ENDOCRINOLOGY OF TRANSPORT 



of plasma protein synthesis all arise from a stimulation of the hepatic 

 capture of the normal amino acids (Noall et al., 1957). This action 

 would make the amino acids more accessible to a variety of hepatic 

 processes. 



Up to the present time, unfortunately, no really satisfactory 

 preparation has been made available for studying transport into liver 

 cells in vitro. The normal hepatic behavior seen in the intact animal 

 toward both potassium ion and amino acids is easily lost on slicing 

 the tissue, on preparation of free cells, or even on perfusion. In 

 contrast, slices of certain other tissues, such as cerebral or renal 

 cortex, have shown interesting transport behavior. 



Vasopressin 



The renal control of the tonicity of the body fluids is assisted 

 by the variable output of the antidiuretic hormone, vasopressin, 

 from the posterior pituitary gland. This octapeptide is believed to 

 decrease diuresis by facilitating the reabsorption of water, which 

 moves under an osmotic gradient through the wall of the distal 

 tubule from a hypotonic fluid in the lumen. This action may well 

 differ from all the rest of those discussed in this section because 

 of the relatively enormous number of molecules handled by the 

 transporting system. 



Fong and associates (1960) have obtained evidence that cir- 

 culating vasopressin becomes fixed to the membrane of the tubular 

 epithelium by a disulfide-exchange reaction, the permeability of the 

 membrane to water being modified in this way. These investigators 

 found that circulating tritium-labeled vasopressin became bound in 

 the rat kidney in such a manner as to appear in a sedimented mem- 

 braneous fraction, from which the tritium was partially released by 

 the action of sulfhydryl compounds but not by electrodialysis. 

 Furthermore, prior treatment of the toad bladder with thiol-blocking 

 agents diminished subsequent fixation of tritium-labeled vasopressin 

 (Schwartz et al, 1960). Figure 30 shows a visualization by these 

 authors of the disulfide-exchange process fixing vasopressin to the 

 plasma membrane. Since the hormone apparently accelerates a 

 saturable (and therefore mediated) entry of sodium ion into the 

 cells of the toad bladder from the mucosal side (Frazier et al, 1962), 

 the diagram probably should not be interpreted as simply an open- 

 ing of pores to molecular migration. The fact that vasopressin must 

 be applied to the serosal (outer) surface of the toad bladder to 



97 



