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Determination of alpha-adrenergic blocking potency.

A M Shepherd¹, C M Kwan, C L Brodie

¹Department of Pharmacology, University of Texas Health Science Center, San Antonio 78284-7764.

Clinical Pharmacology and Therapeutics

|January 1, 1991

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Autonomic blockade, including beta-adrenergic and muscarinic, enhances human sensitivity to phenylephrine. This finding indicates that alpha-adrenergic drug potency can be accurately determined without blocking these homeostatic reflexes.

Area of Science:

Pharmacology
Cardiovascular Physiology

Background:

Determining alpha-adrenergic blocking potency typically involves phenylephrine dose-response curves.
Homeostatic reflexes activated by blood pressure changes can influence these measurements.

Purpose of the Study:

To investigate the impact of autonomic blockade on phenylephrine's dose-response relationship in humans.
To assess if blocking homeostatic mechanisms affects the determination of alpha-adrenergic blocking potency.

Main Methods:

Phenylephrine dose-response curves were analyzed under various conditions: control, propranolol/atropine infusion, prazosin blockade, and combined blockade.
Measurements included threshold dose, slope, and dose required for a 20 mm Hg blood pressure increase.

Main Results:

Related Experiment Videos

Propranolol-atropine infusion significantly decreased phenylephrine's threshold dose and the dose for a 20 mm Hg increase, while increasing the dose-response slope.
Autonomic blockade did not alter the determination of prazosin's alpha-adrenergic blocking potency.

Conclusions:

Beta-adrenergic and muscarinic blockade increase sensitivity to phenylephrine by modifying its dose-response curve.
The alpha-adrenergic blocking potency of drugs like prazosin can be reliably determined irrespective of blocking homeostatic blood pressure regulation.