Characterization of alpha2-adrenoceptor subtypes in pregnant human myometrium.

The aim of the present investigation was to determine which subtypes of the alpha2-adrenoceptors are being expressed in the human pregnant myometrium at term pregnancy. In radioligand binding studies, the specific binding of [3H]rauwolscine to human myometrial membranes was specific and of high affinity with Kd of 2.8 +/- 0.6 nMand Bmax of 95 +/- 5 fmol/mg protein. Results from competition for the binding of [3H]rauwolscine using subtype-selective ligands, oxymetazoline (alpha2A-subptype), chlorpromazine (alpha2B-subtype) and prazosin (alpha2B-alpha2C-subtype), suggested that the alpha2A- and alpha2B-subtypes are being co-expressed. In order to examine if also the alpha2C-subtype is being expressed we used an optimal concentration of oxymetazoline or chlorpromazine which would block the high-affinity site, equivalent to the alpha2A- and alpha2B-subtype respectively. Competition curves of both oxymetazoline and chlorpromazine still showed a significantly better fit using a two-site model, suggesting that the alpha2C-subtype also is being expressed. The expression of alpha2C-subtype mRNA was confirmed using reverse transcription-polymerase chain reaction on mRNA isolated from myometrial biopsies. In conclusion, our results suggest that all three subtypes of alpha2-adrenoceptors are being coexpressed in the human myometrium at term pregnancy and that alpha2-expression is dominated by the alpha2A-subtype.