Alpha 1-adrenoceptors in the corneal endothelium.
The potent alpha 1-adrenoceptor antagonist, [3H]prazosin, exhibited high affinity, specific and reversible binding to intact rabbit, bovine and human corneal endothelial cells in culture. The binding of 1 nM [3H]prazosin to rabbit cells reached a steady-state level within 10 min at 37 degrees C. Under these conditions, approximately 50% of the [3H]prazosin bound was specific. The level of specific [3H]prazosin binding was concentration-dependent, but Rosenthal analysis indicated that [3H]prazosin bound to at least two sites. One site exhibited a high affinity for [3H]prazosin (Kd = 0.2 nM), but a relatively low binding capacity (Bmax = 175 fmol bound mg-1 protein); the other site showed a relatively low affinity for the radioligand (Kd = 85 nM), but a much higher binding capacity (1280 fmol mg-1). Several known alpha 1-adrenoceptor antagonists and agonists competitively inhibited [3H]prazosin binding at the high affinity site when incubated with the radioligand. The relative potencies of these competing ligands were generally consistent with their binding affinities for alpha 1-adrenoceptors in other tissues. Phenylephrine stimulated the rate of hydrolysis of phosphatidylinositol 4,5-bisphosphate to inositol 1,4,5-trisphosphate by 63% in these cells. This stimulation was inhibited by 52% if phentolamine was also present during the incubation. These data indicate that corneal endothelial cells have alpha 1-adrenoceptors which can modulate polyphosphoinositide turnover in this tissue.