Fig. 4. The effects of ryanodine on InsP₃-evoked [Ca²⁺]_i transients and STOCs. At a holding potential of -20 mV (iv) InsP₃ () increased [Ca²⁺]_i (ii); ryanodine (50 µM) significantly reduced the InsP₃-evoked Ca²⁺ transients (i,ii). Activation of RyR by caffeine (Caff, 10 mM, iii) increased [Ca²⁺]_i (ii). A second application of Caff some 60 seconds later almost abolished both the [Ca²⁺]_i transient, presumably by depleting the SR store and the InsP₃ response () leaving only the artefact (ii). Because the InsP₃-evoked Ca²⁺ transient was blocked after caffeine in the presence of ryanodine, InsP₃ receptors and RyR may share a common Ca²⁺ store. i is a summary of eight experiments.