SKELETAL-MUSCLE BLOOD-FLOW IN HUMANS AND ITS REGULATION DURING EXERCISE

Regional limb blood flow has been measured with dilution techniques (c ardio-green or thermodilution) and ultrasound Doppler. When applied to the femoral artery and Vein at rest and during dynamical exercise the se methods give similar reproducible results. The blood flow in the fe moral artery is similar to 0.3 L min(-1) at rest and increases linearl y with dynamical knee-extensor exercise as a function of the power out put to 6-10 L min(-1) (Q = 1.94 + 0.07 load). Considering the size of the knee-extensor muscles, perfusion during peak effort may amount to 2-3 L kg(-1) min(-1), i.e. similar to 100-fold elevation from rest. Th e onset of hyperaemia is very fast at the start of exercise with T-1/2 of 2-10 s related to the power output with the muscle pump bringing a bout the very first increase in blood flow. A steady level is reached within similar to 10-150 s of exercise. At all exercise intensities th e blood flow fluctuates primarily due to the variation in intramuscula r pressure, resulting in a phase shift with the pulse pressure as a su perimposed minor influence. Among the many vasoactive compounds likely to contribute to the vasodilation after the first contraction adenosi ne is a primary candidate as it can be demonstrated to (1) cause a cha nge in limb blood flow when infused i.a., that is similar in time and magnitude as observed in exercise, and (2) become elevated in the inte rstitial space (microdialysis technique) during exercise to levels ind ucing vasodilation. NO appears less likely since NOS blockade with L-N MMA causing a reduced blood flow at rest and during recovery, it has n o effect during exercise. Muscle contraction causes with some delay (6 0 s) an elevation in muscle sympathetic nerve activity (MSNA), related to the exercise intensity. The compounds produced in the contracting muscle activating the group III-IV sensory nerves (the muscle reflex) are unknown. In small muscle group exercise an elevation in MSNA may n ot cause vasoconstriction (functional sympatholysis). The mechanism fo r functional sympatholysis is still unknown. However, when engaging a large fraction of the muscle mass more intensely during exercise, the MSNA has an important functional role in maintaining blood pressure by limiting blood flow also to exercising muscles.