Muscle sympathetic nerve responsiveness to static contraction is not altered under hypoxia

The purpose of this study was to determine if response in muscle sympathetic nerve activity (MSNA) to static muscle contraction alters or not during peripheral chemoreceptor stimulation with hypoxia. MSNA was recorded from the tibial nerve using a microneurographic technique in seven healthy subjects in a sitting position. They performed a static handgrip exercise (SHG) for 2 min under normobaric normoxia and hypobaric hypoxic conditions corresponding to an altitude of 4,000 m (460 mmHg). MSNA represented as burst rate and total sympathetic nerve activity (TSNA) (burst rate X mean burst amplitude) at rest increased by 7.7 bursts/min and by 60%, respectively, compared to those under normoxia. During the exercise, MSNA increased over control values before exercise both under normoxia and hypoxia. TSNA increased during the first and second minute of SHG by 19% (p greater than 0.05) and 35% (p less than 0.05) in normoxia and by 15 (p greater than 0.05) and 34% (p less than 0.05) in hypoxia over the control value, respectively, while the absolute intensity of MSNA during SHG was higher under hypoxia. The response of MSNA to SHG which showed algebraic sums was the same under normoxia and hypoxia, that might relate to consist of afferent pathways independent from carotid chemoreflex.