Sympathetic Vasoconstrictive Responses during Exercise- or Drug-Induced Vasodilatation

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Abstract
The effect of exercise on the vasoconstrictive response to sympathetic stimulation was studied by observing the changes in perfusion pressure in dogs' hindlimbs perfused at constant flow. During steady-state simulated exercise, a time-dependent increase occurred between 10 and 30 minutes in the magnitude of response to lumbar trunk stimulation or carotid sinus hypotension. This was present (although to a reduced Degrees) in drug-induced vasodilatation and in exercising limbs perfused at constant pressure. The time-dependent change was not due to (1) repetition of stimulus, (2) artifact from muscle electrodes, (3) circulating vasoactive substances, (4) skin circulation, or (5) pH, PCO2, or temperature of the perfusing blood. When norepinephrine was used as the vasoconstrictive stimulus, no time-dependent change was noted. After stabilization, the frequency-response curve of exercising muscle exceeded that at rest at all blood concentrations of norepinephrine and at all save the lowest frequencies of stimulation. It was concluded that vascular smooth muscle can develop greater tension from greater initial length. However, during exercise some factor which operates between the sympathetic nerve ending and the receptor site on vascular smooth muscle initially tends to depress this enhanced ability to develop tension.