血管加壓素微量注入最後區引起大鼠心肺功能降低及其可能的訊息傳導路徑

Abstract

最後區位於延腦的背面，缺乏血腦障壁，且富含血管加壓素接受器。以血管加壓素興奮最後區的神經元，會降低運動所引發的升壓作用。將血管加壓素微量注射到延腦腹外側會抑制呼吸、也會調節心血管的功能。本論文研究的目的是評估將血管加壓素注入到最後區是否會抑制呼吸與血壓，並研究其作用的可能機制。實驗選用Wistar品系的雄鼠，以氨基甲酸乙酯(urethane)麻醉，進行股動、靜脈插管，分別用來測量血壓與注射藥物，之後進行氣管插管，並切斷兩側迷走神經，經麻痺後接上人工呼吸器。然後將動物固定於腦定位儀，分離膈神經並記錄其活性，動物在維持高氧、正常二氧化碳濃度情況下進行實驗，少數實驗則是在高氧與提高二氧化碳濃度的情況下進行。微量注射三種不同劑量的血管加壓素，分別是1.5×10-5, 3.0×10-5 和 4.5×10-5 IU等劑量。本論文共完成四個實驗，觀察血管加壓素是否經由V1A接受器來抑制呼吸和血壓，並進一步研究這種抑制作用是否經由一連串細胞內的訊號傳遞作用，促使鉀離子孔道關閉，引起神經元去極化而使電壓閘門鈣離子孔道打開所引起。第一個實驗結果是將血管加壓素注入最後區，發現會抑制呼吸(包括膈神經高度降低與呼氣時間延長)與降低血壓，這種抑制作用呈現劑量反應，且是經由V1A接受器來達成；第二個實驗結果顯示血管加壓素注入最後區對呼吸與血壓的抑制作用，會被兩側孤獨徑核預處理利多卡因(lidocaine)或氯化鈷所阻斷，暗示抑制作用可能是藉由最後區投射到孤獨徑核的神經徑路來完成，不僅如此，注射血管加壓素所引起的抑制作用還會因二氧化碳濃度增加而減弱；這些結果顯示血管加壓素作用於最後區V1A接受器，主要是經由孤獨徑核進而抑制呼吸與血壓。 第三個實驗是要研究血管加壓素引起降壓作用的機制，也就是找出神經元內所發生的訊息傳導作用，在這個實驗，我先以V1A和磷脂酶C抑制劑 (PLC inhibitor)證明血管加壓素的作用是藉由V1A接受器和磷脂酶C，然後以微量注射技術，在相同的注射位置注入 BAPTA-AM (為一種胞內與胞外 Ca++螯合劑)或乙二醇-雙-(2-氨基乙基)四乙酸 (EGTA; 為一種胞外 Ca++螯合劑)，證明血管加壓素的降壓作用是因為Ca++從胞外進入胞內，使胞內Ca++增加所引起，在這個實驗還進一步利用各種不同的Ca++孔道阻斷劑，證明是Ca++經由 L- 型與P-/Q-型的Ca++孔道進入細胞，從另一組動物所得結果顯示，血管加壓素引起的降壓作用會被PKC預處裡所阻斷，但神經元外的鉀離子增多或以鉀離子孔道阻斷劑阻止鉀離子從細胞擴散出來，會模擬血管加壓素的降壓作用，這可能是神經元內的鉀離子增多、發生去極化所引起；第四個實驗是比較麩胺酸與血管加壓素作用於最後區產生降壓作用的機制是否相同，雖然麩胺酸的降壓作用也是使神經元內的Ca++增加，但是增加的機制卻不一樣，麩胺酸引起神經元內Ca++增加是動員細胞本身內部所儲存的Ca++，而不是從胞外進入，因此，與麩胺酸比較，血管加壓素引起Ca++進入神經元的作用是相當具有專一性的。這四個實驗所得結果充分說明血管加壓素的作用是，活化V1A接受器，經由細胞內的訊號傳遞作用，也就是經由PLC-DAG-PKC的路徑而關閉鉀離子孔道，促使神經去極化、電位升高，於是打開了L-型與P-/Q-型的Ca++電壓閘門孔道，讓Ca++進入神經細胞，而引起降壓作用。

