Quantitative Thermogenesis of Brown Fat
in Hibernation and Cold Adaptation 1
R. E. SMITH, BARBARA A. HORWITZ 2 , and Y. IMAr3
With 3 Figures
Abstract
Brown fat thermogenesis, relative to that of the intact animal, was estimated
quantitatively in cold-adapted rats and arousing ground squirrels. The caloric
output of the rat interscapular pad was calculated from temperature differences
between arterial and venous flows through the pad. Extrapolating from these
data, the total brown fat accounted for 8.2 % of the rat's heat production at 4 dc.
In the arousing C. lateral is, brown fat thermogenesis was estimated by approximating the in vivo q02 with rates measured in vivo. Estimates of the heat from this
tissue ranged from 10 % initially to 5 % as the squirrels approached normothermic
body temperatures. In view of the assumptions involved in these calculations, the
7.5 % average is considered a minimal estimate of the tissue's contribution in
support of arousal from deep hibernation.
Thermal evolution from the brown fat has been well demonstrated in
arousing hibernators [5, 6, 12, 14], in cold-exposed neonates [1, 8] and in
cold-exposed adult non-hibernators [3, 4]. To allay assertions [10, 11] that
in rats this heat is insignificant because the brown fat/body weight is small,
the brown fat thermogenesis relative to total heat output was re-examined
in cold-exposed rats and in arousing ground squirrels (c. lateralis).
Rats exposed to 5 °C for 3-4 weeks and returned to 26°C for 2 weeks
were anesthetized (Na Pentabarbitol, 80 mg/kg body wt.). Copper-constantan thermocouples (TC) were placed in the interscapular pad and over
the fat pad on the outer and inner skin surfaces. TC were also fixed upon
the thoraco-dorsal artery and the deep central venous outflow, and one
was inserted 6 em into the colon. In the hibernating ground squirrels, TC
were acutely inserted into the right axillary brown fat and colon. These
animals were placed in a closed-chamber volume meter and the oxygen
consumption continuously recorded at a constant chamber temperature.
1 Supported in part by NASA Research Grant NGR-05-004-035 and USPHS
Research Grant HD-03268-01.
2 Postdoctoral fellow, U.S.P.H.S. (1-F2-GM-13,445-01).
3 Present address: Department of Physiology, Kyoto Prefectural University of
Medicine, Kyoto, Japan.
in Hibernation and Cold Adaptation 1
R. E. SMITH, BARBARA A. HORWITZ 2 , and Y. IMAr3
With 3 Figures
Abstract
Brown fat thermogenesis, relative to that of the intact animal, was estimated
quantitatively in cold-adapted rats and arousing ground squirrels. The caloric
output of the rat interscapular pad was calculated from temperature differences
between arterial and venous flows through the pad. Extrapolating from these
data, the total brown fat accounted for 8.2 % of the rat's heat production at 4 dc.
In the arousing C. lateral is, brown fat thermogenesis was estimated by approximating the in vivo q02 with rates measured in vivo. Estimates of the heat from this
tissue ranged from 10 % initially to 5 % as the squirrels approached normothermic
body temperatures. In view of the assumptions involved in these calculations, the
7.5 % average is considered a minimal estimate of the tissue's contribution in
support of arousal from deep hibernation.
Thermal evolution from the brown fat has been well demonstrated in
arousing hibernators [5, 6, 12, 14], in cold-exposed neonates [1, 8] and in
cold-exposed adult non-hibernators [3, 4]. To allay assertions [10, 11] that
in rats this heat is insignificant because the brown fat/body weight is small,
the brown fat thermogenesis relative to total heat output was re-examined
in cold-exposed rats and in arousing ground squirrels (c. lateralis).
Rats exposed to 5 °C for 3-4 weeks and returned to 26°C for 2 weeks
were anesthetized (Na Pentabarbitol, 80 mg/kg body wt.). Copper-constantan thermocouples (TC) were placed in the interscapular pad and over
the fat pad on the outer and inner skin surfaces. TC were also fixed upon
the thoraco-dorsal artery and the deep central venous outflow, and one
was inserted 6 em into the colon. In the hibernating ground squirrels, TC
were acutely inserted into the right axillary brown fat and colon. These
animals were placed in a closed-chamber volume meter and the oxygen
consumption continuously recorded at a constant chamber temperature.
1 Supported in part by NASA Research Grant NGR-05-004-035 and USPHS
Research Grant HD-03268-01.
2 Postdoctoral fellow, U.S.P.H.S. (1-F2-GM-13,445-01).
3 Present address: Department of Physiology, Kyoto Prefectural University of
Medicine, Kyoto, Japan.