THE PITUITARY GROWTH HORMONE 



361 



changes are not explicable solely in terms of decreased protein and 

 amino-acid catabolism, but rather by an acceleration of amino-acid up- 

 take by the tissues of the body, was demonstrated by the elegant 

 studies of Russell ( 1953, 1955 ) . All these findings lead to the view that 

 growth hormone, in some manner, stimulates protein synthesis from 

 amino acids. 



In an attempt to devise a system which would permit a more direct 

 approach to the study of the role of growth hormone in protein synthe- 

 sis, the incorporation of C^^-labeled leucine by isolated diaphragms re- 

 moved from normal rats, hypophysectomized rats, and hypophysecto- 

 mized rats treated with growth hormone was investigated ( Kostyo and 

 Knobil, 1959a). It was found that hypophysectomy decreased the in 

 vitro incorporation of the amino acid into the protein of diaphragm, 

 and that growth-hormone administration to such animals restored the 

 incorporation of labeled leucine into the diaphragm protein to normal 

 (Figure 3). It remained to determine whether the addition of growth 

 hormone in vitro to diaphragms excised from hypophysectomized rats 

 would restore toward normal the reduced capability of such tissues to 

 incorporate labeled amino acids into protein. In a series of preliminary 



240 



200 



z 



UJ 

 O 



tr 



Q. 



d> 



2 



160 



120 



a: 



" 80 



40- 



Figure 3. The effect of hypo- 

 physectomy and growth-hor- 

 mone treatment on the incor- 

 poration of leucine-2-C^'* into 

 the protein of isolated rat dia- 

 phragm in vitro. (From Kos- 

 tyo and Knobil, 1959a.) 



T* 



(5) 



(10) 



(5) 



NORMAL HYPOX. HYPOX. 



-f- 



G.H. 

 (|mg/day/4dQys) 



*Mean±S.E. 



(n) Number of Hemidiaphrogms 



