3D ultrastructural organisation of calcium release units in the avian sarcoplasmic reticulum.

Excitation-contraction coupling in vertebrate hearts is underpinned by calcium (Ca2+) release from Ca2+ release units (CRUs). CRUs are formed by clusters of channels called ryanodine receptors on the sarcoplasmic reticulum (SR) within the cardiomyocyte. Distances between CRUs influence the diffusion of Ca2+, thus influencing the rate and strength of excitation-contraction coupling. Avian myocytes lack T-tubules, so Ca2+ from surface CRUs (peripheral couplings, PCs) must diffuse to internal CRU sites of the corbular SR (cSR) during centripetal propagation. Despite this, avian hearts achieve higher contractile rates and develop greater contractile strength than many mammalian hearts, which have T-tubules to provide simultaneous activation of the Ca2+ signal through the myocyte. We used 3D electron tomography to test the hypothesis that the intracellular distribution of CRUs in the avian heart permits faster and stronger contractions despite the absence of T-tubules. Nearest edge-edge distances between PCs and cSR, and geometric information including surface area and volume of individual cSR, were obtained for each cardiac chamber of the white leghorn chicken. Computational modelling was then used to establish a relationship between CRU distance and cell activation time in the avian heart. Our data suggest that cSR clustered close together along the Z-line is vital for rapid propagation of the Ca2+ signal from the cell periphery to the cell centre, which would aid in the strong and fast contractions of the avian heart.