To assess whether pregnancy might influence the functionality and expression of human myometrial beta(2)- and beta(3)-adrenoceptors (beta(2)- and beta(3)-AR), we performed functional, binding, Western blot, and molecular biology experiments in human nonpregnant and near-term pregnant myometrium. Inhibition of spontaneous contractions induced by a beta(3)-AR agonist, SR 59119A, was significantly greater in pregnant, compared with nonpregnant, myometrial strips (E'(max) = 61 +/- 5% vs. 44 +/- 5% for pregnant and nonpregnant myometrium, respectively), whereas salbutamol, a beta(2)-AR agonist, was significantly less efficient in pregnant, compared with nonpregnant, myometrium (E(max) = 29 +/- 4 vs. 54 +/- 8%). Although two populations of binding sites corresponding to beta(2)- and beta(3)-AR were identified in both nonpregnant and pregnant myometrium, we found a clear predominance of the beta(3)-AR subtype. Moreover, beta(3)-AR binding sites were up-regulated 2-fold in myometrium at the end of pregnancy. Both beta(2)- and beta(3)-AR mRNA were expressed in human nonpregnant and pregnant myometrium. Contrary to beta(2)-AR, the expression of the beta(3)-AR transcripts and immunoreactive proteins was increased in pregnant, compared with nonpregnant, myometrium. Such compelling data suggest a predominant role for beta(3)-AR in the regulation of human myometrium contractility, especially at the end of pregnancy, which might have important consequences for the clinical management of preterm labor.

1 In order to compare the b 2 -and b 3 -adrenoceptor (b-AR) desensitisation process in human nearterm myometrium, we examined the influence of a pretreatment of myometrial strips with either a b 2 -or a b 3 -AR agonist (salbutamol or SR 59119A, respectively, both at 10 mM, for 5 and 15 h) on the relaxation and the cyclic adenosine monophosphate (cAMP) production induced by these agonists. 2 To assess some of the mechanisms potentially implicated in the b-AR desensitisation process, we studied the influence of such treatment on the number of b 2 -and b 3 -AR binding sites, the b 2 -and b 3 -AR transcripts expression and the phosphodiesterase 4 (PDE4) activity. 3 Salbutamol, but not SR 59119A, concentration-response curve (CRC) was shifted by a 15 h salbutamol preincubation, with a significant difference in Àlog EC 20 values (6.3170.13 vs 5.5870.24, for control and 15 h salbutamol pretreatment, respectively, Po0.05). Neither salbutamol nor SR 59119A CRCs were modified after a 15 h preincubation with SR 59119A. 4 A 15 h exposure of myometrial strips to salbutamol significantly reduced the salbutamol-induced (0.6070.26 vs 1.5470.24 pmol mg À1 protein, Po0.05), but not the SR 59119A-induced, cAMP production. No decrease in cAMP production was observed after a 15 h SR 59119A exposure. 5 A 15 h salbutamol exposure of myometrial strips significantly reduced the b 2 -but not the b 3 -AR binding site density, whereas no decrease in the number of b 2 -and b 3 -AR binding sites was observed after a 15 h SR 59119A treatment. 6 Neither PDE4 activity nor the b 2 -and b 3 -AR mRNA expression levels were affected by salbutamol or SR 59119A treatments. 7 Our results indicate that b 3 -AR, but not b 2 -AR, are resistant to the agonist-induced desensitisation. In our model, b 2 -AR desensitisation is mediated by a decreased number of b 2 -AR that was not explained by transcriptional regulation of the receptor.

BACKGROUND AND PURPOSEMuscarinic and adrenergic G protein-coupled receptors (GPCRs) are the targets of rare peptide toxins isolated from snake or cone snail venoms. We used a screen to identify novel toxins from Dendroaspis angusticeps targeting aminergic GPCRs. These toxins may offer new candidates for the development of new tools and drugs. EXPERIMENTAL APPROACHIn binding experiments with 3 H-rauwolscine, we studied the interactions of green mamba venom fractions with a2-adrenoceptors from rat brain synaptosomes. We isolated, sequenced and chemically synthesized a novel peptide, r-Da1b. This peptide was pharmacologically characterized using binding experiments and functional tests on human a2-adrenoceptors expressed in mammalian cells. KEY RESULTSr-Da1b, a 66-amino acid peptide stabilized by four disulphide bridges, belongs to the three-finger-fold peptide family. Its synthetic homologue inhibited 80% of 3 H-rauwolscine binding to the three a2-adrenoceptor subtypes, with an affinity between 14 and 73 nM and Hill slopes close to unity. Functional experiments on a2A-adrenoceptor demonstrated that r-Da1b is an antagonist, shifting adrenaline activation curves to the right. Schild regression revealed slopes of 0.97 and 0.67 and pA2 values of 5.93 and 5.32 for yohimbine and r-Da1b, respectively. CONCLUSIONS AND IMPLICATIONSr-Da1b is the first toxin identified to specifically interact with a2-adrenoceptors, extending the list of class A GPCRs sensitive to toxins. Additionally, its affinity and atypical mode of interaction open up the possibility of its use as a new pharmacological tool, in the study of the physiological roles of a2-adrenoceptor subtypes. AbbreviationsBRET, bioluminescence resonance energy transfer; EPA, 5-(N-ethyl-N-isopropyl)-amiloride; GPCR, G protein-coupled receptor; RBS, rat brain synaptosomes BJP British Journal of Pharmacology

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