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Vol. 55, Issue 3, 462-472, March 1999

Suramin and Suramin Analogs Activate Skeletal Muscle Ryanodine Receptor via a Calmodulin Binding Site

Markus Klinger, Michael Freissmuth, Peter Nickel, Margit Stäbler-Schwarzbart, Matthias Kassack, Josef Suko, and Martin Hohenegger

Institute of Pharmacology, University of Vienna, Vienna, Austria (M. Kl, M.F., J.S., M.H.); and the Institute of Pharmaceutical Chemistry, University of Bonn, Bonn, Germany (P.N., M.S.-S., M. Ka)

Contraction of skeletal muscle is triggered by the rapid release of Ca2+ from the sarcoplasmic reticulum via the ryanodine receptor/calcium-release channel. The trypanocidal drug suramin is an efficient activator of the ryanodine receptor. Here, we used high-affinity [3H]ryanodine binding to sarcoplasmic reticulum from rabbit skeletal muscle to screen for more potent analogs of suramin. This approach resulted in the identification of NF307, which accelerates the association rate of [3H]ryanodine binding with an EC50 = 91 ± 7 µM at 0.19 µM calculated free Ca2+. In single-channel recordings with the purified ryanodine receptor, NF307 increased mean open probability at 0.6 µM Ca2+ from 0.020 ± 0.006 to 0.53 ± 0.07 with no effect on current amplitude and unitary conductance. Like caffeine, NF307 exerts a very pronounced Ca2+-sensitizing effect (EC50 of Ca2+ shifted ~10-fold by saturating NF307 concentrations). Conversely, increasing concentrations of free Ca2+ sensitized the receptor for NF307 (EC50 = 14.6 ± 3.5 µM at 0.82 µM estimated free Ca2+). The effects of NF307 and caffeine on [3H]ryanodine binding were additive, irrespective of the Ca2+ concentration. In contrast, the effects of calmodulin, which activates and inhibits the ryanodine receptor in the absence and presence of Ca2+, respectively, and of NF307 were mutually antagonistic. If the purified ryanodine receptor was prebound to a calmodulin-Sepharose matrix, 100 µM NF307 and 300 µM suramin eluted the purified ryanodine receptor to an extent that was comparable to the effect of 10 µM calmodulin. We conclude that NF307 and suramin interact directly with a calmodulin binding domain of the ryanodine receptor. Because of its potent calcium-sensitizing effect, NF307 may represent a lead compound in the search of synthetic ryanodine receptor ligands.

Copyright © 1999 by The American Society for Pharmacology and Experimental Therapeutics


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