Characteristics of sarcomere shortening in single frog atrial cardiac cells during lightly loaded contractions.

Abstract

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.

Effect of external force on relaxation kinetics in single frog atrial cardiac cells.

“…In this case, the cell shortens and extends during a twitch in a lightly loaded and relatively isotonic fashion, since the end of the cell in the suction pipette is relatively free to move (see Fig. 8 and Tarr et al, 1981a). In this type of experiment, the end of the cell attached to the glass beam is not wrapped around the beam.…”

“…The techniques of cell preparation, the preparation and calibration of cantilevered glass force beams, the method of attachment of a single cell to the glass beams, the method of recording and determining sarcomere lengths and force development during a twitch contraction, and the methods used to electrically stimulate the cells have been reported previously (Tarr and Trank, 1976;Tarr et al, 1979Tarr et al, , 1981aTarr et al, , 1981bTarr et al, , 1981c. Briefly one end of a single cell is attached to a glass force beam of known compliance; the displacement of this beam is proportional to the force supported by the cell.…”

“…The data are recorded on a closed circuit TVvideo tape system and analyzed using the stop frame capability of the video tape recorder in combination with a double TV cursor (see Tarr et al, 1979). By such an analysis, the time course of sarcomere length changes, cell segment length changes, and force development which occur during a twitch contraction can be analyzed (see Tarr et al, 1981aTarr et al, , 1981bTarr et al, , 1981c. For sarcomere length determinations, the length occupied by a small group of sarcomeres (usually 6-10) is determined and an average sarcomere length is calculated.…”

Bursts of Openings in Transmitter-Activated Ion Channels

“…If perfusion is not possible, the tissue is minced, and the pieces are agitated in the dispersion medium (pancreas islet cells: Kostianovsky et al, 1974;smooth muscle: Singer and Fay, 1977;heart: Hume and Giles, 1981;Tarr et al, 1981; Isenberg and Kl6ckner, 1982). Generally, the free cells are harvested after some time, and the remaining pieces are reincubated in fresh enzyme solution for several more periods of digestion.…”

“…The first trials of tissue dissociation were mainly based on the use of trypsin, and trypsin seems still sufficient for the isolation of snail neurons (Kostyuk et at., 1974) and frog atrial cardiac cells (Tarr et at., 1981). However, proteolytic enzymes are a dangerous tool because they can affect membrane receptors (Lee et at., 1977).…”