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  • Title: Beta-adrenoceptor antagonist activities and binding affinities of timolol enantiomers in rat atria.
    Author: Karhuvaara S, Kaila T, Huupponen R.
    Journal: J Pharm Pharmacol; 1989 Sep; 41(9):649-50. PubMed ID: 2573714.
    Abstract:
    S-Timolol is an effective anti-glaucoma drug, but has potentially hazardous side effects. Recently, R-timolol, also, has been reported to be effective in lowering elevated intraocular pressure. In the present study, the beta-adrenoceptor antagonist activities and binding of R- and S-enantiomers of timolol have been examined on rat atrial preparations. The beta-antagonistic activities were investigated using spontaneously beating rat heart atria. Both timolol enantiomers inhibited (-)-isoprenaline-induced chronotropic action competitively. S-Timolol was about 54 times more potent than R-timolol. The apparent binding affinities of timolol enantiomers to beta 1- and beta 2-adrenoceptors were determined by a radioligand binding assay using (-)-[125I]iodocyanopindolol (ICYP) as a marker and CGP 20712 A as a beta 1- and ICI 118,551 as a beta 2-adrenoceptor antagonist. Both enantiomers of timolol inhibited ICYP binding in nanomolar concentrations with Hill coefficients near unity. Neither enantiomer showed selectivity between beta 1- and beta 2-adrenoceptors, but R-timolol was approximately 30 times less active than S-timolol. It is concluded that R-timolol is a relatively potent non-selective beta-adrenoceptor blocking agent, but may possibly exert a more localized beta-adrenoceptor action in the eye than S-timolol, thus improving the safety of ocular timolol therapy.
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