Growth hormone function in strains of mice selected for large and small size

The gene (dw) causing hypopituitary dwarfism was transferred by repeated backcrosses into strains of mice differing genetically in growth-rate through previous selection. The dwarf mice lack growth hormone, and the purpose was to find out if the differences in growth-rate between the strains were in any part due to differences in their growth hormone status – amount of hormone or tissue sensitivity. In the absence of growth hormone, i.e. in the dwarfs, the strains still grew at different rates, proving that growth hormone status was not the only cause of their differences. The effect of substituting the dw gene was, however, greater in the large strain than in the unselected control, and less in the small strain than in the control. The growth differences between the strains were therefore in part due to growth hormone. Tissue sensitivity in the three strains was compared by their responses to graded doses of exogenous growth hormone. The large and control strains did not differ, but the small strain had a lower sensitivity. The results suggest that the increased growth-rate of the large strain is partly due to an increased amount, or activity, of its circulating growth hormone, while the reduced growth-rate of the small strain is in part due to a reduced sensitivity of its target organs to growth hormone.