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Biomedical subjects

M S Spach

Publications and source records attributed to M S Spach.

At least 19 recordsLinked to original sources
PubMed1992

Distribution of gap junctions in dog and rat ventricle studied with a double-label technique.

To assess the distribution of gap junctions in relation to the cardiac myocyte surface in paraffin sections of dog and rat ventricle, the sarcolemma was labeled with wheat germ agglutinin (WGA1) and gap junctions were labeled with antibodies to cardiac muscle gap junction protein connexin43. WGA labeled all of the myocyte sarcolemma, including that in intercalated discs and transverse tubules. Sarcolemmal WGA labeling was often interrupted at the sites of gap junctions, which were found both at the extreme ends of myocytes and along the length of adjacent myocytes. Small gap junctions predominated at plicate transverse portions of the intercalated disc; larger and sometimes ribbon-like gap junctions predominated at longitudinal portions. The longitudinal portions of the intercalated disc often extended over multiple sarcomere lengths, with ribbon-like gap junctions and linear arrays of smaller gap junctions arranged in parallel overlying successive sarcomeres. Morphometric study showed that ribbon-like gap junctions were relatively infrequent in both dog and rat left ventricular epimyocardium, and that animals with larger myocytes tended to have smaller gap junctions. In dog left ventricular epimyocardium, neither myocytes nor their larger gap junctions were randomly oriented with respect to perimysial separations; myocytes were usually somewhat flattened with their maximal diameters parallel to the separations, whereas large gap junctions were least often oriented parallel or perpendicular to the separations. Overall, the data indicate that myocyte geometry influences gap junction size and distribution; the double-label technique is ideally suited for the further exploration of that influence.

Animals↗
PubMed1992

Cellular Vmax reflects both membrane properties and the load presented by adjoining cells.

This study was designed to test the hypothesis that the electrical load seen at a microelectrode impalement site is sensitive to the direction of propagation of the approaching wavefront as a reflection of an altered spatial relationship between the impalement site and the surrounding microscopic electrical boundaries located up- and downstream. These boundaries correspond to the different sizes and shapes of the impaled and surrounding cells as well as to the distribution of the associated electrical connections between the cells. The effects of changes in these geometric relationships on maximum rate of rise of transmembrane potential (Vmax) were investigated in canine ventricular muscle by measuring Vmax in different cells while the direction of propagation was changed from along the longitudinal axis to the transverse axis of the fibers or the direction of conduction was reversed along either of these axes. Comparison of the Vmax values for longitudinal propagation (LP) and transverse propagation (TP), each in one direction, showed that TP Vmax was significantly greater than LP Vmax (P < 0.001). However, the values of Vmax were different from cell to cell during LP (93-139 V/s) and TP (110-181 V/s). The absolute values of LP Vmax and TP Vmax at the same site varied independently of each other, e.g., some of the lowest LP Vmax values occurred at the same site as the highest TP Vmax values. Furthermore, at the same site, Vmax changed considerably when propagation was maintained along the longitudinal axis but the direction of conduction was reversed. Similar prominent changes in Vmax occurred when the direction was reversed along the transverse axis.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

A multidimensional model of cellular effects on the spread of electrotonic currents and on propagating action potentials.

This study was designed to develop a two-dimensional cellular model of uniform anisotropic muscle and to determine how irregularities of shape and variations in size of cardiomyocytes influence the passive (electrotonic) spread of currents at a microscopic level. A secondary purpose was to determine how the passive transfer of impressed currents across the gap junctions is related to the charge flow across the gap junctions during active propagation of depolarization. The decrease in electrotonic Vm with distance at a large size scale was described by a single exponential in both the longitudinal and transverse directions, as occurs in a continuous anisotropic medium. At a microscopic level, however, the falloff of Vm with distance was directionally different. Longitudinally, Vm decreased primarily along the length of cells, with small step-like decreases at the intercalated disks. Transversely, Vm was more nearly isopotential throughout each cell, and most of the decay in Vm occurred as large step-like decreases across the borders of the cells. Different gap junctions were used for charge flow for longitudinal versus transverse electrotonus. Remarkably similar results were obtained for propagating action potentials, i.e., different gap junctions were used for longitudinal versus transverse conduction. A major implication of the results is that it may be possible to gain information about the different longitudinal and transverse effects of the nonhomogeneous distribution of the cellular connections by improved measurements of propagation at a microscopic level.

