent decreases in the respiratory quotient were observed in both groups. However, the lipolytic, ketogenic and thermic responses were reduced in cirrhotic patients. Reduced metabolic responses were more pronounced in hyperinsulinaemic patients. 5. We conclude that resting energy expenditure is variable but may be substantially increased in patients with ethanol-induced cirrhosis. In contrast, the thermic effect of adrenaline is reduced in cirrhotic patients. Variations in resting energy expenditure are strongly associated with variations in body cell mass, which explains 65% of its variability. No parameter of body composition is predictive for thermogenesis.

1. Studies on human brown adipose tissue require specific molecular probes. A human genomic library has been screened with a complementary DNA corresponding to the uncoupling protein (UCP) of rat brown adipose tissue mitochondria. 2. Two recombinant phages were isolated, carrying genomic sequences of human UCP. From them we have subcloned a 0.5 kilobase fragment. This fragment, H-Ucp-0.5, contained two intronic regions and two exonic regions. Exonic regions encoded a sequence of 84 amino acids which exhibited a strong homology with central domain at rat UCP. The organization of H-Ucp-0.5 was confirmed by SI mapping analysis. 3. A Southern analysis suggested that the gene is single type in the human, as it is in rodents. 4. In Northern analysis experiments, H-Ucp-0.5 detected a specific 1.8 kb mRNA in human brown adipose tissue obtained from six patients with phaeochromocytoma and from one patient with a hibernoma. This molecular probe is a new, sensitive and reliable tool with which to study human brown adipocytes.

1. A solid-phase radioimmunoassay is described for the estimation of the uncoupling protein content of human brown adipose tissue mitochondria, as an index of thermogenic capacity. 2. The concentration of inner mitochondrial membrane uncoupling protein was measured in brown adipose tissue samples from 48 individuals who died suddenly. 3. The uncoupling protein content of axillary adipose tissue was greater than that of perirenal adipose tissue. 4. Variations in brown adipose tissue uncoupling protein content, which would be consistent with changing thermogenic requirements and capacity, were observed in different groups of subjects. Significantly lower concentrations were found in adults and in pre-term and stillborn infants than in older infants and children.

1. This study was designed to investigate the thermogenic effect of intravenously administered nutrition with glucose (given with a fixed nitrogen intake of 12.5 g daily as amino acids) as the principal source of energy. The protocol was designed so that each patient received their energy intake in five consecutive periods of 3 days with intakes ranging from 6650 to 17100 kJ/day with increments or decrements of 2600 kJ. 2. Thermogenesis from administered glucose was evident between levels of energy supply of 6650 kJ/day and 17100 kJ/day. The progressive rise in oxygen consumption and carbon dioxide production accounted for a total of 31% of the additional glucose which was administered. The net rate of fat synthesis from glucose reached a maximum 147 g/day at an energy supply of 14 500 kJ/day. 3. This study suggests that both fat synthesis and the associated obligatory thermogenesis is the main component of diet-induced thermogenesis in response to glucose intakes in excess of 150 kJ day −1 kg −1 . If the energy cost of fat synthesis (fat associated obligatory thermogenesis) is taken to be 22% of the total energy of the increase in glucose supplied, then only 9% (31–22%) of the glucose can be accounted for by adaptive thermogenesis.

1. The influence of β-adrenoceptor-blockade on skeletal muscle was studied in ten healthy males with propranolol, atenolol and pindolol randomly given for 8 days each in a cross-over double blind test. After 7 days on each drug, muscle function was tested by an isokinetic dynamometer. Thermogenesis in biopsy samples taken from vastus lateralis muscle after a low grade exercise was studied after 8 days on each drug by direct calorimetry with a perfusion microcalorimeter. 2. Before drug administration, a median heat production rate of 0.67 mW/g of muscle was measured. This value was significantly reduced by 25% during propranolol, but no significant change was found during atenolol or pindolol administration. 3. Peak torque decline during isokinetic endurance test changed significantly in knee flexor but not in extensor muscles, from 15% to 27% after propranolol and from 15% to 23% after pindolol. Maximum dynamic strength was unaltered. 4. Our data suggest that blockade of sympathetic β 2 -receptors decreases thermogenesis in human skeletal muscle and impairs isokinetic endurance.

