Outflow after Isotope Injections 213 



tion during rest starts out at a high rate but as the duration of the 

 rest period proceeds, the increments of iodide accumulated de- 

 crease until eventually a steady state is reached which is presum- 

 ably dictated by a balance between the rate of active secretion of 

 iodide into the cells and passive back-diffusion across the cells. 

 Towbin and Perkins (1958) have reported that at the beginning of 

 stimulation, the iodide concentration in the venous blood leaving 

 the dog parotid may rise transiently above the arterial level. This 

 phenomenon seems rather similar to that discussed above for 

 potassium and can perhaps be accounted for in a similar manner. 

 Due to this loss into the blood, the amount of iodide that appears 

 in the saliva transient underestimates the total amount actually 

 accumulated during the rest period. 



OUTFLOW PATTERNS AFTER CLOSE ARTERIAL INJECTION 

 OF ISOTOPES 



If a small amount of an isotope is injected rapidly into the arterial 

 supply of a salivary gland which is secreting at a constant rate 

 (and is otherwise in a steady state), the outflow pattern of the 

 substance in the saliva can give useful information about the pro- 

 cess of secretion in the gland. In this procedure, the gland is ex- 

 posed only briefly to the labelled material since after passing 

 through the gland the isotope is rapidly diluted in the animal's 

 blood and body fluids. Before saliva which has acquired isotope 

 from the blood can reach the exterior it must displace out the 

 secretion that is already more distal to it. Thus, the more distal 

 is the site at which the isotope can cross the blood into the saliva, 

 the earlier will it appear in the saliva collected (Fig. 10. 11). In an 

 actual experiment some of the delay in appearance of the isotope 

 in the saliva is also due to the time taken for the injected material 

 to travel to the gland in the blood as well as the time taken to 

 traverse the dead space of the main salivary duct and cannula. In 

 the average experiment these delays amount to about 2-3 seconds 

 or approximately 10-15 /d./g. From histological sections, the 

 lumen of the ducts has been estimated to occupy a volume of 

 some 50-100 /d./g (Burgen, 1956). The time resolution possible 

 with the method as developed by Burgen, Terroux and Gonder 

 (1959) is usually better than 0-5 second (= 2 /d./g)- The method 

 is therefore capable of resolving entry sites whose spatial separa- 

 tion is not more than 5 per cent of the total duct volume. Optimal 



