Changes in plasma glucose and lipid concentrations during treadmill exercise in domestic fowl

Energy supply during nocturnal endurance flight of migrant birds: effect of energy stores and flight behaviour

BACKGROUND

Migrating birds fly non-stop for hours or even for days. They rely mainly on fat as fuel complemented by a certain amount of protein. Studies on homing pigeons and birds flying in a wind-tunnel suggest that the shares of fat and protein on total energy expenditure vary with flight duration and body fat stores. Also, flight behaviour, such as descending flight, is expected to affect metabolism. However, studies on free flying migrant birds under natural conditions are lacking.

METHODS

On a Swiss Alpine pass, we caught three species of nocturnal migrant passerines out of their natural migratory flight. Since most night migrants start soon after dusk, we used time since dusk as a measure of flight duration. We used plasma concentrations of metabolites of the fat, protein, and carbohydrate metabolism as indicators of relative fuel use. We used flight altitudes of birds tracked with radar and with atmospheric pressure loggers to characterize flight behaviour.

RESULTS

The indicators of fat catabolism (triglycerides, very low-density lipoproteins, glycerol) were positively correlated with body energy stores, supporting earlier findings that birds with high fat stores have a higher fat catabolism. As expected, plasma levels of triglycerides, very low-density lipoproteins, glycerol and ß-hydroxy-butyrate increased at the beginning of the night, indicating that nocturnal migrants increased their fat metabolism directly after take-off. Surprisingly, fat catabolism as well as glucose levels decreased in the second half of the night. Data from radar observations showed that the number of birds aloft, their mean height above ground and vertical flight speed decreased after midnight. Together with the findings from atmospheric pressure-loggers put on three species, this shows that nocturnal migrants migrating over continental Europe descend slowly during about 1.5 h before final landfall at night, which results in 11-30% energy savings according to current flight models.

CONCLUSIONS

We suggest that this slow descent reduces energy demands to an extent which is noticeable in the plasma concentration of lipid, protein, and carbohydrate metabolites. The slow descent may facilitate the search for a suitable resting habitat and serve to refill glycogen stores needed for foraging and predator escape when landed.

Glucosensing and glucose homeostasis: from fish to mammals

This review is focused on two topics related to glucose in vertebrates. In a first section devoted to glucose homeostasis we describe how glucose levels fluctuate and are regulated in different classes of vertebrates. The detection of these fluctuations is essential for homeostasis and for other physiological processes such as regulation of food intake. The capacity of that detection is known as glucosensing, and the different mechanisms through which it occurs are known as glucosensors. Different glucosensor mechanisms have been demonstrated in different tissues and organs of rodents and humans whereas the information obtained for other vertebrates is scarce. In the second section of the review we describe the present knowledge regarding glucosensor mechanisms in different groups of vertebrates, with special emphasis in fish.

Blood metabolite levels in normal and handicapped pied flycatchers rearing broods of different sizes

We measured levels of select metabolites (glucose, triglycerides, free fatty acids, glycerol, uric acid) and corticosterone in the blood plasma of adult pied flycatchers Ficedula hypoleuca while they were rearing broods whose sizes were modified experimentally. We also made it more difficult than normal for some pairs of birds to forage by removing certain wing and tail feathers (handicapping them). Both procedures have been shown previously to change parental workload. We did this in order to determine if the birds alter their use of nutrients in response to differences in their workload. Metabolite levels were not influenced by handicapping or brood size. However, the concentration of free fatty acids in the plasma of females and of triglycerides in the plasma of males was directly related to the frequency with which the adults fed their nestlings. These findings suggest that the two sexes have different ways of coping with the work associated with rearing the brood: females apparently undergo brief daily fasts while feeding their chicks, whereas males take more time to feed themselves while providing food for their young, and spend more time doing so as their workload increases. The flycatchers exhibited high concentrations of uric acid and corticosterone in the blood plasma; corticosterone and glycerol were positively correlated in females; and corticosterone and triglyceride levels were negatively correlated in males; all of which suggest that gluconeogenesis provides some of the energy required for their parental activities.