The area postrema (AP) is located at the dorsal surface of the medulla, lacks of blood-brain barrier, and have abundant AVP receptors. AVP-induced activation of neurons in the AP has been demonstrated to attenuate the exercise-evoked pressor effect. Microinjection of AVP into the ventrolateral medulla can produce inhibition on respiration and modulation on cardiovascular functions. The present study was aimed to evaluate whether activation of AVP receptors in the AP could modulate cardiopulmonary functions and to examine the mechanism. Male Wistar rats were anesthetized with urethane (1.2 g/kg, i.p.). The femoral artery and vein were catheterized for monitoring blood pressure (BP) and drug administration. Tracheostomy, bilateral cervical vagotomy, paralyzation, and artificial ventilation were performed. The rat was then placed in a stereotaxic instrument with a prone position. The phrenic nerve was separated and its activity (PNA) was monitored at normocapnia in hyperoxia and hypercapnia if needed. Microinjection of various doses of AVP (1.5×10-5, 3.0×10-5 and 4.5×10-5 IU) into the AP was performed. There were four projects completed in the present dissertation to determine that AVP microinjection into the AP could produce inhibition on the PNA and BP via the V1A receptor and that a putative signaling pathway from PLC-DAG-PKC to inactivation of potassium channels and in turn to activate the voltage-gated calcium channels might have been activated. In the first project, the microinjection of AVP into the AP produced a dose-dependent inhibition on the PNA reflecting a decrease of phrenic amplitude and an elongation of expiratory period (TE) immediately and also a decrease in BP. The cardiopulmonary inhibition caused by AVP was totally abolished by the pretreatment of AVP V1A receptor antagonist. Results obtained from the second project showed that this cardiopulmonary modulation induced by AVP could be completely reversed by a pretreatment of lidocaine and/or CoCl2 at the both sides of the nucleus tractus solitarius (NTS), suggesting that AVP-producing cardiopulmonary modulation might be mediated through a neural pathway projecting from the AP to the NTS. Moreover, respiratory inhibition evoked by AVP was significantly attenuated by hypercapnia. These results strongly suggest that AVP V1A receptors in the AP may participate in the modulation of cardiopulmonary functions through the activation of V1A receptors and the pathway connected to the NTS. The third project was designed to search the putative signal transduction pathway for AVP-induced decrease in blood pressure. To elucidate this putative signaling pathway, response of the BP to calcium influx into the AP neurons caused by AVP was examined in adult Wistar rats. In the third project, we firstly demonstrated that hypotension induced by AVP was totally abolished by V1A antagonist, U73122 (phospholipase C blocker), and by BAPTA-AM (Ca++ chelator), suggesting that an increasing intracellular Ca++ is essential for AVP-induced hypotension. We then confirmed that this hypotension induced by AVP was completely abolished by EGTA (extracellular Ca++ chelator) and various Ca++ blockers such as nifedipine, nimodipine (L-types Ca++ blockers), and omega-conotoxin MVIIC (P/Q type Ca++ blocker), but not by omega-conotoxin GVIA (N-type Ca++ blocker). Finally, we verified that AVP-induced hypotension was blocked by calphostin C (protein kinase C inhibitor) and mimicked by an increase in intracellular K+ ions that was also reversed by EGTA, suggesting that Ca++ influx through voltage-gated calcium channels is essential for AVP-producing hypotension. The fourth project was aimed to confirm that Ca++ influx was specific for AVP-induced hypotension. This conclusion was based on the results that glutamate-induced hypotension was reversed by BAPTA-AM but not by EGTA or V1A antagonist. These results suggest that AVP-induced hypotension depends on Ca++ influx through a PLC-DAG-PKC signal pathway to inactivate K+ channels that may depolarize AP neurons to activate L- and P/Q-type Ca++ channels. It may also provide new insights into establishing a relationship between the signal pathway and physiological functions.