Hydroxylated xestospongins block inositol-1,4,5-trisphosphate-induced Ca²⁺ release and sensitize Ca²⁺-induced Ca²⁺ release mediated by ryanodine receptors

Inositol-1,4,5-trisphosphate receptors (IP₃Rs) and ryanodine receptors (RyRs) often coexist within the endoplasmic/sarcoplasmic reticulum (ER/SR) membrane and coordinate precise spatial and temporal coding of Ca ²⁺ signals in most animal cells. Xestospongin C (XeC) was shown to selectively block IP₃-induced Ca²⁺ release and IP ₃R-mediated signaling (Gafni et al., 1997). We have further studied the specificity of xestospongin structures possessing ring hydroxyl (-OH) substituents toward IP₃R, RyR, and ER/SR Ca²⁺-ATPase (SERCA) activities. XeC potently inhibits IP₃R, weakly inhibits RyR1, and lacks activity toward SERCA1 and SERCA2. XeD (9-OH XeC), 7-OHXeA, and araguspongin C isolated from the marine sponge Xestospongia species also inhibit IP₃-mediated Ca²⁺ release and lack activity toward SERCA. However, these hydroxylated derivatives possess a unique activity in that they enhance Ca²⁺-induced Ca²⁺ release from SR vesicles by a mechanism involving the sensitization of RyR1 channels within the same concentration range needed to block IP₃-induced Ca²⁺ release. These results show that xestospongins and related structures lack direct SERCA inhibitory activity, as suggested by some previous studies. A new finding is that XeD and related structures possessing a hydroxylated oxaquinolizidine ring are IP₃R blockers that also enhance Ca ²⁺-induced Ca²⁺ release mediated by RyRs. In intact cells, the actions of XeD are blocked by ryanodine pretreatment and do not interfere with thapsigargin-mediated Ca²⁺ mobilization stemming from SERCA block. Hydroxylated bis-oxaquinolizadine derivatives isolated from Xestospongia species are novel bifunctional reagents that may be useful in ascertaining how IP₃Rs and RyRs contribute to cell signaling.