Diurnal patterns of rats determined by calorimetry under controlled conditions

The metabolic basis of dual periodicity of feeding in rats

This article examines how the depletion and replenishment of various energy stores give rise to periodic eating and how constant body-energy levels are maintained over time.

Measures of the energy expended throughout the 24-hour feeding pattern in rats indicate that two different energy stores (one of small capacity and one of large) determine two superimposed feeding periodicities: one from meal to meal (prandial), the other from day to night (nycthemeral). The article reviews how experimental overrepletion or overdepletion of gastrointestinal content, blood glucose, or body fats affect food intake. These data suggest that gastrointestinal content determines both meal size and meal-to-meal periodicity. Other evidence indicates that glucose uptake rate in tissues, which is modulated by fat synthesis and fat mobilization, affects the periodic onset of feeding and the difference between nocturnal and diurnal postprandial satiety.

There follows an examination of the neuroendocrine bases for the interacting mechanisms governing energy input and output balance and of the role of the ventromedial hypothalamus in body-fat regulation and the lateral hypothalamus in feeding.

Energy cost and thermoregulation of unrestrained rats during exercise in the cold

The effect of cold ambient temperature on the energy cost and thermoregulation during exercise was studied in 18 male Sprague-Dawley rats accustomed to running for 2 weeks. For measuring T(b) and ensuring an unrestrained exercise performance, the rats were implanted with intraperitoneal telemetric radio transmitters. The rats ran at three submaximal exercise intensities (10, 15 and 20 m.min-1) at different T(a) values (22 degrees, 0 degree, -10 degrees and -20 degrees C) for 30 min. VO2 and Tb were continuously measured. During exercise at 0 degree C a 100% substitution of thermoregulatory heat production by exercise thermogenesis was observed. At T(a) values below 0 degree C the physical strain of exercise increased considerably: Vo2 increased 34-61% at -10 degrees C and 51% at -20 degrees C compared to the corresponding exercise intensity at T(a) 22 degrees C. A partial substitution of thermoregulatory heat production by exercise thermogenesis was observed in the cold. At -10 degrees C the cost for thermoregulation during exercise was 28% and at -20 degrees C 31% lower than at rest at the corresponding T(a) values. The rat could not maintain normal Tb during exercise below 0 degrees C. The T(b) dropped 1.5-1.8 degrees C at -10 degrees C and 2.5 degrees C at -20 degrees C. It is concluded that exercising below 0 degree C at submaximal levels (below 50% of VO2max) partially substitutes for thermoregulatory costs but is thermally unfavorable for the small-sized rat.

Cold tolerance and metabolic heat production in male C57BL/6J mice at different times of day

Nine-month-old male C57BL/6J mice were subjected to three-hour cold stress tests (partial restraint at 6 degrees C) at 9:00 a.m. or at 1:00 p.m. Tests were repeated three times at two-week intervals at the same time of day. Body temperature was measured by colonic thermoprobe, and metabolic heat production was measured by indirect calorimetry during each test. All mice showed habituation to repeated cold exposures (an improvement of cold tolerance across tests) due to an increase in metabolic heat production. The levels of metabolic heat production were similar during morning and afternoon testing; however, mice tested in the afternoon had consistently poorer cold tolerance, which indicated increased heat loss. Increased heat loss in mice of similar body weight and presumably similar body composition, suggests that there is less effective cold-induced skin vasoconstriction during the afternoon. We hypothesize that the compromised skin vasomotor response during the afternoon cold exposure results from competing effects of vasodilation due to local autoregulation stimulated by a circadian reduction of cardiac output during the sleep phase, and vasoconstriction due to the cold stress.

Thermal biology of the laboratory rat

The purpose of this paper is to thoroughly review the literature and present a data base of the basic thermoregulatory parameters of the laboratory rat. This review surveys the pertinent papers dealing with various aspects of the thermal biology of the laboratory rat, including: metabolism, thermoneutrality, core and brain temperature, thermal tolerance, thermal conductance and insulation, thermoregulatory effectors (i.e., thermogenesis, peripheral vasomotor tone, evaporation, and behavior), thermal acclimation, growth and reproduction, ontogeny, aging, motor activity and exercise, circadian rhythm and sleep, gender differences, and other parameters. It is shown that many facets of the thermoregulatory system of the laboratory rat are typical to that of most homeothermic species. However, is several instances the rat exhibits unique thermoregulatory responses which are not comparable to other species.

Interaction of diet and toxicity--the future role of purified diet in toxicological research

There is a growing awareness of the importance of diet as a determinant of the toxicity of many compounds. This paper briefly reviews some of the ways in which diet affects toxicity, and draws some conclusions regarding the ideal diet for toxicological investigations. Then the two competing types of diet, stock and purified (including those frequently called semi-synthetic and semi-purified), are described and their characteristic strengths and weaknesses are discussed. Stock diets are very variable commodities, may be nutritionally poorly balanced, and also contain many non-nutritive components that influence toxicity. The formulation and preparation of purified diet are discussed. It is concluded that investigations of the mechanisms of toxicity, as well as studies of absorption, distribution and metabolism of toxic compounds, could benefit from the use of well-defined purified diets.