Long-duration bed rest modifies sympathetic neural recruitment strategies in males and females

To understand the impact of physical deconditioning with head-down tilt bed rest (HDBR) on the malleability of sympathetic discharge patterns, we studied a) baseline integrated muscle sympathetic nerve activity (MSNA; microneurography) from 13 females participating in the WISE-2005 60-day HDBR study (Female, retrospective analysis), b) integrated MSNA and multi-unit action potential (AP) analysis in 13 males (Male) performed on data collected at baseline and during physiological stress imposed by end-inspiratory apnea, in a new 60-day HDBR study, and c) a repeatability study (Control, n = 6, retrospective analysis, 4 weeks between tests). Neither baseline integrated burst frequency nor incidence were altered with HDBR (both P>0.35). However, baseline integrated burst latency increased in both HDBR studies (Male: 1.35 ±0.02 to 1.39 ±0.02 s, P<0.01; Female: 1.23 ±0.02 to 1.29 ±0.02 s, P<0.01), while Controls exhibited no change across two visits (1.25 ±0.02 to 1.25 ±0.02 s, Group-by-Time Interaction: P=0.02). With the exception of increased AP latency (P=0.03), Male baseline AP data did not change with HDBR (all P>0.19). The change in AP frequency on going from baseline to apnea ( 94 ±25 to 317 ±55 AP/min, P<0.01), and the number of active sympathetic clusters per burst ( 0 ±0.2 to 1 ±0.2 clusters/burst, P=0.02), was greater post- compared to pre-HDBR. The change in total clusters with apnea was 0 ±0.5 pre- and 2 ±0.7 clusters post-HDBR (P=0.07). These data indicate that 60-days HDBR modified discharge characteristics in baseline burst latency and sympathetic neural recruitment during apneic stress.

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