A QUANTITATIVE DESCRIPTION OF THE SHORT TERM CHANGES IN THE FORCE OF CONTRACTION OF RABBIT PAPILLARY MUSCLE WITH THE PATTERN OF STIMULATION

Postextrasystolic mechanical restitution in closed-chest dogs. Effect of heart failure

BACKGROUND

Postextrasystolic mechanical restitution (MRPES) is thought to be an expression of intracellular Ca2+ handling by cardiac sarcoplasmic reticulum (SR). Since congestive heart failure is characterized by abnormal intracellular Ca2+ homeostasis, we sought to delineate MRPES behavior before and after the production of heart failure to obtain insights into the relation between altered mechanical performance and Ca2+ handling.

METHODS AND RESULTS

Ten dogs instrumented with left ventricular (LV) micromanometers and piezoelectric dimension crystals were studied under control conditions; 6 dogs also were studied after tachycardia heart failure (THF) produced by rapid LV pacing for 4 weeks. After priming at a basic cycle length of 375 ms, test pulses were delivered at fixed extrasystolic intervals (ESIs; 300, 375, or 450 ms) and graded postextrasystolic intervals (PESIs). Postextrasystolic mechanical response was assessed using single-beat elastance. MRPES curves were constructed by expressing normalized mechanical response as a function of the PESI. Control MRPES was a monoexponential function whose time constant (TC) and PESI-axis intercept (PESI0) increased significantly (P < .01) with increases in the antecedent ESI. THF significantly slowed MRPES kinetics at each antecedent ESI (P < .025), increased normalized maximal contractile response (CRmax, P < .01), and shortened PESI0 (P < .025). Increases in the TC and CRmax were most pronounced with the smallest antecedent ESI (percent control postextrasystolic TC 363.7 +/- 60.5%, ESI of 300 ms versus 139.0 +/- 15.1%, ESI of 450 ms, P < .005; percent control CRmax 128.6 +/- 4.9%, ESI of 300 ms versus 104.9 +/- 1.0%, ESI of 450 ms; P < .005).

CONCLUSIONS

MRPES is much less dynamic in THF: The failing heart operates at lower levels of contractile performance after higher stimulation frequencies and cannot increase its speed of contractile recovery to compensate for higher heart rate. Prolongation of MRPES kinetics is consistent with depression of SR Ca2+ release mechanisms in THF and implicates this site in the loss of the capacity of the failing heart to maintain mechanical performance with tachycardia.

Evaluation of left ventricular mechanical restitution in closed-chest dogs based on single-beat elastance

The mechanical restitution of the left ventricle of closed-chest dogs was modeled as a monoexponential relation, using peak single-beat elastance as a measure of contractile strength. Data were obtained from nine dogs chronically instrumented with three sets of piezoelectric diameter gauges to assess ventricular volume and high-fidelity manometers to measure pressure. Mechanical restitution curves were obtained during both atrial and ventricular pacing in six dogs. The time constant of mechanical restitution was 64.3 +/- 11.4 msec for atrial and 122.2 +/- 26.3 msec for ventricular paced runs (p less than 0.01). These values are smaller than those previously reported from isolated hearts or isolated muscle segments. Although the time of onset of mechanical restitution was longer for atrial than for ventricular runs (255.1 +/- 14.3 versus 225.1 +/- 9.6 msec, p less than 0.05), the plateau to which mechanical function rose was not different. During ventricular pacing, fusion beats were noted in four dogs. The magnitude of the mechanical contribution of these fusion beats fell to a nadir at approximately 250 msec, suggesting that intracellular calcium available for crossbridge interaction is dropping during this time. In three additional dogs, the time constant of postextrasystolic restitution was found to vary depending on the preceding extrasystolic interval. Thus, mechanical restitution of the ventricle is a dynamic process that can be assessed using an elastance-based approach in the in situ heart.

Postextrasystolic potentiation of the isolated canine left ventricle. Relationship to mechanical restitution

We established, for the isolated, isovolumically beating canine left ventricle, a comprehensive description of postextrasystolic contractile strength (dP/dtmax) as a function of extrasystolic and postextrasystolic stimulus intervals. In contrast to previous studies of postextrasystolic beats in in situ hearts, these isolated ventricles contracted isovolumically so that dP/dtmax was not affected by fluctuations in preload and afterload and was therefore considered to be a reliable index of intrinsic contractility. With the interval preceding extrasystoles constant, postextrasystolic contractile strength increased monoexponentially to a plateau as the interval preceding postextrasystoles lengthened, with a mean time constant (+/- SD) of 182 +/- 44 msec (n = 53). The onset of this increase in postextrasystolic contractile strength coincided with repolarization of the extrasystolic action potential. With the interval preceding postextrasystoles held constant and long (1200 msec), postextrasystolic contractile strength decreased according to a monoexponential function as the interval preceding extrasystoles lengthened [mean time constant (+/- SD) of 176 +/- 18 msec (n = 10)]. These phenomena could be quantitatively summarized by a single equation description of postextrasystolic contractile strength which involved monoexponential functions with one time constant. The mathematical form of this description led us to a simple interpretation of these phenomena in terms of currently proposed excitation-contraction coupling models of the heart.

