NO modulates norepinephrine release in human skeletal muscle: implications for neural preconditioning.

The purpose of this study was to estimate muscle interstitial norepinephrine (NE) levels during exercise and to determine whether nitric oxide (NO) modulates NE release in the skeletal muscle in humans. We measured interstitial dialysate concentrations of NE with two microdialysis probes inserted into the forearm. Probes were perfused with saline and the NO synthesis inhibitor N(G)-monomethyl-L-arginine (L-NMMA), respectively. Dialysate samples were collected during two sequential 20-min intense dynamic handgrip periods, preceded by 40-min baseline periods. On a different day, forearm ischemia was performed instead of the first exercise period. Exercise increased dialysate NE from 172 +/- 42 to 270 +/- 45 pg/ml (83% increase, P < 0.02, n = 6). Probes perfused with L-NMMA had a 136 +/- 39% greater dialysate NE compared with probes perfused with saline (225 +/- 25 vs. 125 +/- 25 pg/ml, P < 0.001, n = 9). The exercise-induced increase in NE (125 +/- 52%) was attenuated if preceded by exercise (34 +/- 34%) or ischemia (40 +/- 36%; P = 0.06, n = 6), suggesting a neural preconditioning effect. This attenuation was not observed in probes perfused with L-NMMA. We propose that NO modulates NE release in skeletal muscle, that ischemic exercise increases muscle interstitial NE, and that this increase can be attenuated by a preconditioning effect mediated in part by NO.