Effect of nitric oxide on release of glutamate in the subretrofacial nucleus (SRF) during the exercise pressor reflex in cats

The subretrofacial nucleus (SRF) has been known to play a crucial role in the expression of the exercise pressor reflex. Previously, we have reported that the release of glutamate (Glu) in the SRF was increased during muscle contraction in anesthetized cats. In this study, static muscle contraction of the triceps surae for 4 min was induced by electrical stimulation of L7 and S1 ventral roots. Endogenous release of Glu and citrulline (Cit) from the SRF was recovered by microdialysis and measured by HPLC. The microdialysis probes were also used to deliver L-arginine and L-NAME to test the effect of nitric oxide (NO) on release of Glu in the SRF and on the cardiovascular responses during muscle contraction. During control, muscle contraction significantly increased mean arterial pressure (MAP) from 98±8 to 151±9 mmHg, and the extracellular concentration of Glu from 610±120 to 1280±290 nM. Dialyzing 2 mM L-arginine into the SRF increased basal Cit concentration from 260±50 to 760±210 nM (P<0.05). During contraction after L-arginine, the increases in MAP and Glu concentration were significantly attenuated (86±3-124±6 mmHg and 300±60-460±100 nM, respectively). Dialysis of 0.5 mM L-NAME into the SRF decreased Cit concentration from 340±40 to 180±20 nM (P<0.05). During contraction after dialyzing L-NAME, the increases in MAP and Glu concentration were significantly potentiated (93±6-154±9 mmHg and 520±80-1290±380 nM, respectively). These results suggest that endogenous NO modulates the cardiovascular responses to static muscle contraction by affecting the release of Glu in the SRF.