In anticipation of prolonged manned space flights, NASA has sponsored
research related to metabolism depression. The daily food requirements,
for example, of astronauts during a voyage of several months can
constitute a major portion of the weight and storage capacity of the
spacecraft. A somewhat promising and fundamental approach to this
problem is the reduction of the astronauts’ daily metabolic
requirements. It has been suggested that astronauts on prolonged space
missions be put in a state of suspended animation until their
destination is reached. Though this sounds fantastic, 10 years ago no
cell had been frozen to cryogenic temperatures and survived. Today it is
commonplace for tissues to be frozen, stored at low temperatures, and
thawed and then to maintain their viability and function.
Animal metabolism may be depressed by reducing body temperature, as in
hibernation and hypothermia. Other means by which metabolism can be
lowered include drugs and electronarcosis. Hibernation is a nonstressful
state and results in a great decrease in metabolism. However, human
beings are not hibernators, and much research is needed before the
mechanism of hibernation is understood, and the possibility of inducing
it in humans evaluated. Hypothermia is the direct cooling of the body to
temperatures where metabolism is substantially depressed. Extracorporeal
circulation systems combined with cooling are in routine use in most
medical centers throughout the world. Hypothermia is not an ideal
solution, however, since general body hypothermia is a stressful
condition. Pharmacologic induction of hypothermia can be accomplished by
such drugs as chlorpromazine and harbamil. Other drugs can be used to
depress metabolism, but all have some disadvantage.
In recent years there has been a growing interest in electronarcosis,
the induction of sleep by an electric current. Although potentially
valuable, this method is far from routine application.
Outstanding advances have been made in metabolism suppression. Recent
progress in the biochemistry and physiology of hibernation and
hypothermia have shown that the oxygen requirements of individual
mammals, organs, and tissues can be reduced. When the chemical
composition of the blood and the cardiac output are sufficient to meet
cellular requirements, regulatory mechanisms remain effective and animal
survival is assured. In contrast, when oxygen transport is interrupted,
a reduction in cellular activity occurs and regulation is impaired. In
induced hypothermia, the low temperature slows the rates of all
processes and modifies the action of metabolites and other substances.
This in itself is not harmful, as shown by the true hibernating animal
(e.g., ground squirrel), but will become disastrous as soon as anoxia
and chemical imbalance begin to develop.
The phenomenon of natural hibernation is being investigated in the
laboratory in the hope that the unusual tolerance of hibernating animals
to reduced metabolism and low body temperature may some day be produced
artificially in ordinary laboratory animals and man. Experiments with
the ground squirrel, a typical hibernator, show that the artificially
cooled ground squirrel does not tolerate such long periods of low body
temperature as does a naturally hibernating animal.
Other studies of the brown adipose tissue (fat), which is present in
most hibernating mammals, show it to be essential to hibernation.
Indications that brown fat has a thermogenic role in rats exposed to low
temperatures suggest that this may be the case in true hibernators
([ref.199]). Arousal of the hibernating animal by cold is triggered by
sympathetically activated thermogenesis in areas of brown fat so
located, relative to the vasculature, that the heat is transferred to
areas of the body concerned with normal metabolic and nervous activity.
Soviet work comparing various depressed metabolic states and resistances
to acceleration shows deep winter hibernation to be most effective,
followed by deep hypothermia, and drug narcosis as the least effective.
Experimental evidence is being accumulated to show that hibernation and
hypothermia somewhat protect animals against radiation. Clinical studies
on irradiation of cancer patients indicate that lowering the body
temperature reduces cellular metabolism and thus decreases tissue
sensitivity to gamma radiation ([ref.200]).
The use of prolonged hypothermia, hibernation, drugs, and
electronarcosis appears to hold some potential for reducing astronauts’
metabolic requirements. If one or mote of these methods become
practical, human requirements for food and oxygen could be drastically
reduced. Simultaneously, these methods may afford radiation protection
and acceleration tolerance.