NCIt definition : A synthetic imidazoline with alpha-adrenergic antagonist activity. As a competitive
alpha-adrenergic antagonist, phentolamine binds to alpha-1 and alpha-2 receptors,
resulting in a decrease in peripheral vascular resistance and vasodilatation. This
agent also may block 5-hydroxytryptamine (5-HT) receptors and stimulate release of
histamine from mast cells.;