Cardiovascular responses to caloric restriction and thermoneutrality in C57BL/6J mice.

We utilized variations in caloric availability and ambient temperature (T(a)) to examine interrelationships between energy expenditure and cardiovascular function in mice. Male C57BL/6J mice (n = 6) were implanted with telemetry devices and housed in metabolic chambers for measurement of mean arterial pressure (MAP), heart rate (HR), O(2) consumption (VO(2)), and locomotor activity. Fasting (T(a) = 23 degrees C), initiated at the onset of the dark phase, resulted in large and transient depressions in MAP, HR, VO(2), and locomotor activity that occurred during hours 6-17, which suggests torporlike episodes. Food restriction (14 days, 60% of baseline intake) at T(a) = 23 degrees C resulted in progressive reductions in MAP and HR across days that were coupled with an increasing occurrence of episodic torporlike reductions in HR (<300 beats/min) and VO(2) (<1.0 ml/min). Exposure to thermoneutrality (T(a) = 30 degrees C, n = 6) reduced baseline light-period MAP (-14 +/- 2 mmHg) and HR (-184 +/- 12 beats/min). Caloric restriction at thermoneutrality produced further reductions in MAP and HR, but indications of torporlike episodes were absent. The results reveal that mice exhibit robust cardiovascular responses to both acute and chronic negative energy balance. Furthermore, we conclude that T(a) is a very important consideration when assessing cardiovascular function in mice.

Central leptin infusion attenuates the cardiovascular and metabolic effects of fasting in rats.

The role of reduced leptin signaling in the regulation of cardiovascular responses to negative energy balance is not known. We tested the hypothesis that central infusion of leptin would attenuate the cardiovascular and metabolic responses to fasting. Male Sprague-Dawley rats, instrumented with telemetry devices and intracerebroventricular cannulas, were housed in metabolic chambers for continuous (24 hours) measurement of dark-phase (active) and light-phase (inactive) mean arterial pressure, heart rate, oxygen consumption, and respiratory quotient. Rats received central infusions of either saline (0.5 microL/h) or leptin (42 ng/h) for 6 days through osmotic pumps and were either fed ad libitum or were fasted for 48 hours followed by refeeding for 4 days. In ad lib animals, continuous intracerebroventricular leptin infusion significantly reduced caloric intake, body weight, and respiratory quotient compared with saline controls while having no effect on mean arterial pressure or heart rate. Fasting reduced mean arterial pressure, heart rate, oxygen consumption, and respiratory quotient in rats receiving saline infusions. Fasting-induced reductions in mean arterial pressure were specific to the active phase and were not attenuated by central leptin infusion. In contrast, intracerebroventricular leptin, at a dose that had no cardiovascular effects in ad lib control animals, completely prevented fasting-induced decreases in light-phase heart rate and oxygen consumption and blunted fasting-induced reductions in dark-phase heart rate and oxygen consumption. The results are consistent with the hypothesis that reductions in central leptin signaling contribute to the integrated cardiovascular and metabolic responses to acute caloric deprivation.

Cardiovascular and metabolic responses to fasting and thermoneutrality are conserved in obese Zucker rats.

