The effect of preload and Ca++-ions on the time-course of the isometric force and on the force-velocity relation: is Vmax dependent on the number of activated cross-bridges?

The maximum shortening velocity of the isolated, externally unloaded papillary muscle of cats, as can be obtained by quick-releases in the initial phase of isometric contractions, is clearly dependent on the Ca++-concentration of the bath solution as well as on the preload: In the range of 0.55 m-mole/1 to 4.4 m-mole/1 a doubling of the Ca++-concentration induces a mean increase of 23% in Vmax; in a range of muscle length assumed to be of physiological relevance, a rise in the extension of 10% of Lmax results in a mean augmentation of 20% in Vmax. In order to reveal possible differences in the effect of calcium and various degrees of filament overlap, firstly, isometric mechanograms were measured under low Ca++-concentration at the muscle length of Lmax. By correspondingly lowering the muscle extension under increased Ca++-concentration the reproduction of almost the entire rising phase of these isometric mechanograms was obtained. Both these experimental conditions result in congruent force-velocity relations and consequently in identical values for Vmax. It must therefore be concluded that there exists an inner relationship between the initial time-course of the isometric force and the force-velocity relations measured during this period. The interpretation of this fact involves weighing the hypothesis of an inner frictional force dependent on velocity against that of a maximum shortening velocity dependent on the number of activated cross-bridges. The resultant consequences for the estimation of the contractile qualities of the myocardium are discussed.