Initiation, duration and dissipation of diet-induced changes in sympathetic nervous system activity in the rat

Thermogenic capacity is antagonistically regulated in classical brown and white subcutaneous fat depots by high fat diet and endurance training in rats: impact on whole-body energy expenditure

This study investigated the regulation of thermogenic capacity in classical brown adipose tissue (BAT) and subcutaneous inguinal (SC Ing) white adipose tissue (WAT) and how it affects whole-body energy expenditure in sedentary and endurance-trained rats fed ad libitum either low fat or high fat (HF) diets. Analysis of tissue mass, PGC-1α and UCP-1 content, the presence of multilocular adipocytes, and palmitate oxidation revealed that a HF diet increased the thermogenic capacity of the interscapular and aortic brown adipose tissues, whereas exercise markedly suppressed it. Conversely, exercise induced browning of the SC Ing WAT. This effect was attenuated by a HF diet. Endurance training neither affected skeletal muscle FNDC5 content nor circulating irisin, but it increased FNDC5 content in SC Ing WAT. This suggests that locally produced FNDC5 rather than circulating irisin mediated the exercise-induced browning effect on this fat tissue. Importantly, despite reducing the thermogenic capacity of classical BAT, exercise increased whole-body energy expenditure during the dark cycle. Therefore, browning of subcutaneous WAT likely exerted a compensatory effect and raised whole-body energy expenditure in endurance-trained rats. Based on these novel findings, we propose that exercise-induced browning of the subcutaneous WAT provides an alternative mechanism that reduces thermogenic capacity in core areas and increases it in peripheral body regions. This could allow the organism to adjust its metabolic rate to accommodate diet-induced thermogenesis while simultaneously coping with the stress of chronically increased heat production through exercise.

Phenotyping small animals as models for the human metabolic syndrome: thermoneutrality matters

It is standard practice in preclinical biomedical research to house mammalian model organisms at an ambient temperature substantially below the thermoneutral zone. These experimental studies are performed using animals that are chronically challenged by mild cold stress. This condition increases food intake, metabolic rate, sympathetic activity, blood pressure and heart rate. Furthermore, this condition alters the behavioral and physiological responses to drug administration, energy restriction and overfeeding. This paper will review these observations, which must be understood in the context of phenotyping small mammals to enhance our understanding of the biology of human disease.

Prdm16 determines the thermogenic program of subcutaneous white adipose tissue in mice

The white adipose organ is composed of both subcutaneous and several intra-abdominal depots. Excess abdominal adiposity is a major risk factor for metabolic disease in rodents and humans, while expansion of subcutaneous fat does not carry the same risks. Brown adipose produces heat as a defense against hypothermia and obesity, and the appearance of brown-like adipocytes within white adipose tissue depots is associated with improved metabolic phenotypes. Thus, understanding the differences in cell biology and function of these different adipose cell types and depots may be critical to the development of new therapies for metabolic disease. Here, we found that Prdm16, a brown adipose determination factor, is selectively expressed in subcutaneous white adipocytes relative to other white fat depots in mice. Transgenic expression of Prdm16 in fat tissue robustly induced the development of brown-like adipocytes in subcutaneous, but not epididymal, adipose depots. Prdm16 transgenic mice displayed increased energy expenditure, limited weight gain, and improved glucose tolerance in response to a high-fat diet. shRNA-mediated depletion of Prdm16 in isolated subcutaneous adipocytes caused a sharp decrease in the expression of thermogenic genes and a reduction in uncoupled cellular respiration. Finally, Prdm16 haploinsufficiency reduced the brown fat phenotype in white adipose tissue stimulated by β-adrenergic agonists. These results demonstrate that Prdm16 is a cell-autonomous determinant of a brown fat-like gene program and thermogenesis in subcutaneous adipose tissues.

