NO formation in nucleus tractus solitarii attenuates pressor response evoked by skeletal muscle afferents

We have previously shown that static muscle contraction induces the expression of c-Fos protein in neurons of the nucleus tractus solitarii (NTS) and that some of these cells were codistributed with neuronal NADPH-diaphorase [nitric oxide (NO) synthase]-positive fibers. In the present study, we sought to determine the role of NO in the NTS in mediating the cardiovascular responses elicited by skeletal muscle afferent fibers. Static contraction of the triceps surae muscle was induced by electrical stimulation of the L7 and S1 ventral roots in anesthetized cats. Muscle contraction during microdialysis of artificial extracellular fluid increased mean arterial pressure (MAP) and heart rate (HR) 51 ± 9 mmHg and 18 ± 3 beats/min, respectively. Microdialysis of L-arginine (10 mM) into the NTS to locally increase NO formation attenuated the increases in MAP (30 ± 7 mmHg, P < 0.05) and HR (14 ± 2 beats/min, P > 0.05) during contraction. Microdialysis of D-arginine (10 mM) did not alter the cardiovascular responses evoked by muscle contraction. Microdialysis of N^G-nitro-L-arginine methyl ester (2 mM) during contraction attenuated the effects of L-arginine on the reflex cardiovascular responses. These findings demonstrate that an increase in NO formation in the NTS attenuates the pressor response to static muscle contraction, indicating that the NO system plays a role in mediating the cardiovascular responses to static muscle contraction in the NTS.