The primary purpose of the study was to test the hypothesis that reduced leptin signaling is necessary to elicit the cardiovascular and metabolic responses to fasting. Lean (Fa/?; normal leptin receptor; n = 7) and obese (fa/fa; mutated leptin receptor; n = 8) Zucker rats were instrumented with telemetry transmitters and housed in metabolic chambers at 23 degrees C (12:12-h light-dark cycle) for continuous (24 h) measurement of metabolic and cardiovascular variables. Before fasting, mean arterial pressure (MAP) was higher (MAP: obese = 103 +/- 3; lean = 94 +/- 1 mmHg), whereas oxygen consumption (VO(2): obese = 16.5 +/- 0.3; lean = 18.6 +/- 0.2 ml. min(-1). kg(-0.75)) was lower in obese Zucker rats compared with their lean controls. Two days of fasting had no effect on MAP in either lean or obese Zucker rats, whereas VO(2) (obese = -3.1 +/- 0.3; lean = -2.9 +/- 0.1 ml. min(-1). kg(-0.75)) and heart rate (HR: obese = -56 +/- 4; lean = -42 +/- 4 beats/min) were decreased markedly in both groups. Fasting increased HR variability both in lean (+1.8 +/- 0.4 ms) and obese (+2.6 +/- 0.3 ms) Zucker rats. After a 6-day period of ad libitum refeeding, when all parameters had returned to near baseline levels, the cardiovascular and metabolic responses to 2 days of thermoneutrality (ambient temperature 29 degrees C) were determined. Thermoneutrality reduced VO(2) (obese = -2.4 +/- 0.2; lean = -3.3 +/- 0.2 ml. min(-1). kg(-0.75)), HR (obese = -46 +/- 5; lean = -55 +/- 4 beats/min), and MAP (obese = -13 +/- 6; lean = -10 +/- 1 mmHg) similarly in lean and obese Zucker rats. The results indicate that the cardiovascular and metabolic responses to fasting and thermoneutrality are conserved in Zucker rats and suggest that intact leptin signaling may not be requisite for the metabolic and cardiovascular responses to reduced energy intake.

Influence of food and water availability on undirected singing and energetic status in adult male zebra finches (Taeniopygia guttata).

The songs of adult male zebra finches are termed "directed" and "undirected," depending on the social context in which they occur. Females elicit directed song, whereas undirected song is not addressed to a particular conspecific and even occurs at high levels in social isolation. We tested the hypothesis that the production of undirected song is more sensitive to a brief period of food deprivation than a comparable period of water deprivation. The hypothesis was based on prior findings suggesting that song production is energetically expensive and that food deprivation constitutes a more serious energetic challenge to zebra finches than does water deprivation. Two days of food or water deprivation were imposed on several groups of birds that provided song production data and a variety of energetic measures; normative data obtained in a baseline period when food and water were available ad libitum provided a standard for comparison. Singing, which occurred exclusively in the light phase of the day, was reduced at the onset of food deprivation, ceased completely within 4 h, and did not occur at all on the second day. When water was removed, the birds showed a slower and less substantial reduction in daily song production across the 2 days of deprivation. Energetic measures indicated that food deprivation was a greater energetic challenge than water deprivation. Our results demonstrate that undirected song in zebra finches is sensitive to nonsocial environmental factors that pose an energetic challenge and raise new questions about how birds calibrate their level of song production to the availability of nutrients in the environment.

Cardiovascular and metabolic responses of hypertensive and normotensive rats to one week of cold exposure.

Challenges to energy homeostasis, such as cold exposure, can have consequences for both metabolic and cardiovascular functioning. We hypothesized that 1-wk cold exposure (4 degrees C) would produce concurrent increases in metabolic rate (VO(2); indirect calorimetry), heart rate (HR), and mean arterial blood pressure (MAP) measured by telemetry. In the initial hours of change in ambient temperature (T(a)), both spontaneously hypertensive rats (SHRs) and normotensive Sprague-Dawley rats showed rapid increases (in cold) or decreases (in rewarming) of VO(2), HR, and MAP, although the initial changes in MAP and HR were more exaggerated in SHRs. Throughout cold exposure, HR, VO(2), food intake, and locomotor activity remained elevated but MAP decreased in both strains, particularly in the SHR. During rewarming, all measures normalized quickly in both strains except MAP, which fell below baseline (hypotension) for the first few days. The results indicate that variations of T(a) produce rapid changes in a suite of cardiovascular and behavioral responses that have many similarities in hypertensive and normotensive strains of rats. The findings are consistent with the general concept that the cardiovascular responses to cold exposure in rats are closely related to and perhaps a secondary consequence of the mechanisms responsible for increasing heat production.

Concurrent reductions in blood pressure and metabolic rate during fasting in the unrestrained SHR.

