Passive vasodilatation of the hind limb after sympathetic preganglionic stimulation.

1. The post-vasoconstriction dilatation (PVCD) observed after release of carotid clamping has been attributed to baroreceptor stimulation. However, PVCD has been observed in baroreceptor deafferented animals following cessation of stimulation of the diencephalon or of chemoreceptor fibres.2. The effects of preganglionic (ventral roots) and post-ganglionic (sciatic nerve) stimulation upon the vascular resistance of the hind limb of anaesthetized cats and dogs was investigated. The limb was vascularly isolated and perfused at constant flow with the animal's own arterial blood. The animals were atropinized and neuromuscular block was obtained with gallamine triethiodide.3. PVCD was observed upon cessation of stimulation of intact preganglionic fibres or of their peripheral stump.4. PVCD was dependent upon the existence of neurogenic vasomotor tone in the vessels, but independent from baroreceptor influences.5. PVCD was never observed after cessation of stimulation of the post-ganglionic fibres.6. Similar results were obtained after removal of the skin of the perfused limb.7. The possibility that the post-vasoconstriction dilatation observed in these experiments is due to the existence of a preganglionic inhibitory system is discussed.