自發性高血壓老鼠（spontaneously hypertensive rat）高血壓神經機制之探討

Abstract

神經性因素於自發性高血壓之發生及維持均扮演關鍵性角色，本論文使用“中風型自發性高血壓老鼠”(storke-prone spontaneously hypertensive rat, SHRSP）及“自發性高血壓老鼠”(spontaneously hypertensive rat, SHR）針對高血壓老鼠神經機制中下列重點進行實驗：(1）血流動力變數中神經及非神經因素之相對重要性。（2）控制交感神經活性之神經細胞於延腦中之分佈（distribution）及對刺激之反應能力(reactivity）之比較。（3）延腦中重要降壓或抑制性交感神經元基本活性（tonic activity）之大小。本實驗使用雄性12至16星期大之SHRSP、SHR及其正常血壓對照組老鼠Wistar Kyoto鼠（WKY)，以尿酯（urethane）及氯酸醣（α-chloralose）腹腔注射麻醉。第一部份實驗，記錄主大動脈上升支血流、血壓及心跳，並以hexamethonium (HX,5mg/kg i.v.）阻斷神經節傳導，以HX加nitroprusside (NP,0.07mg/kg i.v.）使血管作最大放鬆，結果顯示休息狀態下，神經成份（HX阻斷成份）是維持三種老鼠血壓及全身周邊血管阻力指數（TPRI)之最主要成份，而神經阻斷及血管放鬆後之剩餘成份（結構性成份）是維持心輸出量指數（CI）之最主要成份。SHRSP之TPRI之神經性成份及CI之剩餘成份約顯著大於WKY（各大於WKY 75%及47%）。SHR之CI之剩給成份亦顯著（52%）大於WKY。在第二部份實驗，麻醉動物先切去感壓接受器神經，並微注射30nl之麩胺酸（L-glutamate,Glu,10mM）入腹側區延腦（ventro lateral medulla, VLM）及背中區延腦（dorsomedial medulla, DMM）以定位控制血管運動及交感神經活性神經元分佈區域，另以10至50nl之三種濃度之Glu (0.1,1及10mM）於VLM及DMM之最大反應點作劑量、反應關係（dose-response relationship）實驗，結果顯示高血壓老鼠，尤其是SHRSP，當Glu刺激VLM或DMM反應點時，其血壓及腎神經活性（renal nerve activity, RNA）對刺激之反應能力較WKY為強，其閾值劑量較低，最大反應值較高，而三種老鼠延腦之血壓及交感神經活性控制區之分佈形式（distribution pattern）相似，第三部份實驗，以紅藻胺酸（kainic acid , KA,10mM）微注射(30至50nl）入尾端背中區延腦（caudal dorsomedial medulla, CDMM)，破壞其降壓或交感神經抑制性神經元，結果顯示破壞後三種老鼠之升壓及升RNA反應無顯著差異。綜合上述結果，可作如下結論：於高血壓形成之早期，SHRSP延腦中血管運動神經元對外來或內在產生之刺激反應較強烈，以致交感神經活性較高，對全身周邊血管阻力控制較強，同時心臟組織結構對升高之血壓作良性結構調適(structural adaptation)，產生較大之心輸出量，而形成高血壓。至於同齡之SHR，上述因素亦均存在，但以心臟結構調適產生較大心輸出量為高血壓之主因，而延腦中血管運動神經元之分佈及CDMM交感神經元之基本活性於三種老鼠間無顯著差異，應與高血壓無關。

Neural factor plays a critical role in the development and maintenance of essential hypertension. The stroke-prone spontaneously hypertensive rat (SHRSP) and the spontaneously hypertensive rat (SHR), two of the most widely used animal models of essential hypertension, were used in the present project. Three experiments were performed. They were designed to clarify the following important questions in the neural mechanism of hypertension:(1) the relative importances of neural factors in maintaining the hemodynamic variables of hypertensive and normotensive (Wistar Kyoto rats, WKY) rats, (2) the distributions of medullary vasomotor neurons and the reactivity of these neurons to a chemical excitant, L-glutamate (Glu) in the three strains of rats, and (3) the tonic activity of the sympathoinhibitory (depressor) area, the caudal dorsomedial medulla (CDMM), in the three strains of rats. All of the experiments were performed on 12 to 16 week old SHRSP SHR and WKY. Rats were anesthetized with α-chloralose and urethane (60 mg and 450 mg/kg, respectively, i. p. ). In the first experiment, the aortic blood flow, systemic arterial blood pressure and heart rate were recorded at resting control state, ganglionic blockade condition (hexamethonium, HX, 5 mg/kg, i.v.) and maximal vasodilatation condition (nitroprusside, NP, 0.07 mg/kg, i.v.). The results showed that the neural component (HX blockable component) was the major contributing factor for blood pressure and total peripheral resistance index (TPRI). The residual component (structural component) after intravenous injections of HX and NP was the dominant factor for cardiac index (CI). In SHRSP, the neural component of TPRI and structural component of CI were significantly higher than those of WKY (>75 % & 47%, respectively). In SHR, the structural component of CI was significantly larger than that of WKY (>52%). In the second experiment, 30 nl of Glu (10 mM) was injected into the ventrolateral medulla (VLM) and dorsomedial medulla (DMM) of baroreceptor denervated and anesthetized rats to map the distribution of the vasomotor points in these areas. For the dose- response experiment, the responses of renal sympathetic nerve activity (RNA) and blood pressure to Glu (1-500 pmole) stimulations at the most reactive loci in the VLM and DMM were recorded. The results indicated that the reactivity of RNA and blood pressure in hypertensive rats, especially in the SHRSP, was increased, so that their maximum response were larger and the minimum amounts of Glu needed to produce a noticeable change were smaller. The distributions of vasomotor reactive points in the VLM and DMM of the three strains of rats were similar. In the third experiment, chemical lesion was performed by serial 30 to 50 nl of kainic acid (10 mM) microinjected bilaterally into CDMM. The results showed that the increases in blood pressure and RNA were similar among the three strains of rats. Based on the results mentioned above, we suggest that in the early stage of hypertension of SHRSP, the medullary vasomotor neurons are more sensitive to external or internal stimuli, therefore, the sympathetic nerve activity and total peripheral resistance were enhanced. At the same time, the cardiac structural adaptation has been developed to overcome the increased afterload, which increases the CI and results in hypertension. For the SHR, the above mentioned contributing factors of hypertion also exist, however, the cardiac structural adaptation was the major factor for the heightened blood pressure. The distributions of medullary vasomotor neurons and the tonic activity of CDMM were similar among the three strains of rats, these may not relate to the development of hypertension.