The Anabolic Effects of Pituitary Growth Hormone

Further evidence that the rapid effect of growth hormone on protein synthesis in the diaphragm involves only certain proteins has been obtained in experiments (6) in which the proteins of the heavy centrifugal fraction were fractionated with water, salts and detergents. In these studies, the heavy centrifugal fraction was prepared from diaphragms that were incubated for various periods with BGH and Rc-labeled leucine. 

Another early effect of growth hormone on skeletal muscle which can be detected following either the injection of the hormone (8,9) or its addition to medium bathing isolated muscle (2,10,11), is the stimulation of the net transport of certain amino acids into the intracellular compartment of the tissue. Recent studies (4) using the isolated rat diaphragm and the nonutilizable amino acid, a-aminoisobutyric acid (AIB), have indicated that the stimulatory effect of growth hormone on the transport mechanism is due to the enhancement of amino acid influx rather than to the retardation of amino acid efflux from the cells. The effect is not immediate but becomes evident at about the time (20 minutes of incubation) when a significant stimulation of protein synthesis has occurred (4). 

The finding that there is a delay in the on-set of the transport phenomenon has suggested that certain proteins formed as a result of the action of growth hormone on protein synthesis might mediate the transport response. Experiments by Knobil and his co-workers (2) provided the first evidence in favor of this hypothesis. This group incubated diaphragms of 

Breuer and Florini (13) have obtained evidence suggesting that the stimulatory effect of growth hormone on the enzyme is not due to an increase in the ability of the DNA of the muscle to serve as the template for the polymerase reaction. 

These investigators isolated chromatin from the nuclei of leg muscles of hypophysectomized rats that had received an injection of PGH 18 hours previously. This is the point in time at which the enzyme shows maximal activation. This chromatin was then added to a reaction mixture containing an excess of E. coli RNA polymerase (fraction 3) prepared by the method of Chamberlain and Berg (16) and an excess of the nucleoside triphosphates with UTP being labeled with They found that the incorporation of 

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