act on adrinergic receptors. alpha-1 receptors increase IP3 which increases vascular tone (vasoconstriction). beta-1 receptors activate G proteins that increase cAMP levels, resulting in increased myocardial contractile force.
dobutamine (also a vasodilator)
norepinephrine, phenylephrine (pure)
hypotension secondary to vasodilation (can last for hours due to long half life), thrombocytopenia (much less often with milrinone).
PDE-3 vs. beta agonists
PDE-3 not direct positive chronotropes (better for tachy pts/CAD). In CHF b-receptors are frequently down-regulated to to high endogenous chatecholamine compensatory mechanisms, so PDE-3 inhibitors are clinically much stronger.