Seasonal changes in metabolic capacity and norepinephrine thermogenesis in the alaskan red-backed vole: Environmental cues and annual differences

SEASONAL CHANGES IN METABOLIC CAPACITY AND NOREPINEPHRINE THERMOGENESIS IN THE ALASKAN RED-BACKED VOLE: ENVIRONMENTAL CUES AND ANNUAL DIFFERENCES Institute

DALE D. FEIST and PETER R. MORRISON of Arctic Biology, University of Alaska, Fairbanks, Alaska 99701, U.S.A. (Receiced

18 December

1980)

Abstract-l. Wild red-backed voles (Clethrionomys rutilus) were tested for maximum metabolic rate (M,,,) and for metabolic response to norepinephrine (MNE) in September, November and January. During the same period, voles born and raised in the laboratory were acclimated (for 3.5 months) in the following groups: (I) +20 C and 24 hr light daily (LD 24:O): (2) +20 C and LD 4:20: (3) gradual change from +5 C and LD 14: 10 to - 5 C and LD 4:20; (4) gradual change from +5 C to - 5 C and continuous LD 4:20. and tested for M,,,. 2. During acclimatization of wild voles from September to January M,,,, increased 42”,, to Oz.g-‘.hrm’. Peak wjinter 23.05 + 1.19ml 02.g-‘~hr-’ and MN,, increased 59”,, to 14.12 f.0.86ml M “>.I” and MNB in these wild voles were lower than found in a previous winter. 3. In voles acclimated to +20 C and either long or short daily light period M,,,,,, remained unchanged. In voles acclimated to increasing cold and decreasing or short light period M,,, increased 39% to 20.0ml 02.g-‘.hr-‘. 4. The results indicate that the magnitude of seasonal change of M,,, and MNE may vary from year to year and suggest that cold is essential to stimulate these seasonal changes in red-backed voles.

INTRODUCTION Seasonal increases in maximum metabolism (M,,,) and norepinephrine stimulated nonshivering thermogenesis (NE-NST) during acclimatization to winter have been reported for several wild rodents (Lynch, 1973; Rosenmann, et ~1.. 1975: Feist & Rosenmann, 1976). Laboratory experiments in which rodents were exposed to different photoperiods and temperatures suggest that these seasonal changes in thermogenit capacity may be induced or mediated by changes in photoperiod and/or temperature (Lynch 1973; Heldmaier & Hoffman, 1974; Lynch et al., 1978; Hagelstein & Folk, 1978; Heldmaier & Steinlechner, 1980). For example, golden hamsters and Djungarian hamsters exhibit an increase in the thermogenic tissue brown fat during exposure to a short daily photoperiod (LD 8: 16) (Hoffman et al., 1965; Heldmaier & Hoffman, 1974). Djungarian hamsters increase NE-NST during exposure to a short day (LD 8: 16) or to natural photoperiod in the fall (Heldmaier & Steinlechner, 1980). Cold plus natural photoperiod further increases NE-NST in the Djungarian hamster. Whitefooted mice exposed to a short day photoperiod (LD 9: 15) at 23°C exhibit greater NST compared to those on a long day photoperiod (Lynch et al., 1978). In contrast, short daily photoperiod alone does not induce changes in NE-NST in the white rat (Hagelstein & Folk, 1978). Cold (5-8-C) is necessary to induce increased NE-NST in these rats. In earlier studies we showed that M,,, and NE-NST increase remarkably in Alaskan red-backed voles from summer to winter (Rosenmann et (II., 1975; Feist & Rosenmann, 1976). In the present study we have examined the effects of photoperiod length 697

and temperature on M,,, in the Alaskan red-backed vole and have compared responses of laboratory acclimated voles with changes in M,,, and NE-NST found in wild voles during fall and winter. The objectives of the present study were (1) to assess environmental factors which may trigger or mediate seasonal in red-backed voles, changes in M,,, and NE-NST and (2) to determine the time course for changes in during acclimatization from sumM maxand NE-NST mer to winter.

