I read this book the same weekend that the Packers took on the Rams, and the experience of the latter event, obviously, colored my judgment. Although I abhor anything that smacks of being a handbook (like, \"How to Earn a Merit Badge in Neurosurgery\") because too many volumes in biomedical science already evince a boyscout-like approach, I must confess that parts of this volume are fast, scholarly, and significant, with certain reservations. I like parts of this well-illustrated book because Dr. Sj6strand, without so stating, develops certain subjects on technique in relation to the acquisition of judgment and sophistication. And this is important! So, given that the author (like all of us) is somewhat deficient in some areas, and biased in others, the book is still valuable if the uninitiated reader swallows it in a general fashion, realizing full well that what will be required from the reader is a modulation to fit his vision, propreception, adaptation and response, and the kind of problem he is undertaking. A major deficiency of this book is revealed by comparison of its use of physics and of chemistry to provide understanding and background for the application of high resolution electron microscopy to problems in biology. Since the volume is keyed to high resolution electron microscopy, which is a sophisticated form of structural analysis, but really morphology in a modern guise, the physical and mechanical background of The instrument and its ancillary tools are simply and well presented. The potential use of chemical or cytochemical information as it relates to biological fine structure , however, is quite deficient. I wonder when even sophisticated morphol-ogists will consider fixation a reaction and not a technique; only then will the fundamentals become self-evident and predictable and this sine qua flon will become less mystical. Staining reactions (the most inadequate chapter) ought to be something more than a technique to selectively enhance contrast of morphological elements; it ought to give the structural addresses of some of the chemical residents of cell components. Is it pertinent that auto-radiography gets singled out for more complete coverage than other significant aspects of cytochemistry by a high resolution microscopist, when it has a built-in minimal error of 1,000 A in standard practice? I don't mean to blind-side (in strict football terminology) Dr. Sj6strand's efforts for what is \"routinely used in our laboratory\"; what is done is usually well done. It's just that …

Methods in Enzymology.

The cell biology of caveolae is a rapidly growing area of biomedical research. Caveolae are known primarily for their ability to transport molecules across endothelial cells, but modern cellular techniques have dramatically extended our view of caveolae. They form a unique endocytic and exocytic compartment at the surface of most cells and are capable of importing molecules and delivering them to specific locations within the cell, exporting molecules to extracellular space, and compartmentalizing a variety of signaling activities. They are not simply an endocytic device with a peculiar membrane shape but constitute an entire membrane system with multiple functions essential for the cell. Specific diseases attack this system: Pathogens have been identified that use it as a means of gaining entrance to the cell. Trying to understand the full range of functions of caveolae challenges our basic instincts about the cell.

The Caveolae Membrane System

The hypothesis of a Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR) is supported by experiments done in skinned cardiac cells (sarcolemma removed by microdissection). According to this hypothesis, the transsarcolemmal Ca2+ influx does not activate the myofilaments directly but through the induction of a Ca2+ release from the SR. The stimulus gating CICR is not a small change in free Ca2+ concentration (delta[free Ca2+]) outside the SR but a function of the rate of this change (delta[free Ca2+/delta t]). The initial relatively fast component of the transsarcolemmal Ca2+ current would trigger Ca2+ release; the subsequent slow component, perhaps corresponding to noninactivating Ca2+ channels, would load the SR with an amount of Ca2+ available for release during subsequent beats. Inactivation of CICR is caused by the large increase of [free Ca2+] outside the SR resulting from Ca2+ release, which inhibits further release. This negative feedback helps to explain that CICR is not all or none. During relaxation the Ca2+ reaccumulation in the SR is backed up by the Ca2+ efflux across the sarcolemma through Na+-Ca2+ exchange and the sarcolemmal Ca2+ pump. Computations of the Ca2+ buffering in the mammalian ventricular cell and of the systolic transsarcolemmal Ca2+ influx do not support the alternative hypothesis that this influx of Ca2+ is large enough to activate the myofilaments directly. Yet the hypothesis of a CICR can be challenged because of many problems and uncertainties related to the preparations and methods used for skinned cardiac cell experiments.

