The Dynamics of the Inotropic Change Produced by Altered Pacing of Rabbit Papillary Muscle

Abstract

The time course of the active state, the force‐velocity relationship and the series compliance of rabbit papillary muscles were studied at different inotropic states produced by altered pacing of the muscles within the range 18 to 72 stimulations per minute. The inotropic intervention produced a shift of the force‐velocity curve, without markedly changing the shape of the curve, suggesting that Vmax, the capacity to produce motion, and Pmax the capacity to produce tension, were affected to the same degree. Intensification of the active state in response to an increase in stimulation frequency was associated with a steeper rise of the active state and an earlier attainment of maximum. The series compliance was unaffected by the inotropic intervention. The results are taken as evidence (1) that the two parameters of the active state, Vmax and Pmax, are governed by a common control mechanism in the excitation‐contraction coupling and (2) that the inotropic effect is due to altered kinetics of this control function. The nature of the mechanism governing the active state is discussed in relation to the slicling‐filament theory of muscular contraction.

Left ventricular force-velocity relations measured from quick volume changes

“…(2) is also valid for the case of non-isotonically (i.e. approximately isometrically) contracting muscle, as in the present study [8][9][10]21]. Note that for VcE = 0: F = Fo.…”

The instantaneous force-velocity relationship as an index of the contractile state in cardiac muscle

“…The parallel shift of V,,.,x and P,, produced by altered stimulation frequency has been demonstrated previously (5). Figure 3 Data from project by Mattiazzi, Nilsson, and Edman. that adrenaline has the same effect on the force-velocity curve as induced by altered stimulation rate.…”

“…By repeating the release procedure with different loads it is possible to collect force-velocity data at a given time, for instance at the moment of the peak of the isometric twitch. Such data form, without exception, a hyperbolic curve of the same general shape as is obtained in skeletal muscle (4,5,6,12). The data points obtained in this way also represent, to within 1-2 %, the same length of the muscle unit.…”

“…the muscle is released at different times and its ability to shorten against the same small load is recorded at these instances. Experiments on papillary muscles from rabbit (4,5) and fetal human (8) hearts have shown that the ability of the myocardium to produce shortening velocity rises slowly and that maximum activity is reached prior to the peak of the conventional isometric twitch, i.e. 150-250 msec after the stimulus depending on the temperature and stimulation frequency used.…”

“…For instance, there is a phasic contractile response in the heart rather than a steady tetanic tension, which means that all measurements, to be meaningful, have to be related to the particular time during the contraction at which the measurements are carried out. Furthermore, the time course of activation is critically dependent on the length of the muscle, and equally important, the myocardial activity may be affected by length perturbations of the muscle (see below) that may be introduced by the recording technique itself.Perhaps the most relevant index of myocardial activity is provided by the force-velocity relation, and great efforts have been made to define this relationship in isolated heart muscle preparations (2,14,4,5,6,3,9). However, there is still some disagreement as to the true shape of the force-velocity curve in cardiac muscle and the way in which this curve is altered during the activity period and by inotropic interventions.…”