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

Abstract

SUMMARY. The effects of external force on relaxation kinetics were investigated in isolated single frog (Rana catesbeiana) atrial cells. We found that force decay occurred at a maximum and constant rate for a significant portion of auxotonic relaxation, and this rate was linearly related to the peak force developed during auxotonic contraction. The slope of the linear relationship between the maximum rate of auxotonic force decay and peak auxotonic force was not affected by changes in the level of contractile activation produced by activating the cell with different stimulus durations. The rate of force change during auxotonic contraction and relaxation in the isolated cell is directly related to the average sarcomere velocity within the cell. Thus, the results indicate that during auxotonic relaxation the velocity of sarcomere extension is directly related to the peak auxotonic force, and sarcomere extension, during relaxation, is therefore affected by external force. The direct effect of external force on relaxation kinetics was confirmed by the observation that force changes imposed on the cell during relaxation immediately altered the velocity of the extending cell from any given length. However, data are also presented which demonstrate that rapid sarcomere extension occurs during relaxation under conditions where external forces are negligible. Thus, rapid sarcomere extension during relaxation does not require large external forces, and internal forces must play a role in sarcomere extension during relaxation. An explanation is given for these apparently contradictory results. (Ore Res 52: 161-169, 1983) RELAXATION has been defined as the process by which muscle returns to its initial tension and length following contraction (Hill, 1949). Accordingly, an understanding of relaxation requires, as a minimum, information about the time course of decay of the force-generating capacity (P o ) of the muscle as well as information about how external and internal forces affect sarcomere, cell, and tissue extension during relaxation.Recent experiments have given some insight into the factors that affect the time course of P o decay in mammalian cardiac muscle. For example the rate of "isometric" tension decay during relaxation appears to be directly related to the magnitude of the peak force generated during the contraction (Krueger and Strobeck, 1978;Tamiya et al., 1977). Also, it has been clearly demonstrated that sarcomere length directly affects the duration of contractile activation in mammalian cardiac muscle as well as the rate at which contractile activation is removed during relaxation (Krueger and Farber, 1980). In contrast, the factors that affect tissue and/or sarcomere lengthening during relaxation are not well understood, and in some cases the results appear to be contrary to those expected. For example, one might expect that the velocity of tissue extension during relaxation would be proportional to the force tending to restore the muscle to its original rest length. Thus, it might be expected that...

Voltage-tension relations in single frog atrial cardiac cells.

“…Also the rate of relaxation following the peak of contraction appeared to be faster in Method 2 where the external loads were essentially zero compared to Method 1 where external loads were significant (see Figure 3). These results are similar to results reported previously by Tarr et al 21 and are related to the load dependence and length dependence of sarcomere extension during relaxation in frog atrial cells. We do not presently think that any significance should be attached to the apparent dip in the contraction wave form between the phasic and tonic component which was sometimes observed using Method 2 (see Figure 3, lower left) since such dips were not seen routinely (see Figures 6 and 7).…”

Measuring Single Cardiac Myocyte Contractile Force via Moving a Magnetic Bead

“…Obviously, this may cause a measurement error. In addition, attaching myocytes to the force transducer is also a challenging job (Tarr et al, 1983). Finally, external loading force (i.e., the reacting force from filament to cell) cannot be adjusted and controlled once the filament transducer is fabricated.…”

Adaptation of flexible polymer fabrication to cellular mechanics study

“…Here, the force measurement in isolated cardiac myocytes is demonstrated. Considering that the contraction force of cardiac myocytes is about a few N, 7,8 and the lateral dimension is about 20 m ϫ 100 m, 12 cantilevers with aspect ratios of 2:1 were manufactured and employed herein. The resulting cantilever is 4 m tall.…”