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-Receptor agonist activity of phenylephrine in the human
forearm
Department of Anesthesiology and General Clinical Research Center, Mayo Clinic, Rochester, Minnesota 55905
Phenylephrine is generally regarded as a "pure"
1-agonist. However, after treatment of the forearm with
the
-adrenergic-blocking drug phentolamine, brachial artery infusion
of phenylephrine can cause transient forearm vasodilation. To
determine whether this response was
-receptor mediated,
phenylephrine, phentolamine, and propranolol were infused into the
brachial arteries of six healthy volunteers. Forearm vascular
conductance (FVC) was also calculated and expressed as arbitrary units
(units). Infusion of phenylephrine by itself (0.5 µg · dl
forearm volume
1 · min
1) caused a
sustained decrease (P < 0.05) in FVC from 3.5 ± 0.7 to 0.9 ± 0.2 units (P < 0.05). Infusion of
the
-blocker phentolamine increased (P < 0.05)
baseline FVC to 5.7 ± 1.3 units. Subsequent infusion of
phenylephrine after
-blockade caused FVC to increase (P < 0.05) for ~1 min from 5.7 ± 1.3 to a peak
of 13.1 ± 1.8 units. Propranolol had no effect on baseline flow,
and subsequent phenylephrine infusion after
- and
-blockade
caused a small, but significant, sustained decrease in FVC from
5.1 ± 1.0 to 3.6 ± 0.8 units. There were no systemic
effects from the infusions, and saline infusion at the same rate
(1-2 ml/min) had no forearm vasoconstrictor or dilator effects.
These data indicate that in humans phenylephrine can exert transient
2-vasodilator activity when its predominant
-constrictor effects are blocked.
vasoconstriction; adrenergic receptors; blood flow
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