108 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



Evans'"'' showed, these are precisely the conditions under which the heart is me- 

 chanically most inefficient. This mechanical inefficiency implies that the cardiac mus- 

 culature is dissipating a good deal of its energy in the form of heat. The contribution 

 of the brain as a source of heat has not been studied, but it may be of some impor- 

 tance, as nervous tissue has a high meta1)olic rate and the waking hibernator is 

 essentially a heart-lung-brain preparation. 



The recording of muscle action potentials early during the process of arousal with- 

 out any visible movement of the animal may reflect an increased muscular tonus. 

 Fairfield'" in her studies of hypothermia in infant rats felt that this tensing was the 

 cause of the augmentation of oxygen consumption in these animals. Indeed, Swift,^^° 

 Dill and Forbes"^ and Barbour, et a/./*- all reported that muscular rigidity and 

 tonus were capable, in man and rats, of increasing the metabolic rate without actual 

 shivering. There remains the possibility that heat may be produced in muscle with- 

 out a corresponding amount of mechanical work being performed. Although the 

 oxygen uptake of a tissue has usually been found to be proportional to the work 

 done by that tissue, Loomis and Lipmann^^" have shown that these two phenomena 

 are separable, since they found that the drug dinitrophenol uncouples the oxygen- 

 consuming from the w^ork-producing process. In the curarized arousing hibernator 

 a similar reaction might take place, with some unknown factor in the situation play- 

 ing a role similar to that of dinitrophenol, allowing the muscle to consume oxygen 

 and produce heat without doing an}- mechanical work. 



Metabolism. The oxygen consumption of arousing golden hamsters shows a 

 measurable increase 20 minutes after the initial stimulus and continues to climb, at 

 first slowly, then very sharply, until a maximum is reached in about 160 minutes. ^^ 

 This maximum, which is a definite over-shoot, is followed by a decline to a more 

 normal oxygen consumption. Though exaggerated by the annoyance which the 

 awakened animal displays towards the thermocouples used for measurement of its 

 cheek pouch and rectal temperatures, this over-shoot in oxygen consumption dur- 

 ing the last hour of the waking process is also ol)served in animals unencumbered 

 by such devices. The great burst of oxygen consumption which accompanies the 

 process of arousal has been noted in all hibernators which have been studied, 

 and Benedict and Lee'"^ indicated that the peak of oxygen consumption during 

 arousal in the woodchuck is greater than that obtained under conditions of maxi- 

 mum exertion in the normal animal."** 



It is generally agreed that glycogen is the source of energy during arousal,"^- "^ 

 although Benedict and Lee,**" from their RQ determinations, believed that the com- 

 bustion of fat supplied the energy. Using chemical and histochemical methods, 

 Lyman and Leduc'°- have recently re-investigated this prol)lem. Liver and muscle 

 glycogen, which average the same in the awake and hibernating hamster, diminish 

 rapidly during the process of arousal. Blood sugar remains essentially normal dur- 

 ing this time, though it rises to hyperglycemic levels in a few animals. In animals 



***E. F. Adolpli and J. Richmond (J. Appl. Physiol. <?.• 48, 1955) found a close relationship 

 between oxygen consumption and esophageal temiK-rature in ground squirrels during waking, 

 while colonic temperature showed little correlation. They concluded that the rate of oxygen con- 

 sumption is largely governed by the temperature of some tissues near the esophagus. Maximal 

 breath frecpiency occurred at a lower esophageal temperature than maximal oxygen consumption. 

 This suggests to them that breath frequency was governed by the temperature of the lungs or 

 chest more than by temperature of the brain. 



