Use of GH4C1 cell variants to demonstrate a non-spare receptor model for thyrotropin-releasing hormone action

Thyrotropin-releasing hormone (TRH) stimulates maximally both the release of previously synthesized prolactin and the de novo synthesis of prolactin by GH4C1 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 GH4C1 cell variants with different numbers of TRH receptors. GH4C1 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 GH4C1 cells. We also found that occupancy of the TRH receptor by the analogue, N3im-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 Kd and ED50 for TRH do not necessarily result from spare receptors in GH4C1 cells.