1. Relative changes in blood flow and vascular resistance in arm and leg muscle during head-up tilt at 45° were studied in eight patients with complete cervical spinal cord transection and in 13 healthy volunteers.
2. Muscle blood flow was measured by the local 133Xe washout method.
3. In forearm muscle kept at heart level blood flow remained constant in the tetraplegic patients during head-up tilt, in contrast to that seen in the normal subjects, where blood flow decreased by 30%. In the dependent leg muscle, head-up tilt caused a decrease in blood flow of 46% and 40% in the patients and normals, respectively. Abolition of the local veno-arteriolar axon reflex, by inducing local counter-pressure to prevent venous distension in the dependent leg muscle, reduced the decrease in blood flow to 24% and 23%, respectively. Thus, the vascular response to head-up tilt differed significantly in forearm muscle between the two groups, whereas no difference was seen in the leg muscle.
4. The absence of the vasoconstrictor response in forearm muscle indicates that postural sympathetic reflexes to this region depend on sympathetic reflexes integrated in centres located rostrally to the spinal cord. The results further suggest that local veno-arteriolar axon reflexes as well as spinal reflexes contribute to the observed vasoconstriction in the leg muscle.