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.