Fasting produces multiple cardiovascular, metabolic, and behavioral responses. To examine the interrelationship between these responses, male spontaneously hypertensive rats (SHR; n = 8) implanted with cardiovascular telemetry devices were housed in metabolic chambers at 23 degrees C for 22-h daily measurements of physiological variables. The experimental apparatus was designed so that ingestive behavior was detected by photobeams and locomotion was detected by a load sensor. Cardiovascular and metabolic status were determined as both a function of the circadian cycle (12-h dark and 10-h light), as well as during periods of inactivity (no ingestion and minimal locomotion) within the dark and light phases. Data were obtained during baseline, 48-h of caloric deprivation, and 6 days of refeeding. Fasting produced significant reductions in mean arterial pressure (dark: -9.2+/-1.3 from 143.7+/-3.7 mm Hg; light: -8.6+/-1.8 from 140.1+/-3.7 mm Hg), heart rate (dark: -43.4+/-5.2 from 330.0+/-5.2 beats/min; light: -27.4+/-5.2 from 294.0+/-5.2 beats/min), and oxygen consumption (dark: -5.0+/-0.6 from 20.6+/-0.3 ml x min(-1) x kg (0.75); light: -2.7+/-0.2 from 14.9 +/-0.2 ml x min(-1) x kg(0.75)). Analysis of inactive periods during both light and dark phases revealed that these reductions were not dependent on behavioral effects. We conclude that fasting produces concurrent and interrelated reductions in cardiovascular and metabolic function in the SHR. The merging of cardiovascular telemetry, indirect calorimetry, and behavioral monitoring provides a powerful approach for investigation of the integrative physiological responses to energetic challenges.

Shivering and digestion-related thermogenesis in pigeons during dark phase.

The pigeon's main source of regulated heat production, shivering, is especially likely to be used for thermoregulation during the dark phase of the day when there is little heat from locomotor activity. However, food stored in the pigeon's crop is digested during the night, and digestion-related thermogenesis (DRT) will provide heat that should decrease the need for shivering to maintain body temperature (Tb). We investigated the conditions under which DRT alters the occurrence of nocturnal shivering thermogenesis in pigeons. In fasting experiments, in which DRT was minimal, variations in pectoral shivering were closely related to the kinetics of nocturnal Tb when the ambient temperature (Ta) was moderate (21 degrees C). In that case, shivering was low while Tb fell at the beginning of the night, moderate during the nocturnal plateau in Tb, and strong during the prelight increase in Tb. Similar kinetics of nocturnal Tb occurred when Ta = 28 degrees C, but shivering was negligible throughout the dark phase. In restricted feeding experiments, nocturnal DRT was varied by providing different amounts of food late in the light phase. When Ta = 21 degrees C, 11 degrees C, and 1 degrees C, nocturnal Tb and O2 consumption were directly related to the amount of food ingested. However, nocturnal shivering tended to decrease as the food load increased and was significantly reduced at the higher loads. Because nocturnal shivering did not become more efficient in producing heat as the size of the food load increased, we conclude that nocturnal DRT decreased the need for shivering thermogenesis.

Shivering thermogenesis in the pigeon: the effects of activity, diurnal factors, and feeding state.

Shivering (electromyographic activity of the pectoral muscle), oxygen consumption, and body temperature were measured from undisturbed pigeons for periods of several weeks, and segments from the midparts of each phase of the light-dark cycle were compared at various ambient temperatures and feeding regimes. Behavior was recorded with a video camera. None of the observed types of behavior (e.g., walking, preening, feeding, drinking, pecking, defecation) induced spurious electrical activity in the pectoral muscle. On the other hand, none of these behaviors directly inhibited ongoing shivering. There was no difference in the mean level of shivering between the light (L) and dark (D) phases of the day in any of the conditions, although body temperature was 2 degreesC higher during L. Measurements of integrated electromyogram (EMG) with high temporal resolution (28 samples/s) showed that, at 1 degreesC, shivering in the pectoral muscle was present for more than 98% of the time. Plots of oxygen consumption against root mean square EMG were obtained in each condition by a filtering procedure that excludes data points in which oxygen consumption is affected by motor activity. These plots showed that the increase in heat production induced by a unit increase in pectoral EMG was lower in D than in L and that it was further lowered by fasting. The amplitude spectra of raw EMG signals were similar in all conditions. Spectra of demodulated (rectified, low-pass filtered) EMG showed a distinct rhythmicity around 8 Hz at 21 degreesC that was further enhanced by fasting but absent at 1 degreesC. This suggests that the degree of synchronization and pattern of recruitment of motor units are specific for various temperatures and feeding regimes, and may partly explain the variable relation between heat production and muscle electrical activity. The results emphasize the advantages of long-term measurements for understanding the control of thermogenesis in birds.

