THE PITUITARY BODY 



Little is known as to the accuracy with which the growth- 

 promoting hormone can be assayed. Quantitative assay — 

 even by the standards of biological assay — is difficult for a 

 number of reasons. The presence of anterior pituitary hor- 

 mones, other than that promoting growth, may interfere with 

 the response. For example, the presence of the thyrotropic 

 hormone probably would interfere with the growth-response 

 in animals like the guinea pig; in the rat, this is less important 

 because the normal rat's thyroid appears not to be easily 

 stimulated by this hormone. If the assay technique requires 

 the continuation of injections for a long period, or if the ani- 

 mals are repeatedly used by the technique of short-term in- 

 jections, the hormone, which appears to be protein-like, may 

 not produce the maximum possible effect because of the pro- 

 duction of antibodies or "antihormones" (as postulated by 

 Collip and others). Our knowledge of the relationship be- 

 tween the quality of the diet and the degree of the response is 

 still imperfect. Moreover, injected rats gain weight even if 

 the diet is restricted; it is possible that the response might be 

 less pronounced but more constant under such conditions. It 

 would be desirable, if there were agreement as to the most 

 suitable frequency and total number of injections in an ani- 

 mal like the normal adult female rat, to undertake assays 

 under the following conditions: (i) to employ rats of the 

 same race, age, and approximate weight; (2) to use such rats 

 for assay only once (despite statements to the contrary, it is 

 not known to what extent the response is modified by the use 

 of animals more than once); (3) to use a sufficient number of 

 animals for one dose-level (perhaps thirty); (4) to employ 

 moderate doses which are clearly submaximal (in many of the 

 reported assays, the doses appear to be maximal or supra- 

 maximal); (5) to determine the relationship between the dose 

 and the response by observing the effects of multiples or frac- 

 tions of a dose so that a "unit" could be defined; and (6) to 

 inject, in the performance of routine assays, one group of rats 



[ 104 I 



