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

Area of Science:

• Cardiovascular Physiology
• Autonomic Nervous System Regulation
• Pharmacology

Background:

• Head-up tilt (HUT) is a common physiological stressor used to assess autonomic cardiovascular regulation.
• Beta-adrenergic blockade can alter vascular responses to physiological challenges.
• Understanding the mechanisms of vasoconstriction during HUT is crucial for managing cardiovascular conditions.

Purpose of the Study:

• To investigate the effect of propranolol (a beta-receptor blocker) on forearm blood flow regulation during 30-degree head-up tilt.
• To differentiate between centrally mediated and peripherally mediated vasoconstrictor mechanisms during beta-blockade.

Main Methods:

• 15 healthy subjects underwent 30-degree head-up tilt before and during propranolol treatment.
• Local 133Xe washout technique was used to estimate subcutaneous and skeletal muscle blood flow.
• Proximal nerve blockade with lidocaine was employed in some experiments.

Main Results:

• Head-up tilt induced greater vasoconstriction in both subcutaneous and skeletal muscle during propranolol treatment compared to baseline.
• Lidocaine blockade abolished the vasoconstrictor response in subcutaneous tissue, suggesting neurogenic mediation.
• Local propranolol administration in skeletal muscle did not affect the tilt-induced vasoconstriction.

Conclusions:

• The augmented vasoconstriction during head-up tilt with propranolol is primarily mediated by centrally elicited (baroreceptor) and neurogenically mediated impulses.
• This response is not due to the unmasking of peripheral alpha-receptors.
• Central autonomic pathways play a significant role in regulating vascular tone during beta-adrenergic blockade under orthostatic stress.