RU-3117 A STEROIDAL COMPOUND WITH HIGH-AFFINITY FOR SIGMA-SITES IN RAT TESTIS MEMBRANES

RU 3117 belongs to a new series of steroids which exhibited a high rel ative binding affinity (RBA) for (+) [H-3]PPP sites in rat testis memb ranes; its RBA was about 40 times higher than that of progesterone. Fu rthermore, it is devoid of any binding to classical steroid receptors; therefore in order to study its binding parameters on rat testis memb ranes it was tritiated. [H-3]RU 3117 bound at least two distinct sites with Ka values of 0.4+/-0.06x10(9) M(-1) and 1.3+/-0.2x10(7) M(-1). U sing this marker, competition studies with cold haloperidol showed tha t a part of this binding was haloperidol-sensitive, whereas another pa rt was haloperidol-resistant. Interestingly, progesterone described as a sigma ligand competes with [H-3]RU 3117 binding, with an RBA of 1.6 %. When haloperidol was preincubated (250 nM) with rat testis membrane s, in order to mask the sigma sites, we observed that DTG (1,3-di-O-to lylguanidine) and haloperidol displayed a very low RBA (<0.1%) and wer e not able totally to displace the [H-3]RU 3117 binding up to 50 mu M. Furthermore, benztropine exhibited a significant RBA of 19% but its d isplacement curve showed a plateau (500-50,000 nM). These results show ed that part of the haloperidol-resistant sites was benztropine sensit ive but another part was displaced neither by haloperidol nor by benzt ropine. The presence of these remaining binding sites was confirmed by preincubating a mixture of haloperidol and benztropine with testis me mbranes. Under these conditions, [H-3]RU 3117 displayed a Ka of 1.0+/- 0.01x10(7) M(-1), and we observed that these sites were recognized, up to now, only by the steroids RU 1968 and RU 54173 which are also devo id of any binding to classical nuclear steroid receptors. Copyright (C ) 1996 Elsevier Science Ltd.