Ch. Holubarsch

Connective tissue content and myocardial stiffness in pressure overload hypertrophy A combined study of morphologic, morphometric, biochemical, and mechanical parameters

SummaryWe investigated samples of left ventricular myocardium from Goldblatt II (4 and 8 weeks after operation) and spontaneously hypertensive rats (SHR; 40 and 80 weeks old) by histological and morphometric methods. From the same hearts, the distensibility of the left ventricular papillary muscle was analyzed by means of resting tension curves, and the collagen content of the whole left ventricular wall was determined by means of hydroxyproline concentration.In all groups, myocardial fibrosis was observed to accompany myocardial hypertrophy. The severity of fibrotic lesions increased with the duration of hypertension, and, in late stages, degenerative changes of cardiac myocytes were found. Morphometric determinations and chemical analysis of the hydroxyproline concentration revealed a decrease in myocardial muscle content, which was paralleled by an increase in collagen content when compared to the respective controls.In general, morphometric and chemical findings correlate with increased myocardial stiffness observed during mechanical measurements in isolated papillary muscle preparations from the same hearts. Differences were found, however, between chemical analysis and mechanical measurements in the 40-week-old SHR group, which may result from different patterns of collagen distribution between interstitium, perivascular spaces, and the walls of blood vessels.The comparison between histological, morphometric, chemical, and physiological data shows that (1) cardiac hypertrophy of Goldblatt and SH-rats is accompanied by myocardial fibrosis, and (2) changes in passive elastic properties of myocardium is better reflected in morphometric than in chemical analysis.ZusammenfassungLinksventrikuläre Myokardproben von Goldblatt-II-(4 und 8 Wochen post operationem) und spontanhypertensiven Ratten (SHR; Alter 40 und 80 Wochen) wurden histologisch und morphometrisch untersucht. Von denselben Herzen wurden zudem die Dehnbarkeit des linksventrikulären Papillarmuskels mittels Ruhe-Dehnungs-Kurven sowie der Kollagenanteil der gesamten linken Ventrikelwandung mittels Hydroxyprolinkonzentration bestimmt. Neben der zu erwartenden Hypertrophie der Myozyten fanden wir in beiden Hypertrophiemodellen eine Myokardfibrose. Das Ausmaß der Fibrose nahm mit der Dauer des Bestehens der Hypertension zu. In späteren Studien wurden zusätzlich degenerative Veränderungen an Myozyten beobachtet.Morphometrische Bestimmungen und Hydroxyprolinkonzentration weisen einen Anstieg des Kollagengehaltes des druckbelasteten linken Ventrikels nach. Morphometrische und chemische Ergebnisse stehen im Einklang mit der myokardialen Dehnbarkeit—gemessen am isolierten Papillarmuskel derselben Herzen. Diskrepanzen, die im 40-Wochen-Stadium der SHR-Gruppe auftreten, müssen auf Unterschiede im Verteilungsmuster des Kollagens auf Interstitium, perivaskuläre Räume und Wände der Blutgefäße zurückgeführt werden. Der Vergleich histologischer, chemischer und physiologischer Ergebnisse zeigt 1., daß die Herzhypertrophie bei Goldblatt- und SH-Ratten von Myokardfibrose begleitet ist, und 2., daß Veränderungen der passiv-elastischen Eigenschaften des Myokards besser vom morphometrischen als vom chemischen Resultat widergespiegelt werden.

