High affinity C10-Oeq ester derivatives of ryanodine. Activator-selective agonists of the sarcoplasmic reticulum calcium release channel.

The plant alkaloids ryanodine and dehydroryanodine are specific and potent modulators of the sarcoplasmic reticulum calcium release channel. In the present study, acidic, basic, and neutral side chains esters of these diterpene compounds were prepared and their pharmacologic activities were assessed. Binding affinities of the novel C10-Oeq ester derivatives for the sarcoplasmic reticulum Ca2+ release channel were evaluated with sarcoplasmic reticular vesicles prepared from rabbit skeletal muscle. Kd values of the derivatives varied 500-fold, ranging from 0.5 to 244 nM. In comparison, Kd values for ryanodine and dehydroryanodine were 4.4 nM and 5.4 nM, respectively. Basic substituents at the C10-Oeq side chain terminus produced the highest affinity derivatives (Kd values from 0.5 to 1.3 nM). Neutral and/or hydrophobic side chain derivatives exhibited intermediate affinities for the high affinity ryanodine receptor site (Kd values from 2.5 to 39 nM), whereas a derivative with a terminal acidic group had the lowest affinity (Kd value > 100 nM). Certain of the higher affinity C10-Oeq derivatives were evaluated more extensively for their pharmacologic activity on the sarcoplasmic reticular Ca2+ release channel. Both channel activating (opening) and deactivating (closing) actions were assessed from the ability of the ryanoids to alter Ca2+ efflux rates from skeletal junctional sarcoplasmic reticular vesicles that had been passively loaded with Ca2+. The natural Ryania secondary metabolites ryanodine, dehydroryanodine and esters E and F, all exhibit antithetical concentration-effect curves, indicating both activator and deactivator actions. In contrast, the semi-synthetic C10-Oeq esters selectively activate the Ca2+ release channel. Half-maximal concentrations for such activation (EC50 act) ranged from 0.87 microM to 4.2 microM, compared with an EC50 act of 1.3 microM for ryanodine. These derivatives were also evaluated for their ability to augment ATP-dependent CA2+ accumulation by cardiac junctional sarcoplasmic reticular vesicles, an effect that results from deactivation of the Ca2+ release channels. None of the derivatives tested was able to significantly augment Ca2+ accumulation, further substantiating their inability to deactivate the sarcoplasmic reticular Ca2+ release channel. Additionally, these derivatives functionally antagonized the action of ryanodine to close the Ca2+ release channel. The results presented demonstrate that these C10-Oeq ester derivatives of ryanodine and dehydroryanodine bind specifically to the SR Ca2+ release channel, selectively activate the channel, and, although they fail to effect channel closure, they nevertheless functionally compete with ryanodine at its low affinity (deactivator) site(s).