Membrane systems of guinea pig myocardium: Ultrastructure and morphometric studies

Abstract

The structure and quantitative contribution of membrane systems (transverse‐axal tubular system [TATS] and sarcoplasmic reticulum [SR]) have been investigated in the heart of the adult guinea pig. Although previous quantitative studies have been made of guinea pig myocardium, this is the first such study that has utilized tissue in which membrane system elements were clearly identified by selective staining (in this case by the osmium‐ferrocyanide [OsFeCN] postfixation method). Both membrane systems are highly developed in ventricular cells, but a TATS is essentially absent from atrial myocytes. The ventricular TATS consists principally of large‐bore elements which may be oriented transversely, axially, or obliquely, making numerous anastomoses with one another to form a highly interconnected system of extracellular spaces that penetrate to all myoplasmic depths of the ventricular cell. The cell coat that lines the lumina of these tubules is structured, containing fibrillar structures that run along the length of the tubule. The volume fraction (Vv) of the ventricular TATS is low (2.5–3.2%), in consideration of the qualitative prominence of the TATS in these cells. The relative total population of sarcoplasmic reticulum is higher in the atria (Vv of 10–11%) than in the ventricles (Vv of ca. 8%). In all guinea pig myocytes, several major structural divisions of SR can be discerned, which include network SR, junctional SR, corbular SR, and cisternal SR. Junctional SR (J‐SR) in the atrial cells is limited almost exclusively to peripheral saccules of junctional SR (PJSR), whereas both interior J‐SR and PJSR are present in the ventricle. Two distinct morphological types of PJSR appear in atrial cells, including both flattened and distended saccules, the latter resembling PJSR of lower vertebrate heart. Spheroidal bodies of SR with opaque contents (corbular SR) are prominent at or near Z‐line levels of the sarcomeres of atrial and ventricular cells. Cisternal SR is likely a subset of network SR, but some examples appear related to rough endoplasmic reticulum. An overall impression obtained from this study is that guinea pig atria are composed of structurally primitive cells, whereas the ventricular cardiac muscle cells are more highly developed entities. Freeze‐fracture replicas were prepared by Ms. Bonnie Sheppard of the Central Electron Microscope Facility of the University of Virginia School of Medicine. We are grateful as well to Ms. Lisa Kremer for her skillful preparation of the electron micrographs used for the stereology studies reported in this communication. Special thanks go to Mr. Guenter Lamprecht of Carl Zeiss, Inc., Thornwood, New York, for providing access to the Zeiss EM 902 instrument, and to Mr. Jean‐Marc Theler (Department of Physiology, University of Virginia School of Medicine) for valuable discussions concerning aspects of this research.