R. Jacob

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.

Alterations of mechanical parameters in chemically skinned preparations of rat myocardium as a function of isoenzyme pattern of myosin

SummaryMyofibrillar ATPase activity, maximum unloaded shortening velocity, and isometric tension development were evaluated in left ventricular preparations of 5-week-old rats with a high endogeneous level of thyroid hormones and hypothyroid rats after 4-week treatment with propylthiouracil (PTU). The range of possible alterations of the above functional parameters was defined in relation to myosin isoenzyme distribution. The mechanical behaviour of the ventricular preparations was investigated in native myocardium as well as in the glycerinated state.The essential result of the present study is that alterations of myofibrillar ATPase activity and mechanical vmax, evaluated in glycerinated preparations, are limited to a well-defined range of similar magnitude for both functional parameters: 32–40% of maximum values (obtained from rat myocardium with homogeneous myosin V1). Isometric tension was only insignificantly decreased in glycerinated preparations of the PTU-treated group.The alteration in the apparent maximum shortening velocity of native myocardium (v0) was of the same magnitude as changes in vmax of chemically skinned preparations. Physical training revealed a shift in the direction of V1-type myosin with increased ATPase activity and shortening velocity; aging and pressure overload showed an opposite effect. The documented mechanical alterations do not contradict an adaptational interpretation of the myosin isoenzyme redistribution in pressure-induced hypertrophy.ZusammenfassungUm den Bereich möglicher Änderungen der funktionellen Parameter in Abhängigkeit vom Myosin Isoenzymmuster zu bestimmen, wurden myofibrilläre ATPase-Aktivität, maximale lastfreie Verkürzungsgeschwindigkeit und isometrische Spannungsentwicklung linksventrikulärer Myokardpräparationen folgender Tierkollektive gemessen: 5 Wochen alte Ratten mit hohem endogenem Thyroxinspiegel und hypothyreote Ratten nach 4wöchiger Behandlung mit Propylthiouracil (PTU). Das mechanische Verhalten der Ventrikelpräparate wurde im nativen Zustand und nach Glyzerinextraktion untersucht.Das wesentliche Ergebnis dieser Arbeit ist, daß Veränderungen der lastfreien Verkürzungsgeschwindigkeit — vmax — gemessen in glyzerinisierten Präparaten—und der myofibrillären ATPase-Aktivität auf einen definierten Bereich gleicher Größenordnung begrenzt sind: 32–40% des Maximalwerts, jeweils gemessen im Myokard mit homogenem Myosin V1-Typ. Die isometrische Spannungsentwicklung war nur insignifikant verändert; −9% im PTU-behandelten Kollektiv.Die Veränderung des apparenten Wertes der maximalen lastfreien Verkürzungsgeschwindigkeit — v0, gemessen in nativen Myokardpräparaten — war von gleicher Größenordnung wie die Änderungen des vmax-Wertes in chemisch, gehäuteten Präparaten. Durch körperliches Training war eine Umverteilung der Isoenzyme in Richtung Myosin V1 bei gleichzeitiger Zunahme von ATPase-Aktivität und Verkürzungsgeschwindigkeit (v0) festzustellen; steigendes Lebensalter und Druckbelastung führten zu entgegengesetzten Effekten.Die gezeigten mechanischen Veränderungen widersprechen nicht der Annahme, daß die Umverteilung der Myosin-Isoenzyme bei druckbedingter Hypertrophie einen adaptiven Vorgang darstellt.

Myocardial function in different models of cardiac hypertrophy. An attempt at correlating mechanical, biochemical, and morphological parameters

