IDENTIFICATION OF ENDOTHELIN RECEPTOR SUBTYPES IN RAT CILIARY BODY USING SUBTYPE-SELECTIVE LIGANDS

The endothelins are important vasoactive ocular peptides and there is some evidence that they may modulate intraocular pressure. We investig ated the existence and localization of endothelin receptor subtypes us ing subtype selective ligands in rat ciliary body. Scatchard transform ation of saturation binding experiments revealed that the K-D, and B-m ax,,, for [I-125]ET-1,, and [I-125]ET-3 to, membranes from ciliary bod y were 41.7 +/- 9 pM and 236 +/- 20 fmol mg(-1) protein and 37.8 +/- 0 .4 pM and 160 +/- 2.0 fmol mg(-1) protein, respectively. Competitive e xperiments in the presence of cyclic pentapeptide BQ123 (selective for ETA receptors) and BQ3020 (selective for ETB receptors), demonstrated the existence of ETA and ETB receptors in a ratio of 35:65. Cross-lin king of [I-125]ET-1 and [I-125]ET-3 to, ciliary body membranes resulte d in the labeling of two bands with apparent molecular masses of 52 an d 34 kDa, suggesting that ETA and ETB receptors have similar molecular mass. The 34 Kda band is a proteolytic degradation product of the 52 Kda band. Autoradiographic results show that specific [I-125]ET-1 bind ing sites, displaced by BQ123 and BQ3020, are localized to the ciliary epithelium, supporting the idea that ETA and ETB subtype receptors ex ist in this tissue. (C) 1998 Academic Press Limited.