Titration of total binding sites for growth hormone in rabbit liver. Quantitative modifications of these sites during pregnancy.

The present work outlines various kinetic parameters of the interaction between bGH (bovine growth hormone) and a receptor located on the membranes of rabbit liver. A dissociation procedure, which strips the hormone off its receptor has been worked out, by varying MgCl2 concentrations and times of contact; exposure to 4 M MgCl2 for 15--30 min was found optimal for dissociation, without denaturing the receptor, as shown by the possibility of rebinding the hormone to the desaturated membranes. This method has been applied to titrate growth hormone receptors in rabbit liver, during pregnancy and lactation. The first half of pregnancy is characterized by a gradual increase of receptor levels and low saturation by endogenous hormone; conversely at parturition, there occurs a striking increase in receptors, which, however, may be demonstrated only after desaturation. During the lactational period, the receptors reamin in part saturated. The results suggest that growth hormone levels increase at parturition and during lactation in the rabbit and hence may play an important role during lactation. The factors which modulate receptor levels at the same period remain unknown.