Adaptive and Pathological Alterations in Experimental Cardiac Hypertrophy

Based on investigations of various models of experimental cardiac hypertrophy (renal hypertension, spontaneous hypertension, aortic stenosis, swimming training, thyrotoxicosis), an attempt has been made to characterize adaptive and pathological alterations that are inherent to or accompany the process of hypertrophy. In principle, the designation of a process as adaptive is rooted in a teleological point of view and implies that the basic tendency of the respective structural and functional alterations is appropriate for coping with the altered functional requirements. This does not mean, however, that such alterations are favorable under all conditions and in all stages of hypertrophy. Since organisms generally reveal relatively stereotypic reaction patterns, the terms “adaptive” and “pathological” are not mutually exclusive in the final analysis. In the chronically pressure-loaded ventricle, nearly all alterations are ambiguous (myocardial mass increase, prolongation of the action potential, overproportional increase of intracellular contractile material, decrease of myofibrillar ATPase activity). The altered ATPase activity, which is based on a shift in the isoenzyme pattern of myosin in the direction of isoenzyme V3, is accompanied by a decrease in unloaded shortening velocity but an increase in the efficiency of tension development, as is reflected in reduced oxygen consumption (per wall stress and heart rate) of the whole heart under isovolumetric conditions. This change in the elementary contractile process and the myofibrillar ATPase activity need not be interpreted a priori as negative. However, the ability to adapt to other types of loading, e.g., physical exertion with corresponding increase in heart rate, is limited by the specialization for coping with enhanced pressure load. The term “overadaptation” should be reserved for stages and degrees of hypertrophy in which the negative effects of double-faced alterations predominate. Rapid, excessive increase in pressure loading, as well as long-term hemodynamic overloading, leads to degenerative alterations of the myocardium. At the level of the whole ventricle, structural dilatation results in a decreased cardiac efficiency. Fibrosis of the ventricular wall, the pathogenesis of which is not always unequivocal, is also a negative factor for mechanical performance. Since there are pronounced degrees of hypertrophy without connective tissue increase, e.g., in thyrotoxicosis, fibrosis and accompanying decreased distensibility of the myocardium apparently are not necessarily involved in the development of hypertrophy. Ischemically induced alterations stemming from vasculopathy should be distinguished from hypertrophy-induced changes. The adaptive alteration of the heart in swim-trained rats, which involves an increase in myofibrillar ATPase activity and a shift in the myosin isoenzyme pattern in the direction of V1, leads to an increase in functional capacity at all levels and is in agreement with the generally accepted concept of contractility.

Myothermal economy of rat myocardium, chronic adaptation versus acute inotropism.

By means of rapid planar Hill type antimony-bismuth thermophiles the initial heat liberated by papillary muscles was measured synchronously with developed tension for control (C), pressure-overload (GOP), and hypothyrotic (PTU) rat myocardium (chronic experiments) and after application of 10(-6) M isoproterenol or 200 10(-6) M UDCG-115. Economy of force production was analyzed by the ratio of initial heat versus developed tension-time integral. This ratio was found to be reduced by 34% in GOP and by 43% in PTU myocardium (P less than 0.01, respectively) indicating increased economy of force production. In contrast, isoproterenol increased initial heat versus tension-time integral by 70% (P less than 0.01) indicating reduced economy of force production. No change in this ratio was found for UDCG-115. The presented data indicates that long and short term modulation of myocardial energetic costs of force generation is possible. The basic mechanisms for these myocardial alterations are discussed.

Force generation in experimental tetanus, KCl contracture, and oxygen and glucose deficiency contracture in mammalian myocardium.

We studied the amount and time-course of tension recovery after quick releases (0.25 mm) during experimental tetanus, potassium chloride contracture (KCl), hypoxic contracture (H) as well as joint glucose and oxygen deficiency contracture (HG) in left ventricular papillary muscles of rat and right ventricular papillary muscles of cat myocardium. Both in experimental tetanus and KCl contracture, the tension recovery was finished within 200 ms after the release, and was 77% and 60% of initial tension fall, respectively.. The Q10 value for the time constant of half of recovery tension was 2.5, and the time constants differed by the same factor between rat and cat myocardium. In H or HG contracture, we never could find any significant tension recovery process during the first 200 ms after a release, neither in late nor in very early stages of contracture tension generation. However, when longer observation periods (90s) after a release were monitored, a slow tension recovery was observed which was at least 18% of initial tension fall.This data indicates fast, calcium-mediated cross-bridge cycling in experimental tetanus and KCl contracture. In contrast, the very slow tension recovery in H and HG contracture, which is consistent with recent myothermal data, can be interpreted as rigorlike cross-bridges with a very slow cycling rate, a long time of attachment in force-generating position and low energy turnover.

Elastic and contractile properties of the myocardium in experimental cardiac hypertrophy of the rat Methodological and pathophysiological considerations

Two essential parameters of myocardial mechanics, the diastolic stress-strain relationship and the maximum unloaded shortening velocity have been analysed in the pressure-hypertrophied myocardium of Goldblatt rats and compared with the alterations appearing in other models of hypertrophy (spontaneous hypertension, aortic stenosis, swimming training).

