Abstract
The aim of the present investigation was to determine which subtypes of the α<sub>2</sub>-adrenoceptors are being expressed in the human pregnant myometrium at term pregnancy. In radioligand binding studies, the specific binding of [<sup>3</sup>H]rauwolscine to human myometrial membranes was specific and of high affinity with K<sub>d</sub> of 2.8 ± 0.6 nM and B<sub>max</sub> of 95 ± 5 fmol/mg protein. Results from competition for the binding of [<sup>3</sup>H]rauwolscine using subtype-selective ligands, oxymetazoline (α<sub>2</sub>A-subptype), chlorpromazine (α<sub>2</sub>B-subtype) and prazosin (α<sub>2</sub>B-α<sub>2</sub>C-subtype), suggested that the α<sub>2</sub>A- and α<sub>2</sub>B-subtypes are being co-expressed. In order to examine if also the α<sub>2</sub>C-subtype is being expressed we used an optimal concentration of oxymetazoline or chlorpromazine which would block the high-affinity site, equivalent to the α<sub>2</sub>A- and α<sub>2</sub>B-subtype respectively. Competition curves of both oxymetazoline and chlorpromazine still showed a significantly better fit using a two-site model, suggesting that the α<sub>2</sub>C-subtype also is being expressed. The expression of α<sub>2</sub>C-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 α<sub>2</sub>-adrenoceptors are being coexpressed in the human myometrium at term pregnancy and that α<sub>2</sub>-expression is dominated by the α<sub>2</sub>A-subtype.
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