Vigilance states and body temperature during the circadian cycle in fed and fasted pigeons (Columba livia).

Fasting induces nocturnal hypothermia in pigeons. Slow-wave sleep (SWS) and paradoxical sleep (PS) are associated with reduced heat production in pigeons. The possibility that fasting-induced nocturnal hypothermia is related to increased SWS and PS was examined by comparing body temperature (Tb) and vigilance states when pigeons were fed and fasted. The results showed that when Tb is decreasing near the beginning of the dark phase, the percentage of total recording time (%TRT) spent in SWS and PS was elevated in fasting due to increased frequency of episodes and increased duration of PS episodes. When Tb was low during the middle segment of the dark phase, SWS was elevated in fasting due to increased episode duration. However, fasting did not alter PS, which increased in %TRT across the segment due to increased episode frequency. When Tb was rising during the final hours of dark, SWS remained elevated in fasting and %TRT in SWS and PS was relatively high. SWS and PS may promote the fasting pigeon's entry into, and maintenance of, nocturnal hypothermia.

Nocturnal digestion, cloacal excretion, and digestion-related thermogenesis in pigeons (Columba livia).

Two indicators of nocturnal digestive activity were identified in pigeons. Experiment 1 showed that a sizable amount of food empties from the crop while pigeons are inactive during the night. Experiment 2 showed that the number and volume of cloacal droppings during the night were directly related to the volume of food consumed during the day. The temporal pattern of cloacal droppings in the night was systematically related to features of the nocturnal body temperature (Tb) curves, suggesting that excretory activity is thermogenic in its own right and/or that it is a marker for a thermogenic process in the upper digestive tract. Questions about the relationship between digestion-related thermogenesis and shivering thermogenesis during the night in birds are highlighted by these findings.

Photoperiod duration and energy balance in the pigeon.

Energy balance, and daily rhythms in feeding activity, body temperature (Tb), metabolic rate (O2 consumption), and RQ (CO2/O2) that affect that balance, were studied in pigeons when the duration of the photophase gradually lengthened (LP group) or shortened (SP group) from an initial starting point at LD 12:12. The end point of change for the LP group was LD 21:3, and for the SP group was LD 3:21. Standard laboratory conditions were in effect (moderate ambient temperature; ad lib food and water). On LD 12:12, energy balance was positive (the ratio of gross energy intake to energy expenditure approximated 1.25). In the light phase, a bimodal pattern of feeding was accompanied by elevated levels in Tb, O2 consumption, and RQ; in the dark phase, Tb and O2 consumption fell at lights-off, and prior to lights-on there were anticipatory rises in both measures and a drop in RQ. Energy balance was remarkably constant over a wide range of photoperiods, but at the shortest photoperiods energy balance became more positive (approximately 1.45) because energy intake increased without much change in energy expenditure. Changes in the daily rhythms of the various measures provided some bases for understanding the changes in energy balance. Analysis of the Tb rhythms indicated that the circadian system of the pigeon appears to be capable of adjusting to a wide range of photoperiods. It is suggested that the increase in energy balance at short photoperiods may occur because of inadequate feedback from nutritional and metabolic signals, or may reflect anticipatory winter seasonal adjustments triggered by photoperiod duration.

Coupling between light- and food-entrainable circadian oscillators in pigeons.