Vagal tone dominates autonomic control of mouse heart rate at thermoneutrality

It is generally accepted that cardiac sympathetic tone dominates the control of heart rate (HR) in mice. However, we have recently challenged this notion given that HR in the mouse is responsive to ambient temperature (Ta) and that the housing Tais typically 21–23°C, well below the thermoneutral zone (∼30°C) of this species. To specifically test the hypothesis that cardiac sympathetic tone is the primary mediator of HR control in the mouse, we first examined the metabolic and cardiovascular responses to rapid changes in Tato demonstrate the sensitivity of the mouse cardiovascular system to Ta. We then determined HR in 1) mice deficient in cardiac sympathetic tone (“β-less” mice), 2) mice deficient in cardiac vagal tone [muscarinic M2receptor ( M2R−/−) mice], and 3) littermate controls. At a Taof 30°C, the HR of β-less mice was identical to that of wild-type mice (351 ± 11 and 363 ± 10 beats/min, respectively). However, the HR of M2R−/−mice was significantly greater (416 ± 7 beats/min), demonstrating that vagal tone predominates over HR control at this Ta. When these mice were calorically restricted to 70% of normal intake, HR fell equally in wild-type, β-less, and M2R−/−mice (ΔHR = 73 ± 9, 76 ± 3, and 73 ± 7 beats/min, respectively), suggesting that the fall in intrinsic HR governs bradycardia of calorically restricted mice. Only when the Tawas relatively cool, at 23°C, did β-less mice exhibit a HR (442 ± 14 beats/min) that was different from that of littermate controls (604 ± 10 beats/min) and M2R−/−mice (602 ± 5 beats/min). These experiments conclusively demonstrate that in the absence of cold stress, regulation of vagal tone and modulation of intrinsic rate are important determinants of HR control in the mouse.

Feast or famine: the sympathetic nervous system response to nutrient intake

: 1. The use of tritiated norepinephrine (NE) to measure the turnover rate of NE in sympathetically innervated organs was pioneered in the laboratory of Julius Axelrod. This technique provides an organ specific assessment of sympathetic activity, integrated over a 24 h period, in free living laboratory animals. As such it has proved useful in estimating changes in sympathetic outflow in different physiologic and patho-physiologic states. 2. Studies employing NE turnover techniques in laboratory rodents have demonstrated conclusively that fasting suppresses and overfeeding stimulates the sympathetic nervous system (SNS). These changes in sympathetic activity also occur in humans. 3. Diet-induced changes in SNS activity are regulated by insulin-mediated glucose uptake and metabolism in central neurons sensitive to insulin and located anatomically in the ventro-medial hypothalamus. The regulation is imposed by descending inhibition of tonically active sympathetic brainstem centers. 4. Diet-induced changes in SNS activity mediate changes in energy production known as dietary thermogenesis. The capacity for dietary thermogenesis serves as a potential buffer against weight gain.5. Insulin stimulated SNS activity contributes to obesity-related hypertension. The insulin resistance of obesity, and consequent hyperinsulinemia, drives sympathetically mediated thermogenesis, restoring energy balance at the expense of SNS over activity. The association of obesity and hypertension, therefore, may be the unintended consequence of mechanisms recruited in the obese to limit further weight gain.

Unlike mice, dogs exhibit effective glucoregulation during low-dose portal and peripheral glucose infusion

Portal infusion of glucose in the mouse at a rate equivalent to basal endogenous glucose production causes hypoglycemia, whereas peripheral infusion at the same rate causes significant hyperglycemia. We used tracer and arteriovenous difference techniques in conscious 42-h-fasted dogs to determine their response to the same treatments. The studies consisted of three periods: equilibration (100 min), basal (40 min), and experimental (180 min), during which glucose was infused at 13.7 micromol.kg(-1).min(-1) into a peripheral vein (p.e., n = 5) or the hepatic portal (p.o., n = 5) vein. Arterial blood glucose increased approximately 0.8 mmol/l in both groups. Arterial and hepatic sinusoidal insulin concentrations were not significantly different between groups. p.e. exhibited an increase in nonhepatic glucose uptake (non-HGU; Delta8.6 +/- 1.2 micromol.kg(-1).min(-1)) within 30 min, whereas p.o. showed a slight suppression (Delta-3.7 +/- 3.1 micromol.kg(-1).min(-1)). p.o. shifted from net hepatic glucose output (NHGO) to uptake (NHGU; 2.5 +/- 2.8 micromol.kg-1.min-1) within 30 min, but p.e. still exhibited NHGO (6.0 +/- 1.9 micromol.kg(-1).min(-1)) at that time and did not initiate NHGU until after 90 min. Glucose rates of appearance and disappearance did not differ between groups. The response to the two infusion routes was markedly different. Peripheral infusion caused a rapid enhancement of non-HGU, whereas portal delivery quickly activated NHGU. As a result, both groups maintained near-euglycemia. The dog glucoregulates more rigorously than the mouse in response to both portal and peripheral glucose delivery.

