Structural basis for the ascending limb of left ventricular function

Midwall sarcomere lengths near maximal have been reported at left ventricular end-diastolic pressures at the upper limits of normal, but an ascending limb of cardiac function can exist at much higher filling pressures. Accordingly, an analysis was made of sarcomere length distributions across the left ventricular wall over a range of end-diastolic pressures between 2 and 20 mm Hg. In 11 dogs, significant ascending limbs of ventricular function were documented at filling pressures between 2 and 12 mm Hg and between 12 and 20 mm Hg. Nine hearts were arrested and rapidly fixed in diastole with intracoronary glutaraldehyde perfusion, and tissues from five sites equally distributed across the left ventricular free wall were examined by electron microscopy. At filling pressures at the lower end of the normal range (2, 6, and 12 mm Hg), an uneven distribution of sarcomere lengths across the wall was noted: mean sarcomere lengths were shortest at the subendocardial layer, longest at a site between the subendocardial and the midwall layers, and then progressively shorter toward the epicardium. All differences were highly significant. At an end-diastolic pressure of 20 mm Hg, the difference in sarcomere lengths between layers was small; sarcomere lengths decreased only slightly from endocardium to epicardium. Despite maximal sarcomere lengths just inside the midwall in hearts fixed at 12 mm Hg (2.253µ), sarcomere lengths in other layers-the subendocardial and the subepicardial regions-were increased substantially from 2.129 and 2.183µ, respectively, at 12 mm Hg to 2.283 and 2.247µ at 20 mm Hg ( P < 0.001 and P < 0.002, respectively). Therefore, sarcomeres from all layers of the wall increase in length up to a filling pressure of 12 mm Hg; however, as filling pressure is increased further, only short sarcomeres from the subendocardial and the subepicardial layers are recruited. The latter mechanism forms the ultrastructural basis for the ascending limb of normal ventricular function at elevated left ventricular filling pressures.

Ultrastructure of the atrial, ventricular, and Purkinje cell, with special reference to the genesis of arrhythmias

An examination of the anatomy of the atrial, ventricular, and Purkinje cells reveals that the internal composition of all cardiac myofibers is qualitatively the same: all have a single nucleus, sarcomeric and mitochondrial units, and a well-developed sarcoplasmic reticulum. There are important differences, however, in the extent and distribution of the cell membrane and its derivatives in the myofiber. The presence or absence of a transverse tubular system and the variation in the number and type of intercellular linkages explain at least some of the characteristic electrical and functional properties of individual types of cells. The overall pattern of cellular organization in working atrial, ventricular, and conducting tissue is reviewed, and possible anatomic bases for current theories of normal and abnormal impulse generation and conduction in the heart are discussed.

Do morphogenetic tissue rearrangements require active cell movements? The reversible inhibition of cell sorting and tissue spreading by cytochalasin B

Previous studies have indicated that cell sorting and tissue spreading are caused by cell combination-specific differences in intercellular adhesive energies, acting in a system of motile cells. We wished to determine whether these adhesive energies could drive cell rearrangements as well as guide them. Accordingly, aggregates of intermixed embryonic cells were cultured in solutions of the drug cytochalasin B (CCB) at a concentration shown to inhibit the locomotion of cells on a solid surface. In addition, spherical aggregates of several kinds were cultured in mutual contact under similar conditions. Both cell sorting and tissue spreading were found to be inhibited. The prompt release of this inhibition upon removal of the CCB showed that the inhibited cells were not merely injured. Moreover, aggregation experiments showed that CCB did not prevent cells of several kinds from initiating mutual adhesions. In fact, heart cell aggregation was enhanced by CCB. We conclude that interfacial forces, originating outside the cell, act together with forces originating inside it in bringing about the morphogenetic movements of cell sorting and tissue spreading. We propose the term "cooperative cell locomotion" to describe translational movements of cells arising from such a combination of intrinsic and extrinsic forces.

Structures of physiological interest in the frog heart ventricle

Two structures of physiological interest in frog heart ventricles have been examined in detail: (a) the layer of endothelial cells which encloses each bundle of heart fibres, and (b) the sarcoplasmic reticulum (SR) inside the heart fibres. Some additional observations on fibre sizes and types have been made.

Movement across the endothelial cell layer of molecules (molecular or ionic size ≤ 12.5 nm) occurs through narrow clefts separating each endothelial cell from its neighbour. This conclusion results from experiments made with the extracellular markers ferritin and horseradish peroxidase.

