Chihiro Nosaka (1), Hitoshi Ishikawa (2), Isao Haruno (2), Takeshi Yoshitomi
(2), Hiroshi Kase (1), Satoshi Ishikawa (2) and Yoshiteru Harada (3,*)
(1) Department of Biochemistry, Pharmaceutical Research Institute, Kyowa
Hakko Kogyo Co., Ltd., Shizuoka 411, Japan
Departments of (2) Ophthalmology and (3) Pharmacology, Kitasato University
School of Medicine, Sagamihara 228, Japan
(*) To whom correspondence should be addressed.
Abstract: We previously suggested the presence of functionally
atypical endothelin (ET) A receptors in the rabbit iris sphincter. Here,
we further characterized the ET receptor by a radioligand-receptor binding
study utilizing a membrane fraction of the rabbit iris. In addition, we
functionally confirm the presence of an atypical ETA receptor
in the iris dilator similar to that in the iris sphincter. In binding experiments,
[125I]ET-1 was completely displaced by ET-3 in a biphasic fashion,
but only partially by BQ-123 and ETB ligands. In the presence
of RES-701, ET-3 and sarafotoxin (SRTX)-b completely displaced [125I]ET-1
in a monophasic fashion, but with shallow slopes. Moreover, ET-1, ET-3 and
SRTX-b completely displaced [3H]BQ-123 with IC50 values
of 0.8, 81 and 4.4 nM, respectively, but with slopes of ET-3 and SRTX-b
being again shallow. In iris dilator muscles, ET-3 showed lower and SRTX-b
showed higher contractile activities than ET-1. SRTX-c was inactive. BQ-123
more preferentially antagonized ET-3 and SRTX-b than ET-1, with the Schild
plot slope of SRTX-b being shallow. Thus, functional experiments suggested
the presence of atypical ETA receptors in the iris dilator similar
to the iris sphincter. However, the binding experiments suggested the presence
of rather typical ETA- and ETB-like receptors. Therefore,
we apparently failed to show ET binding sites corresponding to functionally
atypical ETA receptors.
Keywords: Rabbit iris sphincter and dilator, Atypical endothelin receptor,
BQ-123, Sarafotoxin, Radioligand-receptor binding