Modification of catecholamine-induced changes in heart function by food restriction in rats

In view of the common practice of dieting for weight reduction, the influence of severe food restriction (about 25% of ad libitum intake) on adrenergic mechanisms was studied. Cardiac norepinephrine and epinephrine concentrations as well as plasma norepinephrine levels, were increased upon feeding a restricted diet to rats for 14 days in comparison with control rats that ingested about 30 g food/ day. Bradycardia as well as characteristic electrocardiographic abnormalities, including prolongation of the QRS and QT intervals, were observed in food-restricted rats. Diet-restricted rats did not develop ventricular arrhythmias in response to epinephrine injections as readily as control rats. Depression in both + dP/dt and -dP/dt of the heart in situ as well as reductions in the inotropic responses to epinephrine were evident in diet-restricted rats. Beta-adrenergic binding studies revealed a significant decrease in receptor density, but the dissociation constant for binding was also depressed in the food-restricted rat heart. Downregulation of the beta-adrenergic receptors in the heart may explain the lack of an epinephrine-induced increase in contractile force development as well as arrhythmias in food-restricted rats. These data demonstrate that severe food restriction has marked effects on adrenergic mechanisms and heart function, and thus some caution should be exercised at early periods of this therapy for weight reduction.

Effects of long-term dietary restriction on cardiovascular function and plasma catecholamines in the rat

To examine the relationship between heart function and plasma catecholamines upon food restriction, normal adult rats were fed 12 g or 6 g food/day for 14 days and 12 g food/day for 28 days. Food-restricted rats exhibited bradycardia, hypotension, and decreased rates of cardiac contraction (+dP/dt) as well as relaxation (-dP/dt) at 14 (12 or 6 g food/day) and 28 (12 g food/day) days. Plasma norepinephrine and epinephrine levels were significantly elevated in the 6 g food/day group at 14 days, whereas in the 12 g food/day group, plasma norepinephrine was elevated at 14 days but was significantly decreased at 28 days. Heart norepinephrine and epinephrine concentrations were elevated at both 14 and 28 days of food restriction in the 12 g food/day group as well as at 14 days in the 6 g food/day group. Thus, dietary restriction appears to result in depressed indices of heart function, while the circulating levels of catecholamines were elevated at early stages.

Depressed immune response in malnourished rats correlates with increased thymic noradrenaline level

Depressed immune response is well documented in protein-calorie malnutrition (PCM). Also, central and peripheral noradrenaline (NA) activities have been reported to be increased in malnourished animals. Since increases in central and peripheral NA may inhibit immune function, it is possible that malnutrition-induced immunodepression could be mediated by noradrenergic hyperactivity. To address this hypothesis the effect of malnutrition on cell-mediated immune response, as well as on NA levels of the median eminence, spleen and thymus was studied in PCM rats. Decreased lymphoproliferative response and IL-1 production by mononuclear macrophages was observed in PCM. Besides, increased NA concentration was detected in thymuses of PCM rats, while unchanged levels of this neurotransmitter were observed in median eminence and spleen. These data suggest a positive correlation between malnutrition-induced immunodepression and sympathetic noradrenergic activity in thymus, an organ implicated in immune cell differentiation during early development.

