M. F. Wendt-Gallitelli

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

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

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

Rhythm-dependent role of different calcium stores in cardiac muscle: X-ray microanalysis.

. 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.

Time course of electron microscopic alterations in the hypertrophied myocardium of Goldblatt rats

Summary1.In guinea pig ventricular strips and isolated cells, 0.1 mM LaCl3 blocks contractility and shortens the action potential (AP) in less than 2 min (“early La-effect”). After 30 min, it prolongs the APs which trigger slow contractions (“late La-effect”). These results confirm earlier reports.2.X-ray microprobe analysis shows that La initially displaces only a small fraction of that Ca which is superficially bound to the sarcolemma. But, since this Ca is completely removed by Ca-free solutions within 2 min, we suggest that La blocks contractility not by displacing superficial Ca but by blocking the Ca inward currentiCa. Blocking ofiCa is analyzed with voltage clamp experiments. It is not La-specific, and can also be observed with other calcium channel blockers as well. WheniCa has been blocked, the membrane can still generate 100–200 ms long plateaus via the sodium inward currentiNa.3.During the late La-effect, the cells internalize La. Intracellular La is detected by x-ray microprobe analysis in cryosections of frozen muscles and as La-precipitates in EM images from freeze substituted preparations. Simultaneously, the cytosol gains Na and Ca, but the plasmalemmal and sarcoplasmic reticulum (SR) membranes are no longer occupied by Ca but by La. The late La-effect on the prolongation of the AP is La-specific. In the absence of extracellular La, it can be induced by pressure injection of La into the cytosol. The long APs are based on an additional inward current which is insensitive to Ca-removal, is inactivated by holding potentials of −40 mV, and is TTX-sensitive. We suggest that the current flows through a fraction of original Na-channels that is modified by i.c. La with respect to inactivation and selectivity.4.We attribute the late re-occurrence of contractility to activator Ca entering from the bath. Ca-entry might be mediated via enhanced Na/Ca-exchange whose rate is increased by the i.c. Na-load. In addition, Ca may enter through the La-modified Na-channels due to their impaired selectivity. Since i.c. La is known to interfere with the Ca-sequestration by the SR, it is expected to impair relaxation.

Presystolic calcium-loading of the sarcoplasmic reticulum influences time to peak force of contraction. X-ray microanalysis on rapidly frozen guinea-pig ventricular muscle preparations

Abstract Goldblatt rats with a mean blood pressure of 190 mmHg show a left ventricular hypertrophy (4 and 8 weeks after operation) of 40 to 50% in comparison to controls. Sarcomere length is not increased significantly. Stereological measurements on myocardial cell components show that contractile mass is significantly increased between 4 and 8 weeks after operation in hypertrophied hearts, partially at the expense of mitochondria. The volume occupied by mitochondria decreases, while mitochondrial structure remains normal during the early stage. The T- and sarcotubular systems also increase. Golgi complex is enlarged and consists of dilated cisternae. Disarrangements of myofibrils are observed. The 24-week group of hypertrophied hearts shows high heterogeneity: in some animals vessel wall alterations and diffuse fibrosis are observed. The hearts of the remaining operated rats show quantitative morphological changes characteristic of the compensatory stage without evidence of degenerative alterations. Physiological alterations indicate that maximum isometric tension development increases in the early compensatory stage of hypertrophy; this fact can be partially explained by the increased mass of contractile structures. The greatest functional impairment with regard to isometric tension development and V max is observed in those 24-week animals showing evident histological alterations outside the muscle cell. The increase in the sarcotubular system and the persistence of decreased V max under high Ca 2+ concentration suggests that not all changes in mechanical parameters can be attributed to alterations in excitation-contraction coupling.

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

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.

Electron probe x-ray microanalysis and cryoultramicrotomy of unstained myocardial sarcoplasmic reticulum, in situ and fragmented

Abstract Under extremely different stimulation patterns such as paired-pulse stimulation, or stimulation after prolonged rest, papillary muscles of guinea-pig show differences in contraction. An early high contraction (360 ms to peak tension) results from paired stimulation at short basic stimulus interval, whereas a small “late” contraction normally occurs (550 ms to peak tension) at a stimulus interval of several minutes (rested state contraction) [Figure 1(a)]. The marked differences in the rate of rise and time to peak of tension in the two cases suggest that different calcium stores releasing calcium in a different mode could be responsible for the different pattern of contraction [1, 7]. We have examined this question by looking for calcium stores involved in such states of contraction with the aid of crytotechniques and electron probe microanalysis of chemically untreated shock frozen preparations of guinea-pig papillary muscle.