The Biology of Senescence 



mammalian pre-imaginal period. It is a primary stimulator of 

 protein anabolism and somatic growth. The change from a 

 protein-building and nitrogen-retaining economy, and the 

 negative specific acceleration of growth, are two of the most 

 evident correlates of senescence (Mayer, 1949). The adminis- 

 tration of growth hormone 'confers strangely youthful propor- 

 tions on the nitrogen, fat and water components of the body, 

 even in old animals' (Asling et al., 1952). Change in specificity 

 of tissue response to growth hormone certainly appears to occur 

 in some mammals, and this change coincides with the attain- 

 ment of maturity and the appearance of a fresh anabolism- 

 maintaining mechanism. The experimental work of Young in 

 England and Li in America suggests that before a critical time, 

 injected growth hormone induces only protein anabolism — 

 after that time, it also induces diabetes. This is the pattern in 

 man, the kitten (Cotes, Reid and Young, 1949) and the dog 

 (Campbell et al., 1950) but not in the rat (Bennett, Li and 

 Evans, 1948) or, apparently, the mouse (Moon et al., 1952) 

 which respond by continued growth. That the change in speci- 

 ficity involves endogenous as well as exogenous hormone is 

 evident from the occurrence of diabetes in association with 

 spontaneous acromegaly. Evidence for the existence of a separ- 

 ate diabetogenic principle is not very impressive (Raben and 

 Westermeyer, 1952; Young, 1953). Complete ablation of the 

 anterior lobe in adults leads to failure of growth but not, in 

 general, to other acceptable evidences of senility (in rats) 

 though this cannot be shown from the life-table. 



In experimental studies, even highly purified growth hor- 

 mone administered to rats produces decreasing effects upon 

 nitrogen retention and upon growth after repeated administra- 

 tion (Whitney et al., 1948). These experiments, however, have 

 invariably been carried out with heterologous (usually ox) 

 hormones, and, as in the case of antigonadotrophic effects, no 

 physiological importance can be attached to the apparent 

 increase in tissue resistance. 



Of the other hormones concerned in growth and differentia- 

 tion, the pituitary thyreotropic hormone appears in most mam- 

 mals which have been studied (rats — Turner and Cupps, 1938; 

 rabbits — Bergman and Turner, 1941; mice — Adams and 



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