Lutropin Receptors from Male and Female Tissues: Different Responses to a Lutropin Receptor Binding Inhibitor

Abstract

Summary

An inhibitor for lutropin receptor site binding (LH-RBI), which strongly inhibited the binding of ¹²⁵I-labeled ovine lutropin ([¹²⁵I]oLH) to ovarian LH receptors, did not inhibit the [¹²⁵I]oLH binding to testicular LH receptors. Preincubation of the LH-RBI with [¹²⁵I]oLH did not affect the binding of preincubated [¹²⁵I]oLH to ovarian LH receptors. No inhibition of [¹²⁵I]oLH binding to testicular LH receptors was observed even when the concentration of LH-RBI was significantly increased or when the testicular LH receptors were first incubated with LH-RBI prior to the addition of [¹²⁵I]oLH and a second incubation. Scatchard analysis revealed that the dissociation constant of [¹²⁵I]oLH binding was essentially the same in the presence or absence of LH-RBI. The results suggest that: (i) the lutropin receptor of ovaries, but not of testes, has a specific LH-RBI binding site in addition to the lutropin binding site, and (ii) the binding of the LH-RBI produces an “allosteric” type of inhibition to the binding of lutropin at the lutropin binding site.

We thank Miss Kathleen Harris for her excellent technical assistance and Miss Debra Starr for typing the manuscript.

This study was supported in part by the following: American Cancer Society Grant No. PDT-41D; USPHS, NIH Grant No. CAO5831-15 and AM-09801; Contract No. HD-9-2221; and Robert A. Welch Foundation (G-147).