Two experiments were conducted with a restricted feeding schedule to determine whether pigeons have a separate food-entrainable oscillator (FEO) and to assess the coupling strength between the FEO and the light-entrainable oscillator (LEO). In the baseline condition, the body temperature (Tb) and O2 consumption of two pigeons increased prior to light onset (LD 12:12 cycle) and food presentation (at the ninth hour of the light phase). In one experiment, when the LD cycle was phase delayed or advanced by 4 h while feeding time remained unchanged, the Tb and O2 rises prior to light-onset showed the expected delaying or advancing transients, but the rises prior to feeding also delayed or advanced for several days before returning to their proper phase position. In the second experiment, food was presented at 23.5-h intervals for 10 days while the LD cycle continued with a 24-h period. Entrainment to the LD cycle was unaffected, and Tb and O2 consumption continued to rise prior to the changing feeding time, but with a reduced lead time. When the feeding time was subsequently delayed by 5 h, Tb and O2 consumption with regard to the LD cycle were unaffected, but delaying transients occurred until both measures reentrained to the new feeding time. The results provide support for the existence of a separate FEO in pigeons and indicate asymmetrical coupling between the LEO and FEO, with the former having a stronger effect on the latter.

Daily cycles in body temperature, metabolic rate, and substrate utilization in pigeons: influence of amount and timing of food consumption.

Pigeons lived in individual chambers where instantaneous metabolic rate (MR; indirect calorimetry), body temperature (Tb), and substrate utilization (RQ) were measured 24 times each hour throughout the 12h:12h light:dark cycle. The amount of food consumed influenced the amplitude of the MR and Tb cycles, primarily by affecting the dark-phase segment of the cycle: when food was consumed ad lib, low-amplitude daily cycles in MR and Tb occurred in which levels in the dark phase were lower than in the light; during reduced food intake in restricted feeding or in fasting, high-amplitude cycles occurred primarily because nocturnal hypometabolism and hypothermia developed; in restricted feeding, the level of MR and Tb during the dark-phase segment of the cycle was directly related to short-term variation in amount consumed. The timing of food consumption primarily affected the light-phase segment of the MR and Tb cycles: when feeding was restricted to a time late in the light phase, these measures became depressed early in the light phase, and then greatly elevated near the scheduled time of feeding. This distinctive light-phase pattern developed quickly after the restricted feeding schedule began and may reflect the influence of a circadian food-entrainable oscillator. RQ indicated carbohydrate utilization for most of the 24-h cycle during ad lib feeding and in restricted feeding. However, approximately 2 h before the first feeding bout of the day, the RQ cycle indicated a sizable shift towards lipid utilization, which terminated after the bout was completed. There was a smaller, more transient, decrease in RQ near the time of the light-dark transition, which may imply cessation of digestive activity in preparation for the nocturnal decrease in Tb. During fasting, RQ indicated lipid utilization throughout the entire cycle. Whole-day energy expenditure by pigeons in these laboratory circumstances was shown to be closely related to the changes in within-day cycles associated with variations in the amount and timing of food intake.

Evidence for a separate food-entrainable circadian oscillator in the pigeon.

In Experiment 1, four pigeons lived in a metabolic chamber on a 12h:12h LD cycle where they maintained a reduced body weight by consuming a daily ration of food presented at the eighth hour of the photophase. Body temperature (Tb) and oxygen consumption (Vo2) increased prior to the daily feeding. The possibility that a food-entrainable oscillator timed these anticipatory responses was tested by four manipulations, conducted in successive phases, each of which involved eliminating the regularly scheduled food presentation, which is the putative entraining stimulus for such an oscillator, while the 12h:12h LD cycle remained in effect. The manipulations, and their outcomes, were: when fasting was imposed for 3 days, the anticipatory responses continued to occur; when ad lib feeding was allowed for 11 days, the anticipatory responses were mostly eliminated; when fasting was reimposed for 5 days, there was evidence that the anticipatory responses reoccurred; and, when the time of the daily feeding was phase-shifted earlier in the photophase for 8 days, anticipatory responses persisted at the original feeding time and simultaneously developed at the new feeding time. In the first phase of Experiment 2, key pecking by two pigeons produced food only during hours 9-11 of the daily photophase (12h:12h LD). In this condition, Tb increased and key pecking occurred in anticipation of the daily period of food availability. Evidence for a food-entrained oscillator was sought in a second phase when constant dim light (LL) was imposed without changing the hours of food availability.(ABSTRACT TRUNCATED AT 250 WORDS)

Ingestive behavior and body temperature of pigeons during long-term cold exposure.