Action Potentials↗
PubMed1989

Structure of canine Bachmann's bundle related to propagation of excitation.

Impulse propagation and histology were studied in adult and neonatal canine Bachmann's bundle. Both showed nonuniform electrical anisotropy: effective longitudinal conduction velocity (theta L) markedly exceeded effective transverse conduction velocity (theta T), and extracellular potential waveforms with transverse propagation were polyphasic. An age difference in theta L (0.80 m/s neonate, 1.31 m/s adult) was found; it could be largely accounted for by a difference in myocyte diameter (4.7 microns neonate, 17.1 microns adult). Close apposition of myocytes in the neonate and development of transverse tubules in the adult may have influenced theta L at each stage. Perimysial septa separated fascicles in both neonatal and adult bundles; however, endomysial septa between individual myocytes were completely developed only in adult bundles. Thus perimysial septa were much more responsible for nonuniform anisotropy and low theta T than endomysial septa. Fascicle diameter and length were greater in the adult, which may have affected transverse propagation. Specialized conduction system cells could not be identified.

Aging↗
PubMed1989

Multiple regional differences in cellular properties that regulate repolarization and contraction in the right atrium of adult and newborn dogs.

Recent studies of isolated cardiac myocytes have generated the need for detailed information about regional electrophysiological differences in the atrium. We measured the spatial distribution of action potentials in adult and newborn canine right atria. Multiple regional differences in action potential shape and duration were found. The multiple regional differences produced an overall simple pattern: the longest action potentials occurred in the area of the sinus node, and the action potential duration decreased with increasing distance from the sinus node area. To account for the overall pattern, we tested factors considered important in causing atrial action potential shape differences (e.g., electronic interactions). None of the factors tested accounted for the regional differences. We then found regional differences in the responses to pauses, which suggested that differences in the properties of individual cells accounted for the regional repolarization differences. If so, genetic regulation of the regional differences may produce the overall pattern on a developmental basis. Experiments in newborn atria showed that only in the upper crista was the spatial pattern similar to that of the adult; there was little variability in action potential shape and duration in the other areas. As a further test for associated regional differences in cell properties, we examined for differences in the isoform expression of troponin T (TnT1, TnT2, TnT3, and TnT4), a protein important in excitation-contraction coupling. In adults, the greatest proportion of TnT1 occurred in the area of the sinus node, and its proportion decreased with increasing distance from the sinus node area in association with a relative increase in the proportion of TnT2. In newborn atria the relative amount of TnT1 was greatest in the upper crista (similar to adult), but little difference was found in the distribution of the isoforms in the other regions. The correspondence between the regional differences in repolarization and in the expression of the troponin T isoforms in adult and newborn atria suggests that 1) cellular ionic mechanisms vary regionally to coordinate differences in action potential configuration with differences in cell properties that regulate contractility and 2) genetic expression of the systems that regulate repolarization and mechanical cellular properties are under similar developmental and regional control.

Action Potentials↗
PubMed1989

Interaction of inhomogeneities of repolarization with anisotropic propagation in dog atria. A mechanism for both preventing and initiating reentry.

Having found the regional differences in right atrial action potentials shown in an accompanying article, we tested two seemingly paradoxical hypotheses: 1) The spatial pattern of repolarization provides a protective mechanism against reentry, and 2) repolarization inhomogeneities interact with anisotropic discontinuous propagation to produce reentry. Measurement of multidimensional refractory periods demonstrated an anisotropic distribution within large bundles with the longest refractory periods in the medial upper crista terminalis (sinus node area), a distribution similar to that of action potential durations. Also, discontinuities of repolarization were found at muscle bundle junctions. Early premature impulses originating in the sinus node area propagated throughout the right atrial preparations without conduction disturbances or reentry. Conversely, early premature impulses that originated at sites distal to the sinus node area resulted in localized conduction block at multiple sites, which frequently produced complex conduction changes and reentry. The critical nature of the site of origin of a premature impulse in initiating reentry was related to locations where the steepest repolarization gradients occurred: within anisotropic bundles in the direction of highest axial resistance (across fibers) and at muscle bundle junctions that represented localized discontinuities of axial resistance. The multiple conduction abnormalities at localized sites interacted to produce different types of reentry at a larger size scale (25 mm2 to several cm2). In each case, neither repolarization inhomogeneities (leading circle concept) nor anisotropic discontinuous propagation was the only "mechanism" involved. That is, reentry at a macroscopic size scale occurred as a result of a combined repolarization-anisotropic discontinuous propagation mechanism.