A diffusion equation describing the movement of solutes into and out of the fibre bundle has been derived using several geometrical parameters, such as the length and width of the clefts and the size of the extracellular aqueous space inside the bundle, all of which were determined from electron micrographs of the tissue.

The theoretical solution for a stepwise change of external calcium concentration gives a halftime of 2.3 s (± 0.8 s, S.D. of 13 bundles) for diffusion equilibrium at the surface of the heart fibres; this value, however, is likely to be an overestimate, by some 20-30 %, on account of several systematic errors which are described.

The sarcoplasmic reticulum in heart fibres consists of a network of thin tubules which partially encircle the myofibrils at Z-line level and also form occasional longitudinal connexions. Branches extend to peripheral regions of the cell and terminate in close apposition to the inner surface of the cell membrane. The volume of the SR is estimated to be approximately 0.5% of the myofibrillar volume of the cells.

Cross-sectional areas of heart fibres (and also their shapes) vary considerably, from less than 2 to more than 100 µm2 (average 17.4 µm2).Fibres of large size and small surface/volume ratio contain many fewer myofibrils and more glycogen granules than fibres of the same size but larger surface/volume ratio.

Physiological implications of these results are discussed.

The disposition, morphology and innervation of cardiac specialized tissue in the guinea-pig

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A fraction of the ventricular myocardium that has the specificity of the cardiac beta-adrenergic receptor

The 78,000 x g microsomal fraction of canine ventricular myocardium effected a 20-fold concentration of [(3)H]norepinephrine from a 10(-9) M solution. The [(3)H]norepinephrine bound was displaced by physiologic concentrations of the beta-adrenergic catecholamines isoproterenol, epinephrine, and norepinephrine, in that order, which is the order of effectiveness of their actions on the force and rate of cardiac contraction. Alpha-adrenergic compounds did not displace [(3)H]norepinephrine until concentrations four orders of magnitude greater were reached. The beta-adrenergic blocker propranolol displaced at 10(-6) M, whereas the alpha-adrenergic blocker phentolamine was inactive. Metabolites of the catecholamines did not compete for this binding site. It is concluded that, on the basis of specificity and affinity of binding, these microsomal particles are likely to contain the beta-adrenergic receptor.

Sterological measurements of cardiac ultrastructures implicated in excitation-contraction coupling

Electron micrographs of osmium-fixed left ventricles from 200-g female rats were analyzed by stereological techniques. By the use of equations developed by H. Sitte it was possible to determine volume fractions of organelles and absolute membrane areas per unit cell volume for cellular membrane systems implicated in excitation-contraction coupling. The fractions of cell volume were: mitochondria 0.34, myofibrils 0.481, T-system 0.012, total sarcotubules 0.035, other 0.13. The membrane areas per unit cell volume (mum(2)/mum(3)) were: external sarcolemma 0.27, external sarcolemma + T-system 0.34, total sarcotubules 1.3. Diads made up 0.08 of sarcotubular volume and 0.12 of sarcotubular membrane area. 0.14 of the external sarcolemmal membrane area was involved in diadic complexes with underlying subsarcolemmal cisterns.

Studies of cardiac muscle with a high permeability to calcium produced by treatment with ethylenediaminetetraacetic acid

Thin strips of frog ventricle were isolated and bathed for 15 min in a solution containing 140 mM KCl, 5 mM Na(2)ATP, 3 mM EDTA, and 10 mM Tris buffer at pH 7.0. The muscle was then exposed to contracture solutions containing 140 mM KCl, 5 mM Na(2)ATP, 1 mM MgCl(2), 10 mM Tris, 3 mM EGTA, and CaCl(2) in amounts to produce concentrations of free calcium from 10(-4.8)M to 10(-9)M. The muscles developed some tension at approximately 10(-8)M, and maximum tension was achieved in 10(-5)M Ca(++). They relaxed in Ca(++) concentrations less than 10(-8)M. The development of tension by the EDTA-treated muscles was normalized by comparison with twitch tension at a stimulation rate of 9 per min before exposure to EDTA. In 10(-5)M Ca(++) tension was always several times the twitch tension and was greater than the contracture tension of a frog ventricular strip in KCl low Na-Ringer. Tension equal to half-maximum was produced at approximately 10(-6.2)M Ca(++). Intracellular recording of membrane potential indicated that after EDTA treatment the resting potential of cells in Ringer solution with 10(-5)M Ca or less was between 5 and 20 mv. Contracture solutions did not produce tension without prior treatment with EDTA. The high permeability of the membrane produced by EDTA was reversed and the normal resting and action potentials restored in 1 mM Ca-Ringer. Similar studies of EDTA-treated rabbit right ventricular papillary muscle produced a similar tension vs. Ca(++) concentration relation, and the high permeability state reversed with exposure to normal Krebs solution.

