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

1 The aim of this study was first, to characterize alpha(2)-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 [H-3]-rauwolscine binding was inhibited by five different compounds with an order of affinity characteristic of the one described for alpha(2)-adrenoceptors (yohimbine greater than or equal to 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 [H-3]-rauwolscine inhibition binding curves performed in the presence of oxymetazoline (alpha(2A)-selective), ARC239, prazosin or chlorpromazine (alpha(2B)- and alpha(2C)-selective) indicated that pregnant human and rat myometrium contain at least two pharmacologically distinct alpha(2)-adrenoceptor subtypes (alpha(2A), alpha(2B) and/or alpha(2C)). RNA blot analysis with probes specific for each cloned human and rat alpha(2)-adrenoceptor subtype demonstrated that alpha(2A)- and alpha(2B)-subtypes were present in both species but alpha(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 alpha(2A)-adrenoceptors at mid-pregnancy whereas, at term, alpha(2A)- and alpha(2B)-subtypes density reached approximately the same level (alpha(2A):alpha(2B) ratio = 73:27 at mid-pregnancy and = 43:57 at term). In addition, quantification of alpha(2A)- and alpha(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 alpha(2)-adrenoceptors in pregnant human and rat myometria and an alteration of the alpha(2A)-/alpha(2B)-subtypes expression pattern during rat pregnancy. Such observations lead us to suggest a multiple role for alpha(2)-adrenoceptors in regulating specific functions of myometrium throughout the time course of pregnancy.