Contracture type and fibrosis type of decreased myocardial distensibility. Different changes in elasticity of myocardium in hypoxia and hypertrophy.

Resting length-tension relationships were measured in rat trabecular muscle strips under control (O2) and contracture (N2) conditions as well as in preparations of Goldblatt II (8-week stage) and control rats. For the evaluation of the diastolic elastic properties, diastolic stress at lmax, stress-strain relationships, the relation between tangent modulus \(\frac{{\Delta \sigma }}{{\Delta \varepsilon }}\) and stress σ, and the function In stress σ = f (strain e) were calculated and plotted. It was shown that the hypoxic contracture corresponds to the calcium-caffeine contracture — an experimental contracture model already investigated earlier. In contrast to these contracture-induced alterations, Goldblatt II hypertrophied myocardium corresponded to the fibrosis type. This type was already analyzed in an experimental model of myocardium-tendon tandem preparation. Additional investigations of hydroxyproline concentration of the same biological material showed in contrast to other studies that the collagen content may already be increased in this early stage of hypertrophy. Thus, altered distensibility of Goldblatt myocardium is certainly primarily due to increased connective tissue content.

Prospects of X-ray microanalysis in the study of pathophysiology of myocardial contraction

X-ray microanalysis was used to compare chemically untreated cryosections of quick-frozen myocardial tissue in “caffeine contracture” with cryosections of normal muscle. Our goal was to find out if it is possible by means of this method to detect changes in the calcium compartmentalization of the myocardial cell occurring by changes in its functional state. While it is possible to quantitate calcium in the cisternae of sarcoplasmic reticulum of the control muscle preparation, calcium could never be detected in these compartments of caffeine-contracted muscles.

Diastolic tension of rat cardiac muscle during deficiency of oxygen and glucose. Stress-strain relationships and reversibility.

SummaryIn the isometrically contracting rat trabecular muscle strip preparation, contracture tension was induced either by 10 min of hypoxia (low contracture tension) or by 30 min of joint oxygen and glucose withdrawal (severe contracture tension) at optimum length. Using stepwise 0.1 mm releases, length-tension relationships were recorded under control and contracture conditions in both types of contracture. Stress-strain relationships were evaluated as well as the linear function of the tangent elastic modulus Δδ/Δε versus stress δ. The slope of the Δδ/Δε-f(δ) function was not significantly changed in the low contracture compared to control conditions (n=8, n.s.) but showed a significant 42% increase in the severe contracture (n=8; p<0.001). When reversibility tests were performed by reoxygenation after 10 min hypoxia and reperfusion with oxygen and glucose after 30 min of glucose and oxygen withdrawal, low contracture tension was completely abolished after 15 min, whereas 38% of severe contracture was maintained even after 60 min of reperfusion. Additional morphological studies revealed uniform sarcomere lengths in low contracture, and two populations of sarcomere lengths in the severe contracture. — On the basis of the present and earlier results, a model of contracture tension generation is proposed in which cross-bridges between actin and myosin cycle at a very low rate and are attached in a force-generating position for a long time.ZusammenfassungAm isometrisch schlagenden Rattenmyokardtrabekel wurden Kontrakturen sowohl durch 10minütige Hypoxie (niedrige Kontrakturspannung) als auch durch 30minütigen gleichzeitigen Sauerstoff- und Glucoseentzug (hohe Kontrakturspannung) am Optimum des Längen-Spannungs-Diagramms induziert. Durch schrittweises Entdehnen um jeweils 0,1 mm wurden diastolische Längen-Spannungs-Beziehungen unter Kontrollbedingungen und Kontrakturbedingungen für beide Kontrakturtypen aufgenommen. Sowohl Stress-strain-Beziehungen als auch die lineare Funktion zwischen dem tangentiellen E-Modul

Evaluation of elasticity by means of length-tension relationships in a model of isolated ventricular myocardium from rat and cat papillary muscle under conditions of contracture

\frac{{\Delta \sigma }}{{\Delta \varepsilon }}