Cardiac muscle. Its ultrastructure in the finch and hummingbird with special reference to the sarcoplasmic reticulum

Cardiac muscle fibers of the hummingbird and finch have no transverse tubules and are smaller in diameter than those of mammalian hearts. The fibers are connected by intercalated discs which are composed of desmosomes and f. adherentes; small nexuses are often interspersed. As in cardiac muscle of several other animals, the junctional SR of the couplings is highly structured in these two birds but, in addition, and after having lost sarcolemmal contact, the junctional SR continues beyond the coupling to extend deep into the interior of the cells and to form belts around the Z-I regions of the sarcomeres. This portion of the sarcoplasmic reticulum, which we have named "extended junctional SR," and which is so prominent and invariant a feature of cardiac cells of hummingbirds and finches, has not been observed in chicken cardiac cells. The morphological differences between these species of birds may be related to respective differences in heart rates characteristic for these birds.

Transitional cardiac cells of the conductive system of the dog heart. Distinguishing morphological and electrophysiological features

Cardiac cells with distinctive electrophysiological and morphological features were found at the junctional region between Purkinje and ventricular cells of the dog heart. The electrophysiological exploration of these "transitional" cells revealed action potentials markedly different in configuration from those generated by Purkinje or by ventricular cells. The impaled cardiac cells which generated transitional action potentials were identified in serial sections and studied with the light and the electron microscopes. The transitional cells were found to be characterized cytologically by: (a) their subendocardial location, (b) their small diameter, (c) the absence of T system and sarcoplasmic reticulum, and (d) the lack of intercalated discs under the light microscope and the sparsity of specialized intercellular junctions under the electron microscope. Purkinje, transitional, and ventricular cells were found to be joined by gap junctions permeable to lanthanum. A quantitative difference in the extent and distribution of specialized intercellular junctions may be one of the factors responsible for the slow velocity of conduction characteristic of the Purkinje-ventricular junctional region.

Ventricular nuclei-DNA relationships with myocardial growth and hypertrophy in the rat

Rat ventricles, ranging from 306 to 1999 mg, were obtained from normal animals of different ages and from animals subjected to chronic aortic constriction. The concentration of DNA in the paired ventricles (right and left) was found to be closely related to the concentration of nuclei in the left ventricular papillary muscles. Muscle nuclei represented only 10 to 15% of this population of nuclei. In young animals (phase 1), the total ventricular content of muscle nuclei, nonmuscle nuclei, and of DNA were increasing with ventricular growth. In the adult rat (phase 2), the total ventricular content of DNA and of both muscle and nonmuscle nuclei remained relatively constant with ventricular growth. In the enlarged hearts (phase 3), there was a further increase in total ventricular DNA but there was no further increase in the total number of muscle nuclei. Spectrophotometric studies (Feulgen stain), showed that 88% of the muscle nuclei belonged to a single ploidy class (probably diploid). No relation could be demonstrated between the extent of nuclear polyploidy and the weight of the ventricles. It was concluded that polyploidy was not a significant factor in the increased total DNA of phase 3. The increased ventricular DNA of phase 3 was explained by the proliferation of the nonmuscle nuclei.

A study of the T system in rat heart

The technique of extracellular space tracing with horseradish peroxidase is adapted for labeling the transverse tubular system (T system) in rat heart. In rat ventricular muscle the T system shows extensive branching and remarkable tortuosity. The T system can only be defined operationally, since it does not display specific morphological features throughout its entire structure. Owing to branching of the T system, a sizable proportion of the apposition between the T system and L system (or closed system) occurs at the level of longitudinal branches of the T system and is not restricted to the Z line region. The regions of apposition between the T system and L system are analyzed in rat ventricular muscle and skeletal muscle (diaphragm) and compared with the intercellular tight junctions (nexuses) of heart muscle by the use of a photometric method. The over-all thickness of the nexus is significantly smaller than that of T-L junctions in both cardiac and skeletal muscles. The thickness of the membranes of the T and L systems are not significantly different in the two muscles, but the gap between both membranes is larger in the heart. In atrial muscle the following two types of cells are found: (a) those cells with a well-developed T system in which the tubular diameter is quite uniform and the orientation predominantly longitudinal and, (b) cells with no T system, but with a well-developed L system. Atrial cells possessing a T system are richly provided with specific granules and show little micropinocytotic activity, whereas cells devoid of T system show intense micropinocytotic activity and few specific granules. The possible functional implications of these findings are discussed.