SummaryBased on mechanical, biochemical and electron microscopic studies performed in the same stage of experimental cardiac hypertrophy, an attempt is made to define the significance of individual factors responsible for the alterations in myocardial function. Using swimming rats, it is demonstrated that a load-induced increase in cardiac mass is not necessarily connected with an impairment of contractile capability on a cellular level. Yet, also, the reduction of specific ATPase activity and unloaded shortening velocity in pressure-induced hypertrophy (Goldblatt rats; aortic stenosis) seems to be the expression of adaptation rather than of cellular damage, at least in the earlier stages. Although there are distinct indications of alterations in Ca-dependent activation and deactivation, in the Goldblatt model electromechanical coupling does not seem to be the main cause of altered contraction parameters. The correlation between specific ATPase activity of actomyosin and unloaded shortening velocity as well as the persistance of decrease in shortening velocity, also under optimal electromechanical coupling conditions, point to an inner relationship between the two values. A discrepancy between unloaded shortening velocity on the one hand and developed tension on the other is mainly due to an increased content of contractile structures. In later stages, an increased connective tissue content influences both isometric and isotonic parameters.ZusammenfassungAuf der Grundlage mechanischer, biochemischer und morphologischer Untersuchungen, die im gleichen Stadium einer experimentellen Herzhypertrophie durchgeführt wurden, wird versucht, die anteilmäßige Bedeutung einzelner Faktoren für die Änderungen des Kontraktionsablaufs abzugrenzen. Am Beispiel schwimmtrainierter Ratten wird zunächst gezeigt, daß nicht jeder Hypertrophieprozeß zwangsläufig mit einer Beeinträchtigung der Myokardfunktion auf zellulärer Ebene verbunden ist. Aber auch die Minderung der lastfreien Verkürzungsgeschwindigkeit und ATPase-Aktivität bei druckbedingter Hypertrophie (Goldblatt-Ratten, artifizielle Aortenstenose) ist, wenigstens in frühen Stadien, eher Ausdruck einer Anpassung an veränderte Belastungsbedingungen. Obwohl sichere Hinweise für Veränderungen im Bereich der elektromechanischen Kopplung vorliegen, scheinen diese, speziell beim Goldblatt-Modell, nicht die Hauptursache für die veränderte Mechanik zu sein. Die Korrelation zwischen lastfreier Verkürzungsgeschwindigkeit und spezifischer ATPase-Aktivität sowie die Persistenz einer verminderten lastfreien Verkürzungsgeschwindigkeit auch unter optimalen elektromechanischen Kopplungsbedingungen weisen auf eine innere Beziehung zwischen beiden Größen hin. Diskrepanzen zwischen lastfreier Verkürzungsgeschwindigkeit und entwickelter Spannung sind vorwiegend auf einen erhöhten Gehalt an kontraktilen Strukturen zu beziehen. In späteren Stadien beeinflußt ein erhöhter Bindegewebsgehalt sowohl die isometrischen als auch die isotonischen Meßwerte.

Pressure-Volume Relations, Elastic Modulus, and Contractile Behaviour of the Hypertrophied Left Ventricle of Rats with Goldblatt II Hypertension*

SummaryIn young male Wistar rats, an increase in systolic blood pressure to above 200 mm Hg was induced by constricting one renal artery (Goldblatt II). This led to cardiac hypertrophy with an increase in left ventricular weight of about 40% after 4 weeks as compared with controls of the same age. Four and 8 weeks after the operation, the systolic and enddiastolic pressure-volume relations of the left ventricle were determined under isovolumetric conditions in open-chest Goldblatt rats and in control animals of the same age. The systolic and diastolic wall stress and the tangential elastic modulus were calculated, assuming a thick-walled sphere.The diastolic pressure-volume curves were shifted to greater volumes after 4 weeks, apparently due to a temporary augmentation of blood volume. However, no significant difference between the diastolic pressure-volume curves of Goldblatt and control ventricles was found after 8 weeks. The isovolumetrically developed pressure was found to be increased in both stages of Goldblatt hypertension at the optimum of the pressure-volume diagram (maximum distance between end-diastolic and systolic pressure-volume curves).The diastolic wall stress rose only after 4 weeks in the hypertrophied ventricles due to the additional volume load. For a given diastolic wall stress, the elastic modulus tended to higher values in the Goldblatt hearts. Sarcomere length was measured after 8 weeks at the same end-diastolic transmural pressure at which the heart works in the closed chest. There was no significant difference between sarcomere length in the left ventricles of Goldblatt rats (1.99±0.03 μm) and control animals

