PROSSEB 



cold in a minnow ( Umbra ) but not in Fundulus (Berg et al., 1959). 

 Temperature effect on the thyroid of amphibians is negligible. How- 

 ever, seasonal variations in thyroid activity of both fish and amphi- 

 bians, probably associated with photoperiod, are considerable. The 

 adrenals of poikilothermic vertebrates produce corticosteroids 

 which seem to function primarily in potassium and sodium balance; 

 no role in carbohydrate metabolism has been found in poikilotherms 

 (Jones etal., 1959). The amount of hydroxycorticosteroid in the blood 

 of a fish may be increased after swimming but no response to tem- 

 perature stress has been reported (Jones et al., 1959). Changes in 

 metabolic enzymes in compensation for temperature occur in yeast 

 (Precht, 1956) and in invertebrates where thyroxin and iodinated 

 tyrosines and corticosterone do not function as they do in homeo- 

 therms. Also, the biochemical changes of poikilotherms in tem- 

 perature adaptation can be either an increase or a decrease in 

 specific enzymes. It seems likely that the acclimation of poikilo- 

 therms is either a direct effect of temperature on enzyme forming 

 systems or an indirect enzyme induction due to differential utiliza- 

 tion of substrates at different temperatures. Thus there is little 

 similarity in the metabolic acclimationof poikilotherms and homeo- 

 therms. 



DIFFICULTIES AND METHODS IN ACCLIMATION MEASUREMENT 



The analysis of biochemical mechanisms of temperature accli- 

 mation is beset with many difficulties. The identification of limiting 

 steps involves extrapolation to the intact animal from measurements 

 on tissue slices, homogenates, isolated mitochondria, and purified 

 enzymes. Such extrapolation is difficult and based on several as- 

 sumptions. It is not possible to provide in vitro conditions which dup- 

 licate in all respects those under which an enzyme functions in vivo . 

 Balance of organic as well as inorganic ions, concentrations of co- 

 factors and hormones cannotbeduplicated, nor can spatial organiza- 

 tion, as of particulates in a cell. An important part of acclimation 

 involves regulation by the neuroendocrine system. Yet the integrated 

 system can be analyzed only by taking it apart. One method of 



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