Effect of vasoconstrictor tone on pressure responses to epinephrine and norepinephrine

Increasing sympathetic activity by either carotid occlusion or raising intracranial pressure increases the incidence and magnitude of the depressor response to both epinephrine and norepinephrine in dogs whose total circulation is maintained at a constant rate by a perfusion pump. Decreasing sympathetic activity by raising carotid sinus pressure reduces the incidence of this response to these drugs. Similarly, in the hindlimb perfused at a constant rate, the depressor response to these agents increased in frequency and magnitude during carotid occlusion or electrical stimulation of the sympathetic chain. This enhancement of the depressor effects seems to be a peripheral effect rather than a direct inhibitory effect of the catecholamines upon transmission through sympathetic ganglia.