Growth hormone-releasing factor binding sites in rat anterior pituitary membrane homogenates: modulation by glucocorticoids.

Specific high affinity binding sites for growth hormone releasing factor (GRF) were described in rat anterior pituitary homogenates with use of the analog [His1-mono-125I-Tyr10,Nle27]-hGRF(1-32)-NH2 as radioligand. Computerized analysis of competition experiments indicated one class of specific high affinity binding sites with a dissociation constant of 0.19 nM. The relative binding affinities of rGRF, hGRF(1-40) and various analogs correlated well with their in vitro biological potencies. Further, the number binding sites was drastically decreased after adrenalectomy; chronic dexamethasone treatment of these animals restored GRF binding capacity to control without changing binding affinity. These results may in part explain the enhanced responsiveness of the somatotroph after dexamethasone treatment.