An orderly lattice of axial tubules which interconnect adjacent transverse tubules in guinea-pig ventricular myocardium

Abstract

Electron microscopy of guineapig heart muscle (ventricular) shows that large-diameter axial tubules are present in between the myofibrils. There is free communication between the lumina of these axial tubules and the transverse tubules. This transverse-axial tubular system forms an interconnected orderly network of “intracellular” tubules lined with basement membrane and open to the bulk interstitial fluid. Micropinocytotic-like vesicles are found along the axial tubules as well as the transverse tubules. Electron-opaque diffusion tracers, such as horseradish peroxidase (diameter of about 55 Å) and colloidal thorium dioxide (diameter of 60 Å) readily enter into these tubules. This tubular lattice occupies a large fraction of the cell volume (> 7.7%), and this fraction increases in hypertonic solutions due to swelling of the transverse-axial system. A network of closed or small SR sarcotubules into which tracer does not penetrate are also present surrounding the myofibrils, and often this system is in close proximity to the transverse-axial system. The transverse-axial tubular system is absent in the guinea-pig atrial muscle, but these cells do contain a closed SR sarcotubular system. Thus, in the ventricular cells the myofibrils and mitochondria have close access to a space that is continuous with the bulk interstitial fluid and which may be of similar cationic composition; such an arrangement should facilitate excitation-contraction coupling.