Beta Receptors Beta receptors have been further subdivided into beta1 and beta2 receptors. It should be pointed out that beta3 and beta4 receptors have recently been isolated, cloned and characterized. The beta3 receptor may be involved in regulating the metabolism of fatty acids. This receptor could be the site of antiobesity drugs in the future. The functions of the beta4 receptor remain to be discovered. For the purposes of this material, we will focus on the beta1 and beta2 receptors only. The classification of beta receptors is based on the interaction of a series of drugs with these receptors. The ability of epinephrine, norepinephrine and isoproterenol to increase the force of myocardial contraction was examined and the dose-response curves shown below were obtained. Equilibrium dissociation constants for these ligands were ISO, 80 nm, E, 800 nM, and NE, 1000 nM. Thus, the rank order of affinities for the beta receptor in the heart is ISO>E>NE. A beta receptor with these characteristics is referred to as a beta1 receptor.