EFFECT OF URETHANE ADMINISTRATION 823 



If the acetate carbon is incorporated into organic compounds by path- 

 ways other than those of oxidation and carbon dioxide fixation, increased 

 cataboHsm will act in a similar way on the incorporation of carbon- 14 

 into the tissue constituents. Thus labelled acetate administered may be 

 very appreciably diluted by endogenous l)ody acetate^^^ formed in the 

 course of catabolic processes. Increased catabolism cun lluis be expected 

 to promote dilution and to diminish the specific activity of llic labelled 

 acetate. 



A decreased catabohsm, as produced, for example, by the administra- 

 tion of urethane, can be expected to act in the opposite direction, slowing 

 clown the decrease in the specific activity of carbon dioxide and acetate 

 with time and thus promoting incorporation of carbon-14 into tissue 

 constituents. 



Muscular exercise and urethane administration may influence the rate 

 of incorporation of carbon-14 into tissue constituents by acting on other 

 metabolic steps than those mentioned above; such an influence is indi- 

 cated by the observed increase following muscular exercise in the car- 

 bon-14 content of the fatty fractions of the muscles and in some experi- 

 ments also in those of the intestinal mucosa. Nevertheless, the dilution 

 effect is presumably to a large extent responsible for the changes observed 

 in the incorporation of carbon-14 following muscular exercise and urethane 

 administration. 



The carbon-14 content of the average total tissue is found to be about 

 twice as large after the lapse of 20 min as after the lapse of 4 • 5 hr. 

 This is mainly due to the fact that the proportion of non-metabolized 

 acetate in the tissue is very much larger after 20 min than after 4 • 5 hr, 

 and also that fatty components present, being renewed at a very 

 rapid rate, are more active after 20 min than after 4 • 5 hr. The pro- 

 bability of carbon-14 taken up by the protein being lost again within 

 a few hours is small, and correspondingly the average tissue protein 

 activity after the lapse of 20 min is about one-third only of the value 

 observed after the lapse of 4 • 5 hr. 



References 



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