MATERIALS AND METHODS Wild Alaskan red-backed voles (Clethrionomys rutilus dawsoni) were taken from typical taiga forest of mixed birch and spruce during late September. mid-October and mid-November of 1976 and early January 1977. Voles were caught in Sherman live traps and transported to the laboratory for testing of M,,, or NE-NST on the same morning of capture (Feist & Rosenmann. 1976). Red-backed voles born and raised in the laboratory were housed individually in cages with wood chips and soft tissue paper for bedding on a daily light period of 12 hr (LD 12: 12) at 20-C. They were fed oats, pelleted commercial mouse chow. lettuce, carrots and water ad /i&turn. These voles were subdivided into 4 groups and maintained m chambers with controlled environmental conditions as follows for 3.5 months (15 wk) from September to early January: (1) +ZO”C and 24 hr light daily (LD 24:O); (2) t2O”C and LD 4:20; (3) gradual change from +5’C and LD 14: IO to -5°C and LD 4:20; and (4) gradual change from +5’C to -5°C and continuous LD 4:20. Daily light period (which included 1 hr of twilight before and 1 hr after daylight hr) and/or temperature were adjusted weekly for groups (3) and (4) to roughly simulate average seasonal outdoor ambient conditions at 65”N latitude. Voles were

DALE D. FEIST and PETER R. MORRISON

698 Table

I. Effect of temperature

and photoperiod

Group

on maximum

metabolism

September

(M,,,)

in red-backed

voles January

November

-I.

t20 c LD 24:0

2.

+20 c LD 4:20

3.

+5to -5c LD 14:lO to LD 4:20 +5 to -5 c LD 4:20

4.

M,,,, ml MeP Body wt. M m**, ml Met Body wt. M ma.r ml Met Body wt, M rn”X.ml Met Body wt.

O,/g’hr g O,/g.hr g O,/g.hr g OZ/g’hr g

14.40 k 0.64(12) 8.84 24.1 k 1.9 14.40 + 0.64 (12)” 8.84 24.1 + 1.9 14.40 + 0.64(12) 8.84 24.1 + 1.9 14.40 + 0.64 (12)” 8.84 24.1 + 1.9

13.63 + 0.86(9) 8.56 28.0 f 2.6 12.93 k 0.49 (9) 8.38 28.5 k 1.9 16.67 + 0.59(11)* 10.41** 25.5 + 1.9 17.99 * 0.75 (lo)** 10.91** 23.1 f 1.5

13.21 * 0.82 (5) 8.54 30.0 * 2.3 14.11 + 0.71 (IO) 8.82 25.9 i 2.1 18.83 i 0.41 (9)* 11.22* 20.8 ?_ 0.9* 19.98 k 0.47 (7) Ii.40 17.7 + 0.x*

“Mm,, for a sample of 12 voles taken from all voles prior to separation into the 4 groups. Sample size given in brackets. bMet. multinle of standard metabolic rate f3.8 ml O2 (9-O “. hr-‘I]: Hart (1971); Rosenmann & Morrison (1974). *P < 0.05. **P < 0.01. November different from September: January different from November.

tested for M,,, in late September, mid-November and early January. Maximum metabolic rate was determined by measuring oxygen consumption with a closed-circuit automatic manometric system (Morrison, 1951). Values were taken first in air to define ordinary resting metabolic rate and in a mixture of 20% O2 in He, to define M,,, as described earlier (Rosenmann & Morrison, 1974; Rosenmann ef al., 1975) using low ambient temperatures of 0 to - 10°C. The crtterion for M,,, was 6-l 1 min of the highest metabolism followed by a decline. Noreprinephrine stimulated nonshivering thermogenesis was estimated by measuring oxygen consumption with a closed-circuit automatic manometric system at + 15°C (also at + 10 and +25”C in some cases) before and after subcutaneous injection of 2 mg NE per kg body weight in a volume of 0.30 ml per 25 g body weight (Feist & Rosenmann, 1976). Resting metabolism (RMR) at these temperatures was calculated from 3 minimum values or about 12-18 min of minimum values during 1 hr before injection with NE or saline. Control animals were injected with saline instead of NE. NE-NST was expressed as the metabolic rate after NE(MNE). M,, was calculated by subtracting the increase in 0, consumption above RMR after saline injection from the total O2 consumption after NE injection [i.e. M,, = RMR + response to NE above RMR (Feist & Rosenmann. 1976)]. Values are expressed as the mean or the mean k SEM. The statistical significance of the difference between mean values was determined by the student’s t-test (Simpson et al., 1960).

