Abstract 16897: Dual Beam Imaging at Nanoscale Indicates Transverse Tubular Involvement in Fatty Acid Delivery in Human Ventricular Myocytes
It is well recognized that transverse tubules (t-tubules) assure electric synchronization by allowing propagation of action potentials from the sarcolemma into the depth of cytosolic space of myocytes by the mechanism of calcium-induced calcium-release. Using 3D reconstruction of a human myocyte, we investigated the possibility that t-tubules additionally serve to synchronize metabolic substrate delivery by providing a direct structural channel for fatty acids (FA) to travel deep within the myocyte and therefore avoid a limiting diffusion process. Tissue collected from the left ventricle of a single non-failing human heart was fixed in Karnovsky solution and embedded for electron microscopy. Dual-beam imaging used a focused ion beam for removing thin layers (10nm) of tissue and a scanning electron beam for imaging. In total, 220 serial images were obtained with high resolution (4096x3536, voxel dimension 3.6nmx4.2nmx10nm) that allowed for the identification of ultrastructure throughout the entire ∼15μm x 15μm x 2.2μm volume. Various ad hoc segmentation techniques were applied to reconstruct 2D images into a 3D volume. Reconstruction shows t-tubules penetrating the entire volume, the compactness of the ultrastructure, and lipid droplets enveloped by mitochondrial networks. When sarcolemma, t-tubules, and lipid droplets are reconstructed together, lipid droplets located well beneath the sarcolemma appear to be co-localized with t-tubules. Black, brown, and grey boxes highlight example droplets that have an association with t-tubules, sarcolemma, or neither. Lastly, we calculated the Euclidean distance from the lipid droplet centroid to the closest t-tubule and found a non-random distribution. The necessity of t-tubules for FA transport could have important implications in heart failure because it mechanistically links two distinctly different observations: 1. the principle energy substrate switches from FA to glucose and 2. t-tubular derangement.
- © 2012 by American Heart Association, Inc.