Table 1.

5β-Steroid ligands that bind to the oxytocin receptor (OTR).

Inhibition of [3H] Oxytocin Binding to OTR by Progesterone Derivatives
Steroid Compound	Ki (nM) *
	Rat OTR	Human OTR
Progesterone	19 ± 3	None †
R5020	59 ± 7	None †
RU486	46 ± 5	None †
5β-pregnane-3,20-dione	None †	32 ± 5

* Membranes derived from CHO cells expressing either the rat or the human OTR were incubated with 2 nM [³H] oxytocin and the effect of steroids on oxytocin binding was expressed as the inhibition constant, Ki (mean ± SEM). ^† No inhibition was observable at concentrations up to 10 μM. Taken from [62].