Stretch activation and myogenic oscillation of isolated contractile structures of heart muscle

Summary Glycerinated or freeze-dryed fibre bundles of heart muscles (papillary and trabecular muscles of rabbit or guinea pig) show in ATP-salt solution with about 10^–6M Ca²⁺ an active, delayed tension increment after quick or sinusoidal stretching. The active tension increase is completely different from the passive tension increment caused by stretching of the elastic structures of the muscle; this well known length dependence of tension is also in phase with the length changes (or the tension-phase preceeds the length-phase in visco-elastic bodies). On the other hand, the active tension increase is delayed with respect to the length change; this can be observed very well after rectangular changes in length. The delayed activation of the contractile bonds at stretch and the delayed deactivation at shortening induce the muscle-during sinusoidal length changes in a characteristic frequency range-to produce power output. The frequency range corresponds to the heart beat frequency of the living muscle. Temperature rise and inorganic phosphate accelerate, Mg-ions and ADP retard the contraction speed. Ca-ions influence only the amount of the isometric tension, but not the contractile velocity.Supported by the Deutsche Forschungsgemeinschaft (Grant RU 154/3).