Characterization of the sarcoplasmic reticulum K+ and Ca2+-release channel-ryanodine receptor-in human atrial cells

Since the role of sarcoplasmic reticulum (SR) in the E-C coupling of mammal ian atrial cells has long been a subject of debate, biochemical, electrophy siological and immunological assays were performed in order to define and c ompare the properties of the Ca2+-release channel-ryanodine receptor (RyR)- from atrial and ventricular tissues. Cardiac SR preparations from human, ca nine and ovine tissues were compared using [H-3]ryanodine binding, channel reconstitution into planar lipid bilayers and Western blot analysis involvi ng RyR antibodies. [H-3]ryanodine binding assays revealed a K-d value of si milar to2.5 nM for all investigated cardiac tissues, Bound [3H]ryanodine wa s Ca2+-dependent with similar EC50 values of 0.43, 0.49 and 0.79 muM for hu man atrium, canine ventricle and ovine atrium, respectively. However the de nsity of binding sites was 4.5 times lower in atrial than in ventricular ti ssues. Beyond the presence of selective K+ channels (gamma = 188 pS) record ed in the SR enriched fraction of human atrium, the activity of a large con ducting (gamma = 671 pS) cationic channel was also observed. The latter dis played typical characteristics of Ca2+-release channels which were activate d by 10 muM free [Ca2+] and 2 mM ATP. Western blot analysis revealed the pr esence of the RyR2 isoform in atrial and ventricular samples whereas no imm unoreactivity was detected with specific RyR1 and RyR3 antibodies. Our resu lts, obtained at the molecular level, are consistent with the presence of f unctional SR in human atrial cells. The human atrial Ca2+-release channel d isplays binding and regulating properties typical of the RyR2 isoform, (C) 2000 Academic Press.