Active shortening retards the decline of the intracellular calcium transient in mammalian heart muscle.

When active shortening of the cat papillary muscle was allowed at any time during a contraction, the intracellular concentration of free calcium ions, detected with the calcium-sensitive bioluminescent protein aequorin, was higher than at comparable times in isometric twitches. The difference was not attributable to the differences of length involved or to motion artifacts, and must have been related to the act of shortening or the difference in force development in the two types of contractions. This observation and the phenomenon of shortening deactivation are both consistent with the hypothesis that attachment of cross bridges increases the affinity of the myofilaments for calcium ions.