Comparison of the interaction of equine LH and human chorionic gonadotrophin to equine testicular receptors.

Human chorionic gonadotrophin (hCG) can be used to study horse luteinizing hormone (LH) receptors in stallion testicular tissue. hCG was more stable than horse LH during radioiodination when compared by their abilities to bind to testicular receptor sites. During incubation, neither hormone lost binding activity at 4 degrees C. Horse LH lost binding activity during incubation at 25 degrees C and both hormones lost binding activity at 37 degrees C. Both hormones bound to the same receptor sites which are specific for the hormones. The receptor sites were not degraded when incubated at 4 degrees C for up to 16 h. However, a rapid loss of binding ability occurred at 37 degrees C and a continuous but slower loss at 25 degrees C. Scatchard plots were similar for both hormones. Affinity constants (Ka) of 4 . 5-8 x 10(10) M-1 and receptor site numbers of 3 . 9-7 . 3 x 10(-11) M were calculated from the Scatchard plots, which were linear, indicating a single class of receptors. Similar association and dissociation rate constants were obtained for hCG and horse LH. Association rate constants (K1) between 1 and 32 x 10(5) M-1 sec-1 and dissociation rate constants (K-1) between 1 . 1 and 5 . 6 x 10(-6) sec-1 were observed.