|Systematic (IUPAC) name|
|AHFS/Drugs.com||Micromedex Detailed Consumer Information|
|Usually IV or IM|
|Biological half-life||19 minutes|
|ATC code||C04AB01 (WHO) V03AB36 (WHO)|
|Molar mass||281.352 g/mol|
|(what is this?)|
Non-selective α-blockers can cause a much more pronounced reflex tachycardia than the selective α-1 blockers. Like the selective α-1 blockers, phentolamine causes a relaxation of systemic vasculature, leading to hypotension. This hypotension is sensed by the baroreceptor reflex, which results in increased sympathetic nerve firing on the heart, releasing norepinephrine. In response, the β-1 adrenergic receptors on the heart increase its rate, contractility, and dromotropy, which help to offset the decrease in systemic blood pressure. Unlike the α-1 selective blockers, phentolamine also inhibits the α2 receptors, which function predominantly as presynaptic negative feedback for norepinephrine release. By abolishing this negative feedback phentolamine leads to even less regulated norepinephrine release, which results in a more drastic increase in heart rate.
It also has usefulness in the treatment of cocaine-induced cardiovascular complications, where one would generally avoid Beta-blockers (e.g. metoprolol), as they can cause unopposed alpha-adrenergic mediated coronary vasoconstriction, worsening myocardial ischemia and hypertension. It is important to note that phentolamine is not a first-line agent for this indication. Phentolamine should only be given to patients who do not fully respond to benzodiazepines, nitroglycerin, and calcium channel blockers.
When given by injection it causes blood vessels to dilate, thereby increasing blood flow. When injected into the penis (intracavernosal), it increases blood flow to the penis, which results in an erection.
It may be stored in crash carts to counteract severe peripheral vasoconstriction secondary to extravasation of peripherally placed vasopressor infusions, typically of norepinephrine. Epinephrine infusions are less vasoconstrictive than norepinephrine as they primarily stimulate beta receptor more than alpha receptors, but the effect remains dose dependent.
Phentolamine is marketed in the dental field as a local anesthetic reversal agent. Branded as OraVerse, it is a phentolamine mesylate injection designed to reverse the local vasoconstrictor properties used in many local anesthetics to prolong anesthesia. 
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