L. Ricciardi

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.

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.

Tritiated water (HTO) and inulin spaces in isolated skeletal and cardiac muscles: influence of contractile activity

In isolated cardiac and skeletal muscle inulin space increased significantly after isometric contractions: no significant change (in myocardium) or a less pronounced increase (in skeletal preparations) was found following isotonic responses. The HTO space was uninfluenced by the contractile activity.

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.