IS RAPID RISE IN VASCULAR CONDUCTANCE AT ONSET OF DYNAMIC EXERCISE DUE TO MUSCLE PUMP

We tested the hypothesis that rapid increases in muscle blood flow and vascular conductance (C) at onset of dynamic exercise are caused by t he muscle pump. We measured arterial (AP) and central venous pressure (CVP) in nine awake dogs, eight with atrioventricular block, pacemaker s, and ascending aortic flow probes for control of cardiac output (CO) (2 also had terminal aortic flow probes). One dog had only an iliac a rtery probe. At exercise onset (0 and 10% grade, 4 mph) C and CVP rose to early plateaus, and AP reached a nadir, all in 2-5 s. At 20% grade and 4 mph, C increased continuously after its initial sudden rise. Ti ming and magnitude of initial change in conductance (DELTAC) were inde pendent of CO, AP, work rate (change in grade at constant speed), or a utonomic function (blocked by hexamethonium). Speed of initial DELTAC and its independence from work rate and blood flow ruled out metabolic vasodilation as its cause; insensitivity to AP and autonomic blockade ruled out myogenic relaxation and sympathetic vasodilation as causes of sudden DELTAC. Sensitivity to contraction frequency (not work per s e) implicates the muscle pump. When reflexes were blocked, a large sec ondary rise in C, presumably caused by metabolic vasodilation, began a fter 10 s of mild exercise. When reflexes were intact in mild exercise , C was lowered below its initial plateau by sympathetic vasoconstrict ion, which partially raised AP from its nadir toward its preexercise l evel. Our conclusion is that dynamic exercise has a large rapid effect on C that is not explained by known neural, metabolic, myogenic, or h ydrostatic influences. We propose that muscle relaxation draws blood f rom arteries into veins at exercise onset, and this reduces AP. Venous blood is pumped centrally, raising CVP. The consequent increase in mu scle blood flow, coupled with falling AP and rising CVP, markedly incr eases calculated C, which is actually a ''virtual conductance'' across a pump.