Heterogeneity of α₂-adrenoceptors in human and rat myometrium and differential expression during pregnancy

Abstract

1. The aim of this study was first, to characterize α₂-adrenoceptor subtypes in human and rat pregnant myometrium and second, to investigate the possibility of a differential expression of the putative subtypes according to the stage of pregnancy.

2. In both species, specific [³H]-rauwolscine binding was inhibited by five different compounds with an order of affinity characteristic of the one described for α₂-adrenoceptors (yohimbine⩾clonidine>noradrenaline>phenylephrine>propranolol). Binding affinities (pK_i) for the compounds tested were, in human and rat, respectively: 7.63 and 8.93 for yohimbine, 6.91 and 8.71 for clonidine, 6.23 and 6.09 for noradrenaline, 5.37 and 5.73 for phenylephrine, 4.64 and 4.72 for propranolol.

3. By use of non-linear iterative curve fitting procedures and by fitting the data to a two-site model, analysis of [³H]-rauwolscine inhibition binding curves performed in the presence of oxymetazoline (α_2A-selective), ARC239, prazosin or chlorpromazine (α_2B- and α_2C-selective) indicated that pregnant human and rat myometrium contain at least two pharmacologically distinct α₂-adrenoceptor subtypes (α_2A, α_2B and/or α_2C). RNA blot analysis with probes specific for each cloned human and rat α₂-adrenoceptor subtype demonstrated that α_2A- and α_2B-subtypes were present in both species but α_2C seems to be expressed only in human tissues.

4. In the pregnant rat myometrium, subtype selective compounds competition curves revealed a predominant expression of α_2A-adrenoceptors at mid-pregnancy whereas, at term, α_2A- and α_2B-subtypes density reached approximately the same level (α_2A : α_2B ratio=73 : 27 at mid-pregnancy and=43 : 57 at term). In addition, quantification of α_2A- and α_2B-transcripts by densitometry, following data normalization with an oligo(dT)_12–18 probe, showed a pattern of expression comparable to the one characterized by pharmacological studies.

5. In conclusion, these data demonstrate heterogeneity of α₂-adrenoceptors in pregnant human and rat myometria and an alteration of the α_2A-/α_2B-subtypes expression pattern during rat pregnancy. Such observations lead us to suggest a multiple role for α₂-adrenoceptors in regulating specific functions of myometrium throughout the time course of pregnancy.

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