Unitary Ca(2+) Current through Mammalian Cardiac and Amphibian Skeletal Muscle Ryanodine Receptor Channels under Near-physiological Ionic Conditions

Ryanodine receptor (RyR) channels from mammalian cardiac and amphibian skeletal muscle were incorporated into planar lipid bilayers. Unitary Ca(2+) currents in the SR lumen-to-cytosol direction were recorded at 0 mV in the presence of caffeine (to minimize gating fluctuations). Currents measured with 20 mM lumenal Ca(2+) as exclusive charge carrier were 4.00 and 4.07 pA, respectively, and not significantly different. Currents recorded at 1–30 mM lumenal Ca(2+) concentrations were attenuated by physiological [K(+)] (150 mM) and [Mg(2+)] (1 mM), in the same proportion (∼55%) in mammalian and amphibian channels. Two amplitudes, differing by ∼35%, were found in amphibian channel studies, probably corresponding to α and β RyR isoforms. In physiological [Mg(2+)], [K(+)], and lumenal [Ca(2+)] (1 mM), the Ca(2+) current was just less than 0.5 pA. Comparison of this value with the Ca(2+) flux underlying Ca(2+) sparks suggests that sparks in mammalian cardiac and amphibian skeletal muscles are generated by opening of multiple RyR channels. Further, symmetric high concentrations of Mg(2+) substantially reduced the current carried by 10 mM Ca(2+) (∼40% at 10 mM Mg(2+)), suggesting that high Mg(2+) may make sparks smaller by both inhibiting RyR gating and reducing unitary current.