Cardiac connective tissue is regarded as passive in terms of cardiac electro-mechanics. However, recent evidence confirms that fibroblasts interact directly with cardiac muscle cells in a way that is likely to affect their beat-by-beat activity.To overcome limitations of traditional approaches to exploring these interactions in native tissue, we will build and explore murine models that express functional reporters (membrane potential, Vm; calcium concentration, [Ca2+]i) in fibroblasts, to identify how they are functionally integrated in native heart (myocyte => fibroblast effects). Next, we will express light-gated ion channels in murine fibroblast, to selectively interfere with their Vm (fibroblast => myocyte effects). Fibroblast-specific observation and interference will be conducted in normal and pathologically remodelled tissue, to characterise fibroblast relevance for heart function in health & disease.Based on these studies, we will generate 2 transgenic rabbits (fibroblast Vm reporting / interfering). Rabbit cardiac structure-function is more amenable to translational work, e.g. to study fibroblast involvement in normal origin & spread of excitation across the heart, in pathological settings such as arrhythmogenicity of post-infarct scars (a leading causes of sudden death), or as a determinant of therapeutic outcomes such as in healing of atrial ablation lines (interfering with a key interventions to treat atrial fibrillation).The final ‘blue-skies’ study will assess whether modulation of cardiac activity, from ‘tuning’ of biological pacemaker rates to ‘unpinning’ / termination of re-entrant excitation waves, can be achieved by targeting not myocytes, but fibroblasts.The study integrates basic-science-driven discovery research into mechanisms and dynamics of biophysical myocyte-fibroblast interactions, generation of novel transgenic models useful for a broad range of studies, and elucidation of conceptually new approaches to heart rhythm management.