Physiology & Behavior, Vol. 48, pp. 327-331. ©Pergamon Press plc, 1990. Printed in the U.S.A.

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Contribution of Spontaneous Activity to Daily Energy Expenditure of Adult Obese and Lean Zucker Rats 1 R I C H A R D E. K E E S E Y , A R T U R H. S W I E R G I E L A N D S T E P H E N W. C O R B E T 1 ~

Department o f Psychology, University o f Wisconsin, 1202 West Johnson Street, Madison, WI 53706 R e c e i v e d 11 D e c e m b e r 1989

KEESEY, R. E., A. H. SWIERGIEL AND S. W. CORBETT. Contribution of spontaneous activity to daily energy expenditure of adult obese and lean Zucker rats. PHYSIOL BEHAV 48(2) 327-331, 1990.--The contribution of somatomotor activity to daily energy expenditure was estimated in 10-month-old, weight-stable, obese (fa/fa) and lean (Fa/?) Zucker rats. Total and resting heat production were assessed by recording oxygen consumption and stabilimeter activity each minute for five consecutive days in free-feeding animals. The number of activity counts, as well as their circadian pattern of occurrence, were highly similar in lean and obese groups. Likewise, the percentage of daily energy expenditure committed by the obese rats to activity was nearly identical to that of leans (19.3 vs. 19.7%, respectively). Observing these rats for one additional day under postabsorptive conditions produced similar estimates of their daily expenditure on activity. Thus, unlike prior estimates based on wheel-running behavior, continuous measurement of stabilimeter behavior indicates that both the proportion of daily expenditure on activity by fa/fa rats and its temporal pattern of occurrence are notably normal. Accordingly, it is concluded that maintenance of obesity in weight-stable adult obese Zucker rats does not depend upon a reduced expenditure on activity. Zucker

Obesity

Activity

Energy expenditure

Heat production

ENERGY expended on somatomotor activity represents a significant portion of the dally energy budget. Morrison (12) estimated that spontaneous activity could account for as much as 25% of total energy expenditure in adult rats. It should be recognized that a significant proportion of spontaneous activity represents a nonobligatory expenditure of energy. Thus, changes in spontaneous activity may be involved in the maintenance of energy balance, serving as effectors for compensatory adjustments in expenditure in response to fluctuations in energy intake (17,19). It has often been suggested that reduced expenditure on activity contributes both to the development (7) and maintenance (2,16) of certain forms of obesity. Young ob/ob mice display low levels of activity during the development of their characteristic obesity (4). Even as adults ob/ob mice manifest reduced activity (4), thus raising the possibility that their obesity is at least in part maintained by the energy savings thereby realized (18). Likewise, reduced expenditure on activity is thought to contribute to the maintenance of obesity in the widely-studied Zucker fatty (fa/fa) rat. The Zucker fatty's running wheel activity, for example, is apparently lower than that seen in lean littermates (6,16). A weakness in the argument linking reduced expenditure on activity to obesity is that it is not the energetic cost of activity that prior investigators have measured. Rather, with occasional exception (4), what has been assessed is the frequency and/or pattern of

the activity itself. If the energetic cost of the measured behavior is the same for lean and obese animals, this procedure is acceptable; if not, the actual energy expended on that activity should be the primary issue of concern. The study reported here examines both the activity and activityrelated heat production of adult obese and lean Zucker rats. Older, weight-stable animals were observed in order to focus attention on the maintenance rather than development of obesity in rats. For five consecutive days, minute by minute, stabilimeter activity and concurrent total and resting heat production were assessed in free-feeding animals. One additional day of testing was conducted in the same rats under postabsorptive conditions. Estimates were then made of how total dally energy expenditure was partitioned between resting- and activity-related components. METHOD Subjects were 8 lean (Fa/?) and 7 obese (fa/fa) male Zucker rats, 10-11 months of age at the time of testing. Initially, 8 obese rats were used, but one animal did not maintain its body weight during the period of respirometer testing and its data are not included. Rats were adapted to living in the 4.0 1 Plexiglas cylindrical respirometer chambers for at least five days before the reported measurements were taken. The chambers resided in a

~Supported in part by NIH Grant AM-19944.

