STATIC MUSCLE-CONTRACTION ELICITS A BAROREFLEX-DEPENDENT INCREASE IN GLUTAMATE CONCENTRATION IN THE VENTROLATERAL MEDULLA

In anesthetized cats, static contraction of the hindlimb reflexly incr eases mean arterial pressure (MAP). This cardiovascular adjustment red uced by the arterial baroreflex. Both of these reflex responses are me diated through activation of ventrolateral medullary (VLM) regions. We tested the hypothesis that the concentration of glutamate (Glu) incre ases in the caudal ventrolateral medulla (cVLM) during static hindlimb contractions in anesthetized cats, and that barodenervation reduces t his elevation in Glu levels. Static contractions of the triceps surae muscle of one hindlimb were evoked by electrical stimulation of the pe ripheral ends of cut L7 and S1 ventral roots. After the insertion of t he microdialysis probes and a 3-h recovery period, a 2-min static cont raction increased MAP by 47+/-7 mmHg. The concentration of Glu increas ed from 606+/-189 to 1042+/-228 nM. These results were repeatable in t hat Glu, as well as MAP, rose by a similar amount in two subsequent co ntractions. By contrast, in a subset of cats paralyzed prior to the th ird contraction, neither MAP nor Glu were significantly increased over baseline levels during the third stimulation period. In a third group of cats, hindlimb contraction increased MAP and Glu levels. However, the Glu release was attenuated in subsequent contractions after these cats were barodenervated. During the same periods of stimulation, the denervation accentuated the rise in MAP. These data demonstrate that s tatic contraction of the hindlimb increases the extracellular concentr ation of Glu in the cVLM. Further, our study implicates this neurotran smitter in the baroreflex mediated reduction of the presser reflex res ponse to static muscle contraction.