Rationale: In mammalian ventricular myocytes, T-tubules (T-tub) are essential for synchronous and rapid changes in cytosolic free Ca2+ concentration and, hence, efficient contraction. T-tub form a highly complex, polymorphic network of membrane invaginations extending from the surface to the centre of the cell. T-tub membranes contain various ion channels that facilitate excitation-contraction coupling. Due to the complex nature of the T-tub network they create an area of restricted diffusion, and it has been proposed that this in turn could create a microdomain of ion concentrations that differ from bulk extracellular space. In order to maintain synchronous contractions however there must exist a mechanism of rapid replenishment of T-tub content in addition to passive diffusion which might be too slow to sustain ion homeostasis, especially at higher heart rates. Previously, transmission electron microscopic imaging has shown that T-tubules may undergo deformation as the cardiomyocytes contract which may give rise to a novel mechanisms of partial content exchange based on convection1. Objective: We hypothesise that the contraction and stretch-induced deformation of T-tub is associated with transitions between circular and more elliptical cross-sectional shape, which – in the presence for a near-constant T-tub surface area – gives rise to changes in partial T-tub volume. This would constitute a convection-driven mechanism for ion exchange (in addition to diffusion). We employ EM tomography (EMT) to reconstruct and model the 3D structure of cardiomyocyte T-tub at nano-scopic resolution (4*10-9 m) and to quantify changes in shape of the T-tubular system in as a function of sarcomere length. Methods: Rabbit hearts were chemically fixed and embedded during contracture, cardioplegic arrest and balloon catheter-induced stretch. Samples were subsequently subjected to dual-axis EMT. Datasets were reconstructed and T-tub and sarcomeric Z-disc planes tracked to obtain 3D models. The orientation of the long cross-sectional axis of the T-tub relative to the Z-disc was measured.Results and Conclusion: Using high-resolution 3-D structural modelling of rabbit cardiac T-tub system in different contractile states we provide evidence that mechanical activity of cardiomyocytes induces directional deformation of T-tub shape. This finding supports the suggestion that ion homeostasis within cardiomyocyte T-tub is based on convection-assisted diffusion, ensuring synchronous cardiac activity regulation on a beat-to-beat basis.