R. Minelli

Study and parameters identification of a rheological model for excised quiescent cardiac muscle.

Abstract A non-linear rheological model has been used to represent mathematically the behaviour of isolated quiescent preparations of rat myocardium. The parameters have been identified by means of numerical methods and the experimental responses of the specimens to several stress or strain histories have been predicted by the model. Results obtained show that the model has a good predictive ability and can discriminate among the responses of specimens with different mechanical behaviour (papillary muscles of young and aged rats and papillary muscles contractured in hypertonic solutions).

Relaxation in atrial and ventricular myocardium: activation decay and different load sensitivity

SummaryIsolated atrial and ventricular preparations from rat heart have been compared. In atrial specimens relaxation is faster than in papillary muscles both in isometric and isotonic conditions. In papillary muscles the tension decay occurs earlier in isotonic than isometric contractions and a stretch applied at or after the peak of isometric twitches promotes a faster relaxation: this load dependence of relaxation is less pronounced in atrial specimens. The decay of activation, evaluated from the decline of the muscle shortening ability, is faster in atrium than in ventricle. These findings suggest that the sensitivity of relaxation to the loading conditions might be determined by both the activation decay rate and the cross bridge kinetics.ZusammenfassungIn der vorliegenden Arbeit wird die Mechanik isolierter Muskelpräparate vom Vorhof und Ventrikel des Rattenherzens verglichen. Bei Vorhofpräparaten beginnt der Erschlaffungsprozeß unter isometrischen und isotonischen Bedingungen früher als beim Papillarmuskel. Bei Papillarmuskeln tritt der Abfall der Spannung unter isotonischen Bedingungen früher in Erscheinung als bei isometrischen Kontraktionen; eine Streckung zum Zeitpunkt der isometrischen Gipfelzeit oder danach verursacht schnellere Erschlaffung. Diese Abhängigkeit der Erschlaffung von der Last ist bei Vorhofpräparaten weniger ausgeprägt. Der Rückgang der Aktivierung, bewertet aufgrund der abnehmenden Verkürzungsfähigkeit des Muskels, erfolgt beim Vorhof schneller als beim Ventrikel. Die Ergebnisse lassen vermuten, daß die Abhängigkeit der Erschlaffung von den Belastungsbedingungen sowohl durch die Geschwindigkeit des Aktivierungsrückgangs als auch durch die Querbrückenkinetik bestimmt wird.

The stiffness of parallel elastic elements in rat papillary muscle

SummaryThe dynamic stress-strain relationship of the parallel elastic element (PE) in rat papillary muscle has been investigated.This relationship cannot be correctly estimated by means of the classical diagram: muscle length-stable passive tension of the whole muscle. A new experimental approach has been developed by which the elastic properties of PE may be measured under dynamic conditions.After isometric stabilization of the muscle at a series of preloads, tension changes following quick release were measured. The tension-length relationships measured under these conditions were used to evaluate the elastic behaviour of the parallel elements.The results obtained show that the stiffness of PE is higher than expected and that its maximum elongation never exceeds 10% of the respectiveLo value at all preload levels explored.

Factors modulating the sensitivity of the relaxation to the loading conditions in rat cardiac muscle.

The load sensitivity of the relaxation phase was studied in rat papillary muscle, with isotonic afterloaded contractions and stretches applied after the peak of isometric twitches.The tension decay occurred earlier in isotonic than in isometric contractions. When a central region of the preparation was marked with small stainless steel pins, a lengthening of this region could be shown during relaxation of isometric (fixed end) contractions. This lengthening was earlier and faster in isotonic afterloaded contractions. Therefore the sensitivity of relaxation to load or length changes could be described in the context of the general mechanism of relaxation which takes into account the non uniform behaviour of the muscle and the internal movement during contractions.Interventions which decelerate the activation decay rate had different effects on the load dependence of relaxation. Caffeine addition and substitution of strontium for calcium abolished the load sensitivity while a temperature reduction had no influence on it.

Isometric relaxation in rat myocardium: load dependence and influence of caffeine.

When caffeine plus calcium is added to the perfusing medium, isometric relaxation of rat myocardium is no longer affected by length changes occurring during the twitch. The dependence of isometric relaxation on the initial muscle length is still present and more pronounced after caffeine addition.

Time course and duration of the depressant effect of active shortening in cradiac muscle

Shortening deactivation is studied in isolated preparations of rat cardiac muscle by comparing tension redevelopment in partially fused isometric contractions with quick releases of different extent (“control” and “test” displacements). The active shortening takes place during the ascending phase of the 1st contraction of the couple and the comparison of redeveloped tension is carried out at the peaks of both 1st and 2nd contractions. The depressant effect of active shortening can be demonstrated at the 1st peak and also at the 2nd one, after the restimulation of the muscle.This effect slowly decreases with time and disappears within approximately one second after active shortening took place. By keeping constant the control shortening and changing the test shortening, a clear dependence of depressant effect on the amount of displacement can be shown in the 1st and in the 2nd contraction.

The analysis of some mechanical properties of quiescent myocardium.