Action Potentials↗
PubMed1988

Influence of the passive anisotropic properties on directional differences in propagation following modification of the sodium conductance in human atrial muscle. A model of reentry based on anisotropic discontinuous propagation.

Available models of circus movement reentry in cardiac muscle and of drug action on reentrant arrhythmias are based on continuous medium theory, which depends solely on the membrane ionic conductances to alter propagation. The purpose of this study is to show that the anisotropic passive properties at a microscopic level highly determine the propagation response to modification of the sodium conductance by premature action potentials and by sodium channel-blocking drugs. In young, uniform anisotropic atrial bundles, propagation of progressively earlier premature action potentials continued as a smooth process until propagation ceased simultaneously in all directions. In older, nonuniform anisotropic bundles, however, premature action potentials produced either unidirectional longitudinal conduction block or a dissociated zigzag type of longitudinal conduction (a safer type of propagation, similar to transverse propagation). Directional differences in the velocity of premature action potentials demonstrated that anisotropic propagation was necessary for a reentrant circuit to be contained within an area of 50 mm2, even with very short refractory periods. Quinidine produced Wenckebach periodicity, which disappeared after acetylcholine shortened the action potential. Quinidine also produced use-dependent dissociated zigzag longitudinal conduction in the older, nonuniform anisotropic bundles but not in the young, uniform anisotropic bundles. The electrophysiological consequence was that propagation events differed in an age-related manner in response to the same modification of the sodium conductance. The electrical events at microscopic level showed that conditions leading to obliteration of side-to-side electrical coupling between fibers (e.g., aging and chronic hypertrophy) provide a primary mechanism for reentry to occur within very small areas (1-2 mm) due to a variety of propagation phenomena that do not occur in tissues with tight electrical coupling in all directions.

Action Potentials↗
PubMed1987

Thin collagenous septa in cardiac muscle.

Light and electron microscopy were used to study the structure and distribution of thin collagenous septa (sheets) in dog and rabbit cardiac muscle to determine whether they, like thick collagenous septa, could affect electrical impulse propagation. Generally, thin septa (0.2-0.5 micron) ensheathed myocytes or groups of myocytes for short distances and thicker septa partially or completely ensheathed groups of myocytes for long distances (up to several mm); together, thin, and thick septa divided the myocardial mass into myocyte cords (funicles) of 10-30 micron diameter. Septal architecture varied not only between regions and within regions at different ages but also within single bundles, precluding the assumption that the architecture found in one bundle can be applied to another. Electron microscopy demonstrated that thick septa consisted of many tightly packed collagen fibrils, often with distinct layers running at different angles; thin septa consisting largely of circumferential collagen fibrils. Thin septa in dog ventricular papillary muscle generally contained few and widely spaced collagen fibrils, whereas thin septa in dog atrial Bachmann's bundle contained tightly packed collagen fibrils. In either site, thin septa were rarely breached by nexuses and thus marked sites where lateral intercellular electrical coupling was unlikely. Serial 7 micron cross sections of dog Bachmann's bundle stained by a modification of the picrosirius red technique showed that thin septa sometimes persisted uninterrupted over several myocyte lengths. The results provide evidence that thin septa comprised of tightly packed collagen fibrils may significantly modify impulse propagation transverse to the longitudinal axis of the myocytes.

Animals↗
PubMed1987

Origin of the right pulmonary artery from the ascending aorta.

Origin of the right pulmonary artery from the aorta is a congenital malformation usually associated with serious symptoms in the first year of life and characterized by a poor prognosis. Sixty-five patients with this disorder have been reviewed in the literature, and 95% presented during the first year with signs of congestive heart failure. All had cardiomegaly by radiographic and electrocardiographic examination. An accurate diagnosis was established by cineangiography, and associated cardiovascular anomalies were present in 85%. Origin of the right pulmonary artery from the ascending aorta is much more common than origin of the left pulmonary artery from this vessel (8 to 1). Twenty-three patients were managed without operation with a 30% 1-year survival rate. Among those patients managed surgically, the survival rate was 84% at 1 year. It is now clear that operation should be done as early as possible to prevent irreversible changes occurring in the pulmonary arterial vasculature since microscopic features of pulmonary hypertension have been seen during the first month of life. One patient was operated on at 5 months with correction of the deformity. The pulmonary arterial pressure decreased to normal after operation. This child is now asymptomatic and his 10-year postoperative follow-up is the longest found in the literature.

