Effect of initial sarcomere length on sarcomere kinetics and force development in single frog atrial cardiac cells

We studied sarcomere performance in single isolated intact frog atrial cells using techniques that allow direct measurement of sarcomere length and force. The purpose of this investigation was to determine whether length-dependent alterations in contractile activation occur in the single isolated cardiac cell. This was accomplished by determining the effect of initial sarcomere length on the time course of sarcomere shortening and force development during auxotonic twitch contractions. The results presented in this paper demonstrate that the velocity of sarcomere shortening, the rate of force development, and the magnitude of force development during auxotonic twitch contractions all increase as initial sarcomere length increases over the range of about 2 micrometers to greater than 3 micrometers. These results indicate that the level of contractile activation increases as initial sarcomere length increases. Also, results are presented that indicate that the rate of increase of contractile activation during a twitch contraction also increases as initial sarcomere length increases. These length-dependent effects on contractile activation in conjunction with the slow time course of contractile activation cause the force-velocity-length relationship to be time-dependent: i.e., the velocity of sarcomere shortening at a given sarcomere length and load depends on the time during the contraction when the sarcomere reaches that length. The results suggest that length-dependent alterations in contractile activation may play a major role in the improved contractile performance that accompanies an increase in initial sarcomere length in cardiac muscle.