364 CELLS, TISSUES, AND ORGANISMS 



Hotchkiss, and Knobil, 1959). The minimal effective concentration of 

 growth hormone has been found to be in the neighborhood of one 

 microgram per milHhter of medium (Kostyo, 1960). Bovine and simian 

 grov^'th hormone were equally effective in this regard. Of interest is the 

 finding ( Kostyo, 1960 ) that the dipping of hypophysectomized rat dia- 

 phragms for ten seconds into a growth-hormone solution ( 1 /xgm/ml ) , 

 followed by immediate washing in five changes of buffer, resulted in 

 an elevated transport of labeled AIB; this suggests a form of binding 

 of the hormone resembling that described for insulin ( Stadie, 1954 ) . 



While it has not yet been demonstrated that growth hormone stim- 

 ulates the transport of naturally occurring amino acids, the above evi- 

 dence nevertheless suggests the possibility that the increase in protein 

 synthesis produced by this honnone may be due, at least in part, to an 

 increase in the availability of amino acids to the cell. 



An alternative possibility— that a site of action of growth hormone 

 may reside along the protein biosynthetic chain independently of 

 amino-acid transport— is suggested by the elegant study of Korner 

 ( 1959 ) , in which he found that cell-free preparations of liver obtained 

 from hypophysectomized rats incorporated less labeled amino acid 

 into protein than did those from normal animals when the labeled 

 amino acids were added to the system in vitro. This defect was local- 

 ized at the level of the microsomal particle, and it could be partly cor- 

 rected by the administration of growth hormone to the donor animal. 

 In a subsequent study, however, in which various liver fractions were 

 obtained after injection of radioactive amino acid in hypophysecto- 

 mized animals at varying times before sacrifice, the incorporation of 

 the label into the proteins of nuclei, mitochondria, and soluble frac- 

 tions as well as microsomes was diminished (Korner, 1960a). Once 

 again, these changes could be partly reversed by growth-hormone 

 treatment. Although the complexities of these systems do not yet per- 

 mit firm conclusions regarding the site of action of growth hormone 

 in protein synthesis, their continued study offers much promise of 

 clarification. 



It should be mentioned that growth hormone shares with insulin 

 all of the effects of amino-acid metabolism just discussed. Thus insulin, 

 when added to the isolated rat diaphragm, increases the incorporation 

 of labeled amino acids into protein in the presence or absence of 

 glucose (see Manchester and Young, 1959a, and Wool and Krahl, 

 1959); insulin increases the transport of AIB when added to rat dia- 

 phragm ( Kipnis and Noall, 1958 ) ; and, when administered to hypophy- 

 sectomized rats, it stimulates the in vivo and in vitro incorporation of 

 labeled amino acids into various fractions of cell-free liver preparations 

 (Korner, 1960b). These effects of insulin can be appended to an im- 

 pressive body of evidence which leads to the generally accepted view 



