clenbuterol is an asthma medicine, so I would assume that it hits beta-2 - the one's that, when bound, stimulate the relaxation of the smooth muscle fibers of the broncioles in the lungs, opening the airways, and making for easier breathing. I think yohimbine is an alpha-2 agonist. not sure about ephedrine, but I bet it binds lots of different adrenergic receptors.
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