Caffeine induces Ca2+ release by reducing the threshold for luminal Ca2+ activation of the ryanodine receptor

Abstract

Caffeine has long been used as a pharmacological probe for studying ryanodine receptor (RyR)-mediated Ca2+ release and cardiac arrhythmias. However, the precise mechanism by which caffeine activates RyRs is elusive. Here we investigated the effects of caffeine on spontaneous Ca2+ release and on the response of single cardiac RyR (RyR2) channels to luminal or cytosolic Ca2+. We found that HEK293 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 diseased-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.

Spontaneous Ca2+ sparks and Ca2+ homeostasis in a minimal model of permeabilized ventricular myocytes

“…12. Similarly, we can assume that low-dose caffeine leads to an increase k co accompanied by a smaller decrease in k oc (ϩ symbol) (29). The model predicted that this would cause a decrease in SR [Ca 2ϩ ], an increase in spark frequency, and little change in spark duration, roughly consistent with the results observed by Lukyanenko et al (35).…”

Clinical and molecular characterization of a cardiac ryanodine receptor founder mutation causing catecholaminergic polymorphic ventricular tachycardia

“…25 This is not the case for p.G357S_RyR2, which, in basal conditions, does not produce an increase in the SOICR activity nor an increase in the sensitivity to caffeine of the channel.…”

Reduced Ca2+ Spark Activity after Subarachnoid Hemorrhage Disables BK Channel Control of Cerebral Artery Tone

“…To assess RyR activity, Ca 2+ sparks were examined in the presence of a relatively low concentration (10 mmol/L) of the RyR activator, caffeine. Caffeine lowers the luminal Ca 2+ threshold for RyR activation (Kong et al, 2008), and at micromolar concentrations causes an increase in both Ca 2+ spark frequency and the number of active spark sites in the smooth muscle (Janiak et al, 2001;Wellman et al, 2001). In absolute terms, the number of active Ca 2+ spark sites and whole-cell Ca 2+ spark frequency in the presence of caffeine (10 mmol/L) remained decreased in myocytes isolated from SAH animals (Figures 5A-5C; control: n = 7 cells from 3 animals, SAH: n = 14 cells from 3 animals).…”