Reactions with dye free radicals reveal weak redox properties of drugs.

The calcium release channel (CRC) of the skeletal sarcoplasmic reticulum is rich in thiol groups and is strongly regulated by covalent modification of these thiols. Oxidizing reagents activate the release channel, whereas reducing reagents inhibit the channel. However, most CRC regulators are not thiol reagents. Here, we propose that reversible redox interactions are involved in regulation of the CRC by nonthiol reagents. This hypothesis was tested with several CRC regulators. The local anesthetics tetracaine, procaine and QX-314, which block the CRC, behaved as electron donors in reactions with dye free radicals. In contrast, ryanodine, caffeine, doxorubicin and daunorubicin, compounds known to activate the release channel, all accepted electrons from dye anion radicals. Moreover, release of Ca2+ from SR initiated by doxorubicin (acceptor) was antagonized by local anesthetics (donors). Based on the redox characterization of CRC modulators, we propose a functional model in which channel inhibitors and activators act as weak electron donors and acceptors, respectively, and shift the thiol-disulfide balance within the release protein. The consequence of this model is that, in spite of the chemical diversity of CRC modulators, a common mechanism of channel regulation involves the transient exchange of electrons between the activator/inhibitor and the CRC.