Two powerful reflexes exist capable of mediating the cardiovascular adjustments to(, dynamic exercise; the muscle chemoreflex and the arterial baroreflex. When oxygen delivery to the active skeletal muscles falls below metabolic requirements, metabolites accumulate within the active muscle, stimulating group III and IV afferents which induce an increase in arterial pressure in an attempt to maintain muscle blood flow - termed the muscle chemoreflex. In addition, during dynamic exercise whenever vasodilation in skeletal muscle outstrips cardiac pumping capacities, arterial pressure will fall below the operating point of the baroreflex inducing baroreflex mediated peripheral vasoconstriction and tachycardia. The functional importance of each of these reflexes during dynamic exercise is unclear. Situations exist when these reflexes may be combating each other or when these reflexes may induce cardiovascular responses in the same direction. For example, in patient with peripheral vascular disease, blood flow to active muscle may be restricted inducing muscle chemoreflex mediated increases in arterial pressure. In this setting, the arterial baroreflex would oppose any rise in blood pressure. Alternatively, in patients with cardiac limitations, the exaggerated peripheral vasoconstriction and tachycardia observed during dynamic exercise may stem from activation of the muscle chemoreflex (due to underperfused active skeletal muscle) or via activation of the arterial baroreflex (due to lower than normal arterial pressure) or both. Using two well developed animal models; the conscious dog chronically instrumented to control blood flow to the hindlimb skeletal muscles, and the conscious dog with atrioventricular block in which cardiac output can be controlled on a beat-by-beat basis; this proposal is focused on determining the functional importance of the muscle chemoreflex and ar- terial baroreflex during dynamic exercise. The experiments are designed to activate the muscle chemoreflex and the arterial baroreflex in opposition and concurrently in order to determine the strengths (gains) of each reflex, the efferent mechanisms of action of each reflex, and the extent of interaction between reflexes. A key hypothesis addressed is that the functional importance of each reflex during dynamic exercise is intimately dependent on both the level of cardiac output and the level of dynamic exercise. These studies will provide a firm, quantitative, and mechanistic basis for our understanding of the reflex control of the cardiovascular system during dynamic exercise.
