The variation in tension output with active shortening in rat myocardium.

: The effect of active shortening on tension development has been studied in rat papillary muscle preparations at 20 degrees C. Total tension at the peak of a contraction with a shortening phase is reduced below the value predicted by the isometric length-tension relationship. At the same instantaneous length, total tension is less in contractions with an isotonic phase than in isometric responses and the amount of the tension deficit increases with the extent of shortening. The influence of the time at which shortening takes place, on the amount of tension deficit, has been explored. The increase in [Ca2+]o does not affect the tension deficit. Conversely an increase in [Ca2+]o associated to addition of caffeine to the bathing solution almost abolished the effect of active shortening on tension development. Several possible explanations of the reduction of tension development by shortening are discussed.