Relationship between receptor-binding affinities and TSH-releasing activities of novel TRH analogs in the rat

Specific bindings of 3H-TRH to rat pituitary homogenate and apparent affinities of TRH analogs for binding sites were studied. The dissociation constant for 3H-TRH binding to rat pituitary homogenate was 30 nM at 0 degrees C, and the number of the binding sites was 120 fmoles/mg protein. The apparent affinities of TRH analogs for the binding sites, which were estimated from the ability to displace 3H-TRH from those binding sites, were found to correlate well with their TSH releasing activities. These findings support the idea that the TSH releasing activities of TRH analogs depend almost entirely upon their binding abilities to the TRH receptor in the pituitary.