Although alpha 2 receptors are found on both presynaptic neurons and postynaptic cells, they work mainly as autoreceptors to mediate feedback inhibition of sympathetic transmission.
In addition to neurons, alpha 2 receptors are located in other regions, like pancreatic beta cells and platelets.
The diagram below shows how when activated, these receptors act as inhibitory autoreceptors (they inhibit norepinephine release from adrenergic neurons) and as inhibitory heteroreceptors (they inhibit acetylcholine release from cholinergic neurons).
In addition, activation of alpha 2 receptors on pancreatic beta cells membranes inhibits insulin release.
The metabolic stability and the reactivity of a library of candidate drug compounds have to be assessed for drug metabolism and toxicological studies. Many methods have been proposed for quantitative predictions in drug metabolism; one example of a recent computational method is SPORCalc .  If the chemical structure of a medicinal compound is altered slightly, this could slightly or dramatically alter the medicinal properties of the compound depending on the level of alteration as it relates to the structural composition of the substrate or receptor site on which it exerts its medicinal effect, a concept referred to as the structural activity relationship (SAR). This means that when a useful activity has been identified, chemists will make many similar compounds called analogues, in an attempt to maximize the desired medicinal effect(s) of the compound. This development phase can take anywhere from a few years to a decade or more and is very expensive. 
Beta-blockers that are used clinically can be divided into two classes: 1) non-selective blockers (block both β 1 and β 2 receptors) , or 2) relatively selective β 1 blockers ("cardioselective" beta-blockers). Some beta-blockers have additional mechanisms besides beta-blockade that contribute to their unique pharmacologic profile. The two classes of beta-blockers along with specific compounds are listed in the following table. Additional details for each drug may be found at . The clinical uses indicated in the table represent both on and off-label uses of beta-blockers. For example, a given beta-blocker may only be approved by the FDA for treatment of hypertension; however, physicians sometimes elect to prescribe the drug for angina because of the class-action benefit that beta-blockers have for angina.