Stimulatory influence of D(-)3-hydroxybutyrate feeding on sympathetic nervous system activity in the rat

To examine the effect of ketone body utilization on sympathetic nervous system (SNS) activity, norepinephrine (NE) turnover was measured in heart and interscapular brown adipose tissue (IBAT) of rats fed diets enriched with D(-)3-hydroxybutyrate (3OHB), the naturally occurring isomer of hydroxybutyrate. Isoenergetic substitution of 3OHB for chow for 4 days increased cardiac [3H]NE turnover (P < .025), with a slightly less marked (P < .06) effect in IBAT. When [3H]NE turnover was measured in rats fed chow diets supplemented with 3OHB or sucrose and compared with that in animals fed chow alone, [3H]NE turnover rates in heart and IBAT were similar in the ketone-supplemented and chow-fed groups. Animals fed the sucrose-supplemented chow displayed lower rates of [3H]NE turnover in IBAT than rates found in those given the 3OHB-containing chow (-36%, P < .025 in IBAT). In addition, the stimulatory effect on SNS activity of a 4-day exposure to a sucrose-enriched diet after 2 days of fasting was significantly enhanced by concomitant ketone ingestion. Fractional NE turnover in IBAT was increased from 9.3% +/- 1.3%/h in control rats to 14.2% +/- 0.9%/h in rats refed with 3OHB (P < .005). These observations indicate that increased ketone body utilization does not suppress SNS activity and may stimulate it in a manner quantitatively similar to that seen with carbohydrate or fat ingestion.

Differential effects of dietary fats on sympathetic nervous system activity in the rat

Fat feeding stimulates sympathetic nervous system (SNS) activity in rats. To determine if fats vary in their potency as stimulants of the SNS, [3H]norepinephrine ([3H]NE) turnover was measured in heart and interscapular brown adipose tissue (IBAT) of animals fed lab chow diets supplemented with safflower oil, coconut oil, or medium-chain triglycerides (MCT). At 5 days, all three fats accelerated [3H]NE turnover in heart and did so equally, but only when the fat supplement represented an increase in energy intake. However, after 14 days, safflower oil and coconut oil but not MCT increased [3H]NE turnover in heart compared with turnover rates obtained in animals fed isoenergetic amounts of chow. Furthermore, the stimulatory effect of safflower oil on [3H]NE turnover was statistically greater than that seen in animals fed equivalent amounts of coconut oil. In vivo synthesis of NE assessed by accumulation of dopamine (DA) in heart following inhibition of dopamine-beta-hydroxylase (D beta H) was likewise highest in safflower oil-fed rats and lowest in those fed MCT. Thus, sympathetic activation by dietary fat varies among different fats, suggesting a role for fatty acid intake in dietary regulation of the SNS.

Influence of somatostatin analogue (SMS 201-995, octreotide) on blood pressure in adrenocorticotrophin (ACTH) treated rats: role of hyperinsulinaemia in ACTH hypertension

1. The hypothesis that adrenocorticotrophin (ACTH)-induced hypertension is a consequence of steroid-induced hyperinsulinaemia was tested using the somatostatin analogue (sandostatin, octreotide) to inhibit insulin release in Sprague-Dawley (SD) rats (n = 41). 2. Octreotide (20 micrograms, twice daily) did not modify blood pressure, plasma glucose, bodyweight, water and electrolyte balance, or organ weights but inhibited insulin secretion in the SD rat. 3. Compared with sham injection, ACTH-treated (0.5 mg/kg per day) SD rats showed an increase in blood pressure (sham 111 +/- 4 mmHg; ACTH 140 +/- 5 mmHg on treatment day 10 (P < 0.01), organ weights, water intake, urine volume, plasma glucose, insulin and sodium concentrations, and decrease of bodyweight and plasma potassium concentration. 4. Systolic blood pressure in rats treated with combined octreotide and ACTH was similar to that in rats on ACTH alone. Plasma insulin concentration was lower in octreotide + ACTH treated rats than with ACTH treatment alone. There were no differences in body or organ weights, plasma glucose, water or electrolyte balance. 5. Octreotide lowered plasma insulin concentration to the normal range but did not modify ACTH-induced hypertension in SD rats. These data do not support the notion that insulin-mediated alterations in blood pressure are a major mechanism for ACTH-induced hypertension in the rat.

