Inactivation of K+ channels occurs by the interaction of the pore with NH2-terminal sequences. Synthetic peptides corresponding to these sequences were applied to the purified mouse cardiac ryanodine receptor type 2 (RyR2) and single channel activity was recorded in planar lipid bilayers. The wild type (WT) Shaker B NH2-peptide MAAVAGLYGLGEDRQHRKKQ induced a block of the open RyR2 channel in a concentration and voltage-dependent manner, but also when the channel displayed a ryanodine-modified state [Mead et al., J. Membrane Biol., 1998]. In the latter condition, in a 200 mM KCl buffer when the holding potential was at +50 mV, the open probability decreased by a factor of 2 (p=0.001, n=10 channels for control and n=5 for WT peptide) mainly due to a 4-fold reduction (p<0.01, n=10 channels for control and n=5 for WT peptide) of the mean open time in the presence of 20 μM WT peptide. It is noteworthy that the peptide at 20 μM had no effect in a 600 mM KCl buffer, suggesting the importance of charged residues in the blocking effect on RyR2. Furthermore we designed a mutant peptide where the amino acid Alanine in positions 3 and 5 was replaced by Glutamine residues, giving a less hydrophobic feature to the peptide. The mutant peptide at 20 μM in a 200 mM KCl buffer was less effective than the WT peptide, while showing a different mechanism of action. These findings are consistent with the idea that RyR2 contains well organised charged and hydrophobic residues in its conduction pathway and that they may play a general role in the ion translocation mechanisms of the sarcoplasmic reticulum Ca2+ release channel.Acknowledgement: This work is funded by the British Heart Foundation.