The Secretion of Non-electrolytes 223 



extra radioactivity lost into the venous blood during stimulation 

 (Fig. 10.14). It appears that transductal loss of radioactive urea 

 from the saliva could account entirely for the excess amount ap- 

 pearing in the venous blood. This suggests that the permeability 

 of the outer face of the acinar cells for urea is not significantly 

 affected by stimulation. On the other hand, it is possible to calcu- 

 late directly the permeability of both the ducts and the inner face 

 of the acinar cells at different rates of saliva flow (Fig. 10.15). This 



075 



^ 0-fOi 





0B 



01] 

 



01 



Flo 



w 



02 



rate 



0-3 



Fig. 10.15. Changes in the permeability of the acinar cells O O and duct 



cells O — O to urea with change in saliva flow rate. 



Note the large increase in duct permeability and relatively small change in acinar permeability 

 as the flow rate increases. Abscissa: saliva flow rate inml./g. min. Ordinate: arbitrary permeability 

 units (calculated from data given by Burgen and Seeman, 1958). 



calculation shows that the permeability of the inner face of the 

 acinar does not change very much with the rate of stimulation but 

 that there is a considerable increase in the permeability of the ducts 

 proportional to the rate of secretion. It seems quite possible, there- 

 fore, that the lipoid membrane of fixed permeability postulated by 

 Burgen (1956) is the inner cell membrane of the acinar cells and 

 that the membrane of variable permeability is in the ducts. It is 

 highly desirable that these findings should be confirmed by a simi- 

 lar analysis carried out with a non-electrolyte of different physio- 

 chemical properties. Since the evidence obtained by the techniques 

 just described indicates very convincingly that equilibration of 

 urea occurs across the ducts, it is of special interest to see how urea 



