Testicular GnRH receptors--characterization and localization on interstitial tissue

Testicular GnRH membrane receptors were demonstrated using the non-degradable GnRH analog D-Ala6des-Gly10 GnRH ethylamide (D-Ala6) as ligand. Displaceable 125I D-Ala6 binding was present on crude membranes prepared from whole testes and interstitial tissue but not on the fractions from seminiferous tubules. 125I D-Ala6 binding to interstitial tissue was specific as only unlabeled D-Ala6 analog and synthetic GnRH inhibited binding of D-Ala6 tracer. Scatchard analysis of the analog data revealed a single high affinity binding site (Ka = 7 x 10(9) M-1) with a binding capacity of 200 +/- 10 (SE) fmol/mg membrane protein. In vivo treatment of both intact and hypophysectomized adult male rats with synthetic GnRH (6.6 microgram every 8 hr for 3 days) resulted in a 2-fold increase in GnRH binding capacity without change in receptor affinity. These results indicate that specific high affinity GnRH receptors are present only on interstitial tissue membrane fractions and receptor numbers are increased by a direct action of GnRH on the testis.