IT IS well established that the resting tension (stress)-sarcomere length relationships of a variety of intact preparations of cardiac muscle are different from those of skeletal muscle preparations. In contrast to skeletal muscle in which resting tension is negligible at sarcomere lengths below about 2.3 jim (Gordon et al., 1966;Lannergren and Noth, 1973; Moss and Halpem, 1977), mammalian cardiac preparations have relatively high resting tensions at these sarcomere lengths (Spiro and Sonnenblick, 1964), and the resting tension increases markedly at sarcomere lengths beyond 2.3 /im. In nonmammalian cardiac preparations, variable results have been obtained. Starling (1918) found in intact tortoise ventricle that the resting tension remained relatively small over a wide range (approximately 6-fold) of diastolic volumes, which included the rising phase and the relatively broad range (approximately 2-fold) of the active tension-diastolic volume relationship. In contrast, both Winegrad (1974) and Matsubara and Maruyama (1977) found that the resting tension increased significantly over the sarcomere length range of 2.0-2.6 /xm in small bundles of frog atrial tissue. Nassar et al. (1974) found

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.

We studied sarcomere performance in single isolated intact cardiac cells using techniques that allow direct measurement of sarcomere length and force. This investigation dealt primarily with sarcomere performance during twitch contractions under lightly loaded conditions. In such contractions, there was a significant portion of the contraction in which sarcomere shortening occurred at constant velocity over a significant range of sarcomere lengths. The constant velocity phase of shortening was followed by a phase of shortening in which sarcomere velocity decreased markedly. Both the velocity and extent of sarcomere shortening depended on the stimulus parameters used to excite the cell. With threshold stimulation, sarcomere velocities during the constant velocity phase of shortening ranged from 1 to 5.5 micron/sec in different cells and significant slowing of sarcomere shortening began at sarcomere lengths of 1.8-2.0 micron. In contrast, when cells were stimulated with a long duration stimulus (200 msec) of large current strength, sarcomere velocities during the constant velocity phase ranged from 6 to 12 micron/sec, and significant slowing did not occur until a sarcomere length of about 1.6 micron was reached. The threshold stimulus strength-stimulus duration relationship was determined on the single cell, and it was found to be of the type expected for a cell having an intact excitable membrane capable of generating an action potential when depolarized to a fixed voltage threshold. The data presented in this paper give direct evidence that the lightly loaded cardiac sarcomere has a velocity of shortening which depends on the level of contractile activation but is independent of sarcomere length at sarcomere lengths greater than about 1.6 micron.

IN preliminary experiments on single frog atrial cells we found that the rate of force development during auxotonic twitch contractions was relatively constant for a significant portion of the rising phaseFrom the