Induction of somatogenic receptors in livers of hypersomatotropic rats.

Male and female Wistar-Furth rats bearing the pituitary tumor MtT/W15 had serum GH and PRL levels several hundred-fold higher, and immunoreactive somatomedin levels 3-fold higher, than those of controls. The presence of tumor appeared to have no effect on specific binding of radioiodinated bovine GH to liver microsomal membranes in male animals, and to decrease specific binding by 78% in females. However, after desaturating receptors by treatment with 3.2 M MgCl2 for 5 min, bovine GH binding to membranes from tumor-bearing males was 3-fold higher than in normal males, while in tumor-bearing females binding was more than twice as high as in normal females. Competitive binding curves showed the induced receptors to be specific for growth hormones, and thus somatogenic in nature. This study indicates that a high degree of occupancy of GH receptors in hypersomatotropic rat liver does not cause their down-regulation. The mechanism of receptor induction is unknown.