AIMS:Our aim was to characterise the actions of novel BIT compounds with structures based on peptides and toxins that bind to significant regulatory sites on ryanodine receptor (RyR) Ca2+ release channels. RyRs, located in sarcoplasmic reticulum (SR) Ca2+ store membranes of striated muscle, are essential for muscle contraction. Although severe sometimes-deadly myopathies occur when the channels become hyperactive following genetic or acquired changes, specific inhibitors of RyRs are rare. MAIN METHODS:The effect of BIT compounds was determined by spectrophotometric analysis of Ca2+ release from isolated SR vesicles, analysis of single RyR channel activity in artificial lipid bilayers and contraction of intact and skinned skeletal muscle fibres. KEY FINDINGS:The inhibitory compounds reduced: (a) Ca2+ release from SR vesicles with IC50s of 1.1-2.5 μM, competing with activation by parent peptides and toxins; (b) single RyR ion channel activity with IC50s of 0.5-1.5 μM; (c) skinned fibre contraction. In contrast, activating BIT compounds increased Ca2+ release with an IC50 of 5.0 μM and channel activity with AC50s of 2 to 12 nM and enhanced skinned fibre contraction. Sub-conductance activity dominated channel activity with both inhibitors and activators. Effects of all compounds on skeletal and cardiac RyRs were similar and reversible. Competition experiments suggest that the BIT compounds bind to the regulatory helical domains of the RyRs that impact on channel gating mechanisms through long-range allosteric interactions. SIGNIFICANCE:The BIT compounds are strong modulators of RyR activity and provide structural templates for novel research tools and drugs to combat muscle disease.