Unitary Ca²⁺ 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 Ca²⁺ 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 Ca²⁺ as exclusive charge carrier were 4.00 and 4.07 pA, respectively, and not significantly different. Currents recorded at 1–30 mM lumenal Ca²⁺ concentrations were attenuated by physiological [K⁺] (150 mM) and [Mg²⁺] (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 [Mg²⁺], [K⁺], and lumenal [Ca²⁺] (1 mM), the Ca²⁺ current was just less than 0.5 pA. Comparison of this value with the Ca²⁺ flux underlying Ca²⁺ sparks suggests that sparks in mammalian cardiac and amphibian skeletal muscles are generated by opening of multiple RyR channels. Further, symmetric high concentrations of Mg²⁺ substantially reduced the current carried by 10 mM Ca²⁺ (∼40% at 10 mM Mg²⁺), suggesting that high Mg²⁺ may make sparks smaller by both inhibiting RyR gating and reducing unitary current.