Alpha 1-adrenoceptors in human corneal epithelium

Specific binding of the potent, selective alpha 1-adrenoceptor antagonist 3H-prazosin was demonstrated in cultured human corneal epithelial cells. Specific binding of the radioligand was concentration-dependent between 0.5 and 6 nM, with apparent saturation of receptor sites seen at higher concentrations. The cells exhibited a maximum binding capacity for 3H-prazosin of 225 fmol/mg of cellular protein and a dissociation constant of 2 nM. The binding of 3H-prazosin was competitive with known alpha 1-adrenoceptor ligands and was reversible. Epithelium of intact human corneas also exhibited specific 3H-prazosin binding, as did cultures of bovine and rabbit corneal epithelium. The alpha-adrenergic agonist methoxamine significantly stimulated phosphatidylinositol 4,5-bisphosphate hydrolysis, measured as myoinositol trisphosphate accumulation in cultures of human corneal epithelium. This stimulation was inhibited by the presence of prazosin during the assays. These findings indicate the existence of specific, reversible, high-affinity receptors for alpha 1-adrenoceptors that regulate inositol phosphate turnover in human, rabbit, and bovine corneal epithelial cells.