Sakihara Takashi, Watanabe Yohei, Mukuda Takao, Ando Masaaki
Laboratory of Integrative Physiology, Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima, 739-8521, Japan.
J Comp Physiol B. 2007 Nov;177(8):927-33. doi: 10.1007/s00360-007-0191-2. Epub 2007 Jul 27.
Isotocin is a fish analogue of the mammalian hormone oxytocin. To elucidate sites of action of isotocin (IT) in the upper esophageal sphincter (UES) muscle, a key muscle in swallowing, IT was applied after treatment with tetrodotoxin (TTX). Even after blocking nerve activity with TTX, IT relaxes the UES muscle in a concentration-dependent manner, suggesting that IT receptor(s) is present on the muscle cells. Similar relaxation was also obtained by application of 3-isobutyl-1-methylxanthine (IBMX), forskolin (FSK) and 8-bromo-adenosine, 3',5'-cyclic monophosphate (8BrcAMP) after pretreatment with TTX, suggesting that the relaxing effect (postsynaptic action) of IT may be mediated by cAMP. In contrast to such relaxing effect, IT enhanced the UES contraction induced by repetitive electrical field stimulation (EFS). Such enhancement was blocked by an IT receptor antagonist, suggesting that this effect is also mediated by IT receptor(s). Similar enhancement was also induced by IBMX, FSK and 8BrcAMP, suggesting the enhancing effect is also mediated by cAMP. However, no enhancing effect of IT was observed when the muscle was stimulated by carbachol, or after treatment with curare or TTX, denying the postsynaptic modulatory action of IT and suggesting presynaptic action for IT, i.e., accelerating acetylcholine release. Summarizing these results, role of IT in precisely regulating the drinking rate in the seawater eel is discussed.
异催产素是哺乳动物激素催产素的鱼类类似物。为了阐明异催产素（IT）在吞咽的关键肌肉——食管上括约肌（UES）中的作用位点，在用河豚毒素（TTX）处理后应用了IT。即使在用TTX阻断神经活动后，IT仍以浓度依赖的方式使UES肌肉松弛，这表明肌肉细胞上存在IT受体。在用TTX预处理后，通过应用3 - 异丁基 - 1 - 甲基黄嘌呤（IBMX）、福斯可林（FSK）和8 - 溴腺苷3',5'-环磷酸酯（8BrcAMP）也获得了类似的松弛效果，这表明IT的松弛作用（突触后作用）可能由cAMP介导。与这种松弛作用相反，IT增强了重复电场刺激（EFS）诱导的UES收缩。这种增强被IT受体拮抗剂阻断，表明这种作用也由IT受体介导。IBMX、FSK和8BrcAMP也诱导了类似的增强作用，表明增强作用也由cAMP介导。然而，当用卡巴胆碱刺激肌肉时，或在用箭毒或TTX处理后，未观察到IT的增强作用，这否定了IT的突触后调节作用，并提示IT具有突触前作用，即加速乙酰胆碱释放。总结这些结果，讨论了IT在精确调节海鳗饮水速率中的作用。