https://www.physiology.org/doi/pdf/10.1152/ajpcell.1983.245.1.C1

Calcium-induced release of calcium from the cardiac sarcoplasmic reticulum

Adjacent cells share ions, second messengers and small metabolites through intercellular channels which are present in gap junctions. This type of intercellular communication permits coordinated cellular activity, a critical feature for organ homeostasis during development and adult life of multicellular organisms. Intercellular channels are structurally more complex than other ion channels, because a complete cell-to-cell channel spans two plasma membranes and results from the association of two half channels, or connexons, contributed separately by each of the two participating cells. Each connexon, in turn, is a multimeric assembly of protein subunits. The structural proteins comprising these channels, collectively called connexins, are members of a highly related multigene family consisting of at least 13 members. Since the cloning of the first connexin in 1986, considerable progress has been made in our understanding of the complex molecular switches that control the formation and permeability of intercellular channels. Analysis of the mechanisms of channel assembly has revealed the selectivity of inter-connexin interactions and uncovered novel characteristics of the channel permeability and gating behavior. Structure/function studies have begun to provide a molecular understanding of the significance of connexin diversity and demonstrated the unique regulation of connexins by tyrosine kinases and oncogenes. Finally, mutations in two connexin genes have been linked to human diseases. The development of more specific approaches (dominant negative mutants, knockouts, transgenes) to study the functional role of connexins in organ homeostasis is providing a new perception about the significance of connexin diversity and the regulation of intercellular communication.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1996.0001q.x

Connections with connexins: the molecular basis of direct intercellular signaling

Saez, Juan C., Viviana M. Berthoud, Maria C. Branes, Agustin D. Martinez, and Eric C. Beyer. Plasma Membrane Channels Formed by Connexins: Their Regulation and Functions. Physiol Rev 83: 1359-1400,...

Plasma Membrane Channels Formed by Connexins: Their Regulation and Functions

Mathematical techniques can be used to extract quantitative information from tissue electron micrographs of heart muscle With these methods it has been possible to measure the fractions of myocardial cell volume made up of myofibrils, mitochondria, sarcotubules, T system and sarcoplasm, as well as the membrane areas per unit of cell volume of plasma membrane, sarcotubular membrane and mitochondrial cristae The techniques have been used to quantitate the changes in the ultrastructure of myocardial cells in experimental left ventricular hypertrophy and in experimentally induced hypothyroidism before and after treatment with thyroxin These measurements have demonstrated striking and physiologically significant changes in the ultrastructural composition of the cells The pattern of ultrastructural change in ventricular hypertrophy differs in characteristic ways from the pattern during thyroxin-stimulated cardiac cellular growth These observations have suggested certain generalizations about the constraints to which growing myocardial cells are subject New and simple microchemical methods have been developed to determine the cardiac contents of myofibrils and mitochondrial cristae The methods are applicable to samples of heart muscle obtained at autopsy, surgery or biopsy, as well as to experimental material Application of these techniques to the study of experimental left ventricular hypertrophy and thyroxin-stimulated growth of myocardial cells has confirmed and extended the results of quantitative measurements on electron micrographs It is suggested that the approaches described can form the basis of a quantitative electron microscopic and microchemical pathology of heart muscle

Quantitative electron microscopic description of heart muscle cells. Application to normal, hypertrophied and thyroxin-stimulated hearts.

1. Measurements combining the techniques of point counting and line integration were performed on light and electron micrographs of Purkinje fibres from the sheep's heart. The measurements were aimed at determining membrane areas of importance for the cellular electrophysiology of this tissue.



2. The mean volume fractions of the cells occupied by various constituents were: myofibrils, 0·234; mitochondria, 0·103; and nuclei, 0·009. The mean volume fraction of the fibres occupied by the interspaces between the tightly packed cells was 0·0023.



3. The mean fractions of intercellular surface area occupied by junctional specializations were: nexus, 0·17; desmosome, 0·023; and fascia adherens, 0·014.



4. The mean surface to volume ratio of the Purkinje cells and fibres was 0·46 μ−1 which is 11·5 times the value of the surface to volume ratio of a long right circular cylinder 100 μ in diameter.



