Velocity of sarcomere shortening in rat cardiac muscle: relationship to force, sarcomere length, calcium and time.

Abstract

SUMMARY1. The relation between force and velocity was determined in sixteen trabeculae of rat right ventricle as a function of time during a twitch, of sarcomere length and of external Ca2+ concentration, [Ca2+]O. The trabeculae were studied in modifed Krebs-Henseleit solution at 25 'C.2. Force was measured with a semiconductor strain gauge. Sarcomere length was measured with a laser diffraction system. A servomotor system was used in which control could be switched between sarcomere length, muscle length and force.3. Force-velocity relations were derived from load clamps and from contractions in which sarcomere length was initially held constant followed by a quick release and slower release of the sarcomeres at controlled velocity.4. Force-velocity relations were fitted by Hill's equation (Hill, 1938) 6. V0 rose with time during the twitch to a maximum at 25 ms following onset of contraction; P0 was then about 50 % of the maximum that was obtained at 120 ms.7. V0 increased with sarcomere length from zero at a sarcomere length of 1 6 ,sm to a maximum at 1-85,um. Between 1-85 Itm and 2-3 ,m, V0 was constant. At 1-85 Ism, P0 was about 60 % of maximum P0. 8. These results are compatible with the hypothesis that V0 is more sensitive than P0 to the amount of Ca2+ bound to the contractile proteins, and that V0 reaches a maximal value with an amount of Ca2+ bound to the contractile proteins at which P0 has obtained only about 50 % of its maximal value.The experiments were performed at the University of Leiden.

An internal viscous element limits unloaded velocity of sarcomere shortening in rat myocardium.

“…Potential sources of error associated with the use of laser diffraction techniques to measure sarcomere length or V0 are Bragg angle reflection artifacts and inhomogeneity of the muscle preparation. We have recently shown that the possible error in the measurement of sarcomere length and V0 that is due to these artifacts is less than 4 % in our measurements (de Tombe & ter Keurs, 1990c (Daniels et al 1984;. We have previously reported a similar relationship in trabeculae from cat and from hypothyroid rat (de Tombe & ter Keurs, 1991 a, b) showing that the sigmoidal relation between Fo and V0 and…”

“…If peak twitch force at 1 9 pum sarcomere length diminished more than 10 % per hour, the preparation was discarded. The methods employed to measure peak isometric twitch force (Fo), sarcomere length (SL) and unloaded velocity to sarcomere shortening (V0) have been described in detail before (ter Keurs et al 1980b: Daniels et al 1984;de Tombe & ter Keurs. 1990c; cf.…”

“…INTRODUCTION Whether contractile activation has an effect on the unloaded velocity of muscle shortening in skinned skeletal muscle fibres is still a subject of considerable discussion (for a review see Podolin & Ford, 1983). A previous study of the central uniform region of intact right ventricular trabeculae of rat revealed that the unloaded velocity of sarcomere shortening (V1) depends on the concentration of calcium in the bathing medium ([Ca2+]O) up to about 1 mm and then saturates, while isometric twitch force (1o) saturated at about [Ca2+]o = 2-5 mm (Daniels, Noble, ter Keurs & Wohlfart, 1984). That study also revealed that, at saturating levels of [Ca2+]0, V0 is independent of sarcomere length above slack length and that VJ diminishes with decreasing sarcomere length below slack length.…”

Whatever Happened to Measuring Ventricular Contractility in Heart Failure?

“…Early attempts involved the assumption of a model of muscle that had a contractile unit in series with an elastic element, but this was found to be incorrect 2 and a complete characterisation had to wait for the development of the laser diffraction technique for measuring sarcomere length (SL). 3 Sarcomere shortening velocity has an inverse hyperbolic relationship to force (load).…”

“…With an increased contractility (increased Ca 2+ ), velocity increased at all levels of load. 3 An attempt was made to apply the series elastic model to assessing contractility in patients by measuring V max as dP/dt/P, where dP/dt is the rate of rise of left ventricular pressure (LVP) and P is the LVP. Not only was the model wrong, but which P do you take?…”