Fuel use and metabolic response to endurance exercise: a wind tunnel study of a long-distance migrant shorebird

This study examines fuel use and metabolism in a group of long-distance migrating birds, red knots Calidris canutus (Scolopacidae), flying under controlled conditions in a wind tunnel for up to 10 h. Data are compared with values for resting birds fasting for the same time. Plasma levels of free fatty acids, glycerol and uric acid were elevated during flight, irrespective of flight duration (1-10 h). Triglyceride levels, the estimated concentration of very-low-density lipoproteins (VLDLs) and β-hydroxybutyrate levels were lower during flight, while glucose levels did not change. In flying birds, plasma levels of uric acid and lipid catabolites were positively correlated with the residual variation in body mass loss, and lipid catabolites with energy expenditure (as measured using the doubly labelled water method), after removing the effect of initial body mass. The plasma metabolite levels indicate: (i) that the rates of catabolism of lipids from adipose tissue and of protein are higher during flight; (ii) that low ketone body concentrations probably facilitate fatty acid release from adipose tissue; (iii) that low triglyceride and VLDL levels do not indicate the use of an additional pathway of fatty acid delivery, as found in small birds; and(iv) that the relationships between energy expenditure, body mass loss and metabolic pattern suggest that a higher individual energy expenditure entails a higher rate of catabolism of both lipids and protein and not a shift in fuel substrate.

Effect of acute heat exposure on triglyceride transfer to the preovulatory follicles of the laying fowl (Gallus domesticus)

1. The effect of acute heat exposure on triglyceride (TG) transfer to preovulatory follicles was studied in the laying fowl. 2. Heat exposure of laying fowl resulted in a 1.1 degree C rise in body temperature, a 10-fold increase in respiration frequency and mild hypocapnia and hypoxaemia. 3. Plasma and follicular tissue TG concentrations were not significantly affected by heat exposure, but plasma TG specific radioactivity decreased significantly and was negatively correlated with body temperature. 4. The transfer rate of TG to the preovulatory follicles was not affected significantly by hyperthermia. 5. We conclude that nutrient supply to the developing follicles is not compromised in acutely heat-exposed laying fowl.

Secretory pattern of growth hormone, insulin, and related metabolites in growing male turkeys: effects of overnight fasting and refeeding

The effect of an overnight fast on the secretory profile of growth hormone (GH), insulin, and related metabolites [glucose, nonesterified fatty acids (NEFA) and triglycerides (TG)] was determined in two separate experiments with 8- and 9-wk-old male turkeys. Toms were provided feed ad libitum or were fasted 20 h prior to sampling. Blood samples (.6 to .7 mL) were collected every 10 min for 8 h from the toms showing a positive weight gain between cannulation and the initiation of the experiment. Overall and baseline, GH levels were higher for the fasted birds versus the fed birds. The GH peaks for the fasted birds were of shorter duration than those for the fed birds, but were more frequent. The plasma NEFA levels of the fasted birds were higher than those of the fed birds. Plasma TG was lower for the fasted than the fed group. Insulin, which was secreted in a pulsatile fashion, was lower for the fasted versus the fed birds. Fasting produced no change in plasma glucose. A separate group of fasted males was refed after 4 h of sampling; the plasma levels of GH, insulin, and metabolites returned to those observed in birds given feed ad libitum within 30 min of refeeding. There appeared to be no relation between the pulsatile release of GH and the circulating levels of NEFA.

Effects of treadmill exercise on the transfer of plasma triglyceride into the ovary of the laying fowl

1. The effects of 90 min continuous treadmill exercise on the plasma-to-ovary transfer of 14C-labelled triglycerides in laying hens were examined. 2. Exercised birds showed a 4-fold increase in plasma free fatty acid concentration, a 15% decrease in plasma glucose concentration and unchanged plasma triglyceride concentration compared with resting controls. The mean plasma-to-ovary triglyceride transfer rate was approximately halved by exercise. 3. The surface area-specific triglyceride transfer rate was greatest for oocytes weighing 0.25-1.5 g and decreased with increasing oocyte size. The lowest rates were in the small (less than 0.25 g) white oocytes. This pattern was largely unaffected by exercise. 4. Factors that might be responsible for the reduction in triglyceride transfer into the oocytes during exercise include hormonal effects on the permeability of the oocyte envelope, increased competition for plasma lipids by the working muscles and reduced blood flow to the ovary, as a result of the release of vasoconstrictors.

Utilisation of energy substrates in treadmill-exercised domestic fowl (Gallus gallus domesticus): blood glucose

1. Blood glucose concentration, glucose turnover and gas exchange were monitored in adult male domestic fowl at rest and during 90 min of continuous treadmill exercise. 2. During exercise oxygen consumption increased to about three times its resting value. Blood glucose concentration fell by 26% by the end of exercise. 3. Blood glucose turnover was not altered by exercise and oxidation showed only a moderate increase (35%). Glucose oxidation accounted for 23% of the total energy expenditure at rest and this value fell to 10% during exercise. At the same time the plasma free fatty acid concentration rose to three times its resting value suggesting an increased mobilisation of this substrate. 4. It is concluded that during exercise of this relatively moderate intensity glucose is a quantitatively minor fuel for the working muscles.

