BIOLOGICAL TRANSPORT 



the muscle of the rat was shown by Riggs and Walker (1960). 

 These actions may play a part in the growth-promoting action of 

 this hormone. A tendency for the muscle/plasma ratio of a-amino- 

 isobutyric acid to decrease with age has been seen in the rat (Riggs 

 and Walker, 1958) and also in man (Christensen et al., 1958). 



Adrenocorticotrophic hormone 



Eichhorn and associates (1960) have shown that the entry of 

 D-xylose into the adrenal cortex is accelerated under the influence 

 of ACTH. 



General considerations 



These numerous examples suggest that hormones very fre- 

 quently act by modifying the access of metabolites to various cells 

 through effects on transport processes. The simplicity of such a 

 basis of superimposed control is obvious. The author (1960b; 1961) 

 has felt that such widespread action of hormones on transport, often 

 involving the simultaneous change of more than one specific trans- 

 port, supports the concept that transport sites could arise from 

 closely ordered relations between chemical groups in the matrix 

 of the membrane and, further, that transport behavior may be occa- 

 sioned by distortions and translocations of such ordered relations. 



At the same time, the very properties by which hormones 

 bind to the surface of cells may permit them also to bind to soluble 

 enzymes, thereby modifying their catalytic action. Direct actions 

 on soluble enzymes have generally not yet been demonstrated to 

 occur in the intact animal treated with a dose of hormone within the 

 physiological range. But a conclusion that all hormone effects are 

 produced by actions on cell transport would obviously be quite 

 unwarranted. 



Pharmacology of transport 



Attention will be called only briefly to the great number and 

 range of drugs that act on, or may be considered to act on, biologi- 

 cal barriers and barrier activities. One could perhaps safely main- 

 tain that at this time more pharmacological effects are obtained by 

 this route than by action on nonbarrier enzyme systems. 



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