Pituitary binding and internalization of radioiodinated gonadotropin-releasing hormone agonist and antagonist ligands in vitro and in vivo
In rat pituitary gonadotrophs, the rates of binding and endocytosis of two GnRH superagonist analogs, (D-Ala6,Pro9-NEt)GnRH and (D-Lys6,Pro9-NEt)GnRH, were compared with those of the potent antagonist analog (N-acetyl-D-pCl-Phe1,2,D-Trp3,D-Lys6,D-Ala10)GnRH by quantitative electron microscopic autoradiography. In dispersed pituitary cells, the two agonist analogs showed similar binding kinetics and comparable degrees of sequestration, as measured by their resistance to dissociation by low pH buffer. However, quantification of silver grain localization suggested that cellular internalization of the (D-Ala6)GnRH agonist increased more rapidly than that of the (D-Lys6)GnRH analog. These discrepancies, and the finding that a larger amount of the specifically bound /sup 125/I-(D-Ala6)GnRH agonist was removed during glutaraldehyde fixation, indicated that the proportional internalization of this analog was over estimated by quantitative autoradiography owing to loss of cell surface-bound radioligand. We, therefore, employed radioiodinated D-Lys6-substituted analogs to analyze the receptor binding and cellular uptake of GnRH agonist and antagonist derivatives in vivo. After iv injection, a high proportion of the /sup 125/I-(D-Lys6)GnRH agonist was translocated into pituitary gonadotrophs within 60 min, whereas the D-Lys6 antagonist was predominantly associated with the plasma membrane during that time. Four hours after injection of the antagonist, an appreciable proportion of silver grains was associated with intracellular organelles, and this trend increased progressively at later time points. The relatively prolonged cellular processing of the GnRH antagonist is consistent with in vivo binding kinetics, and its slower internalization may reflect the basal rate of GnRH receptor turnover in the cell membrane.
- Research Organization:
- National Institute of Child Health and Human Development, Bethesda, MD
- OSTI ID:
- 6851454
- Journal Information:
- Endocrinology; (United States), Vol. 4
- Country of Publication:
- United States
- Language:
- English
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550201* - Biochemistry- Tracer Techniques