RyR selectivity configurations. Unitary RyR2 currents were measured with 25 mM Ca²⁺ on the luminal side of the channel and 100 mM Cs⁺ on the cytosolic side. Tested drugs were applied to the cytosolic solution. Subconductance states induced by ryanodol, imperatoxin, amitriptyline, and trifluoperazine were examined as was the superconductance state induced by suramin and the normal control full-open state in the absence of drug. The relative permeability of Ca²⁺ and Cs⁺ in these states was calculated using the measured current reversal potential and equation labeled no. 1 in Fatt and Ginsborg (1958). Individual RyR determinations are shown as small filled circles and the mean (±SE) selectivity as larger open circles. Method: Heavy SR microsomes were isolated from rat ventricle muscle as described elsewhere (Copello et al., 1997). Planar bilayers contained a 5:3:2 lipid mixture (50 mg/ml in decane) of phosphatidylethanolamine, phosphatidylserine, and phosphatidylcholine, respectively. Bilayers were formed across an ∼100-µm hole in a Teflon partition separating solution compartments (luminal and cytosolic). All chemicals and drugs were obtained from commercial sources. Ryanodol was generated from high purity commercially available ryanodine. Single channel currents were digitized at 20 kHz and filtered at 1 kHz using a Bessel filter for analysis. Voltage ramps were applied and current reversal potential determined.