Type 1 ryanodine receptor (RyR1) exhibits a markedly lower gain of Ca²⁺-induced Ca²⁺ release (CICR) activity than type 3 ryanodine receptor (RyR3) in the sarcoplasmic reticulum of mammalian skeletal muscle ('selective stabilization' of the RyR1 channel) and this reduction in the gain is largely eliminated by CHAPS (Murayama, T and Ogawa, Y. (2004) Am. J. Physiol. 287, C36-C45). In this study, we investigated whether the hypothesized interdomain interactions within RyR1 are involved in the selective stabilization of the channel using [³H]ryanodine binding, single channel recordings, and Ca²⁺ release from the SR vesicles. Like CHAPS, DP4 (a synthetic peptide corresponding to the Leu²⁴⁴²-Pro²⁴⁷⁷ region of RyR1), which seems to destabilize the interdomain interactions, markedly stimulated RyR1, but not RyR3. Their activating effects were saturable and non-additive. Dantrolene, a potent inhibitor of RyR1 that is used to treat malignant hyperthermia, reversed the effects of DP4 or CHAPS in an identical manner. These findings indicate that RyR1 is activated by DP4 and CHAPS through a common mechanism probably mediated by the interdomain interactions. DP4 greatly increased [³H]ryanodine binding to RyR1 with only minor alterations in the sensitivity to endogenous CICR modulators (Ca²⁺, Mg²⁺, adenine nucleotide). However, DP4 sensitized RyR1 4-6-fold to caffeine in the caffeine-induced Ca²⁺ release. Thus, the gain of CICR activity (expressed as B/B_max) critically determines the magnitude and the threshold of Ca²⁺ release by drugs such as caffeine. These findings suggest that the low CICR gain of RyR1 is important in normal Ca²⁺ handling in skeletal muscle and perturbation of this state may result in muscle diseases, such as malignant hyperthermia.