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

摘要

class="enumerated" style="list-style-type:decimal">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.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 (pKi) 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.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 (α2A-selective), ARC239, prazosin or chlorpromazine (α2B- and α2C-selective) indicated that pregnant human and rat myometrium contain at least two pharmacologically distinct α2-adrenoceptor subtypes (α2A, α2B and/or α2C). RNA blot analysis with probes specific for each cloned human and rat α2-adrenoceptor subtype demonstrated that α2A- and α2B-subtypes were present in both species but α2C seems to be expressed only in human tissues.In the pregnant rat myometrium, subtype selective compounds competition curves revealed a predominant expression of α2A-adrenoceptors at mid-pregnancy whereas, at term, α2A- 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.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.