The Skeletal Muscle Pump During Contractile Transitions

Abstract

The aim of this study was to characterize the contribution of skeletal muscle contraction to the immediate hyperemic blood flow response as well as the continued involvement in matching tissue perfusion to elevated metabolic rates. There exists a substantial reserve for increased blood flow within skeletal muscle in response to dynamic exercise however the interaction of neural regulation, vasoactive metabolites, and mechanical characteristics are incompletely understood. To address questions concerning blood flow in response to transitions from rest to various metabolic rates an isolated canine gastrocnemius in situ model was employed. Seven canines were used for this investigation with the gastrocnemius muscles isolated for isometric contractions with tetanic stimulation. Measures were made for blood flow, blood pressure, force, and near infrared spectrophotometric analyses under conditions of spontaneous blood flow. The following transitions were investigated: from 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 all with spontaneous blood flow response intact. Additionally, an estimation for the blood flow response with no mechanical contribution from the muscle pump was made with determination for the kinetics of the estimate. The time constant (tau) for the blood flow response was not significantly different between the measured flow with contraction (Qwc) and the estimate with no contraction (Qnc) for the 1/3 s stimulation rate (12.8 ± 5.5 s vs 11.8 ± 3.2 s respectively), the transition from the high baseline (1/3 s) to a higher rate (2/3HB) (21.2 ± 3.4 s vs 21.7 ± 4.8 s respectively), from rest to 2/3 s (25.6 ± 12.0 s vs 22.1 ± 1.3 s respectively), or from rest to 1/1 s (16.7 ± 3.0 s vs 22.1 ± 1.3 s respectively). Initially, for this model, there is a positive contribution to total blood flow provided by the contracting skeletal muscle, however this diminishes within the first few contractions. At higher stimulation rates the net effect of the contracting muscle is to limit local blood flow in the exercising muscle. In conclusion, the muscle pump may contribute to local perfusion at exercise onset with diminishing returns as rhythmic contractions continue. In the steady state the main contributions of the muscle pump is to aid in the maintenance of central hemodynamics.