Pigeons well adapted to living in 21 degrees C ambient temperature (T(a)) were continuously exposed to cold T(a) for 6 days (Experiment 1) or 30 days (Experiment 2). The pigeons lived on a 12:12 L:D cycle where they could obtain short access to food and water at any time in the light phase by making 12 keypeck responses. Across the two experiments, the levels of cold exposure were 6 degrees C, 1 degrees C, and 0.6 degrees C. In cold, total daily food intake increased gradually across the first few days of the exposure, and the new level of food intake was inversely related to T(a). The daily ratio of total water drunk to total food eaten averaged approximately 1.3 in 21 degrees C, fell to approximately 1.0 on day 1 of cold exposure, and remained at that level for as long as 30 days. The day-night cycle in core body temperature was not changed by cold exposure. The bimodal temporal pattern of feeding in the light phase, which is characteristic of pigeons in moderate T(a), was preserved in the cold although the absolute level of feeding activity was enhanced. An analysis of the ways individual pigeons achieved total daily food intake by combining a number of feeding episodes with an average amount eaten per episode revealed considerable variation between birds, but a relatively constant feeding style within bird over time. In the colder temperatures used, however, the pigeons all increased the number of feeding episodes per day. The results provide the first detailed analysis of cold-induced changes in ingestive behavior in the pigeon and raise several questions about the behavioral expression of cold-sensitive regulatory processes.

Energetic responses of pigeons during food deprivation and restricted feeding.

Pigeons ate food ad lib, then fasted for several days, and finally ate a controlled amount of food once a day for several months to maintain body weight at 80% of the ad lib value. Whole-body dry heat loss (HL) and core body temperature (Tb) were measured continuously for each pigeon. Thermal conductance (C) was calculated from HL and Tb. Relative to ad lib feeding, 24-h HL was reduced by approximately 50% during fasting and controlled feeding. The majority of energy savings was achieved by lowering C, which appeared to maintain a saturated low value throughout most of the light and dark phases. Therefore, the pigeon's characteristic high Tb in the daily light phase during fasting and food restriction does not necessarily imply high energy expenditure. In the dark phase, the fasting pigeon's characteristically low Tb enhances energy savings already being achieved through lowered C. The daily cycle in Tb, and to a lesser extent in HL and C, was strongly altered by occasional probe variations in the amount of food given at the single daily feeding and by a shift in the time of the daily feeding.

Coping with rising food costs in a closed economy: feeding behavior and nocturnal hypothermia in pigeons.

The pigeon's response to increasing fixed-ratio schedules in a 24-hr closed economy is marked by changes in feeding behavior during the daily light phase and by changes in body temperature during the dark phase. The time course of these responses to increasing behavioral cost of obtaining food is very different. Feeding is most affected immediately, within the first day of exposure to moderate fixed ratios. The number of times the pigeons produce the food hopper each day decreases, and the rate at which they eat from the food hopper (grams per minute) when it is available increases, as the fixed ratio is raised. Body temperature is affected later, falling to progressively lower resting levels during the dark phase as body weight drops at the higher fixed ratios when food intake is reduced. The changes in feeding and in body temperature that occur as the fixed-ratio schedule increases seem to reduce daily energy expenditures, within the constraints imposed by the experiment. The ascending and descending limbs of the bitonic function obtained when total daily operant responding is plotted as a function of fixed-ratio schedule in the closed economy is possibly related to the occurrence of thermoregulatory strategies for energy conservation. The energetic analysis of performances in the closed economy requires consideration of a variety of energetic strategies available to the species being studied.

Twenty-four-hour free-feeding patterns of dogs eating dry food.

In order to provide some detailed information on the feeding behavior of freely-feeding dogs, four adult beagles were maintained in an isolated outdoor environment where food and water were freely available. Observations across an 8-month period indicated that the dogs are episodically (in "meals") and that the total daily intake of food and water changed as the observation period progressed, most likely as a consequence of change in the ambient environmental conditions across seasons. Within-day changes in ambient temperature, and the timing of daily maintenance activities in the animal's runs influenced the temporal distribution of meals with the day.