The contractile state of rabbit papillary muscle in relation to stimulation frequency.

Abstract

SUMMIARY1. The relationship between active force and stimulation frequency (0-25--5/sec) was studied at 36-37o C in isolated papillary muscles of the rabbit.2. The muscle's force producing capability at a given frequency was determined as the isometric twitch response to a test stimulus that was applied at various times after a priming period. The optimum contractile response was obtained at an interval of 0-8 sec between the test pulse and the last stimulus of the priming period.3. The optimum contractile response exceeded the steady-state twitch amplitude at all stimulation frequencies higher than 1/sec. While the steady-state twitch response declined at frequencies higher than 4/sec, the optimum contractile response was steadily increased as the stimulation frequency was raised.4. The optimum contractile response was also determined after priming the muscle with a sinusoidal a.c. pulse (field strength, 10 V (r.m.s.)/cm; frequency, 20 c/s; duration, 2-5 sec). The optimum contractile response obtained after a.c. stimulation was 2-2 times greater than the maximal steady-state response. Its absolute value was 67-3 + 6-1 mN/mm2 (mean + sxE. of mean, n = 6).5. The twitch potentiation produced by priming the muscle at a given frequency decayed exponentially in two phases after optimum contractile response had been attained. The time constants of the two phases, determined after a.c. stimulation, were 2-6 ± 0-8 (n = 4) and 92-0 + 13-3 sec (n = 7), respectively.6. The optimum contractile response determined at various stimulation frequencies was linearly related to the fraction of time during which the cell membrane was depolarized (beyond -40 mV) by the action potentials.

The pressure-volume and stiffness-pressure relations in the isolated rabbit left ventricle.

“…The maximum developed pressure at Volmax, 99.9-150.2 mmHg, corresponds to the maximum wall stress, 20.0-37.8 mN/mm2 (28.2+5.6 mN/mm2) at Lmax, which should be the muscle length at Volmax. These estimated values are comparable with the maximum force obtained in the rabbit papillary muscle which is about 25 mN/mm2 at 95 % Lmax at 2 Hz stimulation (EDMAN and JOHANNSSON, 1976). The active stress in the wall muscle was found to be 6.2+2.1 (3.4-9.4) times greater than resting stress at Lmax, that is, the ratio of resting-tototal stress was 15.4±4.8% (10-23 %).…”

Inotropic effects of Ca2+ channel agonist and antagonists in neuraminidase‐treated left atria of rats

“…We previously reported that the peculiar force-frequency relationship in rat atria was differently affected by aand P-adrenoceptor stimulation, especially at a higher range of stimulation rates (Hattori & Kanno, 1984). Complexly intertwined processes, sometimes acting in opposite directions, determine the actual tension developed at a constant stimulation rate (Chapman, 1979), and changing the stimulation interval may unmask a fundamental step ofcellular Ca2" dynamics (Allen et al, 1976;Edman & J6hannsson, 1976).…”

Contractility of the Frog Ventricular Myocardium in Sodium-free Lithium Solution

“…The latter site provides the activator Ca to be released by the following action potential. Such a process has been postulated mainly in the mammalian myocardium, especially in regard to the Woodworth type staircase phenomenon (WOOD et al, 1969;MORAD and GOLDMAN, 1973;EDMAN and JOHANNSON, 1976).…”