AMINO-ACIDS IN THE MEDULLA-OBLONGATA CONTRIBUTE TO BAROREFLEX MODULATION BY ANGIOTENSIN-II

We investigated the underlying mechanisms of baroreflex alteration pro duced by intravenous angiotensin II (ANG II) by monitoring the release of amino acids from the rostral ventrolateral medulla (VLM) using a b rain microdialysis technique. Reflex changes in heart rate were elicit ed by bolus intravenous injection of phenylephrine (2-40 mu g/kg) befo re and 120 min after the initiation of administration of a subpressor dose of ANG II (5.4 pmol/kg/min) or vehicle. The slope of the regressi on line obtained from changes in mean arterial pressure and heart rate elicited by phenylephrine was used as an index of baroreceptor reflex sensitivity. ANG II administration for 120 min significantly attenuat ed the baroreflex sensitivity(from -0.59 +/- 0.10 to -0.30 +/- 0.08 bp m/mmHg). This attenuation was accompanied with an increase in the rele ase of glutamate and glycine from the VLM (+40% and +20%, respectively ) at 120 min. Glycine perfusion into this area resulted in an attenuat ion of baroreflex sensitivity with a magnitude similar to that obtaine d with infusion of a subpressor dose of ANG II, whereas glutamate perf usion caused a resetting of baroreflex. These results suggest that gly cine and glutamate are involved in cardiovascular regulation in the VL M. Furthermore, the augmented releases of these amino acids may accoun t for the underlying mechanism of ANG Ii-induced attenuation of barore flex function.