Abstract The mechanical behaviour of quiescent rat papillary muscle has been studied by using two different experimental tests. In the first (Displacement Controlled test), predetermined length changes were imposed to the muscle and the corresponding tensions were recorded. In the second (Force Controlled test), a predetermined load was applied to and removed from the specimen and the corresponding length changes were recorded. By means of this procedure different stress-strain relationships, stress relaxation and its reversal, creep and its recovery, were obtained. The tests were conducted in order to study (1) the effects of deformation magnitude, (2) the asymmetries and the hysteretic behaviour and (3) the influence of viscous phenomena (elastic after-effects) duration. Strain rate sensitivity of papillary muscle has not been investigated. Experimental findings are discussed accounting for the possibility of a quantitative separation of elasticity, viscosity and plasticity in quiescent myocardium. The results obtained are examined in view of their application to rheological cardiac muscle models.

Cardiac muscle models for both isotonic and isometric contractions

SummaryThe choice of a suitable muscle model consistent with the mechanical behaviour of Rat papillary muscle at rest or during isotonic or isometric contraction has been considered.Three different preload levels within the ascending limb of the Frank-Starling curve have been used in five papillary muscles.Series elastic (SE) and parallel elastic (PE) length-tension relationships have been evaluated according to five mechanical muscle models using data from systolic and diastolic quick-release manoeuvres.For each preload level and muscle model, the time course of the force-velocity relationship of the contractile element (CE) has been calculated using a PDP 8/L digital computer. Several mechanical and biological characteristics are considered which exclude four of the five selected muscle models. The Maxwell model with a damper seems to be the most suitable mechanical model for Rat papillary muscle.According to the selected muscle model it seems thatVCEmax is dependent on muscle length variation; the reliability of this parameter as an index of myocardial contractility is discussed.

Myocardial glycogen in the heart-lung preparation (rat) during severe haemodynamic work

Zusammenfassung1. Der Glykogen-Gehalt im Rattenmyokard wurde unter zwei verschiedenen maximalen Belastungsbedingungen am Herz-Lungen-Präparat untersucht.2. Eine strenge langdauernde Arbeit verursacht keine Herabsetzung des Glykogen-Gehalts im Myokard, sofern kein Herzversagen eintritt.3. Die Herzdekompensation kann daher in dieser Versuchsanordnung nicht auf eine Entleerung der Kohlenhydratdepots bezogen werden.4. Erst nach Einsetzen der Herzdekompensation tritt eine starke Abnahme des Glykogen-Gehalts ein. Sehr wahrscheinlich ist diese Abnahme auf eine sekundäre Anoxie des Herzens zurückzuführen.Summary1. The glycogen content of cardiac muscle has been investigated during two types of maximum cardiac load in a rat heart-lung preparation.2. Severe prolonged exercise caused no reduction in cardiac glycogen until heart failure developed.3. Heart failure in this preparation cannot therefore ascribed to carbohydrate stores depletion.4. Only after the onset of heart failure a marked reduction in glycogen content was observed. This change was probably due to consequent anoxia of the heart muscle.

Influence of load and work on the high energy phosphates content in the myocardium (rat heart-lung preparation)

ZusammenfassungDie im Herzmuskel vorhandene Konzentration an energiereichen Phosphaten wurde am Herzlungen-Präparat von Ratten unter verschiedenen Herzleistungs- und Herzarbeitsbedingungen bestimmt.Durch zweckmäßige Änderungen des mittleren arteriellen Blutdrucks und des Herzminutenvolumens wurden den Präparaten fünf verschiedene Leistungsniveaus auferlegt (24,9, 31,3, 36,5, 49,7 und 65,9 g meter/min).Es wurden drei verschiedene Werte der Herzarbeit (das heißt 0,182, 0,942, 1,884 kgmeter) (bei unverändertem Leistungsgrad) erhalten, indem man die Präparate jeweils 5, 30, 60 min arbeiten ließ.Die Unterbrechung der mechanischen Leistung der Präparate und die Herausnahme des Herzens zur chemischen Analyse erfolgten immer in Abwesenheit von Herzdekompensationszeichen.Die statistische Analyse der Ergebnisse zeigte, daß es im Rahmen der physiologischen Herzbelastung unmöglich ist, eine Korrelation zwischen Herzleistungs- oder Herzarbeitsgrad und Konzentration von energiereichen Phosphaten im Myokard nachzuweisen.Wenn keine Herzdekompensationszeichen trotz der bedeutenden hämodynamischen Herzbelastung vorhanden sind, wird die Spaltung der energiereichen Phosphate durch die Resynthesevorgänge völlig ausgeglichen.SummaryThe high-energy phosphates content in myocardial tissue has been estimated in the rat heart-lung preparation under various levels of cardiac load and after delivering of various amounts of work.Five cardiac loads (i.e. 24.9, 31.3, 36.5, 49.7, and 65.9 gmeter/min) were imposed to the preparations by suitably varying the mean arterial blood pressure and the cardiac output.Three different amounts of cardiac work (i. e. 0.182, 0.942, 1.884 kgmeter) were obtained (at the same load) by allowing the preparation to work for 5, 30 and 60 min respectively.The preparations were stopped and the heart removed for the chemical analysis in the absence of signs of failure.The statistical analysis of the results showed that, within the physiological range of mean arterial blood pressure and cardiac output, no correlation between the load imposed or the work performed and the content of high-energy phosphate in the heart can be observed.As far as no signs of cardiac failure are present, the high-energy phosphates breakdown is perfectly balanced by resynthesis processes, even when a considerable haemodynamic load is imposed to the heart.