The hypertriglyceridemic rat as a genetic model of hypertension and diabetes

Hypertriglyceridemia was demonstrated in untreated hypertensive patients as well as in animals with genetic and experimental hypertension. The main purpose of the present study was to evaluate the possibility to use the hereditary hypertriglyceridemic (HTG) nonobese rats in hypertensive research. Direct measurement of blood pressure demonstrated significantly higher systolic, diastolic and mean arterial pressures in HTG rats in comparison with control Wistar rats. There was significant positive correlation between blood pressure and plasma triglyceride concentration (r = 0.585, n = 40, p less than 0.001). In addition, there were significantly increased plasma norepinephrine and epinephrine concentrations in HTG rats, suggesting that the stimulation of sympathetic nervous system could be one of the pathogenetic mechanisms involved in the increase of blood pressure of HTG rats.

Effects of energy restriction on norepinephrine turnover and serum glucose and fatty acids in lean mice

Norepinephrine (NE) turnover rate was determined in several tissues of 5-week-old female mice fed a high carbohydrate diet (58% of energy as carbohydrate, 30% fat) either ad lib or restricted to 34 or 24 kJ/day (36 to 50% restriction) presented as 1 or 2 daily meals. When the restricted intakes were divided into 2 equal meals, daily NE turnover did not differ from that of ad lib-fed mice. When the above restricted amounts were provided as a single daily meal at the beginning of the dark period, NE turnover was 38% and 46% lower, respectively, in the heart only compared to ad lib-fed controls. Serum glucose and total free fatty acids were affected by dietary conditions known to produce sympathetic activation (high carbohydrate and high fat diets) and suppression (high protein diet and energy restriction as a single meal), but the changes were unrelated to fractional NE turnover. Thus, the lower NE turnover seen when food intake is restricted is due to the prolonged overnight fast and not due to the lower energy intake per se, and is not associated with serum concentration of glucose or total free fatty acids.

Prostaglandins, brown fat and weight loss

In obesity, a situation is created in which energy intake exceeds energy expenditure. The three components of energy expenditure are resting metabolism, physical activity, and thermogenesis. Increasing attention is being paid to the role of impaired energy expenditure in obesity. Evidence indicates that impairment in activity of the sympathetic nervous system, which stimulates thermogenic processes, contributes to the etiology of obesity. In addition, insulin resistance, a well-recognized metabolic consequence of obesity, appears to interfere with feeding-related, insulin-mediated increases in thermogenesis in brown adipose tissue. This thermogenic defect results in reduced energy buffering by brown adipose tissue leading to deficient energy expenditure and an increased efficiency in weight gain. A unique weight loss program, The Princeton Metabolic Diet Program, is presented. The Program stimulates metabolism by stimulating the sympathetic nervous system and correcting insulin resistance, thereby enhancing thermogenesis in brown adipose tissue. Methods include: 1) alternating diet composition and caloric intake and, 2) the use of nutritional metabolic stimulants. This type of non-toxic therapy, directed at correcting biochemical defects, will enhance metabolic mechanisms and induce weight loss.

The effects of various carbohydrates on sympathetic activity in heart and interscapular brown adipose tissue of the rat

The present studies were undertaken to determine the effect of various carbohydrates on sympathetic nervous system (SNS) activity. Tritiated-norepinephrine (3H-NE) turnover was measured in heart and interscapular brown adipose tissue (IBAT) of rats fed either chow or chow plus 50% caloric supplements of fructose, sucrose, dextrose, or corn starch. Additional studies were performed to examine whether absorption of carbohydrate plays a role in the SNS response, and to determine whether sweet taste in the form of artificial sweeteners may influence SNS activity. After five to ten days on the respective diets, 3H-NE turnover was increased to a similar extent by all carbohydrates tested (from 38% to 160% greater than controls in different studies). Addition of acarbose (which impairs sucrose absorption) to a sucrose-supplemented diet abolished the SNS stimulatory response, whereas cholestyramine (a drug that blocks fat absorption) had no effect. Finally, the addition of saccharin or aspartame to a chow diet failed to alter SNS activity. Thus, caloric supplementation with several carbohydrates, in addition to sucrose, stimulates both cardiac and IBAT SNS activity, absorption of carbohydrate is required for this effect, and noncaloric sugar substitutes do not alter SNS function.

