Thermoregulatory responses in normal and cold acclimated rabbits.

Metabolic rate (M), tissue conductance (k), rectal (Trs ) and mean skin temperature (Ts ) were measured in normal (CONT) and cold acclimated (CA) rabbits after steady state for a given ambient temperature (Ta ) from 5 to 35°C. Below Ta=15°C, T rs of CONT decreased considerably, while T rs of CA was maintained within a normal range. The rate of change in Ts for each degree change in Ta (below Ta=15°Cl was smaller in CA. M, always higher in CA than in CONT at any Ta, increased more than twice the basal value (2.54 W/kg±0.06 SE for CONT and 2.85 W/kg±0.07 SE for CA at Ta=25°C) during cold exposure (Ta= 5°C). Though k was relatively constant at Ta=25-5°C, it increased considerably at Ta~35°C. The value for CA was higher than that for CONT at any Ta. Perfusion of norepinephrine (NE, 3 ,ug/kgomin for 30-min) in curarized rabbits caused bradycardia, which was more conspicuous in CONT. NE caused an increase in M (ca. 18%) in CA without any change in CONT (p<O.01). The increase in M was not limited to the duration of NE perfusion. NE did not change body temperatures except for slight rise of T;, in CA. During cold exposure, body temperatures decreased continuously in the curarized rabbits. M increased slightly during the initial 30-40 min of cold exposure in CA. The results confirmed that the rabbit acclimates to cold by enhancing nonshivering thermogenesis mediated by NE, as well as by improving insulation. INTRODUCTION Rabbits, which have been used in studies of temperature regulation, can survive in severe cold for prolonged periods. In rabbits, nonshivering thermogenesis, mediated through norepinephrine, also appears to be augmented by cold exposures, but the magnitude of the calorigenic response to norepinephrine is not profound. According to Jansky et at.,9) rabbits adapt to cold mainly by reducing heat loss from the skin. However, metabolic acclimation should be involved when the animal is exposed to more severe conditions in which homeothermy can not be maintained by insulation. In this study, rabbits were acclimated to 25°C and DOC, and their thermal and metabolic responses 7]( .tB[ ~ 7<:Received for publication July 14, 1973. 79 80 T.NAGASAKA were measured in hot and cold environments to see how they acclimate to moderate cold. The role of norepinephrine in cold acclimation was also studied. METHODS Young male New Zealand white rabbits (Oryctolagus cuniculus), with an average weight of 2.2 kg at the beginning of this study, were used. The animals, provided ad libitum with water and commercial rabbit food (CR-l. NIHON CLEA Co.). were segregated into control (CONT) and cold acclimated (CA) groups. The CONTs were kept in a temperature controlled room in which ambient temperature was maintained at 25±2°C. Relative humidity (R.H.) in the room was 70-80%. The CA, composed of six rabbits, was placed in a cold environmental chamber (O±I°C, R.H. 90-100%). Before experiment, each group was exposed to its respective environment at least four months. Food consumption and body weight were checked once every week. The average growth rate was 0.38 kg/month in CA and 0.43 kg/month in CONT. Each rabbit was trained repeatedly to rest quietly in a canvas sling prior to the tests. In the canvas sling. the animal was allowed to freely move the foreand hindlimbs without contact with the floor. All experiments were performed in winter (from January to March) from 9: 00-16 : 00 hr. No food was allowed the morning of an experimental day. In the first series of experiments, heat balance of the unanesthetized non· curarized rabbits was determined at an ambient temperature (Ta) of 5± 1, 10±1, 15±1, 25±2 and 35±2°C. At 9: 00 hr, each animal was transferred from each environment into a climate chamber in which temperature was first kept at 25±2°C. The velocity of the air, blowing from the ceiling, was approximately 1 m/sec near the animal. The animal was placed in the canvas sling and was allowed to equilibrate for at least one hour in the climate chamber before measurement. Metabolic rate was measured by a similar method described by Gonzalez et a[.5) A polythylene hood placed over the animal's head was continuously ventilated at the rate of 10-12 L/min by an air pump. The ears protruded from the hood without interfering with blood flow and heat loss from the ears. Oxygen consumption was measured with a paramagnetic O2 analyzer (Beckman E2). Heat production was calculated assuming an RQ of 0.74. 12) Respiratory heat loss was calculated from the rate of respiratory water loss of which measurement method was the same as reported previously.13) Rectal (Tre ) and skin temperatures were continuously recorded by means of copper-constantan thermocouples on an Ohkura potentiometer. The rectal thermocouple. enclosed in vinyl tubing, was inserted to a depth of 10 em. The skin temperatures were recorded from the following four sites: the dorsal surface of the ear (TeL the back (Tba), forelimb (Tit) and hindlimb (Tht). After the fur of these sites was locally removed with a COLD ACCLIMATION IN. RABBITS 81 depilatory, the skin thermocouples were placed with an adhesive tape (BANDAID, Clear Tape). Mean skin temperature (Ts) was calculated as Ts=0.73 Tba + 0.07 Til + 0.08 Thl + 0.12 Te, the formula proposed by Gonzalez et a/.) Heart rate was recorded on a Nikkor polygraph. In a single experiment, the animal was exposed to different ambient temperatures by either increasing or decreasing the chamber temperature in steps. After body temperature or oxygen consumption became constant, all data were collected in a given ambient temperature. In the second series of experiments, thermoregulatory responses to nor· epinephrine (NE) were measured in the curarized rabbits. Either CONT or CA rabbit was brought into the laboratory (of which temperature was kept at 25±2°C) and anesthetized with sodium pentobarbital (25 mg/kg, Lv.). Following anesthesia, a tracheotomy was performed for artificial respiration. A small polyethylene catheter was inserted into the marginal vein of the ear for infusions. After the surgical procedures, the animal was placed in the canvas sling and transferred into the climate chamber, in which temperature was also kept at 25 ± 2°C. One hour after anesthesia, the animal was paralyzed with gallamine triethiodide (1 mg/kg, every 20-min) and resuscitated. After 20,minutes resuscitation period under gallamine triethiodide, 3 pg/kg·min of NE, diluted with normal saline, was perfused intravenously for 30 minutes with an infusion pump (ATOM-LG 2). The volume of the NE solution perfused was 0.17 ml per minute. Oxygen consumption, heart rate and Ts and Tre were also measured every 10 minutes during the experiments. After 150 minutes at To of 25°C, the animals were exposed to an air temperature of 5± 1°C for 70 minutes. Changes in oxygen consumption and Tre were also measured during this cold exposure. RESULTS Body temperatures. Tre and Ts under steady state conditions were plotted as a function of To in Fig. 1. In CONT, Tre, which was 38.94°C±0.1l SE at Ta=25°C, fell considerably in the cold environments. At 5°C, Tre was 37.39°C ±O.18 SE. In CA, Tre was regulated within the range of 38.6-38.3°C at T a of 5-25°C. Tre increased significantly in hot environments in both CONT and CA (p<O.OI). In the 15-35°C range of Ta, Tre of CONT was significantly higher (P<0.05) than that of CA. In CONT, Ts decreased linearly as Tawas decreased. The rate of change in Ts for each degree change in T a was O.35°C. In CA, however, this rate decreased to 0.26°C in the cold environments. Heat production. The metabolic rate (M) of rabbits in conditions tinder present investigation are shown in Fig. 2. M, which was minimum (2.54 W/ kg±O.06 SE for CONT and 2.85 W/kg±O.07 SE for CA) at To=25°C, increased steadily with reduction of To from 25°C to 5°C. The average metabolic rate