Utilisation of energy substrates in treadmill-exercised domestic fowl (Gallus gallus domesticus): blood plasma free fatty acids

1. Plasma free fatty acid (FFA) concentration, FFA turnover and gas exchange were measured in adult male domestic fowl at rest and during 90 min continuous treadmill exercise at a work load of approximately three times the resting metabolic rate. 2. Plasma FFA concentration and turnover increased 7-fold and 4-fold respectively during exercise. Of the FFA turnover 0.54 was oxidised at rest and this increased to 0.70 during exercise. The fraction of the total carbon dioxide production derived from plasma FFA oxidation rose from 0.33 at rest to 0.55 during exercise. 3. It is concluded that plasma FFA are the most important source of energy for the working muscles of domestic fowl during treadmill exercise of this intensity and duration.

Sexual maturity and physical exercise alter fibrinolytic activity in birds (chickens)

1. Sexual maturity in the domestic hen is accompanied by hyperlipidaemia. This study examined effects of hyperlipidaemia on fibrinolytic activity at rest and following prolonged treadmill exercise. 2. In immature hens the mean pooled-plasma fibrinolytic activity was 50 mm2 and following 90 min continuous exercise this rose to 114 mm2. In contrast, fibrinolytic activity in mature hens was undetectable but rose to 51 mm2 after exercise. 3. The fibrinolytic response of mature and immature cocks was comparable to that of the immature hens. 4. The resting plasma triglyceride concentration in mature hens was approximately seven times that of the immature hens, immature and mature cocks. Exercise induced significant increases in plasma non-esterified fatty acid concentrations but had no significant effect on triglyceride or cholesterol levels. 5. The results demonstrate that prolonged exercise in birds, as in man, evokes hyperfibrinolysis. However, the response is impaired in mature females and this may be associated with increased blood lipid levels.

Plasma free fatty acid profile in male and female domestic fowl at rest and after exercise

The concentration and profile of the plasma free fatty acid (FFA) pool was determined in three male and three female domestic fowl before and after a 70 min treadmill run. The fatty acid profile of total food lipids was also determined. At rest, palmitic, stearic oleic, and linoleic acids were the main individual fatty acids in the plasma FFA pool and together constituted 50 to 60% of the total. Shorter chain fatty acids (C10 to C15) made up 24 to 38% of the total. In food lipids C16 and C18 fatty acids constituted more than 95% of total fatty acids, therefore food was not a significant direct source of short chain (less than C16) fatty acids. After exercise, there was an 8 to 10 fold increase in plasma FFA concentration. The proportion of lower chain (less than C16) FFA decreased and the proportion of C16 and C18 FFA, in particular oleic acid, increased. At the end of exercise the profile of the plasma FFA pool was similar to that of the adipose tissue triglyceride. Release of fatty acids from adipose tissue may be a major influence on exercise-induced changes in the plasma FFA profile.

Plasma glucagon and energy substrate responses of domestic fowl to treadmill exercise

Exercise-induced alterations in the concentrations of plasma glucagon-like immunoreactivity (GLI), plasma free fatty acids (FFA) and blood glucose and lactate were measured in separate groups of male and female domestic fowl. There were only small changes in blood glucose and lactate concentrations but plasma FFA and GLI rose by up to 450 and 200% respectively. There was evidence that the GLI response was stronger at higher exercise intensities. It is suggested that the mobilization of FFA for use as energy substrates by the working muscles may be stimulated by the enhanced secretion of glucagon.

Respiratory metabolism of D[U-14C]glucose in hens and cocks during prolonged treadmill exercise

1. The specific activity of expired 14CO2 was measured at rest and during 90 min treadmill exercise following an initial intravenous injection of D[U-14C]glucose. 2. The rate of CO2 production rose 4.5-fold during exercise in cocks but only 2.5-fold in females. The mean respiratory quotient was close to unity at rest and during exercise. 3. Estimated glucose turnover rate rose approximately 3.5-fold during exercise in cocks. Turnover rate did not increase in hens but the fraction of the glucose turnover oxidized to provide energy for the working muscles was increased. 4. It is concluded that carbohydrate sources account for the major fraction of energy expenditure during exercise of this magnitude and duration.