CHARACTERIZATION AND LOCALIZATION OF OXYTOCIN RECEPTORS IN THE RAT TESTIS

In this study oxytocin (OT) receptors have been characterized and loca lized in the testis of the rat using the radioiodinated OT receptor an tagonist I-125-labelled d(CH2)(5) [Tyr(Me)(2),Thr(4),Tyr(9)-NH2]-vasot ocin (OTA). Receptor density and localization have been compared with the rat testis arginine vasopressin (AVP) receptor using the radioiodi nated AVP V-1a receptor antagonist I-125-labelled d(CH2)(5)Sar(7)-AVP and the radioiodinated linear AVP V-1a antagonist I-125-labelled [(C6H 5-CH2CO)-O-methyl-D-Tyr-Phe-Gln-Asn-Arg-Pro -Arg-Tyr-NH2]. I-125-label led OTA bound with high affinity to membrane fractions of the rat test is (K-a = 13.8 +/- 1.25 litres/nmol), mammary tissue (K-a = 20.3 +/- 4 .36 litres/nmol) and uterus (K-a = 27.8 +/- 0.74 litres/nmol). Competi tion studies with various OT and AVP receptor agonists and antagonists confirmed that the binding was to OT receptors. AVP receptors in the testis were found to be identical to AVP V-1a receptors in the liver. The AVP receptor density in the testis was much higher than the OT rec eptor density (109 +/- 12.3 vs 5.2 +/- 0.79 (mean +/- S.E.M.) fmol/mg protein). Autoradiographical localization showed that both OT and AVP receptors were present in the interstitial spaces in the testis consis tent with binding to Leydig cells. AVP receptors were also localized o n the epithelial surfaces of the seminiferous tubules and on testicula r blood vessels. This study has, for the first time, found OT receptor s in the testis of the rat which have similar ligand-binding character istics to mammary and uterine OT receptors. The receptor localizations are consistent with binding to Leydig cells.