Effects of graded LBNP on MSNA and interstitial norepinephrine

Exposure to lower body negative pressure (LBNP) leads to an increased activation of the sympathetic nervous system (SNS) and an increase in muscle sympathetic nerve activity (MSNA). In this study, we examined the relationship between MSNA and interstitial norepinephrine (NEi) concentrations during LBNP. Twelve healthy volunteers were studied (26 +/- 6 yr). Simultaneous MSNA and microdialysis data were collected in six of these subjects. Measurements of MSNA (microneurography) and NEi (microdialysis, vastus lateralis) were performed at rest and then during an incremental LBNP paradigm (-10, -30, and -50 mmHg). MSNA rose as a function of LBNP (P < 0.001, n = 12). The plasma norepinephrine (NEp) concentration was 0.9 +/- 0.1 nmol/l at rest (n = 12). NEi measured in six subjects rose from 5.2 +/- 0.8 nmol/l at rest to 17.0 +/- 1.7 nmol/l at -50 mmHg (P < 0.001). Of note, the rise in NEp with LBNP was considerably less compared with the changes in NEi (Delta21 +/- 6% vs. Delta197 +/- 52%, n = 6, P < 0.015). MSNA and NEi showed a significant linear relationship (r = 0.721, P < 0.004). Activation of the SNS increased MSNA and NEi levels. The magnitude of the NEi increase was far greater than that seen for NEp suggesting that NE movement into the circulation decreases with baroreceptor unloading.