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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 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 that VCEmax is dependent on muscle length variation; the reliability of this parameter as an index of myocardial contractility is discussed.

In order to investigate internal shortening effects upon time course of active state, a study on rat papillary muscle has been carried out. Internal (or CE) shortening was evaluated according to a mechanical muscle model in which parallel elastic (PE), series elastic (SE), contractile (CE) elements are represented. CE force-velocity-shortening-time relationships have been used to describe time course of muscle active state in isometric, completely isotonic, afterloaded, and stopped isotonic contractions. The results obtained suggest that, regardless of type of contraction utilized, the parameters of muscle active state are strongly influenced by the amount of CE shortening.

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.

The elastic and mechanical behaviour of isolated papillar muscle was studied in aged and in young rats. The spontaneous fall in tension (stress relaxation phenomenon) and the duration of the active state of the contractile element during the isometric and isotonic contraction were greater in aged than in young rats. The elastic properties of the elements in parallel and in series and the intensity of the active state in the contractile elements, on the contrary, did not exhibit any significant difference between aged and young rats.