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 mammalian atrial cells has long been a subject of debate, biochemical, electrophysiological and immunological assays were performed in order to define and compare the properties of the Ca2+-release channel—ryanodine receptor (RyR)—from atrial and ventricular tissues. Cardiac SR preparations from human, canine and ovine tissues were compared using [3H]ryanodine binding, channel reconstitution into planar lipid bilayers and Western blot analysis involving RyR antibodies. [3H]ryanodine binding assays revealed a Kdvalue of 2.5 n for all investigated cardiac tissues. Bound [3H]ryanodine was Ca2+-dependent with similar EC50values of 0.43, 0.49 and 0.79 μ for human atrium, canine ventricle and ovine atrium, respectively. However the density of binding sites was 4.5 times lower in atrial than in ventricular tissues. Beyond the presence of selective K+channels (γ=188 pS) recorded in the SR enriched fraction of human atrium, the activity of a large conducting (γ=671 pS) cationic channel was also observed. The latter displayed typical characteristics of Ca2+-release channels which were activated by 10 μ free [Ca2+] and 2 m ATP. Western blot analysis revealed the presence of the RyR2 isoform in atrial and ventricular samples whereas no immunoreactivity was detected with specific RyR1 and RyR3 antibodies. Our results, obtained at the molecular level, are consistent with the presence of functional SR in human atrial cells. The human atrial Ca2+-release channel displays binding and regulating properties typical of the RyR2 isoform.