782 15. EFFECTS OF VARIOUS FACTORS ON USTHIBITION 



these results that the myocardium at 20° while beating spontaneously 

 will not suffer from a significant discrepancy between the energy require- 

 ments for function and the metabolic supply, since the rate of beating 

 will decrease markedly, but that when the heart is driven at a constant 

 rate, there will be a greater energy requirement coupled with a smaller 

 supply. In the latter case the tissue may become more sensitive to anoxia 

 or metabolic inhibitors, while in the former case there would be no neces- 

 sary change due to these factors alone. These points have been brought 

 out to emphasize that it is dangerous to make generalizations regarding 

 the relative behaviors of metabolism and function, and from these gener- 

 alizations to draw conclusions as to the responses of tissues to anoxia or 

 inhibitors at different temperatures. It is, perhaps, especially unjustified 

 to state a priori that the sensitivity of a tissue function to metabolic depres- 

 sion will be less at a lower temperature on the basis of a lessened energy 

 requirement. 



Very few quantitative studies have been made of the effects of anoxia 

 as they are modified by temperature changes. The work of Fuhrman et 

 (il. (1950) is interesting in this connection. The respiration of rat ventricle 

 slices was determined in 100% oxygen, then in 10% oxygen, and again 

 in 100% oxygen, both at 37. 7° and 15°. At 37. 7^ the respiration decreased 

 45.5% when the pure oxygen was replaced with 10% oxygen, while at 15^ 

 the decrease was only 17.6%. Recovery in 100% oxygen at 37.7*^ was 

 54.6% and at 15° it was 82.3%. Thus anoxia was less effective in depressing 

 the myocardial respiration at 15° than at 37.7°, and the irreversible changes 

 that occurred, whereby the respiration could not be restored to its original 

 level, were less at the lower temperature. It may be noted that the control 

 respiration at 15^ is about 20% of that at 37.7°. A possible explanation for 

 the less hyijoxic depression at 15° would be that different fractions of the 

 total respiration show differing sensitivities to lowered oxygen tension, 

 and that a fall in the temperature allows the more resistant type of respi- 

 ration to become relatively more important. However, as pointed out ear- 

 lier in this chapter, the control of respiratory rate may be so complex that 

 it is useless to attempt an explanation without mucli more knowledge than 

 we possess at present. 



Examples of Temperature Effects on Cellular Inhibitions 



(A) Respiratory inhibition by cyanide. The effect of cyanide on the respi- 

 ration of frog muscle varies with tlie temperature in an interesting fashion 

 (Ellinger, 1924). At cyanide concentrations around 1 mJ/, the respiratory 

 inhibition changes very little between 20° and 40° (Fig. 15-7) but as the 

 concentration of cyanide is increased there is a progressively greater fall 

 in the inhibition at the liigh temperatures. Indeed, above 5 \\\M the in- 

 hibition begins to be reversed at all the temperatures. These results may be 