Table

2. Changes

ml 02/g. wt, g ml 0*/g. wt, g

hr

hr

Eflcts

16.23 f 0.61 (21) 9.51 17.7 + 0.5 8.87 + 0.52(18) 5.00 17.0 + 0.09

of temperature and light on M,,,

In late September, immediately before division of voles into separate groups, a sample of 12 animals was tested for M,,,. As shown in Table 1, this value of 14.40ml O2 g-“hr-’ was used as the initial M,,,,, for all 4 groups. The M,,, of both long and short daylight groups exposed to +2o”C (Table 1) remained unchanged throughout the period from September to January. By mid-November, M,,, for both cold exposed groups was significantly increased above September values. M,,, was apparently further increased in both cold exposed by early January but this was statistically significant only for group 3. Mean body weight (Table 1) of cold exposed groups was lower in January than in September or November. As an approach to normalizing for body weight M,,, was calculated as Met [multiple of standard metabolic rate (SMR); SMR = 3.8 ml O2 (g- o.27. hr)- ’ ; Rosenmann & Morrison, 1974; Hart, 19711. As shown in Table 1, M,,, expressed as Met also increased significantly in both cold exposed groups. Seasonal changes in M,,,

and M,,

As shown in Table 2, M,,, expressed as ml O2 g- ‘. hr- ’ increased in wild voles from September to

in maximum metabolism (M,,,) and during seasonal acclimatization September

M,.,: Met” Body MNe: Met Body

RESULTS

(MNfi)

metabolic response to norepinephrine in red-backed voles

9.74 + 0.48 (3) 5.66 19.0 * 1.9

’ Met, multiple of standard metabolic rate (see Table 1). *P < 0.05, **P < 0.01, ***P < 0.01. November different November. Sample size given in brackets.

January

November

October

20.47 + 0.50*** (13) G.30*** 15.8 k 0.67 12.32 + 1.17** (8) 6.86** 16.2 + 0.71

from

September;

January

23.05 + 1.19* (9) 12.29 13.3 + 0.61 14.12 + 0.86 (6) 7.65 14.7 li: 0.82

different

from

Environmental

cues for thermogenesis

November and from November to January. However, body weight declined significantly from September to January. Expressed as Met, M,,, increased from September to November but not significantly from November to January. Comparison of M,,, between cold exposed laboratory groups (Table 1) and wild voles (Table 2) indicates that M,,,, when expressed as ml OZ. g- ‘. hr- ‘, was higher in wild voles from September to January. However, when expressed as Met, the M,,, was similar in cold exposed laboratory voles and wild voles from September to January. Wild voles exhibited an increase in MNt (Table 2) from September to November. The January MNE was not significantly increased above that in November. DISCUSSION

The results indicate that cold is an essential stimulus for seasonal changes in thermogenic capacity in red-backed voles. Decreased daily light period may play an important role in acclimatization (e.g. to prime or to facilitate response to cold) but apparently does not induce changes in thermogenesis in these subarctic voles. Hagelstein & Folk (1978) reported similar findings for white rats (Rattus noroegicus): short daily photoperiod (LD 9: 15) alone did not effect any metabolic changes in rats but short day and cold (8°C) combined to elicit increased NST. Although in the present study we did not measure M,, in the laboratory acclimated voles. our previous studies (Feist & Rosenmann. 1976) suggest that M,, changes proportionally with M,,, during cold acclimation or acclimatization to winter. Thus it is reasonable to assume changes in M,,,,, in the present study reflect similar changes in M,. In contrast to the responses of voles and rats to short day and low temperature, golden hamsters (Mesocricetus auratus) and Djungarian hamsters (Phodopus sungorus) increased brown fat during exposure to a short daily photoperiod (LD 8:16) without cold (23°C) and Djungarian hamsters and white-footed mice (Permyscus leucopus) increased

