Point: the muscle metaboreflex does restore blood flow to contracting muscles.

Point: The muscle metaboreflex does restore blood flow to contracting muscles Imposed reductions in oxygen delivery to active skeletal muscle during dynamic exercise in conscious dogs evoke a powerful pressor response, termed the muscle metaboreflex. During submaximal work rates, the major mechanism used by this reflex is to increase cardiac output (CO), which generates a greater perfusion pressure for blood flow and thereby partially restores the blood flow deficit in the active skeletal muscle. We directly demonstrated this concept in conscious dogs during submaximal treadmill exercise (20). We measured hind-limb blood flow via a blood flow transducer placed on the terminal aorta just proximal to the iliac arteries. In resting dogs, 87% of hindlimb blood flow is to skeletal muscle (6), thus during exercise the vast majority of this flow signal is flow to the skeletal muscle. We implanted a vascular occluder distal to the flow probe and we measured the arterial pressure both above and below the occluder. When we partially inflated the vascular occluder, the resistance to flow in the hindlimb increased (because we measured the pressure above and below the occluder as well as the flow, the resistance could be directly calculated for the occluder and the vascular resistance of the hindlimb; the sum of these resistances in series is the total hindlimb resistance). According to Ohm's Law, if no increase in arterial pressure occurs, then with the imposed increase in hindlimb resistance, hindlimb blood flow must decrease in proportion to the rise in resistance. Indeed, this is what occurred initially with the inflation of the occluder. Flow fell to the level predicted (HLBF P) by the imposed increase in resistance. However, over the next several minutes, the muscle metaboreflex became engaged and a pressor response developed [previous and subsequent studies using this preparation have shown that this pressor response occurs via increases in CO during submaximal workloads (2, 9, 16, 28)]. With this muscle-metaboreflex-induced increase in perfusion pressure, the observed level of hindlimb blood flow (HLBF O) rose substantially above the predicted level and reached steady state at about one-half between the initial level of flow before partial vascular occlusion (HLBF I) and the HLBF P. We calculated the closed-loop gain of the muscle metaboreflex (G CL) as G CL ϭ (HLBF O Ϫ HLBF P)/(HLBF I Ϫ HLBF P). G CL reflects the ability of a reflex to partially restore any perturbation (e.g., the closed …