Rhee J W, Zhang L, Ducsay C A
Center for Perinatal Biology, Department of Physiology, Loma Linda University, School of Medicine, CA 92350, USA.
Eur J Pharmacol. 1998 Aug 28;356(1):9-14. doi: 10.1016/s0014-2999(98)00504-4.
We designed the present study to determine: (1) if phenoxybenzamine can be used as an irreversible blocker for oxytocin receptors, and as such to determine oxytocin affinity, (2) if prolonged hypoxic exposure alters oxytocin receptor coupling efficacy of oxytocin receptors to post-receptor mediated mechanisms in the rat myometrium. Rats were exposed to room air (control), or to continuous hypoxia (10.5% O2) from day 19 through day 21 (2-day exposure). On day 21, one uterine horn was removed and used for in vitro study of myometrial contractile responses to oxytocin, while the other was used for oxytocin receptor analysis. In normoxic tissues, phenoxybenzamine (20 microM) decreased the maximum contractile response (EMAX) to oxytocin (155+/-17 vs. 66+/-19 g s/cm2) and oxytocin binding sites (BMAX: 253+/-35 vs. 114.9+/-21.3 fmol/mg protein). A similar degree of reduction in EMAX and BMAX were observed in hypoxic tissues. The oxytocin dissociation constant (KA) in the normoxic rat was 2.8+/-0.7 nM, which was not different from the chronic hypoxic rat (3.3+/-0.9 nM). Analysis of receptor occupancy-response curves indicated no oxytocin receptor reserve in both normoxic and hypoxic myometrium. However, for a given fraction of the total oxytocin receptors occupied, hypoxic tissue elicited a lower contractile response to oxytocin. We conclude that: (1) phenoxybenzamine is a useful tool to functionally study oxytocin receptor kinetics, (2) prolonged hypoxic exposure does not affect the oxytocin affinity, (3) no spare receptors for oxytocin are present in the rat myometrium, and (4) prolonged exposure to hypoxia decreases oxytocin receptor-effector coupling efficiency in rat myometrium.
（1）苯氧苄胺是否可作为催产素受体的不可逆阻断剂，进而确定催产素亲和力；（2）长时间缺氧暴露是否会改变大鼠子宫肌层中催产素受体与受体后介导机制的偶联效率。从第19天至第21天（2天暴露），将大鼠置于室内空气（对照）或持续缺氧（10.5% O₂）环境中。在第21天，取出一个子宫角用于体外研究子宫肌层对催产素的收缩反应，另一个用于催产素受体分析。在常氧组织中，苯氧苄胺（20 μM）降低了对催产素的最大收缩反应（EMAX：155±17 vs. 66±19 g s/cm²）以及催产素结合位点（BMAX：253±35 vs. 114.9±21.3 fmol/mg蛋白质）。在缺氧组织中观察到EMAX和BMAX有类似程度的降低。常氧大鼠的催产素解离常数（KA）为2.8±0.7 nM，与慢性缺氧大鼠（3.3±0.9 nM）无差异。受体占有率 - 反应曲线分析表明，在常氧和缺氧子宫肌层中均无催产素受体储备。然而，对于占据的总催产素受体的给定部分，缺氧组织对催产素引发的收缩反应较低。我们得出结论：（1）苯氧苄胺是在功能上研究催产素受体动力学的有用工具；（2）长时间缺氧暴露不影响催产素亲和力；（3）大鼠子宫肌层中不存在催产素的备用受体；（4）长时间缺氧暴露会降低大鼠子宫肌层中催产素受体 - 效应器的偶联效率。