5. There are two reasons for the increment in the surface to volume ratio of the fibre (when compared to that of a long right circular cylinder 100 μ in diameter): the multicellular composition of the fibres and the extensive folding of the surface of the cells.



6. After correction for the intercellular nexal area the surface to volume ratio of a long cylindrical fibre 100 μ in diameter was 0·39 μ−1, or about 10 times the value for a long right circular cylinder 100 μ in diameter. The surface to volume ratio of the tissue interspaces in the same fibre was 170 μ−1.



7. It was concluded that the total sarcolemmal area in this tissue is great enough so that the specific membrane capacitance could be about 1 μF/cm2 and the specific membrane resistance 20,000 Ω cm2.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1972.sp009722

The surface area of sheep cardiac Purkinje fibres

• In physiological studies of the major inorganic constituents of heart muscle cells, magnesium (Mg) has until recently been the neglected stepchild. There are several reasons for this neglect. Chemical analysis for Mg was for many years difficult and tedious. The only available radioactive isotope of Mg, ~Mg, is expensive and has a short half-life. Moreover, Mg acts on relatively inaccessible intracellular processes some of which have been identified and measured only in tissue fractions or in purified proteins extracted from cells; the intracellular processes in which Mg is implicated are multiple and themselves poorly understood. The ionized Mg concentration ([Mg]) in the cardiac cell cannot at present be directly determined, and the rate at which Mg is transported into and out of myocardial cells is so much slower than the rates for potassium, sodium, calcium, and chloride that isotopic or electrophysiological measurements of ion transport in the usual in vitro preparations of heart muscle become insentitive or inconvenient. Nevertheless, there are compelling reasons for reexamining the role of Mg in heart muscle at this time. First, recent experiments on skeletal muscle indicate that [Mg + ] may be a critical modulator of the tension with which the contractile apparatus of striated muscle responds to the prevailing ionized calcium concentration ([Ca + ]) (1-5); at the same time, the Mg complex with adenosine triphosphate (MgATP) is the substrate for the enzymatic reactions that underlie the sliding filament mechanism for myofibrillar contraction and relaxation. Second, work with subcellular model systems indicates that Mg participates in many of the most vital oxidative, synthetic, and transport processes of the myocardial cell. Finally, advances in the microchemical and radioactive measurement of Mg as well as in the

Magnesium in heart muscle.

Effects of thyroxin on ultrastructure of rat myocardial cells: a stereological study.

A sequestered fraction of myofibrillar magnesium presumably bound to thin filaments during polymerization of actin was used to determine myofibrillar mass in rat left ventricles and rabbit hearts Myofibrillar volume was also determined by stereological measurements on electron micrographs of rat ventricles Myofibrillar Mg (111 mmoles/kg dry ventricle) can be determined in glycerinated ventricle either by measuring 28Mg-inexchangeable Mg or Mg remaining after extraction with EDTA, KCl, and a nonionic detergent These measurements, combined with the Mg content of myofibrillar suspensions similarly extracted (32 mmoles/kg protein), indicated that myofibrillar mass was 347 g myofibrillar protein/kg dry ventricle In rabbit hearts, increases in myofibrillar Mg per unit dry weight were as follows: auricular appendage < right ventricle < left ventricle ⋍ interventricular septum After production of left ventricular hypertrophy in rats by constriction of the ascending aorta, both myofibrillar Mg and volume increased within 24 hours After 10 or more days, myofibrillar Mg and myofibrillar volume increased proportionately more than tissue dry mass and cell volume, and the ratio of mitochondrial volume to cell volume decreased

Ernest Page

Papers

Quantitative electron microscopic description of heart muscle cells. Application to normal, hypertrophied and thyroxin-stimulated hearts.

The surface area of sheep cardiac Purkinje fibres

Magnesium in heart muscle.

Effects of thyroxin on ultrastructure of rat myocardial cells: a stereological study.

Myofibrillar Mass in Rat and Rabbit Heart Muscle: CORRELATION OF MICROCHEMICAL AND STEREOLOGICAL MEASUREMENTS IN NORMAL AND HYPERTROPHIC HEARTS