1. In adult man, brown fat can be detected in perinephric fat depots by visual inspection, electron microscopy and nucleotide binding to the tissue-specific uncoupling protein. 2. The 32 kDa uncoupling protein is functionally active, showing a nucleotide-sensitive conductance to protons and an uncoupling response to fatty acids. 3. The amount of uncoupling protein in human mitochondria is equivalent to that in a partially cold-adapted guinea pig, indicating some potential for thermogenesis. 4. Respiratory capacity measurements indicate that the total perinephric fat in adult man can only account for one-fivehundredth of the whole-body response to infused noradrenaline. Thus, although brown fat has been found to be quantitatively important in animal studies, considerable caution must be exercised in extrapolating its significance to adult man.

1. The thermic response of five lean and five obese subjects was measured by indirect calorimetry before, and for 157.5 min after a meal of protein, carbohydrate or fat, each of which provided 1.25 MJ. The change in plasma glucose, insulin and (in the case of the carbohydrate meal) the rate of exogenous glucose oxidation was also measured. 2. There was no significant difference between the lean and obese groups in the magnitude of the thermic response to any of the three meals. In both weight groups the response was largest and most prolonged after the protein meal ( P < 0.01). 3. The obese group showed a higher concentration of fasting plasma insulin ( P < 0.01) and a larger increase in plasma glucose ( P < 0.05) after the carbohydrate meal, but there was no significant difference in the oxidation of exogenous glucose when compared with the lean group. 4. Previous studies on dietary-induced thermogenesis in lean and obese subjects have given conflicting results. In general reports of decreased thermogenesis in obese subjects are characterized by either (a) high pre-meal metabolic rates in the obese group, especially in diabetic subjects, or (b) a group classified as ‘normal’ who have been selected for their high thermogenic capacity.

1. Ten male subjects were cooled on three occasions to a rectal temperature of 35°C by immersion to the neck in water at 11·3°C. The subjects were rewarmed for 60 min, once by metabolic heat production alone (shivering), once by inhalation rewarming with spontaneous breathing of saturated air at 47°C (control) and once by inhalation rewarming with ventilation regulated at 40 litres/min by respiring a controlled fraction of CO 2 (hyperventilation). 2. Metabolic heat production was substantially reduced by inhalation rewarming ( P < 0·05), from 913 kJ when shivering to 766 kJ (control) and 613 kJ when hyperventilating. The fall in metabolic heat production was greater than the corresponding respiratory heat gain, which increased from a loss of 41 kJ when shivering to gains of 85 kJ (control) and 169 kJ (hyperventilation). 3. As differences in mean skin temperatures were small (<1·0°C), it is concluded that the lower metabolic heat production in response to increased respiratory heat input must result from more rapid central temperature gains. This conclusion is supported by the relative values of rectal and tympanic temperatures. It was calculated that the percentage of the total heat supply which was donated to the core increased from 13% during shivering to 16% for the control and 23% in hyperventilation. Results imply that respiratory heat input is more efficient than metabolic heat production in elevating central temperature.

1. The thermogenic response and changes in plasma substrates and hormones were tested after a liquid meal in lean, obese and formerly obese women. 2. Subjects with a family history of obesity tested either while obese or after slimming to a normal weight had a thermogenic response, which was only half that of the lean group. 3. The immediate response in plasma glucose and insulin was greater in the lean subjects, but the sustained changes in circulating substrates did not differ in the three groups. Thyroidal hormone concentrations did not alter postprandially: venous noradrenaline levels rose in the obese groups only. 4. The differences in postprandial thermogenesis at rest would reduce the energy requirements of subjects with familial obesity, but they still had a metabolic rate estimated to be nearly 1MJ (240 kcal)/day in excess of the lean group so additional thermogenic defects must exist for familial obesity to be explained solely on a metabolic basis.