Regulation of myofibrillar protein degradation in rat skeletal muscle during brief and prolonged starvation

Myofibrillar protein breakdown during brief and prolonged starvation was assessed in perfused rat skeletal muscle from 8-week-old fat-fed rats that conserve skeletal muscle protein during starvation and survive for 12 to 15 days and age-matched chow-fed rats that do not conserve protein and survive only five to six days. Following the inhibition of protein synthesis with cycloheximide, myofibrillar proteolysis was assessed by measuring the release of 3-methylhistidine from the perfused rat hindquarter while simultaneous measurement of total protein breakdown was assessed by measuring tyrosine release. Myofibrillar proteolysis progressed through three distinct phases during starvation: an early phase occurring within 24 hours in which proteolysis increased in all rats, a middle phase, which took three to five days to develop and during which proteolysis decreased and was present only in fat-fed rats, and a late phase in which proteolysis again increased. Total protein breakdown (ie, tyrosine release) changed little in phase I, decreased in phase II, and increased in phase III. The release of 3-methylhistidine from the perfused hindquarter reflected changes in muscle and urine of intact rats suggesting that data obtained with the perfused hindquarter reflected the in vivo situation. Insulin, amino acids, high concentrations of glucose, indomethacin, or epinephrine as well as adrenalectomy failed to attenuate the increase in 3-methylhistidine release from the perfused hindquarter during brief and late starvation. Free fatty acids and ketone bodies were also without effect in vitro. Refeeding fasting rats for four hours decreased myofibrillar proteolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Sympathoadrenal system and regulation of thermogenesis

The sympathetic nervous system (SNS) plays a critical role in the regulation of mammalian thermogenic responses to cold exposure and dietary intake. Catecholamine-stimulated thermogenesis is mediated by the beta-adrenergic receptor. In the rat brown adipose tissue is the major site of metabolic heat production in response to both cold (nonshivering thermogenesis) and diet (diet-induced thermogenesis). Measurements of norepinephrine turnover rate in interscapular brown adipose tissue of the rat demonstrate increased sympathetic activity in response to both cold exposure and overfeeding. In adult humans, a physiologically significant role for brown adipose tissue has not been established but cannot be excluded. It appears likely that dietary changes in SNS activity are related, at least in part, to the changes in metabolic rate that occur in association with changes in dietary intake.

Effects of dietary carbohydrate, fat, and protein on norepinephrine turnover in rats

To investigate effects of diet composition on rates of norepinephrine (NE) turnover in sympathetically innervated organs, weaning rats were fed for 2 to 21/2 weeks diets varying in carbohydrate (74.2% to 7.4% of total metabolizable energy) and fat (5.2% to 72.0%), or diets varying in protein (9.9% to 39.6%) and carbohydrate (77.8% to 48.1%). Changing the proportions of carbohydrate and fat in the diet, while maintaining similar intakes of energy and all other essential nutrients did not affect rates of NE turnover in heart, white adipose tissue (WAT), liver or pancreas and only minimally affected NE turnover in interscapular brown adipose tissue (IBAT). Decreasing the proportion of protein in the diet from 39.6% to 9.9% accelerated rats of NE turnover in heart (52%), IBAT (20%), WAT (42%), and liver (37%). When rats fed a diet containing 19.8% protein were also given a 10%(wt/vol) sucrose solution to drink for three days, their rates of NE turnover increased in heart (45%), IBAT (17%), liver (71%), and pancreas (55%). This response to sucrose depended on the protein content of the diet, since rats fed a 9.9% protein diet in which rates of NE turnover was already accelerated had no further increase in NE turnover when given the sucrose solution to drink. These data demonstrate that diet composition can affect activity of the sympathetic nervous system, as indicated by changes in rates of NE turnover. Changing the proportion of protein in the diet was more effective in altering NE turnover than changing the proportion of carbohydrate or fat.

