MODULATION OF BAROREFLEX FUNCTION BY ANGIOTENSIN-II ENDOGENOUS TO THECAUDAL VENTROLATERAL MEDULLA

Neurons in the ventrolateral medulla (VLM) mainly determine the tonic sympathetic activity. The caudal VLM (CVLM) relays baroreflex signals to the rostral VLM. We have reported that endogenous angiotensin II (A NC II) contributes to the ongoing activity of the VLM neurons. In the present study, we examined if ANG II endogenous to the CVLM modulates the baroreflex function in anesthetized normotensive Sprague-Dawley ra ts. Changes in renal sympathetic nerve activity (RSNA) in response to changes in mean arterial pressure (MAP) induced by i.v. infusion of ph enylephrine and nitroglycerin were recorded before and after bilateral microinjection of [Sar1, Thr8]-ANG II, antagonist, into the CVLM. The ANG II antagonist injection into the CVLM significantly increased MAP and RSNA by 17.6 +/- 8.0 mmHg (mean +/- S.D.) and 36.3 +/- 18.1%, res pectively. It also significantly increased the baroreflex sensitivity (BS) from -0.49 +/- 0.38 to -0.74 +/- 0.37%/mmHg during nitroglycerin infusion. In contrast, the BS examined by phenylephrine infusion was n ot altered by the pretreatment with ANG II antagonist. Injection of ar tificial CSF affected neither the baseline values of MAP and RSNA nor the BS. These results suggest that ANG II endogenous to the CVLM exert a modulating role in baroreflex control of RSNA.