Contribution Of The Skeletal Muscle Pump To Blood Flow At The Onset Of Contractions

Abstract

There is a substantial reserve for increased skeletal muscle blood flow in response to exercise; however, the interaction of neural regulation, vasoactive metabolites, and mechanical characteristics are incompletely understood. The potential contribution of skeletal muscle contraction, the muscle pump (MP), to exercise hyperemia remains in question. PURPOSE: To determine MP contribution to muscle blood flow during transitions from rest to various metabolic rates. METHODS: Canine gastrocnemius muscle (n=7) was isolated and stimulated to produce rhythmic isometric tetanic contractions. The total blood flow (Qwc) kinetics were fitted by a mono-exponential function (tau), and the blood flow response to contraction with the mechanical contribution of MP removed (Qnc) was calculated. The Qnc calculation estimated blood flow in each contraction cycle as a linear variation of blood flow from a point immediately prior to, until the end of each contraction. The following transitions were investigated with spontaneous blood flow response intact: rest to tetanic contractions of 1/3 s; rest to 2/3 s; rest to 1/1 s; and during the transition from 1/3 s to 2/3s. RESULTS: During 1/3 s stimulation, Qnc exceeded Qwc by the second contraction, indicating a negative contribution of MP to total flow from that point on. During 2/3 s stimulation, Qwc exceeded Qnc for at least six contractions but declined rapidly to a negative contribution beyond 20 s. During 1/1 s stimulation, MP contribution declined from onset but Qwc remained greater than Qnc throughout indicating a positive contribution to total flow. In all transitions tau was not different (p=ns) between Qwc and Qnc: 1/3 s stimulation rate, tau = 12.8 ± 5.5 s vs 11.8 ± 3.2 s, respectively; rest to 2/3 s, tau = 25.6 ± 12.0 s vs 22.1 ± 1.3 s; rest to 1/1 s, tau = 16.7 ± 3.0 s vs 22.1 ± 1.3 s; transition from 1/3 s to 2/3s, tau = 21.2 ± 3.4 s vs 21.7 ± 4.8 s. CONCLUSION: MP likely contributes to local perfusion at exercise onset with diminishing returns as rhythmic contractions continue. The benefit to total skeletal muscle blood flow contributed by MP is dependent on contraction frequency. Supported by BBSRC UK, BB/F019521/1.