Discussion
In this paper, we investigated the activation energy of metabolismE in rats after injection of a previously developed
PITS-composition capable of inducing a daily hypothermia and torpor-like
state in rats , as well as in rats after injection of anesthetic
xylazine, which can reduce body temperature for a few hours. The use of
this anesthetic is based on the fact that it, often in combination with
ketamine, can induce hypothermia in animals whereas dexmedetomidine, an
analog of xylazine, can be used to initiate hypothermia in humans, and
is proposed for protection in the emergency cases or during long-term
space travels .
Data on changes in body temperature and oxygen consumption by rats after
injection of PITS-composition or xylazine were used to determineE by the linear regression slopes with regard to mass-corrected
metabolic rate ln(I/M3/4 ), which was equivalent
to heat production, and the inverse body temperature 1/kT . In
accordance with the WBE-theory, E should be in the range of -0.6
– -0.7 eV. For example, in hibernating mammals E = -0.69 eV .
Our calculations made separately for daily heterotherms and true
hibernators revealed some deviations from the above range: in daily
heterotherms E = -0.57±0.04 eV, and in true hibernators E= -0.80±0.04 eV. This deviations from a classical viewpoint is not
unexpected, especially in the studies of poikilothermic organisms. For
example, in the study of fish, E = - 0.5 eV . Even lower values
were obtained for the marine copepod , rocky-shore eulittoral-fringe
snail (Echinolittorina malaccana ) that experiences fluctuating
temperatures , while the teleost fish study revealed excessively large
values of E = - 0.79 eV .
The presented above significant deviations in E are inexplicable
from the point of view of the WBE-theory , which begins with the
observation that temperature controls metabolism through its effect on
the rate of biochemical reactions. It is known, that the reaction
kinetics depends on temperature according to the Boltzmann factor. In
line with Clarke’s criticisms, while statistical thermodynamics provides
a very successful description of the behavior of a simple system where
temperature is the only variable that changes, organismal metabolism is
very different. Organismal metabolism involves a large number of
physiological processes, each of which interacts with many others .
Although it is generally accepted that changes in temperature should
lead to corresponding changes in metabolism, which was demonstrated in
model systems or in mitochondrial suspension, the question is how
universal the application of this physical principle to processes at the
level of multicellular organisms .
We have found that E in rats injected with PITS-composition was
close to the corresponding value in natural daily heterotherms, but
smaller than that in true hibernators. This suggests that the
PITS-composition is able to initiate in homeothermic organisms, like
rats, a state close to daily heterotherms, which, however, differ from
true hibernation. Indeed, the state of pharmacological torpor that
occurs in rats after a single injection of the PITS-composition lasts
for about 16 hours, which is characteristic of daily heterotherms rather
than of true hibernators, experiencing the state of torpor and
hypothermia during many days and even several months .
When rats were injected with the anesthetic xylazine, their body
temperature also decreased for several hours (the curve half-width = 3
h), however, E was about three-fold less than that after
injection of PITS-composition (E = -0.17±0.071 eV) and did not
correspond to the value for a natural state of hibernation . What can be
the reason for such a significant difference?
For determination of the resting metabolic rate and correct assessment
of E , a stationary metabolic state of animals has to be achieved
. In our experiments, the body temperature of animals after injection of
the anesthetic xylazine constantly changed: initially decreased, and
then increased followed by significant changes in heat production and,
accordingly, the animal state could not be regarded as stationary.
Therefore, although the anesthetized animals in our experiment were
immobile, their metabolism cannot be regarded as the resting metabolism.
In homeothermic animals, the body heat production is the source for the
elevated body temperature compared to the ambient one. The metabolism
plays a leading role in maintaining the body temperature of warm-blooded
animals in our understanding of the mechanisms of homeothermy. Thus,
changes in body temperature lag behind changes in metabolic rate and,
accordingly, the changes in oxygen consumption. Lag compensation by
means of an imaginary numerical shift of oxygen consumption by 1 hour
allows us to obtain corrected Ec = -0.68 eV,
close to the corresponding values for hibernating mammals .
In experiments with PITS-composition, the body temperature initially
decreased and then stabilized for a long period of time in a state of
hypothermia followed by restoration of the initial temperature level.
Thus, in animals under PITS the period of steadily lowered body
temperature accounted for a significant part of the time of
pharmacological torpor, and can be regarded as a stationary resting
state. Oppositely, in anesthetized animals during short-term hypothermia
a stationary state is not achieved.
As mentioned above, in the study of poikilothermic organisms, there may
be significant variability of E . It can be assumed that in this
case, there may also appear a temporary mismatch in temperature and
metabolic rate. Since in the cold-blooded organisms the ambient
temperature is the main factor determining the body temperature and
accordingly the metabolic rate, we suppose that the changes in metabolic
rate may occur later than the changes in body temperature for both
physiological reasons, including the time necessary for changes of heart
rate, respiratory rate, blood vessel conductivity, etc. and even slower
metabolic changes in composition of membrane lipids leading to a
decrease in phase transition temperature of lipids which is coupled with
enhanced cold induction of genes . Thus, if for homeotherms the time lag
of temperature behind metabolism was designated as Δt , then for
poikilotherms the lag of metabolism behind temperature should be denoted
by the opposite sign, as (-)Δt. We assume that as in a case with
homeotherms mentioned above, for poikilotherms the correct calculation
of E should be made taking into account the time shift mentioned
above .