The multiple actions of growth hormone

GH clearly has effects on carbohydrate and lipid metabolism, the protein-synthetic machinery, expression of specific proteins, and cell differentiation and proliferation. For a hormone concerned with the overall control of somatic growth such a wide spread of activities is potentially physiologically meaningful. It is at least possible that GH mediates these various actions via several different intracellular mechanisms and that it will not be possible to identify any one action as of key importance. 

In the few months since the main part of this chapter was written there have been several important developments in GH biochemistry. 

The tertiary structure of recombinant DNA-derived pig GH has been reported [137], The conformation includes a large proportion of a-helix (about 54%), 4 antiparallel a-helices being arranged in a left-twisted helical bundle. Several unrelated proteins also contain 4 a-helices arranged in this way, but the connections in GH are unusual and unlike those found elsewhere. In view of the marked homology between the amino acid sequences of members of the GH-prolactin protein family, it seems likely that a similar tertiary structure will be found in other GHs and in prolactins and placental lactogen. 

Expression of the cDNA for the rabbit GH receptor and of the extracellular domain of the human receptor was obtained in transfected monkey COS-7 cells. The proteins so obtained bound 125I-labelled human GH, and showed a specificity similar to that of the receptors obtained from normal tissues. Thus, the rabbit receptor could bind human GH, bovine GH and to a lesser extent ovine prolactin, while the human receptor bound human GH but not bovine GH or ovine prolactin. This provides strong evidence that these receptors mediate the growth-promoting actions of GH, since biological responses show a similar specificity. 

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