Evidence suggests that the proteins termed TRIC-A and TRIC-B behave as K+ channels in the sarcoplasmic reticulum (SR) of cardiac and skeletal muscle and that there is approximately 10 fold more TRIC-A than TRIC-B present in both muscle types (1-3). We here compare the single channel properties of SR K+ channels derived from wild type (WT) and TRIC-A knockout (KO) cardiac and skeletal muscle SR as we might expect to observe only TRIC-B channels from KO tissue but a more heterogeneous population of channels from WT tissue. Single SR K+ channels from cardiac or skeletal WT and TRIC-A KO SR were incorporated into planar lipid bilayers in symmetrical 210 mM KPIPES, pH 7.2, under voltage-clamp conditions as previously described (4). SR vesicles incorporated in a fixed orientation and the cytosolic side of the channels was voltage-clamped at potentials relative to the luminal side which was held at ground. Mean values ± SEM were compared by Student’s t test. The single channel conductance of SR K+ channels from WT and TRIC-A KO mice were similar for both cardiac (WT=210.1±1.4 pS, n=7; TRIC-A KO=211.8±1.1 pS, n=15) and skeletal muscle (WT=210.8±2.7 pS, n=25; TRIC-A KO=211.1±1.8 pS, n=36). A characteristic of SR K+ channels is that they gate into multiple sub-conductance levels and this was observed for cardiac and skeletal channels from both WT and TRIC-A KO mice, however, the percentage of time that the channels dwelt in a sub-conductance level was higher in channels from TRIC-A KO mice. For example, at -30 mV, the sub-conductance openings contributed only 41% (n=16) towards the total open probability (Po) of skeletal WT channels in comparison to 86% (n=18, p<0.001) of the total Po in skeletal KO channels. The overall Po of single (lone) channels was also significantly lower in channels derived from the skeletal muscle of TRIC-A KO mice (for example, at +30 mV, WT Po=0.22±0.06, n=17 and TRIC-A KO Po=0.09±0.01, n=22; p<0.01). When multiple channels were gating, however, it was found that the channels derived from TRIC-A KO mice behaved in a positively cooperative manner exhibiting higher Po than when a single channel was gating in the bilayer; this was not the case for channels from WT mice. When both TRIC subtypes are present (SR from WT mice), since TRIC-A is much more abundant, it is likely that TRIC-A will predominantly incorporate into the bilayer. When TRIC-A is not present (TRIC-A KO), the remaining SR K+ channels, presumably TRIC-B, gate with lower Po, gate more in sub-conductance open states and exhibit positive cooperativity in their gating. The physiological relevance for the different gating behaviour of these two populations of SR K+ channel is not yet understood but may be linked to the different expression patterns for TRIC-A and TRIC-B in different cell types.