Single Ryanodine Receptor Channel Basis of Caffeine's Action on Ca²⁺ Sparks

Caffeine (1, 3, 7-trimethylxanthine) is a widely used pharmacological agonist of the cardiac ryanodine receptor (RyR2) Ca²⁺ release channel. It is also a well-known stimulant that can produce adverse side effects, including arrhythmias. Here, the action of caffeine on single RyR2 channels in bilayers and Ca²⁺ sparks in permeabilized ventricular cardiomyocytes is defined. Single RyR2 caffeine activation depended on the free Ca²⁺ level on both sides of the channel. Cytosolic Ca²⁺ enhanced RyR2 caffeine affinity, whereas luminal Ca²⁺ essentially scaled maximal caffeine activation. Caffeine activated single RyR2 channels in diastolic quasi-cell-like solutions (cytosolic MgATP, pCa 7) with an EC₅₀ of 9.0 ± 0.4 mM. Low-dose caffeine (0.15 mM) increased Ca²⁺ spark frequency ∼75% and single RyR2 opening frequency ∼150%. This implies that not all spontaneous RyR2 openings during diastole are associated with Ca²⁺ sparks. Assuming that only the longest openings evoke sparks, our data suggest that a spark may result only when a spontaneous single RyR2 opening lasts >6 ms.