362 CELLS, TISSUES, AND ORGANISMS 



experiments ( Kostyo and Knobil, 1959b ) in which the hemidiaphragms 

 from hypophysectomized rats were incubated in a suitable medium 

 containing labeled leucine, it was found that the addition of simian 

 growth hormone to one of a pair of hemidiaphragms, the other serving 

 as a control, significantly increased the incorporation of the amino acid 

 into protein. The minimal effective concentration of hormone was ten 

 micrograms per milliliter of medium. 



Of considerable interest was the observation that the addition of 

 porcine and bovine growth-hormone preparations to the identical sys- 

 tem—preparations which, within the limits of bioassay in the rat, were 

 equipotent with the simian hormone— were relatively ineffective in 

 stimulating the incorporation of labeled leucine into the diaphragm 

 protein. Subsequent experiments designed to clarify this curious phe- 

 nomenon (Brande and Knobil, 1960) have confirmed the above ob- 

 servations and show further that a similar dichotomy between the 

 effect of bovine and simian growth hormone in this system is demon- 

 strable in the case of phenylalanine incorporation. The incorporation 

 of labeled glycine into the protein of hypophysectomized rat dia- 

 phragms on the other hand, is stimulated equally by the addition of 

 simian or bovine growth hormone. The physiological significance of 

 these findings is obscure at present, but they permit the conclusion that 

 highly purified growth-hormone preparations which stimulate the 

 growth of hypophysectomized rats can, when added to an isolated tis- 

 sue under suitable conditions, stimulate the incorporation of some 

 amino acids into the protein of such tissues. Similar observations have 

 been made independently by Manchester and Young ( 1959a ) , who 

 studied the. effect of bovine growth hormone, added in vitro to dia- 

 phragms of hypophysectomized rats, on the incorporation of labeled 

 glycine into protein. It should be noted, however, that growth hor- 

 mone may not enhance the incorporation of all amino acids into pro- 

 tein, since recent experiments have shown that the addition of either 

 simian or bovine growth hormone to hypophysectomized rat dia- 

 phragms in systems identical to the above failed to stimulate glutamic- 

 acid incorporation ( Brande and Knobil, 1960 ) . 



If, with appropriate reservations and assumptions, it is possible to 

 equate, or at least closely relate, amino-acid incorporation into protein 

 with protein synthesis, the question becomes: does growth hormone 

 act either by accelerating the rate of all or a portion of the various 

 intracellular steps involved in the synthesis of protein from amino 

 acids, or does it facilitate the entrance of amino acids into the intra- 

 cellular pool, or both? 



That growth hormone may exert an influence on amino-acid trans- 

 port was suggested by the experiments of Noall et al. ( 1957 ) which 

 showed that a single injection of growth hormone increased the cellu- 



