PROTEIN REQUIREMENTS OF CATTLE: MITCHELL C5 



in some later period of the day, generally the following period, the creat- 

 inine output is with equal regularity decreased, so that the total day's 

 output of creatinine is not appreciably affected by the work performed. 

 HartmannC'^^) found much the same fluctuations in the uric acid and 

 phosphoric acid content of 2-hour urine samples, and suggests that such 

 variations are not due to variations in the production of these compounds, 

 but in the excretory activity of the kidneys as a result of muscular work/ 

 That the excretory activity of the kidney is affected by muscular work 

 has been clearly shown by Wilson and associates, by Dobreff, and by 

 Asher and Weber (*®). 



Impressed by tlie scarcity of experimental investigations planned to 

 determine directly the effect of work on tissue catabolism, and by the dis- 

 cordance among the results obtained, Mitchell and Kruger(^^) have 

 recently performed a number of experiments on rats in which the effect 

 of work on the excretion of endogenous nitrogen in the urine was deter- 

 mined. The results of 19 experiments clearly indicate that muscular effort, 

 either static or motive, may be performed with no appreciable increase in 

 the excretion of total endogenous nitrogen or of creatinine in the urine, 

 and hence presumably with no appreciable increase in the catabolism of 

 muscle tissue. This was true whether the diet was predominantly car- 

 bohydrate in character, or whether it was predominantly fat and contained 

 only traces of carbohydrate. These experiments on rats, as well as the 

 experiment of Thomas on himself, point to the conclusion that an in- 

 creased catabolism of muscle tissue is not an inevitable consequence of 

 increased muscular activity.' It is equally clear, however, that if the diet 

 does not provide sufficient non-nitrogenous nutrients to supply the work- 

 ing muscle with energy, the muscle tissue itself may be sacrificed for this 

 purpose. 



Whether an accelerated breakdown of muscle tissue results from any 

 other contingency during contraction cannot definitely be decided from 

 available information, although several possibilities suggest themselves. 

 Intense muscular work, in which the rate of consumption of non-nitrog- 

 enous nutrients by the muscles greatly exceeds the rate of replenish- 



^ This explanation is supported by the experimental findings of Rakestraw (J. 

 Biol. Chem., 47: 565 (1921)), as well as of Levine, Gordon and Derick (J. Amer. 

 Med. Assoc, 82: 778 (1924)), who have noted consistent increases in the uric acid 

 concentration of the blood during work, possibh^ due to diminished kidney 

 function. 



'In work experiments on dogs. Chambers and Milhorat (J. Biol. Chem., 77: 603 

 (1928)) have more recently shown that the urinary output of nitrogen in fasting 

 was greatly increased by work, but that this increase could he entirely obviated 

 by the administration of carbohydrates. 



