Thyrotropin-releasing hormone-receptor interaction in GH3 pituitary cells

The interaction between TRH and GH3 pituitary tumor cells was studied in monolayer cultures or membrane-containing fractions. In intact cells, the apparent dissociation constant (Kd) was approximately 10 nM, and the total number of binding sites was approximately 1.4 pmol/mg protein at temperatures of 0-37 C. In broken cell preparations, the number of sites occupied at saturating TRH concentrations was reduced by half when the temperature was increased from 0 to 30 C. Linear Scatchard plots were obtained under all conditions. The rate of dissociation of TRH was temperature dependent, and first order plots were nonlinear. The half-times for dissociation at 0 C were over 240 min in cells and membranes, while at 37 C, the half-time values were 24 min (cells) and less than 0.5 min (membranes). Identical dissociation kinetics were obtained by dilution alone or dilution with excess unlabeled hormone. When cultures which had been incubated with TRH were lightly fixed with glutaraldehyde, dissociation at 37 C became immeasurably slow. However, TRH dissociated immediately when sodium dodecyl sulfate, ethanol, or acetone was added, indicating that the tripeptide was not covalently coupled to cell proteins. The data indicate that binding of TRH to high affinity GH3 receptors is not cooperative, and that the majority of TRH bound after short incubations is dissociable.