title: Single ryanodine receptor channel basis of caffeine's action on Ca2+ sparks.
abstract: Caffeine (1, 3, 7-trimethylxanthine) is a widely used pharmacological agonist of the cardiac ryanodine receptor (RyR2) Ca(2+) 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 Ca(2+) sparks in permeabilized ventricular cardiomyocytes is defined. Single RyR2 caffeine activation depended on the free Ca(2+) level on both sides of the channel. Cytosolic Ca(2+) enhanced RyR2 caffeine affinity, whereas luminal Ca(2+) essentially scaled maximal caffeine activation. Caffeine activated single RyR2 channels in diastolic quasi-cell-like solutions (cytosolic MgATP, pCa 7) with an EC(50) of 9.0 ± 0.4 mM. Low-dose caffeine (0.15 mM) increased Ca(2+) spark frequency ∼75% and single RyR2 opening frequency ∼150%. This implies that not all spontaneous RyR2 openings during diastole are associated with Ca(2+) 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.
authors.affiliation: Department of Physiology, Midwestern University, Downers Grove, Illinois, USA.
authors.forename: Maura
authors.initials: M
authors.lastname: Porta
authors.forename: Aleksey V
authors.initials: AV
authors.lastname: Zima
authors.forename: Alma
authors.initials: A
authors.lastname: Nani
authors.forename: Paula L
authors.initials: PL
authors.lastname: Diaz-Sylvester
authors.forename: Julio A
authors.initials: JA
authors.lastname: Copello
authors.forename: Josefina
authors.initials: J
authors.lastname: Ramos-Franco
authors.forename: Lothar A
authors.initials: LA
authors.lastname: Blatter
authors.forename: Michael
authors.initials: M
authors.lastname: Fill
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D020013:Calcium Signaling / Q000187:drug effects*; D002417:Cattle; D015640:Ion Channel Gating / Q000187:drug effects*; D011817:Rabbits; D051381:Rats; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D012996:Solutions
chemical_list: D019837:Ryanodine Receptor Calcium Release Channel; D012996:Solutions; D002110:Caffeine