Fig. 4. Heater organ sarcoplasmic reticulum (SR) vesicles release Ca²⁺ through ryanodine receptor (RyR) channels. (A) After the heater SR vesicles have been loaded with Ca²⁺, a very small Ca²⁺ release event can be evoked by the addition of 10 mmol l^-1 caffeine and 350µmoll^-1 ryanodine. (B) In similarly loaded SR vesicles isolated from toadfish swimbladder fast-twitch muscle, the addition of 10 mmol l^-1 caffeine and 350µmoll^-1 ryanodine resulted in a large Ca²⁺ release event. (C) In the absence of an ATP-regenerating system (creatine phosphate and creatine phosphokinase), a much larger release event is evoked in heater tissue by the addition of 10 mmol l^-1 caffeine and 350µmoll^-1 ryanodine, indicating that the strong presence of the pump in heater tissue limits SR Ca²⁺ release in this assay.