PRESSOR-INDUCED RESPONSES OF THE CAT VENTRAL MEDULLARY SURFACE

We examined ventral medullary surface activity using light reflectance procedures after blood pressure alterations induced by phenylephrine or sodium nitroprusside in 23 pentobarbital sodium-anesthetized cats. Images of reflected 660-nm Light were collected and digitized at 1- to 3-s intervals after baseline and intravenous saline, 5-40 mu g/kg phe nylephrine, or sodium nitroprusside infusion. Carotid sinus nerve dene rvation (CSD) and bilateral vagotomy were performed in five and three animals, respectively, and challenges were repeated. Phenylephrine eli cited a dose-dependent transient blood pressure elevation and reflecta nce increase (interpreted as activity decline) over the entire ventral medullary surface examined. The increase consisted of an initial rapi d transient component, peaking at 45 s, and a 3- to 5-min recovery. CS D enhanced, and vagotomy substantially reduced, the initial transient response to phenylephrine. Sodium nitroprusside-induced lowering of bl ood pressure was associated with decreased reflectance in rostral site s and increased reflectance in caudal regions. CSD abolished a late co mponent and diminished amplitude of an initial rapidly rising componen t of changes induced by nitroprusside, a decline further accentuated b y addition of vagotomy.