Sympathetic Activation to Limb Venous Distension is Preserved during Muscle Metaboreceptor Stimulation

Abstract

Venous saline infusions in an arterially occluded forearm evokes reflex increases in muscle sympathetic nerve activity (MSNA) and blood pressure (BP) in humans (venous distension reflex). Animal studies suggest that venous distension increases the discharge of Group III and IV afferent nerve endings in peripheral veins. Exercise also activates the Group III and IV afferent nerves in exercising muscles. It is unclear if the venous distension reflex contributes to BP regulation during exercise. We hypothesized that muscle metaboreceptor stimulation might alter the venous distension reflex response. BP (Finapres), heart rate (ECG), and MSNA (microneurography) were assessed in 18 young healthy subjects (13 M, 5 F, 26 ± 1 yrs). In trial 1, saline (5% forearm volume) was infused into veins of an arterially occluded arm (non‐handgrip trial). In the second experiment, subjects performed 2 min static handgrip followed by post exercise circulatory occlusion (PECO) of the arm. During PECO, saline was infused into veins of the arm (handgrip trial). In trial 1, infusion increased MSNA (18.5 ± 1.6 to 29.1 ± 2.4 bursts/min, P < 0.001) and BP (P <0.001) as expected. Handgrip significantly raised MSNA (19.8 ± 1.6 to 28.0 ± 1.8 bursts/min, P < 0.001), BP (P < 0.001) and venous lactic acid (1.0 ± 0.1 to 3.1 ± 0.4, mmol/L, P < 0.001). Venous saline infusion during PECO further raised MSNA (prior infusion vs. end of infusion, 29.0 ± 2.4 to 37.7±2.7 bursts/min, P= 0.001) and BP (P < 0.001). The changes in MSNA evoked by infusion during PECO were not significantly different from the rises in the non‐handgrip trial (delta 8.6 ± 1.5 to delta 10.6 ± 1.8 bursts/min, P = 0.258). The BP responses to the infusion were not different in the two trials. These observations indicate that venous distension reflex responses are preserved during the sympathetic activation mediated by the muscle metaboreceptor stimulation. These data may suggest that venous distension reflex contributes to the BP regulation during exercise.Support or Funding InformationSupported by NIH R01 HL144781 (Cui and Sinoway) and UL1 TR002014 (Sinoway), and Pennsylvania Department of Health Tobacco CURE Funds SAP#4100079742 (Cui).