Months of

in voles

NE-NST

699

under similar conditions (Hoffman et ui., 1965: Heldmaier & Hoffman. 1974: Lynch rt L/I.. 1978; Heldmaier & Steinlechner. 1980). Djungarian hamsters also increased NE-NST when exposed to natural photoperiod during fall and winter but without cold (23 C) (Heldmaier & Steinlechner. 1980). Whether redbacked voles would respond similarly to natural photoperiod remains to be shown. It is possible that the short day (LD 4:20) exposure at 2O’C lacked some feature of natural photoperiod and thus did not provide an adequate stimulus even during the fall and winter when we might expect the voles to be especially sensitive. Nevertheless voles did not respond to short day conditions while hamsters did. Thus, for voles shortening day length may not be as important for metabolic acclimatization as in some other rodents. The pattern of change in M,,, and MNE in redbacked voles during acclimatization to winter differs from the pattern previously suggested for certain. other wild voles and mice (Didow & Hayward, 1969; Lynch. 1973). Seasonal M,,, and M,, in red-backed voles (Fig. 1) increased to the highest levels by the coldest period in early January. The annual change in interscapular brown fat mass in Alaskan red-backed voles has been shown to follow a similar pattern during acclimatization to winter (Fig. 1; Sealander. 1972). Evidence for the seasonal pattern of changes in brown fat and NE-NST in other wild voles and mice (Didow & Hayward, 1969; Lynch, 1973) suggested a steep rise in the capacity for thermogenesis in fall to a peak in fall or early winter. For example, Lynch (1973) found in white-footed mice (Perompscus leucopus) in Iowa (USA) that the seasonal maximum in NE-NST occurred by September without any change during subsequent winter despite a continual decrease in ambient temperature. Changes in the thermogenic capacity during acclimatization of red-backed voles to winter do not occur in a single step early in the season but appear to be correlated to ambient temperatures (Figs 1 and 2). The magnitude of the change in M,,, and M,w,

YeOr

Fig. 1. Seasonal changes in maximum metabolism (M,,.) and metabolic response to NE(M,W,). Solid diamonds give M,,, and solid circles give M,, for 1973-74. Open diamonds give M,,, and open circles give M,, for 1976-77. Values given as mean f SEM. Dashed line shows seasonal changes in brown fat (Sealander, 1972). Continuous line shows AT = TB - 7” for 20 yr averages for daily minimum TA by months at Fairbanks. TB is body temperature and TA is outdoor ambient temperature.

DALE D. FEIST and PETER R. MORRISON

700

Health Research Grant ES-00689 from the Institute of Environmental Health Sciences and by Grant GM-10402 from the Institute of General Medical Sciences, United States Health Service, Department of Health, Education and Welfare. -

6

. act

: 5

REFERENCES

. Sept DI~W

\ -40

Fig. 2. of NE as the month. legend method

-30 -20 -10 TEMPERATURE FCl (20 yr mean daily minimum)

0

Relationship between metabolic rate after injection (MNE) and outdoor ambient temperature expressed 20yr mean daily minimum temperature for each Met equals multiple of standard metabolic rate (see Table 1). The regression line calculated by the of least squares has a slope of -0.096 and a correlation coefficient of 0.99.

from fall to winter and the highest M,,, and MNE achieved in winter were lower in red-backed voles in the present study than in voles tested in an earlier winter (Rosenmann et al., 1975; Feist & Rosenmann, 1976). As shown in Fig. 1, M,,, and M,, reached higher levels in the winter of 1973 than in the winter of 1976 (M,,,73 vs M,,, 76, P < 0.01; M,, 73 vs MNE 76, P = 0.05). Body weights of animals tested were similar for both years. Year to year differences in thermogenic capacity may be related to factors such as food supply and substrate (e.g. glucose or fatty acids) availability (Wang, 1980), and ambient temperature of exposure during early growth and development in spring and summer (Lynch et al., 1976; Lacy et al., 1978) or later during fall and winter. Wang (1978, 1980) recently found that Sprague-Dawley rats fasted overnight exhibited a lower M,,, in the cold than normally fed rats. He also found that overnight fasted rats given a mixture of substrates (intragastrically) 1 hr before testing exhibited the highest M max. He concluded that substrate availability limits thermogenesis in severe cold. Whether this applies to red-backed voles and accounts for year to year differences in M,,,remains to be shown. The Lynchs and their coworkers (Lynch et al., 1976; Lacy et al., 1978) found that house mice exposed to cold (SOC) during the first month of life exhibited a greater brown fat mass at 2 months of age than did mice exposed to cold during the second month of life. They also found that rearing mice at 5°C from birth to 3 months of age resulted in permanent increases in NST and brown fat weight. Experiments on red-backed voles to determine the influence of cold exposure during early life on the thermogenic capacity in later life could help explain year to year differences in thermogenic capacity. Acknowledyements~We would like to thank Joanne Groves and Andre& Porchet for technical assistance. This research was supported in part by National Institutes of

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