2i 6 Theories of Secretion 



otherwise are identical in shape as in sodium which is delayed by 

 a further 7 //l. g. Potassium appears at the same time as sodium 

 and reaches a similar peak height but then declines very slowly over 

 a period of 2-3 minutes. Sometimes an inflection can be seen on 

 this curve shortly after the peak has been passed. Iodide is slightly 

 delayed with respect to sodium and potassium by about 2/^L/g, it 

 falls off more slowly than sodium but not nearly as slowly as potas- 

 sium and its height is about ten times as great as the others. Finally, 

 urea is delayed by a further 3 //l./g (see also Fig. 10.16) and falls 

 off rather slowly. 



How can we interpret these results? The volume of saliva se- 

 creted before the appearance of an injected substance indicates the 

 most distal region at which the substance can penetrate into the 

 saliva and we can conclude that all these substances are entering 

 through the duct epithelium for the following reasons. Firstly, the 

 small volume involved which is less than the total duct volume 

 (the latest appearing substances, urea, is delayed by about 20 

 //l./g). Secondly, in the case of the two latest appearing constitu- 

 ents, iodide and urea, there is independent evidence that tubular 

 transport occurs (see pages 157 and 222). This type of experiment 

 then reveals a minimum of five permeability areas in the salivary 

 ducts and suggests that there is considerably more specialization 

 in the physiological behaviour of these structures than had pre- 

 viously been supposed. It might be argued that these delays in 

 appearance do not represent different sites but rather selective 

 retardation in the duct cells. This does not seem a likely hypothesis 

 because it would imply that the take-off points for all these sub- 

 stances are really identical and that only the rates of rise are 

 affected. Similarly, it would be expected that the more delayed the 

 rate of rise, the lower would be the peak height, and the slower 

 the rate of recovery. Careful examination of the results does not 

 give support to any of these features. For instance, in Fig. 10.12 

 at a time when a considerable amount of Br 82 was present in the 

 saliva, there was no trace of Na 22 present. Furthermore, the paral- 

 lelism of the rising phase of these two curves is very striking; there 

 appears to be evidence here of delay but not of other significant 

 differences in behaviour. 



Before proceeding further, we must make it quite clear that 

 experiments of this kind do not directly demonstrate sites for bulk 

 transfer of ions, all they show is sites at which interchange between 



