Modulation of sympathetic nerve activity during posthandgrip muscle ischemia in humans

To evaluate modulation of muscle sympathetic nerve activity (MSNA) during posthandgrip muscle ischemia (PHGMI), subjects performed 2 min of isometric handgrip at 33% of maximal voluntary contraction (MVC) followed by 2 min of PHGMI produced by forearm vascular occlusion. The response to PHGMI was studied in the absence and again during the addition of contralateral rhythmic handgrip (RHG; 40 times/min) at 15% (n = 6) and 30% (n = 10) MVC during the second minute of the PHGMI. Additionally, to isolate the effect of central command, response to PHGMI was studied during attempted RHG after sensory nerve blockade (n = 5). RHG for 2 min at 15 and 30% MVC and attempted RHG for 2 min did not increase MSNA. Isometric handgrip elicited an 130 +/- 48% increase in MSNA (P < 0.05), which was maintained during PHGMI. RHG at 15 and 30% MVC elicited an attenuation of MSNA (-10 +/- 7% and -14 +/- 6%, respectively) when performed during the second minute of PHGMI (P < 0.05). In contrast, attempted RHG did not significantly affect MSNA during PHGMI. The findings demonstrate modulation of MSNA during activation of the muscle metaboreflex. The attenuation of metaboreceptor-mediated increases in MSNA appear to be the result of mechanosensitive muscle afferents and not central command.