Figure 4. Intracellular resting [Ca²⁺]_i and SR Ca²⁺ load comparisons.

A, resting [Ca²⁺]_i was measured using indo-1 during the same normal loading protocol as the Ca²⁺ spark measurements, and Ac-10C had no effect. B, SR Ca²⁺ load was measured by the Ca transient (Δ[Ca²⁺]_i in nm) and by the integral of Na⁺–Ca²⁺ exchange current (∫I_NCX, μmol (l cytosol)⁻¹) upon application of 10 mm caffeine onto the cell through fast solution switch. Figure 2 shows an example recording of caffeine-induced Δ[Ca²⁺]_i and I_NCX. The bar graph summarizes the SR Ca²⁺ loads from different recording conditions. Values are normalized to the mean value for the control cells (see text).