2. Endocrines and Populations 245 



suiting shift of the blood to the musculature and central nervous system. If 

 at the same time the output of ACTH is stimulated, adrenal glucocorticoids 

 will also be increased, and among their effects is an increase in the reactivity 

 of the blood vessels to the effects of epinephrine and norepinephrine. None 

 of the reactions is completely isolated, but the balance between them may 

 shift considerably depending on the nature of the stimulus. There are also 

 temporal relationships. In many ways the immediate and short-lived ac- 

 tivity of epinephrine is mimicked more chronically by the glucocorticoids. 

 The response of the adrenal medulla is immediate, whereas the cortex re- 

 sponds somewhat more slowly and can be sustained for a great deal of 

 time. 



It is worth describing some of the measurable changes effected in a 

 mammal by activation of the adaptive responses. Many of these have been 

 described earlier under actions of the various hormones, but it is appropri- 

 ate to discuss these as measurable changes in such a way that they might be 

 useful in detecting and interpreting the effects of physiologic adaptation to 

 potentially harmful stimuli. Many of the actions of the adrenocortical and 

 other hormones serve specifically to restore the equilibrium of the internal 

 environment after an alarming stimulus. These actions may be a decided 

 disadvantage to the animal when they are prolonged. For example, in- 

 creased secretion of the carbohydrate-active corticoids serves to maintain 

 proper fluid and electrolyte balances and to provide readily available 

 glucose reserves in an emergency, but they also suppress inflammation, 

 granulation, and antibody formation and thereby reduce host resistance to 

 infection. The following list of adaptive responses is by no means complete, 

 those effects having been selected which might prove useful or of basic con- 

 ceptual importance to the investigator wishing to study the effects of 

 physiologic adaptation. 



2. Measurements Indicating Increased Adrenocortical Function 



a. Adrenal weight. This is a presumptive measurement of ACTH activity 

 and response to an acute or chronic distress. Weights are the best available 

 index of adrenocortical activity for many studies. For many long-term 

 changes, weights are much more useful and much less subject to pitfalls 

 than techniques such as changes in adrenal ascorbic acid, lipids, cholesterol, 

 the production of plasma corticosteroids, or circulating eosinophils or 

 lymphocytes. All these measurements are labile and reflect rapid changes 

 in adrenal function: they indicate the status of the animal at the moment 

 of making the measurement, thus they often may reflect nothing more 

 than the process of trapping or handling an animal and may completely 

 obscure longer-term changes, especially with excitable, highl}^ reactive 



