2 1 8 Theories of Secretion 



conclusion that the outward permeability of the ducts to potas- 

 sium is lower than the inward permeability. 



A feature of considerable interest that this technique has re- 

 vealed is that there is a distal region in the ducts where anions are 

 able to enter the saliva but where cations cannot. Since anions can 

 enter, they must also be able to leave the saliva at this point, be- 

 cause in the absence of cation entry anion movement can only 

 occur by anionic exchange. It is pointed out below that a reciprocity 

 between bicarbonate and chloride concentrations in the saliva can 

 be demonstrated that is under the control of the arterial pC0 2 . 

 It is tempting to believe that this distal site is the place where 

 a chloride-bicarbonate exchange occurs. 



Finally, it must be remarked that the permeability of the ducts for 

 water is very great. This is especially interesting in view of the fact 

 that the saliva is quite hypotonic and that, therefore, a considerable 

 osmotic gradient exists across the duct. However, there is evidence 

 in the case of the frog skin and of amphibian eggs that diffusional 

 permeability for water may bear no close relationship to the osmo- 

 tic hydrostatic permeability of the same cells (Prescott and Zeuthen, 

 1953; Koefoed-Johnsen and Ussing, 1953). 



In summary, this technique has demonstrated that the salivary 

 ducts of the dog parotid gland contain a minimum of five func- 

 tional regions across which selective isotope movement can occur. 

 These do not correspond to any known histological characteristic 

 of the duct cells. 



EVIDENCE FROM ASSOCIATION AND DISASSOCIATION 

 IN CHANGES IN COMPOSITION 



Whilst the changes in composition in saliva at different rates of 

 secretion do not, of themselves, provide useful information relating 

 to the mechanism of secretion, when taken together with other 

 observations they can be of value. 



The Immature Dog. In the submaxillary gland of the neonatal 

 puppy, saliva secretion is quite as active as in the adult. This is so 

 despite a surprising degree of histological immaturity. Acini of 

 adult type are almost totally absent and the ducts are large, fre- 

 quently multilayered and quite unlike those of the mature animal 

 (Wechsler, unpublished). The saliva produced is hypotonic al- 

 though less so than the adult but bicarbonate is absent and chloride 

 practically the sole anion. By three weeks of age, the gland appears 



