Stearic acid oxidation in the Alaskan red-backed vole: effects of cold and norepinephrine.

Saturated fatty acids constitute a considerable energy reserve that could convey survival value under chronic cold exposure. It was investigated whether acclimation to cold was associated with a change in the ability of a small microtine rodent, the red-backed vole (Clethrionomys rutilis), to increase the use of saturated fatty acids during thermogenesis. The C-14 labeled stearic acid, a typical saturated fatty acid (FA), was used to determine the rate of utilization through oxidation. In warm acclimated (WA, 20 degrees C) and cold acclimated (CA, 5 degrees C) voles, acute cold exposure (WA at 5 degrees C, -2 degrees; CA at -5 degrees C) and norepinephrine (NE) injection increased metabolic rate (VO2, VCO2) and mobilization of lipid reserves. Acute cold exposure increased percent blood stearate oxidized in a linear fashion with both metabolic rate and stearate concentration in both WA and CA voles. The CA voles at 5 degrees C had increased stearate irreversible loss (1.5x) and oxidation rate (2x) compared to WA voles at 20 degrees C. The CA voles at -5 degrees C increased stearate irreversible loss and oxidation linearly with metabolic rate and blood stearate concentration. In CA voles the contribution made by stearate oxidation to VCO2, increased with the level of cold exposure and NE injection. In contrast, for WA voles stearate irreversible loss and the contribution made by stearate oxidation to VCO2 were unaffected by acute cold exposure. Thus, cold acclimation involves: (1) a modest increase (1.5x) in the use of stearate as a fuel substrate; and (2) a greater stearate oxidation and contribution to overall metabolism during acute cold exposure (-5 degrees C).

Thermoregulation, growth, and reproduction in Alaskan collared lemmings: role of short day and cold.

To assess factors controlling seasonal thermoregulatory and reproductive changes, collared lemmings (Dicrostonyx groenlandicus) were exposed for 16 wk to long day (LD, 22 h light: 2 h dark) and warm (15 +/- 3 degrees C), LD and cold (1 +/- 0.5 degrees C), short day (SD, 4 h light: 20 h dark) and warm, SD and cold or acclimatized to outdoor winter conditions (OUT). Hair length and color, body mass, and food intake were monitored weekly. Resting metabolic rates (RMR) and nonshivering thermogenesis (NST) were estimated several times by measuring oxygen consumption before and after norepinephrine injections. Body composition and reproductive condition were determined at the end of the experiment. SD and OUT groups had a 15.8% lower (P less than 0.01) RMR at 7 degrees C than the LD groups. Lower thermal conductance in SD and OUT animals appears due to molt to white winter pelage, which occurred by week 3 in SD but not in LD groups. Neither SD, cold, nor OUT altered NST or reproductive morphology. SD-exposed lemmings showed 19.2% greater growth than those in LD, resulting primarily from a 29.2 and 15.0% increase in lean and ash components, respectively. Cold exposure increased food intake by 34.7%. Results suggest that the pineal gland, which mediates SD effects, may influence molt and growth but not NST or reproductive morphology.

Effects of cold, short day and melatonin on thermogenesis, body weight and reproductive organs in Alaskan red-backed voles.

This study examined whether cold, short day or melatonin causes reproductive regression and stimulates nonshivering thermogenesis in a subarctic rodent Clethrionomys rutilus. Red-backed voles born and raised at 23 degrees C and 22 h light per day (LD 22:2) at Fairbanks, Alaska (65 degrees N) were exposed in one of six groups to: 1) long day (LD 22:2), 23 degrees C, injected daily with melatonin or saline 2 h before lights out, 2) long day, 3 degrees C, injected daily with melatonin or saline, 3) short day (LD 8:16), 23 degrees C or 3 degrees C. Voles were tested for nonshivering thermogenesis (NST) prior to and after 8 wk exposure. Body weight, testes weight and female reproductive tract weight were assessed after 8 wk in long day and 12 wk in short day. NST was not altered by short day or melatonin but cold (3 degrees C) caused an increase in NST which was similar in long day and short day. Body weight of males and females was not affected by short day but was decreased by melatonin. Short day did not alter mean testes weight (about 20% voles regressed) but reduced mean female reproductive tract weight (more than 40% voles regressed). Melatonin reduced testes weight and female reproductive tract weight (more than 50% of voles of both sexes regressed). The results suggest that in northern red-backed voles: the pineal does not mediate seasonal changes in thermogenic capacity, the pineal may mediate reduction of body weight and regression of reproductive organs but, in addition to day-length, other cues or factors may be important, populations may exhibit variability in sensitivity of reproduction to photoperiod which could allow for opportunistic breeding.

Increased beta-adrenergic receptors in brown fat of winter-acclimatized Alaskan voles.

