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Endocrinology of transport 



General 



We may assume that long before the ductless glands appeared, 

 enzymes and metabolic sequences and cycles were evolved in the 

 interlocking patterns in simple organisms. As the design of the 

 complex multicellular organism was approached, specific agencies 

 acting on certain cells and controlling certain processes had then 

 to be superimposed. Basically, the cells of these organisms have 

 essentially similar metabolic processes. An obvious means of differ- 

 entiation lay in developing somewhat different transport relation- 

 ships to their environment; a means of developing mutual controls 

 lay in developing receptor sites on the surfaces of certain cells, where 

 the fixation of humoral agents could accentuate or inhibit transport 

 processes by which particular solutes gain access to those cells. 

 The difference in the readiness with which sugars gain access to the 

 hepatic cell and to the muscle cell is as remarkable as the difference 

 in their responsiveness to insulin. 



Instead, biochemists have generally been optimistic that the 

 "targets" of hormone action would eventually be identified as en- 

 zymes and enzymatic reactions. Although a number of hormone 

 effects on enzymatic reactions have been observed, these have only 

 in a very few cases corresponded at all well with the physiologic 

 action of the hormone. In the case of a number of hormones, direct 

 actions on enzymes are still unknown. In the meantime, a number 

 of powerful actions on transport, without the obvious mediation 



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