The Secretion of Non-electrolytes 221 



rates of secretion, the concentration of passively distributed non- 

 electrolytes such as urea, thiourea and methenamine rises above 

 the plasma level (Burgen, 1956). Similar increases occur with the 

 electrolytes. Following a brief rest interval, a moderate increase in 

 concentration of protein and iodine occurs in the first few drops 

 of saliva secreted. This increase reaches a maximum after only 2 or 

 3 minutes. A similar increase of total osmolytes occurs in the same 

 few drops (Seeman, unpublished). It has also been noted by many 

 observers that following a period of parasympathetic stimulation, 

 if a small drop remains on the end of the cannula, this can be seen 

 to recede slowly during a brief rest period. All these observations 

 suggest that water reabsorption, possibly under an osmotic gra- 

 dient, proceeds in the duct system. The fact that this leads to 

 concentration of only the first few drops of saliva secreted after a 

 rest period and also the fact that it changes the concentration of 

 urea which is known to cross the epithelium of the proximal part 

 of the duct system, leads us to believe that this process occurs in 

 the extreme distal region of the salivary ducts. 



The sum of the observations in the present section is that one 

 needs to postulate a minimum of five mechanisms that are con- 

 cerned with the concentrations of the four major ions in the saliva. 

 These are : (a) a bi-ionic movement of sodium and chloride, (b) an 

 anion exchange of chloride with bicarbonate, (c) an exchange of 

 sodium with potassium under the control of adrenal corticoids, 

 (d) a potassium secretion depending on the potassium plasma level, 

 and (e) a water reabsorptive process. 



THE SECRETION OF NON-ELECTROLYTES 



In the study of the steady state secretion of ten non-electrolytes, 

 Burgen (1956) found that these could be classified according to the 

 relationship between the saliva concentration and the rate of saliva 

 secretion which they exhibited. For some components (N-ethyl- 

 urea, chloramphenicol, 4-acetamidoantipyrine and methenamine) 

 the concentration was inversely related to flow rate. This relation- 

 ship can be accounted for satisfactorily on the hypothesis that the 

 permeability of the gland for these substances did not change sig- 

 nificantly with alteration in the rate of nerve stimulation. With the 

 other group of substances (urea, thiourea, N-methylurea, manni- 

 tol, creatinine and glycerol) the relation of saliva concentration to 

 flow rate was U-shaped (see Fig. 9.8). The permeability of the 



