Bedrest and muscle reflex responses to exercise

The effect of 14 days of head down (-6°) bedrest (HDBR) on muscle sympathetic nerve activity (MSNA) during static handgrip (SHG) exercise was measured in healthy volunteers. Mean arterial pressure (MAP) (Finapres), heart rate (HR) and MSNA (peroneal nerve) burst frequency (B_f) were measured at rest and at fatigue (-20% MVC) before and following HDBR. The MAP and HR responses to SHG were similar in pre and post HDBR tests. MSNA B_f at rest (n=10) was reduced in the post (16.7±2.6 min^-1), compared with the pre (23.0±2.0 min^-1) tests (P<0.05). End exercise B_f (n=8) (34.3±3.8 min^-1 vs. 23.3±3.7 min^-1; pre vs. post HDBR, respectively) was reduced following HDBR (P<0.05). The reductions in MSNA persisted when B_f was normalized to heart rate, or to the increase above rest. In contrast, the increase in MSNA during a cold pressor test was not altered by HDBR (n=4). To examine if the reduction in exercise MSNA with HDBR was related to metabolic alterations in response to SHG exercise, deep venous lactate, pH and HbO₂ saturation were measured during rhythmic handgrip exercise with +50 mmHg positive pressure about the forearm (n=5). End exercise lactate, pH and HbO₂ saturation were not different after HDBR. Thus, HDBR resulted in a reduced MSNA B_f at rest and attenuated the ability to elevate MSNA during fatiguing exercise. The reductions in MSNA did not appear to be due to alterations in metabolite-sensitive afferent stimulation in the working muscle.