Effect of dietary fat on sympathetic nervous system activity in the rat

Previous studies from our laboratory have demonstrated that dietary intake affects the sympathetic nervous system (SNS); carbohydrate intake, in particular, has been shown to stimulate sympathetic activity. The present studies were undertaken to characterize the effect of dietary fat on SNS activity in the rat. Sympathetic activity was assessed by measurement of norepinephrine (NE) turnover in heart, interscapular brown adipose tissue (IBAT), and pancreas and by excretion of NE in the urine. When fed a fat-enriched diet (50% chow, 50% lard), fractional NE turnover in heart (k) increased from 6.3 +/- 0.6% h in ad lib. fed controls to 14.7 +/- 1.3% h in the high-fat group (P less than 0.001); calculated NE turnover rate increased from 24.5 +/- 2.4 ng/heart per h to 36.8 +/- 3.5 (P less than 0.05). Urinary NE excretion more than doubled after 6 d of the same high fat diet (P less than 0.001). Ganglionic blockade produced a greater effect on NE turnover in fat-fed, as compared with chow-fed animals, consistent with increased sympathetic activity in the fat-fed group. When fat absorption was blocked with a bile acid binding resin (cholestyramine), the same high-fat diet did not increase cardiac NE turnover, indicating that fat absorption is required for the stimulatory effect on sympathetic activity. In another series of experiments, in which chow (and hence protein) intake was held constant, the effect of fat and isocaloric sucrose supplements on NE turnover was assessed in heart, IBAT, and pancreas. The caloric value of the supplements was 50, 100, and 335% of the chow in the different experiments. An effect of fat on NE turnover in heart and IBAT was demonstrable at the lowest level of fat supplement. Fat increased pancreatic NE turnover when added in amounts sufficient to double the caloric intake. The stimulatory effect of sucrose and fat on NE turnover in heart and IBAT was similar. These experiments demonstrate that fat increases SNS activity in the rat and that the magnitude of the effect is similar to that of sucrose. The results imply that fat may contribute to dietary thermogenesis in this species.

Stimulation of thermogenesis by carbohydrate overfeeding. Evidence against sympathetic nervous system mediation

Daily carbohydrate intake of seven men with normal weight was limited to 220-265 g/d for 6 d and then increased to 620-770 g/d for 20 d, while intake of protein, fat, and sodium remained constant. Carbohydrate overfeeding increased body weight by 4.8%, basal oxygen consumption (VO2) by 7.4%, BMR by 11.5%, and serum triiodothyronine levels by 32%. Overfeeding did not affect the thermic effect of a standard meal. Intravenous propranolol reduced the thermic effect of a meal by 22% during the base-line feeding period, and by 13% during carbohydrate overfeeding, but did not affect preprandial VO2. Overfeeding attenuated the rise in plasma glucose and FFA levels induced by infusion of norepinephrine, but had no effect on the increase in VO2 induced by norepinephrine. Overfeeding did not alter 24-h urinary excretion of vanillylmandelic acid, supine plasma catecholamine levels (pre- and postprandial), blood pressure, or plasma renin activity, but increased peak standing plasma norepinephrine levels by 45% and resting pulse rate by 9%. Even though short-term carbohydrate overfeeding may produce modest stimulation of sympathetic nervous system activity in man, the increase in thermogenesis induced by such overfeeding is neither suppressed by beta adrenergic blockade nor accompanied by an increased sensitivity to the thermogenic effects of norepinephrine. These data do not support an important role for the sympathetic nervous system in mediating the thermogenic response to carbohydrate overfeeding.