CARDIOVASCULAR EFFECTS OF MICROINJECTION OF ANGIOTENSIN II AND ITS ANTAGONIST INTO THE ROSTRAL VENTROLATERAL MEDULLA OF SPONTANEOUSLY HYPERTENSIVE RATS

The mean arterial blood pressure (MAP) and heart rate (HR) responses were measured by microinjecting of increasing doses of angiotensin Ⅱ (Ang Ⅱ) and its antagonist, [Sar 1,Thr 8]Ang Ⅱ, into the rostral ventrolateral medulla (RVLM) of spontaneously hypertensive rats(SHR) and Wistar Kyoto (WKY) rats. Microinjection of increasing doses of Ang Ⅱ (pmol:0 1,1 0,10,100) into the RVLM caused a dose dependent pressor and tachycardiac responses in SHR and WKY rats. The increases in both MAP and HR were of similar magnitude in the two strains. Pretreatment of the RVLM with [Sar 1,Thr 8]Ang Ⅱ abolished the MAP and HR responses produced by microinjection of Ang Ⅱ. In contrast, microinjection of [Sar 1,Thr 8]Ang Ⅱ(pmol:0 1,1 0,10,100) elicited a dose dependent depressor and bradycardic responses. The magnitude of the MAP responses was significantly greater ( P 0 01) in SHR than that in WKY rats, while the magnitude of HR responses was not significantly different in the two stains. These results indicated that Ang Ⅱ acts as an excitatory agent in the region of RVLM to modulate the cardiovascular activities in SHR and WKY rats. The cardiovascular effects of Ang Ⅱ may be related to the action of its specific receptors in the RVLM of rats. The activity of Ang Ⅱ in the RVLM of SHR is higher than that of WKY rats. This increased action of Ang Ⅱ may contributed to the generation and maintenance of higher blood pressure in this model of hypertension.