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328 sound-proofed room with a 12:12 light-dark schedule. Ambient temperature was maintained at 27°C, a value within the thermoneutral zone for rats. Animals in the chambers had free access to water. Measurements of oxygen consumption and activity were taken over consecutive 23-hr 20-min long periods. Forty minutes daily were devoted to cleaning of the chambers and resupplying feed and water. The animals had unrestricted access to powdered Wayne laboratory chow with a gross energy content of 4.08 kcal/g (determined by bomb calorimetry) for the first five days. Food intake, corrected for spillage, was measured daily. Midway through the 6th day, food was removed from the chambers. The rats remained in their chambers, although oxygen consumption and activity were not recorded. Recording of oxygen consumption and activity were resumed on the 7th day, for 23 hr 20 min during which time the animals remained food deprived.

Total Heat Production Total whole-body heat production was determined by measuring oxygen consumption by an open-circuit respirometric method. Dried air was moved through the chambers at the rate of 900 ml/min. After exiting the chamber the air was again dried and its oxygen content measured every minute (Applied Electrochemistry S-3A). Knowledge of the rate of air flow through the chamber, measured by a mass flow meter, and of the oxygen concentration in the air both entering and exiting the chamber, allowed each rat's oxygen consumption to be calculated each minute. Total heat production (kcal/day) was then estimated by taking the heat equivalent of oxygen to be 4.85 kcal for each liter of oxygen consumed. This total heat production value was composed of heat production when animals were at rest (whether in absorptive or postabsorptive state) and heat production resulting from motor activity. It did not include thermoregulatory heat production since testing was conducted at a temperature (27°C) presumed to be thermoneutral for rats.

KEESEY, SWIERGIEL AND CORBETT

consumption scores fulfilling these criteria in each 23-hr 20-min long test period were located by a computer-assisted search. The mean of all such values was taken to represent the resting rate of heat production.

Assessment of Heat Production in Fasting Animals Applying the above described methods heat production was also measured for one day in the fasting rats. This permitted heat production to be assessed in the absence of heat of nutrient metabolism or diet-induced thermogenesis that could differ between the lean and obese rats.

Estimation of Activity Related Heat Production The difference between total heat production and resting heat production in each 23-hr 20-min period provided an estimate of the energy expended on spontaneous motor activity. Activity-related heat production was determined by this procedure both in the presence and the absence of heat of nutrient metabolism.

Statistics Intraindividual day-to-day coeffficients of variation in repeated measurements over five consecutive days were low for all rats (the range was 0.56% and 3.20% for oxygen consumption, and 3.84% and 28.19% for activity). Results from the free-feeding phase of the experiment are presented as values averaged over the 5-day period. Comparisons of mean differences between groups were made with Student's unpaired t-test. RESULTS

Body Weight and Food Intake

The chambers rested on four compression springs, with a linear velocity transducer (Trans-Tek) positioned on the chamber lid. Vertical movement of the chamber caused by the animal's movements energized the transducer. This system recorded all movements that involved displacement of the rat's center of gravity, though it did not distinguish between specific activities such as walking, feeding, grooming, etc. Sensitivity of the system was set to register all discernible movements of rat except those associated with breathing. With this arrangement, both the amount and temporal pattern of activity could be specified. Cumulative 1 minute activity counts (A~) and oxygen consumption values (O1) were recorded every minute using a program prepared for an Apple microcomputer.

Both lean and obese rats displayed stable body weights over the days of respirometer testing. The mean weight of the eight lean rats was 490 g at the beginning of the five-day period of testing in the fed rats and 488 g at the end. The 7 obese rats weighed an average of 727 and 723 g at the beginning and end of the same period, respectively. The absence of a significant net change in body mass suggests that energy intake and expenditure were in balance during the 5 days of measurement. During fasting the lean rats lost 24---2 g (4.9%) of their prefasting body weight, and the obese rats lost 25 - 2 g (3.5%) of their body weight. Over the initial 5 days of measurement, the lean rats consumed 16.8 g of lab chow per day and the obese rats consumed 20.2 g of laboratory chow per day. Since the caloric content of the chow was determined to be 4.08 kcal/g, the total feed energy intake in the lean and obese rats was 68.5 and 82.4 kcal/day, respectively. This difference was statistically significant, t(13)=4.658, p

Contribution of spontaneous activity to daily energy expenditure of adult obese and lean Zucker rats.

The contribution of somatomotor activity to daily energy expenditure was estimated in 10-month-old, weight-stable, obese (fa/fa) and lean (Fa/?) Zucke...
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