BIOLOGICAL TRANSPORT 



Migration by more than one process 



As the level of a solute is increased, one frequently finds that 

 the rate does not entirely stop increasing; instead it still continues 

 to increase gradually in a linear fashion, as illustrated in Figure 19. 

 This result shows that at least two processes participate, one satu- 

 rable and one that fails to saturate within the range of study. (The 

 latter probably includes not only diffusion but also, or instead, a 

 hard-to-saturate passive mediation, like the process seen above to 

 participate in the escape of amino acids from cells.) A situation of 

 this kind has been studied for the isolated diaphragm in its uptake 

 of radioactive a-aminoisobutyric acid in the presence or absence 

 of added insulin (see Figure 19) (Akedo and Christensen, 1962b). 



In his analysis, Akedo separated these two components by in- 

 tegrating the equation giving the total flux as the sum of two fluxes: 



I I l I l I L 



4 8 12 16 20 



[a-Aminoisobutyrate] raM 



Figure 19 Uptake by two distinct mechanisms. a-Aminoisobutyr- 

 ate entry into the isolated rat diaphragm in vitro. The upper curve 

 shows the total rate of uptake (corrected for entry into the inulin space) 

 as a function of the extracellular concentration. The middle curve shows 

 the calculated rate of the saturable process; the lower curve, of the 

 process not subject to saturation up to 0.1 M. [Results of Akedo, H., and 

 Christensen, H. N. (1962), /. Biol. Chevu, 237, 1180.] 



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