Growth hormone effects on RNA and protein synthesis in liver

The case for believing that growth hormone exerts its protein anabolic action by stimulating messenger RNA synthesis is summarized. Ribosomes, but not polysomes, from liver of rats have amino acid incorporating abilities in vitro which are directly related to the level of circulating growth hormone in the rat. Growth hormone treatment of rats stimulates labeling of all types of RNA including mRNA, possibly by activating the liver RNA polymerase. Growth hormone can exert some of its stimulating activity even in actinomycin‐treated rats. It is argued that control of rate, as opposed to type, of protein synthesized would be more likely to be exercised at the cytoplasmic rather than the genetic level, and anomalous results are examined in light of this suggestion.