To assess a possible mechanism for the enhanced thermogenesis of cold-acclimated and winter-acclimatized red-backed voles (Clethrionomys rutilus), beta-adrenergic receptors of brown fat were characterized by specific binding of (-)-[3H]-dihydroalprenolol [( 3H]DHA) to isolated brown fat membranes from 23 degrees C-acclimated controls, cold-acclimated (5 wk or 5 mo at 5 degrees C), wild summer, and winter-acclimatized voles. Scatchard analysis to determine the equilibrium dissociation constant (Kd) and the maximum number of binding sites (Bmax) for control brown fat membranes gave a Kd of 4.45 nM [3H]DHA and Bmax of 249 fmol [3H]DHA bound per milligram of protein. beta-Adrenergic agonists competed for specific binding sites with an order of potency typical of the beta 1 subtype of adrenergic receptors: (-)-isoproterenol greater than (-)-norepinephrine greater than or equal to (-)-epinephrine. After cold acclimation for 5 wk or 5 mo, the Kd and Bmax for adrenergic binding sites were similar to those of controls. Brown fat mass was 1.5 times greater than that of controls after 5 wk cold acclimation but similar to controls after 5 mo cold acclimation. Winter voles had 1.7 times higher Bmax and 1.6 times more brown fat than summer voles. Thus seasonal acclimatization to winter in red-backed voles appears to involve an increase in beta-adrenergic receptors in brown fat, but cold acclimation does not. The results suggest quantitative and possibly qualitative differences in neural and hormonal stimulation of brown fat between cold acclimation and winter acclimatization in voles.

Catecholamine-synthesizing enzymes in adrenals of seasonally acclimatized voles.

Tyrosine hydroxylase (TH) and phenylethanolamine-N-methyltransferase (PNMT) activities were assayed in adrenal glands of the following groups of the Alaskan red-backed vole (Clethrionomys rutilus dawsoni): 1) laboratory reared at 20 degrees C and 2) exposed to 5 degrees C for 1, 3, 7, and 28 days; 3) wild, summer acclimatized; 4) wild, fall acclimatized; and 5) wild, winter acclimatized. TH activity in laboratory-acclimated voles exposed to 5 degrees C was increased by 2 times after 3 days and remained elevated after 28 days. PNMT activity in these same voles was increased after 7 days and also remained elevated after 28 days of cold exposure. In wild-acclimatized voles TH activity and PNMT activity in summer were equivalent to levels in 28-day cold-acclimated laboratory voles. In fall, TH activity was increased to 2.5 times the summer value. It decreased by midwinter, but remained elevated above the summer level. In contrast, PNMT activity appeared unchanged from summer through fall and winter. Pregnant summer voles had markedly increased TH activity. Adrenal norepinephrine and epinephrine did not change significantly with cold acclimation or seasonal acclimatization. Thus, acclimatization of wild voles to fall and winter conditions involved aquisition of a greater capacity to synthesize adrenal catecholamines than that produced by exposing laboratory-reared voles to an extended period of cold.

Norepinephrine thermogenesis in seasonally acclimatized and cold acclimated red-backed voles in Alaska.

The calorigenic response (millilitres O2 per gram pre hour) to injected norepinephrine (NE) was compared as an index of nonshivering thermogenesis (NST) in the following groups of the Alaska red-backed vole (Clethrionomys rutilus): (1) summer, (2) fall acclimatized, (3) winter acclimatized, (4) 20 degrees C acclimated and (5) 5 degrees C acclimated. The metabolic response was tested at thermoneutrality (25 degrees C) and during cold exposure (5 degrees C). Winter acclimatized voles showed a significantly greater metabolic response to NE than summer voles at both 25 degrees C and 5 degrees C. In summer or winter voles the total metabolic rate after NE (Mne) was similar at 25 degrees C and 5 degrees C but the fraction of the total caused by exogenous NE was lower at 5 degrees C. Thus, thermogenesis during cold exposure and resulting from exogenous NE appear to be based on the same mechanism, and NE has thermoregulatory significance in these voles. The magnitude of the NE response in winter voles was comparable to he highest values reported for bats and exceeded levels reported for other adult small mammal species. Summer acclimatized voles and those acclimatized to 20 degrees C in the laboratory were comparable in their response to NE but winter acclimatized voles were significantly more sensitive to NE than voles acclimated to 5 degrees C. The seasonal winter peak in MNE coincided with peaks previously found for maximum metabolic capacity (MMAX), maximum brown fat, and the period of coldest temperature in December-January. the ratio of MNE to Mmax was similar throughout the year. The results suggest that small arctic-subarctic rodents have a greater capacity for NE stimulated NST than rodents from temperate latitudes probably because they are acclimatized to colder seasonal condtions.