Augmented vasoconstrictor response to head-up tilt in peripheral tissues during beta-receptor blockade.

Subcutaneous and skeletal muscle blood flow in the forearm during 30 degrees head-up tilt was studied in 15 healthy subjects before and during treatment with propranolol. Relative blood flow was estimated by the local 133Xe washout technique. Head-up tilt elicited greater vasoconstriction in both tissues during beta-receptor blockade as compared to the pretreatment period. Proximal nerve blockade with lidocaine prevented the vasoconstrictor response in subcutaneous tissue to the tilt. In skeletal muscle injection of a low dose of propranolol had no effect on the vasoconstrictor response to tilt. Therefore, the augmented vasoconstrictor response to head-up tilt during beta-receptor blockade is most probably due to centrally elicited (baroreceptor) and neurogenically mediated impulses to resistance vessels in peripheral tissues and not to "unmasking" of peripheral alpha-receptors.