Maximum rate of tension fall during isometric relaxation at end-systolic fiber length in canine papillary muscle.

Abstract

SUMMARYWe measured the characteristics of the decline in tension during isometric relaxation of canine papillary muscle. In the intact heart, relaxation begins with the isovolumic phase, but in experiments on papillary muscle previously reported the isotonic phase preceded the isometric phase during the course of relaxation. In our experiments, however, the isotonic bar was locked at the instant the muscle reached the end-systolic fiber length in order to hold the fiber at that length during the succeeding relaxation process. Therefore, we obtained a relaxation process similar to that occurring in an intact heart. The major re- Jewell and Wilkie 2 studied the mechanics of muscle relaxation using the skeletal muscle of a frog, and observed that tension declined exponentially when the muscle was allowed to change its length, but in the case of true isometric relaxation the tension did not decline exponentially. Parmley and Sonnenblick 3 studied an isolated cat papillary muscle, and observed that tension decayed exponentially in the isometric relaxation phase following an afterloaded contraction. They mentioned that the time constant of exponential isometric tension decay (tj) increased linearly with total load. In a true isometric relaxation following true isometric contraction, they concluded that the time required for tension to fall to one-half its peak value (T -P o /2) was altered by different inotropic agents. Under conditions in which the interventions used produced similar peak contractile force (P o ) and maximum dP/dt, there were, however, significant differences in the decline in tension, depending on the intervention. On the other hand, Cohn et al., 4 using both intact hearts in situ and isolated hearts, concluded that the primary determinants of maximum rate of pressure fall (-dP/dt max ) were the end-systolic fiber length and the contractile state of the cardiac muscle. Weisfeldt et al. 5 conducted experiments similar to those of Cohn et al. 4 and observed that -dP/ dt m ax increased with mean aortic pressure. The seemingly contradictory results of these investigations make it difficult to predict exactly how myocardial relaxation is affected by various factors.

Differential effects of tert‐butyl‐benzohydroquinone, a putative SR Ca2+ pump inhibitor, on isometric relaxation during the staircase in the rabbit and rat ventricle

“…There was a highly significant correlation between both parameters as already observed in isolated preparations from other mammalian species (Tamiya et al, 1977;Pery-Man et al, 1993a,b). By contrast, in control and during exposure to TBQ, there was no such correlation between twitch tension and t1/2 (not shown).…”

“…Indeed, the slope of this relationship has been used as an index of the intrinsic relaxing capabilities of the muscle in isometric conditions (Tamiya et al, 1977;PeryMan et al, 1993a,b). Any manoeuvre (application of TBQ or increase in the stimulation frequency) that decreases the slope of this correlation exerts an 'intrinsic' negative relaxant effect.…”

“…For this reason, we preferred to analyse our data with two different approaches. Studies of the CRC process intend to determine whether changes in relaxation induced by protocols affecting inotropism are, in some way, proportional to variations in twitch tension (Tamiya et al, 1977;Chemla et al, 1986). If coupling varies, then interventions are said to exert an 'intrinsic' relaxant effect, i.e.…”

“…A second mechanism could involve the contraction-relaxation coupling (CRC) process (Chemla et al, 1986;Eichhorn et al, 1992). Indeed, isolated muscle studies have shown that the various parameters characterizing contraction and relaxation were correlated (Tamiya et al, 1977;Chemla et al, 1986). In fact, the higher the Ca2+ transient, the faster its rate of decline (Bers & Berlin, 1995).…”

Effect of Hypoxia on Myocardial Relaxation in Isometric Cat Papillary Muscle

“…In the whole heart, peak -dP/dt, the maximal rate of pressure decline after end ejection, has been applied as an index of myocardial relaxation (13)(14)(15), but its magnitude has been shown to be influenced by developed pressure (14). Similarly, a recent study using isolated papillary muscle applied peak -dT/dt to characterize isometric relaxation and demonstrated that peak -dT/dt is influenced by developed tension (16). In an attempt to normalize peak -dT/dt for the change in tension produced by hypoxia, two additional indices of relaxation were examined: peak -dT/dt divided by the instantaneous tension at which peak -dT/dt occurred, and the maximum value of the continuous division of the rate of tension fall (-dT/dt) by the corresponding instantaneous tension.…”