PARALLEL MODULATION OF SYMPATHETIC NEURAL CONTROL OF BLOOD FLOW AND TISSUE OXYGENATION IN CONTRACTING MUSCLE

Abstract

Metabolic modulation of sympathetic vasoconstriction has been demonstrated in anesthetized rodents using pulsed Doppler to measure hindlimb blood flow velocity and in conscious humans using near infrared spectroscopy (NIRS) to measure regional forearm muscle oxygenation. Whether these techniques provide equivalent information is incompletely understood. PURPOSE: To test the hypothesis that sympathetic neural control of blood flow velocity and tissue oxygenation are attenuated in parallel in contracting skeletal muscle. METHODS: In 8 anesthetized Sprague Dawley rats, we measured blood flow velocity with a Doppler probe on the femoral artery and tissue oxygenation with NIRS optodes placed directly on the surface of the ipsilateral gastrocnemius muscle. Responses to lumbar sympathetic nerve stimulation (LNS) were recorded with the hindlimb at rest and during intermittent tetanic contractions performed at 33% of maximal tension. RESULTS: In resting hindlimb, 1 Hz LNS decreased femoral blood flow velocity (−16 ± 2%) and tissue oxygenation (−27 ± 3%), indicating sympathetic vasoconstriction. In contracting hindlimb, LNS caused minimal changes in femoral blood flow velocity (+2 ± 3%) and tissue oxygenation (−1 ± 3%), indicating metabolic modulation of sympathetic vasoconstriction. Blood flow velocity and tissue oxygenation responses to 1 Hz LNS were highly correlated in both resting and contracting hindlimb (r = 0.87). Similar findings were obtained for 2.5 Hz and 5 Hz LNS. CONCLUSIONS: In rat skeletal muscle sympathetic neural control of tissue oxygenation, like blood flow velocity, is sensitive to modulation by metabolic products of contraction. The strong correlation between responses measured by NIRS and Doppler velocimetry implies that such modulation occurs within the skeletal muscle microcirculation. Under appropriately controlled conditions, tissue oxygenation responses measured by NIRS can be used to provide a sensitive and reliable index of sympathetic vasoconstriction in the skeletal muscle microcirculation. Supported by NIH grant HL64784.