Kyozuka M, Crankshaw D J, Crankshaw J, Berezin I, Kwan C Y, Daniel E E
McMaster University, Department of Neurosciences, Hamilton, Ontario, Canada.
J Pharmacol Exp Ther. 1988 Mar;244(3):1128-38.
To test the hypothesis that functional postsynaptic alpha-2 adrenoceptors exist in rat myometrium, we examined whether specific binding sites for [3H]rauwolscine were present on microsomal membranes from myometrium of nonpregnant, day 16 pregnant and delivering rats and whether an alpha-2 adrenoceptor agonist has functional effects. The myometrium of rats at term undergoes a physiological denervation, confirmed in this study by ultrastructural examination of uterine samples and by [3H]saxitoxin binding studies. Binding sites for rauwolscine of similar Kd (11-15 nM) were present in all groups of myometrium and were localized on plasma membranes. There was no significant change in the density of rauwolscine binding sites in membranes from day 16 animals compared to nonpregnant ones, but a significant fall (38%) from these values at term. Strips of uterine circular or longitudinal muscle from nonpregnant, day-16 or day-22 pregnant rats failed to respond to the selective alpha-2 agonist, BHT-920, in the presence of propranolol; i.e., BHT-920 neither caused contraction nor inhibited contractions induced by oxytocin. BHT-920 did not affect the inhibitory effects of isoproterenol which were antagonized by propranolol. However, it antagonized contractions to norepinephrine in the presence of propranolol with a pKB value of 5.6 to 5.7. These contractions were phentolamine-sensitive. BHT-920 displaced rauwolscine from binding to all groups of myometria (IC50 = 2 to 3 x 10(-6) M) and displaced prazosin (Kd = 0.65 nM) from binding to myometria of nonpregnant rats (IC50 value congruent to 2 x 10(-4) M). Phentolamine also displaced rauwolscine from binding (IC50 = 2 x 10(-8) M). 5-Hydroxytryptamine displaced rauwolscine from binding only at higher concentrations (IC50 greater than 10(-5) M). We conclude that binding sites for alpha-2 adrenoceptor antagonists in myometrial microsomes were located primarily on smooth muscle plasma membrane. A smooth muscle alpha-2 adrenoceptor agonist appeared to occupy a site on muscle with the same affinity as it displayed toward rauwolscine binding site and competitively inhibited effects of an alpha-1 agonist. Our data suggest that alpha-2 adrenoceptor binding sites may exist on smooth muscle without coupling to contractile function, but their occupancy competitively prevents occupancy of alpha-1 agonist receptor activation sites.
为验证大鼠子宫肌层中存在功能性突触后α₂肾上腺素能受体这一假说，我们研究了未孕、妊娠第16天及分娩期大鼠子宫肌层微粒体膜上是否存在[³H]萝芙木碱的特异性结合位点，以及α₂肾上腺素能受体激动剂是否具有功能效应。足月大鼠的子宫肌层会经历生理性去神经支配，本研究通过子宫样本的超微结构检查和[³H]石房蛤毒素结合研究证实了这一点。所有组子宫肌层中均存在Kd值相似（11 - 15 nM）的萝芙木碱结合位点，且定位于质膜上。与未孕大鼠相比，妊娠第16天动物膜上萝芙木碱结合位点的密度无显著变化，但足月时该值显著下降（38%）。在普萘洛尔存在的情况下，未孕、妊娠第16天或第22天大鼠的子宫环形或纵形肌条对选择性α₂激动剂BHT - 920无反应；即BHT - 920既不引起收缩，也不抑制催产素诱导的收缩。BHT - 920不影响异丙肾上腺素的抑制作用，而普萘洛尔可拮抗该作用。然而，在普萘洛尔存在的情况下，它能拮抗去甲肾上腺素引起的收缩，pKB值为5.6至5.7。这些收缩对酚妥拉明敏感。BHT - 920能将萝芙木碱从所有组子宫肌层的结合位点上置换下来（IC₅₀ = 2至3×10⁻⁶ M），并能将哌唑嗪（Kd = 0.65 nM）从未孕大鼠子宫肌层的结合位点上置换下来（IC₅₀值约为2×10⁻⁴ M）。酚妥拉明也能将萝芙木碱从结合位点上置换下来（IC₅₀ = 2×10⁻⁸ M）。5 - 羟色胺仅在较高浓度时（IC₅₀大于10⁻⁵ M）能将萝芙木碱从结合位点上置换下来。我们得出结论，子宫肌层微粒体中α₂肾上腺素能受体拮抗剂的结合位点主要位于平滑肌质膜上。一种平滑肌α₂肾上腺素能受体激动剂似乎以与它对萝芙木碱结合位点的亲和力相同的亲和力占据肌肉上的一个位点，并竞争性抑制α₁激动剂效应。我们的数据表明，α₂肾上腺素能受体结合位点可能存在于平滑肌上，但不与收缩功能偶联，但其占据会竞争性地阻止α₁激动剂受体激活位点的占据。
