Transverse-axial tubular system in guinea pig ventricular cardiomyocyte: 3D reconstruction, quantification and its possible role in K+ accumulation-depletion phenomenon in single cells

K+ accumulation-depletion (AD) phenomena were found in single guinea pig ventricular myocytes using the patch-clamp method in whole cell configuration. We suggest that the cardiomyocyte transverse-axial tubular system (TATS) lumen is the restricted extracellular space where the K+ AD could take place. A three-dimensional (3D) reconstruction of the TATS in a cardiomyocyte segment from serial ultrafine sections was made by three-dimensional isosurface rendering and quantitative data were obtained from the image processing. This original approach of the TATS intricated network gave a new vision of this membrane system; moreover, quantitative data about the tubular membrane importance (52.6% of the total plasma membrane) and its surface area versus the tubular volume fraction (S-TATS/V-TATS = 13.5 mu m(2)/mu m(3)) would fit in the electrophysiological results. The hypothesis whereby this 'extracellular' compartment could play, in single cells, a role as important as that of narrow clefts in the whole heart is discussed.