Fig. 2. Effect of FK506 on Ca²⁺release induced by Ca²⁺. One milligram sarcoplasmic reticulum terminal cisternae vesicles (SRVs) per milliliter was loaded actively with Ca²⁺by addition of 1 mm adenosine triphosphate in the presence of an adenosine triphosphate–regenerative system. Before Ca²⁺loading and challenging with sequential additions of Ca²⁺, SRVs were incubated at 37°C for 40 min in the absence (A ) or presence (B ) of 25 μm FK506. After active Ca²⁺uptake, the extravesicular concentration of Ca²⁺was essentially the same for control and FK506-treated SRVs. The lower arrows indicate single additions of 10 nmol Ca²⁺. The Ca²⁺release induced by seven sequential additions of 10 nmol Ca²⁺was observed in approximately 5 of 12 (40%) SRVs treated with FK506. In contrast, up to eight sequential additions of 10 mmol Ca²⁺produced no Ca²⁺-induced release of Ca²⁺in control SRVs (0 of 14).