Caffeine has long been used as a pharmacological probe for studying RyR (ryanodine receptor)-mediated Ca2+ release and cardiac arrhythmias. However, the precise mechanism by which caffeine activates RyRs is elusive. In the present study, we investigated the effects of caffeine on spontaneous Ca2+ release and on the response of single RyR2 (cardiac RyR) channels to luminal or cytosolic Ca2+. We found that HEK-293 cells (human embryonic kidney cells) expressing RyR2 displayed partial or ‘quantal’ Ca2+ release in response to repetitive additions of submaximal concentrations of caffeine. This quantal Ca2+ release was abolished by ryanodine. Monitoring of endoplasmic reticulum luminal Ca2+ revealed that caffeine reduced the luminal Ca2+ threshold at which spontaneous Ca2+ release occurs. Interestingly, spontaneous Ca2+ release in the form of Ca2+ oscillations persisted in the presence of 10 mM caffeine, and was diminished by ryanodine, demonstrating that unlike ryanodine, caffeine, even at high concentrations, does not hold the channel open. At the single-channel level, caffeine markedly reduced the threshold for luminal Ca2+ activation, but had little effect on the threshold for cytosolic Ca2+ activation, indicating that the major action of caffeine is to reduce the luminal, but not the cytosolic, Ca2+ activation threshold. Furthermore, as with caffeine, the clinically relevant, pro-arrhythmic methylxanthines aminophylline and theophylline potentiated luminal Ca2+ activation of RyR2, and increased the propensity for spontaneous Ca2+ release, mimicking the effects of disease-linked RyR2 mutations. Collectively, our results demonstrate that caffeine triggers Ca2+ release by reducing the threshold for luminal Ca2+ activation of RyR2, and suggest that disease-linked RyR2 mutations and RyR2-interacting pro-arrhythmic agents may share the same arrhythmogenic mechanism.

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