Calcin-induced Ca²⁺ release from skeletal heavy SR vesicles. (A) Typical trace of Ca²⁺ release by calcins. A concentration of 20 nM imperacalcin elicited abrupt Ca²⁺ release from SR vesicles, and further additions had incremental effects only. The fraction of calcin-induced Ca²⁺ release was calculated by adding the Ca²⁺ ionophore A23187 (5 µM). (B) Except urocalcin, all calcins elicited abrupt Ca²⁺ release within 100 nM (n = 3–4). (C) At 100 nM, all calcins except urocalcin elicited Ca²⁺ release to ∼60% of total Ca²⁺ load from heavy SR. Mean ± SEM is shown. **, P < 0.01; calcins versus A23187, t test.