Figure 2. Adenovirally-expressed GCaMP2.2(Low)-FKBP12.6 and untagged GCaMP2.2(Low) are positioned to measure local cleft [Ca²⁺]_Cleft and, respectively, bulk cytosolic [Ca²⁺]_Bulk, and are highly sensitive to Ca²⁺ in intact myocytes.

(A) Example of myocytes infected with adenoviruses expressing untagged GCaMP2.2Low (Left) and GCaMP2.2Low-FKBP12.6 (Right). Bottom panel shows the fluorescence profile along the longitudinal axis of the myocyte for the area shown in the white square in the top panels. Right and Left panels show limited areas from each myocyte; entire myocyte images are in Online Fig. V. (B) Ca²⁺-sensitivity of GCaMP2.2Low-FKBP12.6 in intact cells. A resting myocyte expressing GCaMP2.2Low-FKBP12.6 was imaged first under control conditions (Ba). The cell was then exposed to Ca²⁺-free external solution in the presence of 10 μmol/L ionomycin (Bb). This resulted in a significant fluorescence drop (F_min). Then, the cell was Ca²⁺-overloaded in a 0Na⁺/1 mmol/L Ca²⁺ external solution, which favors Ca²⁺ entry via both ionomycin and reverse mode Na⁺/Ca²⁺ exchange, to saturate the sensor and determine F_max (Bc). The F_max/F_min ratio for this cell is 6.2.