Fig. 3. Heater organ sarcoplasmic reticulum (SR) vesicles sequester Ca²⁺ in an ATP-dependent manner. 0.5 mg ml^-1 of heater tissue SR vesicles were added to a cuvette containing 1.5 ml transport buffer, 10 µmol l^-1 carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP) and 10 µmol l^-1 oligomycin. The extravesicular Ca²⁺ concentration was monitored using the fura-2 fluorescence ratio (340 nm/380 nm). Ca²⁺ uptake was initiated with the addition of 2.5 mmol l^-1 MgATP, and sequential additions of 75 nmol CaCl₂ were used to load the vesicles. (A) The Ca²⁺ ionophore A23187 releases the SR-loaded Ca²⁺. (B) The uptake of Ca²⁺ is blocked by the sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) inhibitor thapsigargin.