The ability of the same compounds to produce bronchodilation was examined and a different set of dose response curves and equilibrium dissociation constants were obtained. The dissociation constants were ISO, 80 nm, E, 800 nM, and NE, 10,000 nM. Notice how the ability to activate the beta receptors is dependent on the structure of the drugs under study. Clearly then the receptor in the lung is different from that in the heart and is referred to as a beta2 receptor. These examples serve to illustrate how the equilibrium dissociation constant is often used as a “finger print” to identify a receptor. Regardless of its location, the same receptor will have the same dissociation constants for agonists and antagonists. As shown here, the differences in equilibrium dissociation constants are an indication of heterogeneity within the main receptor population. As will be illustrated throughout the course, receptor subtypes are routinely exploited in drug development to make ligands that interact selectively with one subtype in preference to another