110
R. E. SMITH, BARBARA A. HORWITZ, and Y. IMAI
in a maximum. The period following the peak MR ("D") was characterized
by an apparent "hunting" phenomenon wherein the rates were erratic but
tended to decrease.
Notably, the maximum MR of the squirrel was attained at an average
T f of 30.7 ± 1.9 °C and well before the colonic temperature reached normal
levels. That the peak rate actually occurred near the time that the posterior
vascular area opened was indicated by the rapid rise in colonic temperature,
and suggests that the tissues in the anterior portion of the body were
primarily responsible for the total heat production during the greater part
of arousal.
Concordantly, the estimated thermogenesis of brown fat (Table 1,
Estimate I) was greatest during the initial stages of arousal (10.4 %) and
became reduced as the peak MR was approached (5.1 %). These values,
Table 1. Average brown/at thermogenic contribution (% of intact animal heat production *)
Arousal
Estimate
Phase
n
I
II
A
(S4)
1O.43±0.37
41.46±1.46
B
(46)
6.7S±0.30
26.99±1.16
C
(65)
5.11 ±0.16
20.29±0.63
D
(40)
6.13±0.26
24.37 ± 1.02
A-C
(195)
7.79±0.25
30.97±0.99
* Means ± S. E. Estimate I calculated from in vitro q02 = S04 ,ul O 2/100 mg
tissue/hr; Estimate II from a q02 = 3402/-11 O2/100 mg tissue/hr as projected
from the differences between the effect of norepinephrine on rabbit brown fat
in vitro and in vivo [2, 7].
however, are dependent upon the in vitro q02 used to approximate the in
vivo metabolism. Since arousal undoubtedly constitutes a systemic stress,
a q02 obtained in vitro in the presence of catecholamines was considered a
better approximation than measurements without these agents. However,
norepinephrine is reported to raise the metabolism of rabbit brown fat in
vivo 3-4 times more than it does in vitro [2, 7]; hence these differences, if
applying also to the ground squirrel, would increase the calculated thermogenesis from brown fat to 41.5 % initially and 20.3 % at the peak metabolic rate (Table 1, Estimate II).
Thus, the values ranging from 10.4 % initially to 5.1 % appear to represent a minimum estimate of the thermal contribution of brown fat in support of arousal at low temperature.
In perspective, however, the thermogenic importance of brown fat,
whether in hibernators or non-hibernators, depends not only upon the
actual amount of heat produced, but its bodily distribution as well. As
R. E. SMITH, BARBARA A. HORWITZ, and Y. IMAI
in a maximum. The period following the peak MR ("D") was characterized
by an apparent "hunting" phenomenon wherein the rates were erratic but
tended to decrease.
Notably, the maximum MR of the squirrel was attained at an average
T f of 30.7 ± 1.9 °C and well before the colonic temperature reached normal
levels. That the peak rate actually occurred near the time that the posterior
vascular area opened was indicated by the rapid rise in colonic temperature,
and suggests that the tissues in the anterior portion of the body were
primarily responsible for the total heat production during the greater part
of arousal.
Concordantly, the estimated thermogenesis of brown fat (Table 1,
Estimate I) was greatest during the initial stages of arousal (10.4 %) and
became reduced as the peak MR was approached (5.1 %). These values,
Table 1. Average brown/at thermogenic contribution (% of intact animal heat production *)
Arousal
Estimate
Phase
n
I
II
A
(S4)
1O.43±0.37
41.46±1.46
B
(46)
6.7S±0.30
26.99±1.16
C
(65)
5.11 ±0.16
20.29±0.63
D
(40)
6.13±0.26
24.37 ± 1.02
A-C
(195)
7.79±0.25
30.97±0.99
* Means ± S. E. Estimate I calculated from in vitro q02 = S04 ,ul O 2/100 mg
tissue/hr; Estimate II from a q02 = 3402/-11 O2/100 mg tissue/hr as projected
from the differences between the effect of norepinephrine on rabbit brown fat
in vitro and in vivo [2, 7].
however, are dependent upon the in vitro q02 used to approximate the in
vivo metabolism. Since arousal undoubtedly constitutes a systemic stress,
a q02 obtained in vitro in the presence of catecholamines was considered a
better approximation than measurements without these agents. However,
norepinephrine is reported to raise the metabolism of rabbit brown fat in
vivo 3-4 times more than it does in vitro [2, 7]; hence these differences, if
applying also to the ground squirrel, would increase the calculated thermogenesis from brown fat to 41.5 % initially and 20.3 % at the peak metabolic rate (Table 1, Estimate II).
Thus, the values ranging from 10.4 % initially to 5.1 % appear to represent a minimum estimate of the thermal contribution of brown fat in support of arousal at low temperature.
In perspective, however, the thermogenic importance of brown fat,
whether in hibernators or non-hibernators, depends not only upon the
actual amount of heat produced, but its bodily distribution as well. As