Abstract
The isoflavones tectoridin (TTR) and 3′-hydroxy TTR (3′-TTR) were isolated from an Ayurvedic herbal preparation Vacä and evaluated for their affinity and effect on ryanodine receptors (RyR) using junctional sarcoplasmic reticulum vesicles (JSRVs). In [3H]ryanodine displacement binding affinity assays, TTR and 3′-TTR exhibited IC50 values of 17.3 ± 1.3 μM (Kd = 6.7 ± 0.4 μM) and 6.6 ± 1.4 μM (Kd = 2.4 ± 0.2 μM), respectively, for fast skeletal muscle RyR (RyR1) compared with an IC50 value for ryanodine of 6.2 ± 0.4 nM (Kd = 2.4 nM). TTR demonstrated a 3-fold higher affinity for cardiac RyR (RyR2) [IC50 value of 5.2 ± 0.6 μM (Kd = 0.95 ± 0.3 μM)] than for RyR1. The displacement isotherms for both TTRs paralleled that for ryanodine, consistent with the notion that all three are likely binding to similar site(s) on the receptors. Calcium efflux from and calcium influx into JSRVs were used to measure function effects of TTRs on binding to RyR. In calcium efflux assays, TTR (up to 1 mM) enhanced the release of 45Ca2+ from JSRVs in a concentration-dependent manner (EC50act of 750 μM). Higher concentrations deactivated (partially closed) RyR1. 3′-TTR had similar effects, but was approximately 2-fold more potent, exhibiting an EC50act value of 480 μM. Using passive calcium influx assays, TTR activated and deactivated RyR1 in a time- and concentration-dependent manner. The aglycone tectorigenin also was effective in displacing [3H]ryanodine from RyR1 but not from RyR2. These results demonstrate that TTRs are capable of interacting at ryanodine binding sites to differentially modulate fast skeletal and cardiac calcium-release channels.
Footnotes
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Send reprint requests to: Keshore R. Bidasee, Ph.D., Department of Pharmacology and Toxicology, 635 Barnhill Dr., MS A417, Indianapolis, IN 46202-5120. E-mail: kbidase{at}iupui.edu
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↵1 This work was supported in part by a grant from the Biomedical Research Committee of the Indiana University School of Medicine (to K.R.B.) and by the Ralph W. and Grace M. Showalter Trust (to H.R.B.).
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2 A preliminary report of this study was presented at the 40th Annual Meeting of the Biophysical Society, Baltimore, MD, February 17–21, 1996, and published in abstract form [(1996)Biophys J70:A281].
- Abbreviations:
- SR
- sarcoplasmic reticulum
- RyR
- ryanodine receptors
- TTR
- tectoridin
- 3′-TTR
- 3′-hydroxy TTR
- RyR1
- skeletal muscle ryanodine receptor
- RyR2
- cardiac muscle ryanodine receptor
- JSRV
- junctional sarcoplasmic reticulum vesicles
- TLC
- thin-layer chromatography
- CV
- crude sarcoplasmic reticular membrane vesicles
- PMSF
- phenylmethyl sulfonyl fluoride
- JSRV
- junctional SR membrane vesicles
- E1c
- CHCl3 extract
- Received December 28, 1999.
- Accepted February 29, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
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