β-Adrenergic receptors in human trabecular meshwork. Identification and autoradiographic localization

β-Adrenergic receptors were identified in slide-mounted sections of human trabecular meshwork by in vitro labeling, light microscopic autoradiography. Autoradiograms were generated after incubation of slide-mounted tissue sections with ¹²⁵I-cyanopindolol, a selective high-affinity probe for β-adrenergic receptors. Experiments in which an excess of either unlabeled Practolol (β₁ selective ligand) or Zinterol (β₂ selective ligand) were included suggested that most of the receptors were of the β₂ subtype. Data from displacement studies using increasing concentrations of the highly specific β₁- and β₂-adrenergic receptors antagonists, ICI-89,406 and ICI-118,551, respectively, confirmed the predominance of the β₂-adrenergic receptors. Similar results obtained with cultured trabecular endothelial cells suggest that the β-adrenergic receptors may be associated with the endothelial cells of the trabecular meshwork in vivo. These results provide anatomic evidence for the hypothesis that β-adrenergic agents improve outflow by direct action on the trabecular meshwork and provide a rationale for the development of more selective β₂-adrenergic agents to increase outflow facility.