Outflow after Isotope Injections 2 1 7 



the ion in the blood and that in the saliva may proceed. Of course, 

 such regions may coincide with ones at which bulk transfer is 

 occurring, but independent supporting evidence must be found 

 for this in the case of each ion under consideration. 



As yet, we have only considered the interpretation of the leading 

 edge of the outflow curve as giving evidence of the most distal locus 

 at which permeation is possible. Can we obtain any evidence as to 

 how far proximal this zone extends and also are we safe in assum- 

 ing that a rapid uninfected fall away of the curve means that no 

 acinar process is involved? For example, there are a number of 

 reasons for believing that water is taken up by the acinar cells 

 during activity and is also excreted into the acinar saliva, but the 

 smooth, rapid decline of the labelled water concentration in the 

 saliva gives no indication that this is the case. The reason for this 

 becomes clear when we consider the experiments of Burgen and 

 Seeman (1958) with radioactive sodium. The submaxillary gland 

 was loaded with radioactive sodium undl it was in a steady state and 

 then was autoperfused with non-radioactive blood. During stimu- 

 lation, the specific activity of sodium in the saliva was never more 

 than a few per cent of that in the gland due to a very extensive 

 radioactive exchange occurring in the salivary ducts. This, together 

 with the countercurrent nature of the duct circulation (page 19), 

 leads to almost complete dilution of the radioactive sodium in the 

 proximal saliva by non-radioactive sodium from the blood. We 

 may generalize fiom this experiment as follows : we can only expect 

 to see evidence of a more proximal process in the outflow pattern 

 if (a) there is no distal process or (b) the proximal process is quan- 

 titatively large and the outward (saliva to blood) permeability of 

 the ducts to this constituent is not great. This brings us to a con- 

 sideration of the late phase of the potassium curve. There is clear 

 evidence here of a proximal process since the potassium secretion 

 in the saliva continues for at least 1-3 ml./g. This duration is so 

 long that it must involve acinar secretion. We have direct evidence 

 in this case also that outward permeability of the ducts to potas- 

 sium is not very great because in unloading experiments (Burgen 

 and Seeman, 1958), the saliva specific activity averaged 70-80 per 

 cent of that in the gland. Indeed, the rate of decline of radioactivity 

 in the saliva in the close arterial experiments resembles very 

 closely the rate of loss of gland radioactivity in the unloading type 

 of experiment. The analysis of rest transients also had led to the 



