Use of GH 4C 1 cell variants to demonstrate a non-spare receptor model for thyrotropin-releasing hormone action

Abstract

Thyrotropin-releasing hormone (TRH) stimulates maximally both the release of previously synthesized prolactin and the de novo synthesis of prolactin by GH 4C 1 rat pituitary cells at concentrations less than those necessary to fully occupy the TRH receptor at equilibrium. We have examined the dependency of maximal TRH-enhanced prolactin release and synthesis on receptor number using GH 4C 1 cell variants with different numbers of TRH receptors. GH 4C 1 cell variants with increased and decreased numbers of TRH receptors were selected by using a morphological response known as stretching which renders the cells more adherent to the tissue culture substrate. We found that maximal TRH-enhancement of prolactin release or synthesis increased proportionally to the number of TRH receptors per cell, indicating that spare receptors do not exist for TRH on these GH 4C 1 cells. We also found that occupancy of the TRH receptor by the analogue, N 3im-methyl-TRH (MeTRH), in contrast to TRH, closely paralleled stimulated prolactin release in a manner consistent with Clark's receptor-occupancy model. We conclude that differences between apparent K d and ED 50 for TRH do not necessarily result from spare receptors in GH 4C 1 cells.