Hormone-treated CHO cells exit the cell cycle in the G2 phase

In order to localize and identify receptor structures, the binding of radiolabeled thyrotropin releasing hormone (TRH) to thyrotropin (TSH)-secreting cells from rat and bovine anterior pituitaries and from a mouse TSH-secreting tumor was studied $\text{in vitro}$. The binding of TRH to rat anterior pituitaries increased linearly with the log of TRH concentration in the incubation medium. Plasma membranes were the only subcellular fractions isolated after incubation from bovine anterior pituitary and the TSH tumor which bound detectable quantities of TRH.