Analysis of Refractoriness to the Effects of Growth Hormone on Amino Acid Transport and Protein Synthesis in Diaphragms of Young Normal Rats*

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Abstract
The in vitro responsiveness of diaphragms from 18-day-old fasted rats to bovine GH (bGH) was studied after the administration of bGH in vivo by determining the accumulation of α-amino-isobutyric acid to measure amino acid transport and the incorporation of phenylalanine into diaphragm protein to assess protein synthesis. bGH in vivo induced a transient stimulation of both processes. Three hours after the in vivo administration of 50 μg bGH iv, the rates of protein synthesis and amino acid transport had returned to prestimulatory levels. Moreover, bGH in vitro (5 μg/ml) had no effect on the two processes at this time, i.e. the muscle was refractory to GH. Five and 7 h after the administration of 50 or 100 fig bGH in vivo, the α-amino-isobutyric acid transport system was completely refractory to bGH in vitro, in contrast to protein synthesis which showed a full response to bGH in vitro. This finding indicates that the duration of refractoriness of amino acid transport to GH was considerably longer than that of protein synthesis. Furthermore, refractoriness of protein synthesis could be overcome by increasing the test dose of bGH in vitro, indicating that refractoriness of protein synthesis to GH only was partial. These results demonstrate that in skeletal muscle tissue from normal animals, GH causes a transient stimulation of protein synthesis and amino acid transport and then induces refractoriness to further stimulation by the hormone. The results further suggest that different mechanisms may be involved in refractory responses of the amino acid transport system and the protein synthetic process. (Endocrinology106: 298, 1980)