Fig. 2. Caffeine reduces the luminal Ca²⁺ threshold level at which spontaneous Ca²⁺ release occurs.

HEK293 cells expressing RyR2 were grown on glass coverslips. Cells were transfected with D1ER cDNA 48 hr before imaging and RyR2 expression was induced 24 hr before imaging. The cells were perfused with KRH buffer containing 2 mM Ca²⁺ and 0, 0.3, 1 or 10 mM caffeine with or without 20μM ryanodine. (A) A representative trace captured using single cell imaging. The dash-lines illustrate the relative luminal Ca²⁺ threshold for spontaneous Ca²⁺ release at each concentration of caffeine. (B) The relative luminal Ca²⁺ threshold for spontaneous Ca²⁺ release at various caffeine concentrations. The threshold for spontaneous Ca²⁺ release is expressed as a percentage of the threshold in the absence of caffeine. Data represents the mean ± SEM of 34 cells from 3 separate experiments.