Evaluation of the force-frequency relationship as a descriptor of the inotropic state of canine left ventricular myocardium

The short-term force-frequency characteristics of canine left ventricular myocardium were examined in both isolated and intact preparations by briefly pertubing the frequency of contraction with early extrasystoles. The maximum rate of rise of isometric tension (Fmas) of the isolated trabeculae carneae was potentiated by the introduction of extrasystoles. The ratio of Fmas of potentiated to control beats (force-frequency ratio) was not altered significantly by a change in muscle length. However, exposure of the trabeculae to isoproterenol (10(-7)M) significantly changed the force-frequency ratio obtained in response to a constant frequency perturbation. Similar experiments were performed on chronically instrumented conscious dogs. Left ventricular minor axis diameter was measured with implanted pulse-transit ultrasonic dimension transducers, and intracavitary pressure was measured with a high fidelity micromanometer. Atrial pacing was performed so that the end-diastolic diameters of the beats preceding and following the extrasystole could be made identical. Large increases in the maximum rate of rise of pressure (Pmas) were seen in the contraction after the extrasystole. The ratio of Pmax of the potentiated beat to that of the control beat was not changed by a 9% increase in the end-diastolic diameter, produced by saline infusion. Conversely, isoproterenol significantly altered this relationship in the same manner as in the isolated muscle. Thus, either in vitro or in situ, left ventricular myocardium exhibits large functional changes in response to brief perturbations in rate. The isoproterenol and length data indicate that the force-frequency ratio reflects frequency-dependent changes in the inotropic state, independent of changes in length.

Force-frequency relationship. A basis for a new index of cardiac contractility?

The way that contractility varies between beats--the force-frequency relationship--was determined for rabbit papillary muscles held at different lengths. When the maximum rate of rise of tension in a contraction, F max , was used as a measure of contractility, the way contractility changed between contractions was independent of length. That is, when the values of F max obtained at one length were multiplied by an appropriate scaling factor, the force-frequency relationship determined at that length was indistinguishable from the force-frequency relationship determined at another length. If, however, the peak tension in a contraction was used as a measure of contractility, the Force-frequency relationship generally was not independent of muscle length. Therefore, it is proposed that the ratio of two values of F max obtained by perturbing the rate of stimulation at any given length should be tried as a length independent index of the inotropic state of the muscle.

The interval-strength relationship in mammalian atrium: a calcium exchange model. I. Theory

A model is developed for predicting the interval-strength relationship in mammalian atrium. The postulates underlying the model relate the intracellular and transmembrane calcium fluxes to changes in contractility. The predictions of the model agree qualitatively with the behavior of atrium for the following patterns of stimulation: (a) constant interval between stimuli, (b) a rest, or period with no stimuli, after the attainment of a steady-state force level, (c) a sudden change in the interval between stimuli, and (d) paired pulse stimulation. The effects of varying several parameters of the model on both the contraction staircases, after a rested-state contraction, and the steady-state interval-strength relationship are examined. Additional considerations are made: (a) estimates are made of the tissue calcium content available for contraction; (b) the physical meaning of the rested-state contraction is discussed; and (c) estimates are made of the proportionality constant between the maximum value of the contractile tension and the amount of calcium released before a contraction.

Cardiac muscle. A comparative study of Purkinje fibers and ventricular fibers

With light and electron microscopy a comparison has been made of the morphology of ventricular (V) and Purkinje (P) fibers of the hearts of guinea pig, rabbit, cat, dog, goat, and sheep. The criteria, previously established for the rabbit heart, that V fibers are distinguished from P fibers by the respective presence and absence of transverse tubules is shown to be true for all animals studied. No evidence was found of a permanent connection between the sarcoplasmic reticulum and the extracellular space. The sarcoplasmic reticulum (SR) of V fibers formed couplings with the sarcolemma of a transverse tubule (interior coupling) and with the peripheral sarcolemma (peripheral coupling), whereas in P fibers the SR formed only peripheral couplings. The forms of the couplings were identical. The significance, with respect to excitation-contraction coupling, of the difference in the form of the couplings in cardiac versus skeletal muscle is discussed together with the electrophysiological implications of the differing geometries of bundles of P fibers from different animals.