Adaptive and Pathological Alterations in Experimental Cardiac Hypertrophy

{\text{[1}}{\text{.97 }} \pm {\text{0}}{\text{.01}} \mu m(\bar x \pm s_{\bar x} ){\text{]}}

Geometric and muscle physiological determinants of cardiac stroke volume as evaluated on the basis of model calculations

. The peak systolic wall stress, calculated from after loaded contractions, was not enhanced. However, the developed isovolumetric stress and the rate of stress development showed a significant increase in Goldblatt rats.These results indicate that the work capacity of the hypertrophied ventricle as a whole is enhanced. This enhancement of the contractile force per unit of cross-sectional area and its first derivative does not, however, allow the conclusion that there is an improvement in the elementary contractile process as the maximum myocardial shortening velocity at zero load (Vmax) is decreased whilst the content of contractile proteins is augmented.

Morphological alterations and their functional interpretation in the hypertrophied myocardium of Goldblatt hypertensive rats

SummaryAction potentials were measured in hypertrophied rat myocardium by means of glass microelectrodes. Left ventricular hypertrophy was induced by experimental chronic coarctation of one renal artery without contralateral nephrectomy. The action potentials of the hypertrophied myocardium exhibit a marked prolongation without any significant changes in the transmembrane resting potential, in the amplitude, or in the maximum upstroke velocity of the action potential. The increase in the half-width of the action potential for the 6-week stage after the surgical procedure is 235%, for the 12-week stage 281%, and for the 24-week stage 314%. The prolongation of the action potentials was more marked when the degree of hypertrophy was more severe.After depression of the fast Na+ inward current either by tetrodotoxin, by augmentation of the extracellular K+ concentration, or by reduction of the extracellular Na+ concentration, action potentials with similar marked broadening were still obtained in hypertrophied myocardium. The results under variation of the extracellular Ca++ concentration and simultaneous inactivation of the fast Na+ channels suggest that the slow transmembrane inward current is primarily carried by Ca++ ions.The prolongation of the action potentials could be one cause of the significant increase in the time to peak force as well as the augmentation of isometric force found in this model of hypertrophied myocardium.ZusammenfassungAktionspotentiale wurden mit Hilfe von Glasmikroelektroden an hypertrophierten linksventrikulären Rattenherzmuskeln abgeleitet. Die Hypertrophie wurde operativ durch die Einengung einer Arteria renalis ohne kontralaterale Nephrektomie erzeugt. Die Aktionspotentiale der hypertrophierten Myokardzelle sind gegenüber den Kontrollherzen durch eine ausgeprägte verbreiterung gekennzeichnet, bei nichtsignifikanten geringfügigen Änderungen im Ruhepotential, in der Amplitude sowie in der maximalen Aufstrichgeschwindigkeit. Sechs Wochen post operationem beträgt die Zunahme der Aktionspotentialhalbwertsbreite 235%, nach 12 Wochen ist der Anstieg 281%, und nach 24 Wochen beläuft sich der Zuwachs auf 314%. Die Verbreiterung der Aktionspotentiale ist im Mittel um so ausgeprägter, je stärker das Ausmaß der Hypertrophie ist.Nach Blockade des schnellen Na+-Einwärtsstroms durch die Gabe von Tetrodotoxin, durch Erhöhung des extrazellulären K+-Konzentration oder durch Reduktion der extrazellulären Na+-Konzentration resultieren im Falle der Hypertrophie gleichfalls stark verlängerte Aktionspotentiale. Die Ergebnisse unter Variation der extrazelluären Ca++-Konzentration bei gleichzeitiger Inaktivierung der Na+-Kanäle lassen vermuten, daß der langsame transmembranäre Einwärtsstrom primär von Ca++-Ionen getragen wird.Die Verbreiterung der Aktionspotentiale kann sicherlich für die signifikante Verlängerung der isometrischen Gipfelzeit sowie die Zunahme der isometrischen Gipfelkraft miverantwortlich gemacht werden, wie sie für das hypertrophierte Myokard der Goldblattratte beschrieben werden.

Mechanics of the isolated ventricular myocardium of rats conditioned by physical training.

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.