Age Factors↗
PubMed1987

Propagating depolarization in anisotropic human and canine cardiac muscle: apparent directional differences in membrane capacitance. A simplified model for selective directional effects of modifying the sodium conductance on Vmax, tau foot, and the propagation safety factor.

As yet there is no model or simulation that accounts for the anisotropic difference in the shape of the upstroke and safety factor of propagating cardiac action potentials: fast upstrokes occur with slow transverse propagation and slow upstrokes occur with fast longitudinal propagation. The purpose of this paper is to demonstrate, however, that a simplified cable model based on directional differences in the effective membrane capacitance predicts in detail the experimentally measured directionally dependent behavior of the upstroke in response to modification of the sodium conductance. Quinidine and lidocaine produced greater relative decreases in Vmax and conduction velocity with longitudinal propagation than with transverse propagation, as predicted on the basis that the shape differences should produce an anisotropic distribution in the membrane uptake of sodium channel binding drugs. The simulation predictions of the effects of positive shifts of the take-off potential due to premature action potentials were also confirmed experimentally: there was a greater relative decrease in conduction velocity, Vmax, and Vamp with a greater increase in tau foot during longitudinal propagation than with transverse propagation. The major anisotropic differences in shape occurred when the take-off potential approached the least negative value that produced a propagated response. The extensive experimental verification of the results of a simplified model based on directional differences of effective membrane capacitance, combined with directional differences in effective axial resistivity, provides an initial quantitative basis for the anisotropic behavior of propagating depolarization in response to modification of the sodium conductance in cardiac muscle.

Action Potentials↗
PubMed1987

Picrosirius red staining of cardiac muscle following phosphomolybdic acid treatment.

When the picrosirius red technique was applied to cardiac muscle sections, intense yellow myocyte staining sometimes obscured thin collagenous septa. The picrosirius red technique was modified to include treatment of the sections in 0.2% (w/v) aqueous phosphomolybdic acid prior to staining. With 1-5 min treatment, cytoplasmic staining was eradicated; diminution of collagen staining occurred only with long treatments at much higher concentrations of phosphomolybdic acid. Using this phosphomolybdic acid-picrosirius red technique, collagenous septa as thin as 0.2-0.5 micron and fine collagen fibers making up the septa were clearly discernible. The technique also worked well on sections stained by other techniques and then destained. The phosphomolybdic acid-picrosirius red technique should be useful in experiments designed to investigate the effects of collagen distribution on the electrical and mechanical behavior of cardiac muscle.

Animals↗
PubMed1986

Relating extracellular potentials and their derivatives to anisotropic propagation at a microscopic level in human cardiac muscle. Evidence for electrical uncoupling of side-to-side fiber connections with increasing age.

Elucidation of the mechanisms of cardiac conduction disturbances leading to reentry will require resolution of the details of multidimensional propagation at a microscopic size scale (less than 200 micron). In practice, this will necessitate the combined analysis of extracellular and transmembrane action potentials. The purpose of this paper is to demonstrate the relationships between the time derivatives of the extracellular waveforms and the underlying action potentials in the experimental analysis of anisotropic propagation at this small size scale, and apply these relationships to human atrial muscle at different ages. The extracellular waveforms and their derivatives changed from a smooth contour during transverse propagation in young preparations to complex polyphasic waveforms in the older preparations. The major problem was to estimate the size and location of small groups of fibers that generated the complex waveforms in the older preparations. We found dissimilarities in the derivatives that distinguished source (bundle) size from the distance of the source to the measurement site. The differences in the extracellular waveforms and their derivatives indicated that there was electrical uncoupling of the side-to-side connections between small groups of fibers with aging. These changes produced a prominent zigzag course of transverse propagation at a microscopic level which, in turn, accounted for the increased complexity of the waveforms. The waveform differences also correlated with the development of extensive collagenous septa that separated small groups of fibers. The electrophysiological consequence was an age-related decrease in the "effective" transverse conduction velocities to the range of the very slow conduction (less than 0.08 m/sec) which makes it possible for reentry to occur in small regions of cardiac muscle with normal cellular electrophysiological properties.

Action Potentials↗